Wednesday, September 17, 2008

Federal Aviation Regulations (FARs)

From Wikipedia, the free encyclopedia

The Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration (FAA) governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal Regulations (CFR). A wide variety of activities are regulated, such as airplane design, typical airline flights, pilot training activities, hot-air ballooning, lighter than air craft, man-made structure heights, obstruction lighting and marking, and even model rocket launches and model aircraft operation. The rules are designed to promote safe aviation, protecting pilots, passengers and the general public from unnecessary risk. They are also intended to protect the national security of the United States, especially in light of the September 11, 2001 attacks.

Organization

The FARs are organized into sections, called parts due to their organization within the CFR. Each part deals with a specific type of activity. For example, 14 CFR Part 141 contains rules for pilot training schools. The sections most relevant to aircraft pilots and AMTs (Aviation Maintenance Technicians) are listed below. Many of the FARs are designed to regulate certification of pilots, schools, or aircraft rather than the operation of airplanes. In other words, once an airplane design is certified using some parts of these regulations, it is certified regardless of whether the regulations change in the future. For that reason, newer planes are certified using newer versions of the FARs, and in many aspects may be thus considered safer designs.

  • Part 1 – Definitions and Abbreviations
  • Part 13 – Investigation and Enforcement Procedures
  • Part 21 – Certification Procedures for Products and Parts
  • Part 23 – Airworthiness Standards: Normal, Utility, Acrobatic and Commuter Airplanes
  • Part 25 – Airworthiness Standards: Transport Category Airplanes
  • Part 27 – Airworthiness Standards: Normal Category Rotorcraft
  • Part 29 – Airworthiness Standards: Transport Category Rotorcraft
  • Part 33 – Airworthiness Standards: Aircraft Engines
  • Part 34 – Fuel Venting and Exhaust Emission Requirements for Turbine Engine Powered Airplanes
  • Part 35 – Airworthiness Standards: Propellers
  • Part 39 – Airworthiness Directives
  • Part 43 – Maintenance, Preventive Maintenance, Rebuilding, and Alteration
  • Part 45 – Identification and Registration Marking
  • Part 47 – Aircraft Registration
  • Part 61 – Certification: Pilots, Flight Instructors, and Ground Instructors
  • Part 65 – Certification: Airmen Other Than Flight Crewmembers
  • Part 67 – Medical Standards and Certification
  • Part 71 – Designation of Class A, Class B, Class C, Class D, and Class E Airspace Areas; Airways; Routes; and Reporting Points
  • Part 73 – Special Use Airspace
  • Part 91 – General Operating and Flight Rules
  • Part 97 – Standard Instrument Approach Procedures
  • Part 101 – Moored Balloons, Kites, Unmanned Rockets and Unmanned Free Balloons
  • Part 103 – Ultralight Vehicles
  • Part 105 – Parachute Operations
  • Part 119 – Certification: Air Carriers and Commercial Operators
  • Part 121 – Operating Requirements: Domestic, Flag, and Supplemental Operations
  • Part 125 – Certification and Operations: Airplanes Having a Seating Capacity of 20 or More Passengers or a Payload Capacity of 6,000 Pounds or More
  • Part 133 – Rotorcraft External-Load Operations
  • Part 135 – Operating Requirements: Commuter and On Demand Operations
  • Part 136 – Commercial Air Tours and National Parks Air Tour Management
  • Part 137 – Agricultural Aircraft Operations
  • Part 141 – Flight Schools
  • Part 142 – Training Centers
  • Part 145 – Repair Stations
  • Part 147 – Aviation Maintenance Technicians Schools
  • Part 183 – Representatives of The Administrator

Regulations of interest

The FARs are comprised of tens of thousands of separate sections, many of which have large numbers of researchers using them on any given day. A few of the regulations particularly interesting to laypersons, relevant to current political issues, or of historical interest are listed below.

Part 23

Part 23 contains airworthiness standards for airplanes in the normal, utility, acrobatic, and commuter categories. It dictates the standards required for issuance and change of type certificates for airplanes in these categories. The Maximum Takeoff Weight of an airplane in the normal, utility or acrobatic category cannot exceed 12,500 lb. The Maximum Takeoff Weight of an airplane in the commuter category cannot exceed 19,000 lb.

This Part has a large number of regulations to ensure airworthiness in areas such as performance, stability, controllability, and safety mechanisms, how the seats must be constructed, oxygen and air pressurization systems, fire prevention, escape hatches, flight management procedures, flight control communications, emergency landing procedures, and other limitations, as well as testing of all the systems of the aircraft. It also determines special aspects of aircraft performance such as stall speed (for single engine airplanes - not more than 61 knots), rate of climb (not less than 300 fpm), take off speed (not less than 1.2 x Vs1), weight of each pilot and passenger (170 lb for airplanes in the normal and commuter categories, and 190 lb for airplanes in the acrobatic and utility categories).

The Cessna 177 and Cirrus SR20 are well-known airplanes types that were certificated to FAR Part 23.

Most of the Federal Aviation Regulations, including Part 23, commenced on February 1, 1965. Prior to that date, airworthiness standards for airplanes in the normal, utility and acrobatic categories were promulgated in Part 3 of the US Civil Air Regulations. Many well-known types of light airplane are type certificated to CAR Part 3, even though they remained in production after 1965. For example, the Cessna 150 and Piper Cherokee are type certificated to CAR Part 3.

Part 25

This Part contains airworthiness standards for airplanes in the transport category. The majority of airplanes up to 12,500 lb Maximum Takeoff Weight are type certificated in the normal, utility or acrobatic categories so most airplanes certificated to Part 25 have Maximum Takeoff Weights greater than 12,500 lb, although there is no lower weight limit.

The Boeing 737 and later types, and Airbus A300 series, are well-known airplane types that were certificated to FAR Part 25.

Most of the Federal Aviation Regulations, including Part 25, commenced on February 1, 1965. Prior to that date, airworthiness standards for airplanes in the transport category were promulgated in Part 4b of the US Civil Air Regulations. The Boeing 707 and 727 are two well-known airplane types that were certificated to CAR Part 4b.

Part 27

This Part contains airworthiness standards for rotorcraft in the normal category. Rotorcraft up to 7,000 lb Maximum Takeoff Weight and 9 or fewer passengers are type certified in this Part.

Examples of rotorcraft certified in this Part are the Schweizer 300 and the Bell 429.

Part 29

This Part contains airworthiness standards for rotorcraft in the transport category. Rotorcraft with more than 20,000 lb Maximum Takeoff Weight and 10 or more passengers must be certified to Category A standards.

An example of rotorcraft certified in this Part is the Sikorsky S-92.

Part 91

Section 91.3(b)

This regulation states that the pilot-in-command is the party directly responsible for, and is the final authority as to, an aircraft being operated.

Additionally, this regulation states that in an emergency requiring immediate action, the pilot-in-command may deviate from any regulation to the extent required to handle the emergency.

Section 91.145

Example of a presidential TFR surrounding Charleston, South Carolina.
Example of a presidential TFR surrounding Charleston, South Carolina.

This section describes a Temporary Flight Restriction (TFR). A TFR is a geographically-limited, short-term, airspace restriction, typically in the United States. Temporary flight restrictions often encompass major sporting events, natural disaster areas, air shows, space launches, and Presidential movements. Before the September 11, 2001 attacks, most TFRs were in the interest of safety to flying aircraft with occasional small restrictions for Presidential movements. Since 9/11, TFRs have been routinely used to restrict airspace for 30 nautical miles around the President, with a 10-nautical-mile (20 km) radius no-fly zone for non-scheduled flights. They are also available to other important people such as presidential and vice-presidential candidates (though Senator John Kerry, a pilot, declined TFRs during the 2004 election).[1] TFRs are deeply unpopular with pilots in the general aviation sector. Large Presidential TFRs frequently close off not only the airport Air Force One is using but nearby airports as well.[2] Others, including the Transportation Security Administration, argue that they are necessary for national security.[3]

Private, Commuter, and Commercial

For pilots, there is an important distinction in the parts that address classes of flight. These parts do not distinguish type of aircraft, but rather type of activity done with the aircraft. Regulations for commuter and commercial aviation are far more intensive than those for general aviation, and specific training is required; flight schools will often designate themselves as "Part 61" or "Part 141," for example.

Part 61 is certification for private pilots, flight instructors and ground instructors.

Part 91 is general operating rules for all aircraft. Part 91, Subpart (K), prescribes operating rules for fractional ownership programs.

Part 141 is a more intensive set of standards for pilot training, based on FAA syllabus and other standards.

Part 133 is external load (helicopter) operations.

Part 135 is a set of rules with more stringent standards for commuter and on demand operations.

Part 121 is scheduled air carrier (commercial aviation).

See also

References

  1. ^ "No TFRs for Kerry campaign". Aircraft Owners and Pilots Association Online (2004-08-03). Retrieved on 2008-08-23.
  2. ^ Temporary flight restrictions: President Bush travels to Rhode Island
  3. ^ Thurber, Matt (2003-02-01). "Meet Big Brother". Aviation Maintenance. Access Intelligence, LLC. Retrieved on 2008-08-23.

External links

Tuesday, September 16, 2008

Pilot report

From Wikipedia, the free encyclopedia

A pilot report or PIREP is a report of actual weather conditions encountered by an aircraft in flight. This information is usually relayed by radio to the nearest ground station. The message would then be encoded and relayed to other weather offices and air traffic service units.

Although the actual form used to record the PIREP may differ from one country to another, the standards and criteria will remain almost the same. At a minimum the PIREP must contain a header, aircraft location, time, flight level, aircraft type and one other field.

The term PIREP is used, also, to describe a defect report entered into an aircraft's technical log by an aircraft's flight crew (in contrast to a defect report entered by maintenance crews).

Included data

Mandatory

  • UA or UUA used to identify the PIREP as routine or urgent.
  • /OV location of the PIREP, in relation to a NAVAID, an aerodrome or geographical coordinates
  • /TM time the PIREP was received from the pilot. Coordinated Universal Time.
  • /FL flight level or altitude above sea level at the time the PIREP is filed. Essential for turbulence and icing reports.
  • /TP aircraft type. Essential also for turbulence and icing reports.

Optional (at least one is required)

Body

The message identifier "UA" is used when the PIREP contains non-hazardous weather information. If the PIREP contains a report of a tornado, funnel cloud, waterspout, severe turbulence, severe icing, hail, or a low level wind shear hazard, the identifier "UUA" would be used.

The location (/OV) can be reported in one of three ways: as a direction and distance from a navigation aid (NAVAID), as a direction and distance from an airport, or as the latitude and longitude of the aircraft.

The time (/TM) used is the UTC time that the PIREP is reported.

The flight level (/FL) is reported as either a three digit value that indicates the altitude of the aircraft above sea level in hundreds of feet or can one of three abbreviations: DURD (during descent or on approach), DURC (during climb or after takeoff) and UNKN (unknown).

Aircraft type (/TP) will be the approved ICAO designator, or UNKN if not reported.

Sky cover (/SK) is used to report the cloud layer amounts and the height of the cloud base. The tops of the cloud layers can also be included, as can more than one layer of cloud. Heights are in hundreds of feet above sea level and are three digits. Abbreviations used in this group are "CLR" (clear), "FEW" (few), "SCT" (scattered), "BKN" (broken) and "OVC" (overcast).

Temperature (/TA) is the air temperature in whole degrees Celsius as a two-digit value, with negative temperatures preceded by a minus (-) sign.

Wind velocity (/WV) must contain both the wind speed and direction. Direction is reported as a three-digit value in whole degrees true and the wind speed in knots also in three digits.

Turbulence (/TB) and the intensity are reported in a PIREP based on the aircraft and occupants reaction to the turbulence. The altitude of the turbulence should be included using three-digit groups. When the top or the base of the turbulence is unknown then the abbreviation BLO (below) or ABV (above) should be used. Turbulence should be reported as LGT (light), MDT (moderate) or SVR (severe). Clear-air turbulence is reported as CAT.

Icing (/IC) is reported by type and the intensity or rate of accretion. The type of ice is reported as "CLR" (clear), "RIME", or "MXD" (mixed). The intensity is reported as "TR" (trace), "LGT" (light), "MDT" (moderate), and "SVR" (severe).

Remarks (/RM) report on other weather conditions that are not covered in the rest of the PIREP may include such things as icing in precipitation, thunderstorms, St Elmo's fire and frontal conditions. There are many other types of weather conditions that could be reported in a PIREP.

The above explanation of PIREP fields is Canadian. The United States version may also include a weather (/WX) group; and other countries may use other groups and measurements.

Examples of PIREPs

These examples are taken from the Canadian MANOBS (Manual of Surface Weather Observations) published by Environment Canada.

UACN10 CYQT 192128
YZ WG
UA /OV YSP 090025 /TM 2120 /FL050 /TP BE99 /SK 020BKN040 110OVC /TA -14 /WV 030045 /TB MDT CAT 060-080 /IC LGT RIME 020-040 /RM LGT FZRA INC

Decoded is
Routine Upper Air, Aircraft report from Thunder Bay, Ontario issued at 2128 UTC on the 19th
YZ is Toronto and WG is Winnipeg. This is the Flight Information Region where the PIREP was issued
Aircraft observation was 25 nautical miles (46 km) east (090 degrees magnetic) of the Marathon, Ontario VOR/DME at 2120 UTC. The aircraft was at 5,000 ft (1,524 m)and is a Beech 99. The clouds were broken at 2,000 ft (610 m) AMSL with tops at 4,000 ft (1,219 m) and an overcast layer at 11,000 ft (3,353 m) AMSL. The temperature is -14 Celsius and the winds are from the NE (030 degrees true) at 45 knots (83 km/h). There is moderate clear air turbulence between 6,000 ft (1,829 m) and 8,000 ft (2,438 m). There is light rime icing between 2,000 ft (610 m) and 4,000 ft (1,219 m). Note this would indicate that the icing is picked up in the cloud. The remarks section says that light freezing rain was encountered in the cloud.

UACN10 CYXU 032133
YZ
UA /OV YUX 09010 /TM 2120 /FL030 /TP C172 /TB MDT /RM MDT TURB BLO 050 CYKF CYXU

Routine message from London Ontario, issued at 2133 UTC on the 3rd
The Flight Information Region is Toronto
The aircraft was 10 nautical miles (18.5 km) east (090 degrees true) of the London VOR at 2120 UTC. The aircraft was at 3,000 ft (914 m) and was a Cessna 172. The pilot reported moderate turbulence. Then in remarks went on to say that the turbulence was below 5,000 ft (1,524 m) between Kitchener/Waterloo and London.

Soliciting PIREPs

In the US, Air Traffic Controllers are required to solicit PIREPs upon request of other facilities or pilots, or when any of the following conditions exists or is forecast in their area:

At least once hourly, terminal controllers must obtain a descent/climb-out PIREP, including cloud information and other related phenomena.

See also

  • AMDAR -- aircraft meteorological data relay -- is a WMO FM-42 code for an automatic meteorological report from an aircraft.
  • AIREP -- one aircraft report encoded according to the AFMAN manual 15-124 pages 32 through 35.
  • ACARS -- Aircraft Communication Addressing and Reporting System
  • RECCO -- Reconnaissance Observation
  • SIGMET
  • AIRMET

External links

Icing conditions

From Wikipedia, the free encyclopedia

In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine as carburetor icing. Inlet icing is another engine-related danger, often occurring in jet aircraft. These icing phenomena do not necessarily occur together. Many aircraft are not certified for flight into known icing—icing conditions which are certain to exist based on pilot reports, observations, and forecasts.

Ice forms when air containing water vapour is cooled below the freezing point of water, which becomes higher with decreasing pressure; ice is likely to form wherever there is reduced pressure, including wings and empennage (where it is known as structural icing), carburetors, pitot tubes, and engine inlets. Ice gradually builds as long as icing conditions exist and if left unchecked results in dangerous conditions. Ice adds to an aircraft's weight and disrupts airflow, increasing the stall speed. In engines, carburetor ice and inlet ice can lead to reduced power or complete engine failure.

Types of structural ice

  • Glaze ice is often clear and smooth. Supercooled water droplets, or freezing rain, strike a surface but do not freeze instantly. Often "horns" or protrusions are formed and project into the airflow.
  • Rime ice is rough and opaque, formed by supercooled drops rapidly freezing on impact. Forming mostly along an airfoil's stagnation point, it generally conforms to the shape of the airfoil.
  • Mixed ice is a combination of clear and rime ice.
  • Runback ice is the result of water freezing on unprotected surfaces. Often forming behind deicing boots or heated leading edges, it was a factor in the crash of American Eagle Flight 4184.

Icing prevention and removal

Several methods exist to reduce the dangers of icing. The first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical.

If icing is expected, deicing fluid can be applied to the aircraft before flight to prevent ice from forming, or from sticking to the surface if it does form. Airliners are often deiced before departure, and the deicing fluid prevents ice buildup for a short time after.

Another common approach is to include heating elements along the leading edges of wings and tailplanes, which can keep the surface temperature above freezing point. A weeping wing has hundreds of small holes in the leading edges and releases deicing fluid on demand to clear the entire wing of ice. Older aircraft and propellers are often equipped with pneumatic deicing boots

Carburetor heat is applied to carbureted engines to prevent and clear icing. Fuel-injected engines are not susceptible to carburetor icing but can suffer from blocked inlets. In these engines an alternate air source is often available.

Note there is a difference between deicing and anti-icing.

See also

Deicing

From Wikipedia, the free encyclopedia
(

De-icing is the process of removing ice from a surface.

Anti-icing is the process of preventing ice from forming on a surface.

De-icing can be accomplished by mechanical methods (scraping); through the application of heat; by use of chemicals, known as icemelters, designed to lower the freezing point of water (various salts or alcohols); or by a combination of these different techniques.

Aircraft

See also: ice protection system

When there are freezing conditions and precipitation, it is critical that an aircraft be de-iced. Failure to do so means the surface of the aircraft's wings will be too rough to provide for the smooth flow of air and thereby greatly degrading the ability of the wing to generate lift, possibly resulting in a crash. If large pieces of ice separate once the aircraft is in motion, they can be ingested into turbine engines or impact moving propellers and cause catastrophic failure. Thick ice can also lock up the control surfaces and prevent them from moving properly. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to dip below the freezing point.

De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of contamination caused by ice or snow.

De-icing on the ground is usually done by spraying aircraft with a deicing fluid such as monopropylene glycol, similar to ethylene glycol antifreeze used in some automobile engine coolants. Ethylene glycol is still in use for aircraft deicing in some parts of the world, but Monopropylene glycol is more common because it is classified as non-toxic, unlike ethylene glycol. Nevertheless, it still must be used with a containment system to capture all of the used liquid, so that it cannot seep into the ground and streams. Even if it is classified as non-toxic, it still has negative effects in nature, as it uses oxygen as it breaks down, causing other life to suffocate. (In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.) Many airports successfully recycle used deicing fluid, separating out water and solid contaminants in order to be able to reuse the fluid.

Though there are several different formulations of deicing fluid, they fall into two basic categories: Heated glycol diluted with water for deicing and snow/frost removal, also referred to as "Newtonian fluids", and unheated, undiluted glycol that has been thickened (imagine half-set gelatin), also referred to as "Non-Newtonian fluids", that is applied as an agent to retard the future development of ice or to prevent falling snow or sleet from accumulating. In some cases both types of fluid are applied, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is referred to as "a two-step procedure".

Inflight ice buildups are most frequent on the leading edges of the wings, tail and engines (including the propellors or fan blades). Lower speed aircraft frequently use pneumatic boots on the leading edges of wings and tail to affect de-icing in flight. The rubber coverings are periodically inflated, causing ice to crack and flake off in the slipstream. Once the system is activated by the pilot, the inflation/deflation cycle is automatically controlled. In the past, it was thought such systems can be defeated if they are inflated too soon; that the pilot must allow a fairly thick layer of ice to form before inflating the boots. More recent research shows “bridging” does not occur with any modern boots.[1]

B-17 bomber. The black strips on the leading edges of the tail, stabilizers and wing are decicer boots made of rubber.
B-17 bomber. The black strips on the leading edges of the tail, stabilizers and wing are decicer boots made of rubber.

Some aircraft may also use electrically heated resistive elements embedded in a rubber sheet cemented to the leading edges of wings and tail surfaces, propeller leading edges, and helicopter rotor blade leading edges. Such systems usually operate continuously. When ice is detected, they first function as de-icing systems, then as anti-icing systems for the duration of flight in icing conditions. Some aircraft use chemical de-icing systems which pump antifreeze such as alcohol or propylene glycol through small holes in the wing surfaces and at the roots of propeller blades, causing the ice to melt and making the surface inhospitable to further ice formation. A fourth system, developed by the National Aeronautics and Space Administration, detects ice on the surface by sensing a change in resonance frequency. Once an electronic control module has determined that ice has formed, a large current spike is pumped into the transducers to generate a sharp mechanical shock, cracking the ice layer and causing it to be peeled off by the slipstream.

Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled off the powerplants and is ducted into a cavity just under the surface to be anti-iced. The warm air heats the surface up to a few degrees above zero, preventing ice from forming on that surface. The system may operate completely autonomously, switching itself on and off as the aircraft enters and leaves icing conditions.

Infrared deicing

Infrared is the transmission of energy by means of electromagnetic waves or rays. Infrared is invisible and travels at the speed of light in straight lines from the heat source (the emitter) to all surfaces and objects (the receivers) without significantly heating the space (air) through which they pass. When infrared waves strike an object, they release their energy as heat. This heat is then either absorbed or reflected by the cooler surface. Infrared energy is continually exchanged between "hot" and "cold" surfaces until all surfaces have reached the same temperature (equilibrium). The colder the surfaces, the more effective the infrared transfer from the emitter. This heat transfer mechanism is substantially faster than conventional heat transfer modes used by conventional deicing (convection and conduction) due to the cooling effect of the air on the deicing fluid spray.

References

See also

Ice protection system

From Wikipedia, the free encyclopedia

Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft flight surfaces while in flight. The effects of ice accretion on an aircraft can cause loss of control, resulting in a catastrophic flight event.

Types of ice protection systems

The pneumatic boot is a rubber device attached to a wing's leading edge, invented by the Goodrich Corporation (previously known as B.F. Goodrich) in 1923. Portions of the boot are inflated to break ice off the boot, de-icing the wing. Rubber boots are used on jets and propeller driven aircraft.

A bleed air system is used by jet aircraft to keep flight surfaces above the freezing temperature required for ice to accumulate (called anti-icing). The hot air is "bled" off the jet engine into tubes routed through wings, tail surfaces, and engine inlets.

Electrical thermal systems use electricity to heat the protected surface. The electric heaters are usually flexible enough to use as anti-icers or de-icers. As a de-icer, the heater melts the ice, the ice no longer sticks to the surface due to aerodynamic forces. As an anti-icer, the heater keeps the surface to the point that the ice does not form.

Electro-mechanical systems use a mechanical force to knock the ice off the flight surface. Typically, actuators are installed underneath the skin of the structure. The actuator is moved to induce a shock wave in the protected surface to dislodge the ice.

A weeping wing system uses a liquid (such as ethylene glycol) to coat the surface and prevent ice from accumulating.

Wing and Flight Surfaces

Ice accumulates on the leading edge of wings, tails, and horizonal stabilizers as an aircraft flies through a cloud containing super-cooled water droplets. Ice forms because super-cooled water droplets in clouds contact the aircraft. This contact imparts energy into the droplet and causes it to change from liquid water to solid water (ice). As the ice layer grows (accretes), it affects the airflow over the affected surface. If the layer grows large enough, it can create lift or handling problems for the aircraft.

Rotary Surfaces

Ice can also accumulate on helicopter rotor blades and aircraft propellers. The accretion causes weight and aerodynamic imbalances that are amplified due to the rapid rotation of the propeller or rotor.

Engine Inlets

Ice accreting on the leading edge (lip) of engine inlets causes flow problems and can lead to ice ingestion. In turbofan engines, laminar airflow is required at the face of the fan. Because of this, most engine ice protection systems are anti-ice systems (prevent build up).

Related Companies

Related accidents

Monday, September 15, 2008

Conventional landing gear

From Wikipedia, the free encyclopedia

The Piper Super Cub is a popular taildragger aircraft.
The Piper Super Cub is a popular taildragger aircraft.

Conventional landing gear describes an undercarriage arrangement consisting of two main weight-bearing wheels forward of the aircraft's centre of gravity, the remaining weight being supported by a tail wheel or skid. The term taildragger is aviation jargon for an aircraft with conventional undercarriage.

History and current use

In early aircraft, a tail skid made of metal or wood was used in place of the tail wheel. In modern aircraft, a small, articulated wheel assembly is attached to the rearmost part of the airframe.

For many years aircraft with tricycle landing gear have been more popular than those with conventional undercarriage because of ease and safety during landing. Tailwheel aircraft are, however, still preferred for specialty applications such as aerobatics, agriculture, and back-country flying; so some taildraggers are still commercially manufactured. Other tail-wheel aircraft are still made in high numbers by individuals who build aircraft from plans or kits. Examples of such tailwheel aircraft are the Vans RV-4, kitfox, and the Murphy Moose.

Several after-market modification companies offer kits to convert many popular nose-wheel equipped aircraft to conventional landing gear. Aircraft for which kits are available include the Cessna 150, Cessna 172 and Piper PA-22 Tripacer.

Some ski-planes also have this "conventional" arrangement for their undercarriage: Two main skis and a tail ski.

Three-point and wheeler landings

A Cessna 150 converted to taildragger configuration by installation of an after-market modification kit.
A Cessna 150 converted to taildragger configuration by installation of an after-market modification kit.
A taildragger by Jodel: the 1965 D140C Mousquetaire
A taildragger by Jodel: the 1965 D140C Mousquetaire

Tailwheel aircraft are landed in two distinct styles. One is referred to as a "three pointer", referring to all three wheels contacting the ground at the same time. Another type of landing taildraggers use is called a "wheeler" or "wheel landing", where the plane is flown onto the main wheels in a more level attitude, until the airspeed bleeds off and the tail stops flying.

Characteristics

Taildraggers will behave just like tricycle gear aircraft when flying. Taildraggers, though, have ground handling characteristics that can be challenging. In a taildragger, the center of mass is located behind the front landing gear, which is an inherently less stable configuration than a tricycle landing gear. If the pilot is not able to keep the longitudinal axis of the aircraft aligned with the runway, then the center of mass of the aircraft will tend to rotate around the landing gear (the highest point of drag, or friction while on the ground) until the center of mass is in front of the highest point of drag. This results in an abrupt turn known as a ground loop, typically causing damage. There is no doubt that more skill is needed to take off and land a tailwheel aircraft, and some translate this to mean that tailwheel aircraft are harder to operate. However, the added skill is a benefit to any pilot, and some nosewheel pilots develop similar levels of skill without ever flying a tailwheel aircraft.

The tailwheel landing gear configuration has benefits when landing in high winds and rough, unimproved runways in that in such applications the sometimes fragile front landing gear could be easily damaged as it is often relied upon to counter any transverse loading, or side loads. A tailwheel aircraft offsets side loads and maintains direction by using control surfaces, which along with the leverage from the structure of the aircraft, are made to handle such loads. In high crosswinds, as a tailwheel aircraft slows and the forces exerted by the control surfaces decrease it may be necessary to simply let the tailwheel aircraft rotate around and point nose-first into the wind. This tends to alarm untrained nosewheel pilots. The use of a locking tailwheel may reduce this weather vane tendency, but it once again adds transverse loads to parts of the aircraft structure that are not typically designed for such loads. Many nosewheel landing gear struts are structurally little more than props to hold up the front of the aircraft, and do not deal well with sideloads. It should be noted that skilled pilots of tricycle landing gear use techniques very similar to tailwheel pilots in order to avoid damaging the front landing gear in challenging situations. Also, single engine tricycle gear aircraft typically have very little clearance between the front propeller and the ground, making operations on rough or unimproved airfields problematic. This can be countered on tricycle gear aircraft by modifying the landing gear to accommodate larger tires and wheels.

Douglas DC-3, a taildragger airliner
Douglas DC-3, a taildragger airliner

While on the ground, visibility over the nose may be reduced due to the pitch attitude of the aircraft, depending on the model and geometry of specific aircraft. But, not all taildraggers have poor forward visibility on the ground. For example, the Cessna 170 taildragger has better visibility over the nose than later model tricycle gear Cessna 172s due to the increased size of the instrument panel in the later model 172s.

Taildragger versions of the same aircraft often have higher useful loads and cruise speeds due to the elimination of the nose gear and its associated drag and weight.

There are experienced tailwheel pilots who insist that the added challenges of a tailwheel aircraft arise largely from a lack of skill that results from poor training when transitioning from a tricycle landing gear aircraft. Most pilots now learn to fly in tricycle gear aircraft (e.g., Cessna 152 and Cessna 172) and only later transition to taildraggers. Since the number of factory-built general aviation aircraft with a tailwheel is fairly low, the numbers of instructors experienced in this type of aircraft are also limited.

Friday, September 12, 2008

Deadstick landing

From Wikipedia, the free encyclopedia

A deadstick landing, also called a dead-stick landing or forced landing, occurs when an aircraft loses all of its propulsive power and is forced to land. The term is a misnomer, as the flight controls in the majority of aircraft are either fully or partially functional, even with no engine power. So it is not the “stick” (flight control actuator) that is “dead”, but rather the engine(s).

All fixed-wing aircraft have some capability to glide with no engine power; that is, they do not sink straight down like a stone, but rather continue to glide horizontally while descending. After a loss of power, the pilot’s goal is to fly the descending aircraft to the most suitable landing spot within gliding distance, and then land with the least amount of damage possible. The area open for potential landing sites depends on the original altitude and the engine-out gliding capabilities of the aircraft.

The success of the deadstick landing largely depends on the availability of suitable landing areas. A competent pilot gliding a relatively light, slow plane to a flat field or runway should result in an otherwise normal landing. A heavier, faster aircraft or a plane gliding into mountains and/or trees could result in substantial damage.

There have been several instances of large jet airliners successfully doing a deadstick landing:

  1. Gimli Glider: An Air Canada Boeing 767, ran out of fuel enroute from Montreal to Edmonton. The plane had insufficient glide range to complete a diversion to Winnipeg, but the crew managed to make a successful dead stick landing at an abandoned airfield at Gimli, where a car rally was underway on the runway.
  2. Air Transat Flight 236: An Air Transat Airbus A330 also ran out of fuel while flying across the North Atlantic, from Toronto to Lisbon. The crew glided the aircraft over 100 miles and made a deadstick landing at a military air base in the Azores.
  3. Hapag-Lloyd Flight 3378: An A310 enroute from Greece to Germany experienced a landing gear problem and subsequent fuel depletion, resulting in a deadstick landing in Vienna.

With helicopters, a forced landing involves autorotation, since the helicopter glides by allowing its rotor to spin freely during the descent thus generating lift.

When a pilot makes an emergency landing of an aircraft that has some or all of its propulsive power still available, it is known as a precautionary landing, or practiced force landing.

See also

Thursday, September 11, 2008

Aircraft engine controls

From Wikipedia, the free encyclopedia

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.

Basic Controls and Indicators

  • Master Switch - Most often actually two separate switches, the Battery Master and the Alternator Master. The Battery Master activates a relay (sometimes called the battery contactor) which connects the output of the battery to the aircraft's main electrical bus. The alternator master activates the alternator by applying power to the alternator field circuit. These two switches provide electrical power to all the systems in the aircraft.
  • Throttle - Sets the desired engine power level. The throttle controls the volume of fuel/air mixture delivered to the cylinders.
  • Pitch Control - Adjusts the Constant Speed Unit, which in turn adjusts the propeller pitch & regulates the engine load as necessary to maintain the set R.P.M.
  • Mixture Control - Sets the amount of fuel added to the intake airflow. At higher altitudes the air pressure (and therefore the oxygen level) declines so the fuel volume must also be reduced to give the correct air/fuel mixture. This process is known as "leaning".
  • Ignition Switch - Activates the magnetos by opening the grounding or 'p-lead' circuit; with the p-lead ungrounded the magneto is free to send its high-voltage output to the spark plugs. In most aircraft the ignition switch also applies power to the starter motor during engine start. In piston aircraft engines, the battery does not generate the spark for combustion. This is accomplished using devices called magnetos. Magnetos are connected to the engine by gearing. When the crankshaft turns, it turns the magnetos which mechanically generate voltage for spark. In the event of an electrical failure, the engine will continue to run. The Ignition Switch has the following positions:
    1. Off - Both magnetos are connected to electrical ground.
    2. Right - The right magneto is connected to its spark plugs. The left magneto is grounded.
    3. Left - The left magneto is connected to its spark plugs. The right magneto is grounded.
    4. Both - Both magnetos are connected to their spark plugs. This is the normal operating configuration.
    5. Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over.
  • Tachometer - A gauge to indicate engine speed in revolutions per minute (RPM) or percentage of maximum
  • Manifold Pressure Gauge – Used to measure the pressure in the intake manifold.
  • Oil Temperature Gauge - Indicates the engine oil temperature
  • Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant.
  • Exhaust Gas Temperature Gauge – Indicates the temperature of the exhaust gas just after combustion. Used to set the fuel/air mixture (leaning) correctly.
  • Cylinder Head Temperature Gauge - Indicates the temperature of at least one of the cylinder heads. Used to set the fuel/air mixture.
  • Carburetor Heat Control - Controls the application of heat to the carburetor venturi area to remove or prevent the formation of ice in the throat of the carburetor as well as bypassing the air filter in case of impact icing.
  • Alternate Air - Bypasses the air filter on a fuel-injected engine

Fuel

  • Fuel Primer Pump - A manual pump to add a small amount of fuel at the cylinder intakes to assist in starting a cold engine. Fuel injected engines do not have this control. For fuel injected engines, a fuel boost pump is used to prime the engine prior to start.
  • Fuel Quantity Gauge - Indicates the amount of fuel remaining in the identified tank. One per fuel tank.
  • Fuel Select Valve - Connects the fuel flow from the selected tank to the engine.

If the aircraft is equipped with a fuel pump:

  • Fuel Pressure Gauge - Indicates the supply pressure of fuel to the carburetor (or in the case of a fuel injected engine, to the fuel controller.)
  • Fuel Boost Pump Switch - Controls the operation of the auxiliary electric fuel pump to provide fuel to the engine before it starts or in case of failure of the engine powered fuel pump.

Propeller

If the aircraft is equipped with adjustable-pitch or constant-speed propeller:

  • Propeller Control - Sets the desired R.P.M. of the propeller governor, which turn regulates the engine load as necessary to maintain the set R.P.M. Once the R.P.M. is set by the pilot, the governor adjusts engine load by increasing or decreasing propeller pitch by using engine oil pressure to move a piston in the propeller hub.
  • Manifold Pressure Gauge - Indicates the absolute pressure in the engine intake manifold between the throttle and the cylinders.

Cowl

If the aircraft is equipped with adjustable Cowl Flaps:

  • Cowl Flap Position Control - Cowl Flaps are opened during high power/low airspeed operations like takeoff to maximize the volume of cooling airflow over the engine's cooling fins.
  • Cylinder Head Temperature Gauge - Indicates the temperature of all cylinder heads or on a single CHT system, the hottest head. A Cylinder Head Temperature Gauge has a much shorter response time than the oil temperature gauge, so it can alert the pilot to a developing cooling issue more quickly. Engine overheating may be caused by:
    1. Running too long at a high power setting.
    2. Poor leaning technique
    3. Restricting the volume of cooling airflow too much.
    4. Insufficient delivery of lubricating oil to the engine's moving parts.

See also

References

Wednesday, September 10, 2008

Inertial navigation system (INS)

From Wikipedia, the free encyclopedia

An Inertial Navigation System (INS) is a navigation aid that uses a computer and motion sensors to continuously track the position, orientation, and velocity (direction and speed of movement) of a vehicle without the need for external references. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial reference platform, and many other variations.

Overview

An inertial navigation system includes at least a computer and a platform or module containing accelerometers, gyroscopes, or other motion-sensing devices. The INS is initially provided with its position and velocity from another source (a human operator, a GPS satellite receiver, etc.), and thereafter computes its own updated position and velocity by integrating information received from the motion sensors. The advantage of an INS is that it requires no external references in order to determine its position, orientation, or velocity once it has been initialized.

An INS can detect a change in its geographic position (a move east or north, for example), a change in its velocity (speed and direction of movement), and a change in its orientation (rotation about an axis). It does this by measuring the linear and angular accelerations applied to the system. Since it requires no external reference (after initialization), it is immune to jamming and deception.

Inertial-navigation systems are used in many different types of vehicles, including aircraft, submarines, spacecraft, and guided missiles. However, their cost and complexity does place constraints on the environments in which they are practical for use.

Gyroscopes measure the angular velocity of the system in the inertial reference frame. By using the original orientation of the system in the inertial reference frame as the initial condition and integrating the angular velocity, the system's current orientation is known at all times. This can be thought of as the ability of a blindfolded passenger in a car to feel the car turn left and right or tilt up and down as the car ascends or descends hills. Based on this information alone, he knows what direction the car is facing but not how fast or slow it is moving, or whether it is sliding sideways.

Accelerometers measure the linear acceleration of the system in the inertial reference frame, but in directions that can only be measured relative to the moving system (since the accelerometers are fixed to the system and rotate with the system, but are not aware of their own orientation). This can be thought of as the ability of a blindfolded passenger in a car to feel himself pressed back into his seat as the vehicle accelerates forward or pulled forward as it slows down; and feel himself pressed down into his seat as the vehicle accelerates up a hill or rise up out of his seat as the car passes over the crest of a hill and begins to descend. Based on this information alone, he knows how the vehicle is moving relative to itself, that is, whether it is going forward, backward, left, right, up (toward the car's ceiling), or down (toward the car's floor) measured relative to the car, but not the direction relative to the Earth, since he did not know what direction the car was facing relative to the Earth when he felt the accelerations.

However, by tracking both the current angular velocity of the system and the current linear acceleration of the system measured relative to the moving system, it is possible to determine the linear acceleration of the system in the inertial reference frame. Performing integration on the inertial accelerations (using the original velocity as the initial conditions) using the correct kinematic equations yields the inertial velocities of the system, and integration again (using the original position as the initial condition) yields the inertial position. In our example, if the blindfolded passenger knew how the car was pointed and what its velocity was before he was blindfolded, and he is able to keep track of both how the car has turned and how it has accelerated and decelerated since, he can accurately know the current orientation, position, and velocity of the car at any time.

All inertial navigation systems suffer from integration drift: Small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which is compounded into still greater errors in position. This is a problem that is inherent in every open loop control system. The inaccuracy of a good-quality navigational system is normally fewer than 0.6 nautical miles per hour in position and on the order of tenths of a degree per hour in orientation.

Inertial navigation may also be used to supplement other navigation systems, providing a higher degree of accuracy than is possible with the use of any single navigation system. For example, if, in terrestrial use, the inertially tracked velocity is intermittently updated to zero by stopping, the position will remain precise for a much longer time, a so-called zero velocity update.

Control theory in general and Kalman filtering in particular provide a theoretical framework for combining information from various sensors. One of the most common alternative sensors is a satellite navigation radio, such as GPS. By properly combining the information from an INS and the GPS system, the errors in position and velocity are stable GPS/INS.

History

Inertial navigation systems were originally developed for rockets. American rocket pioneer Robert Goddard experimented with rudimentary gyroscopic systems. Dr. Goddard's systems were of great interest to contemporary German pioneers including Wernher von Braun. The systems entered more widespread use with the advent of spacecraft, guided missiles, and commercial airliners.

One example of a popular INS for commercial aircraft was the Delco Carousel, which provided partial automation of navigation in the days before complete flight management systems became commonplace. The Carousel allowed pilots to enter a series of waypoints, and then guided the aircraft from one waypoint to the next using an INS to determine aircraft position. Some aircraft were equipped with dual Carousels for safety.

Inertial navigation systems in detail

INSs have angular and linear accelerometers (for changes in position); some include a gyroscopic element (for maintaining an absolute angular reference).

Angular accelerometers measure how the vehicle is rotating in space. Generally, there's at least one sensor for each of the three axes: pitch (nose up and down), yaw (nose left and right) and roll (clockwise or counter-clockwise from the cockpit).

Linear accelerometers measure how the vehicle is moving in space. Since it can move in three axes (up & down, left & right, forward & back), there is a linear accelerometer for each axis.

A computer continually calculates the vehicle's current position. First, for each of the six degrees of freedom (x,y,z and θ x, θ y and θ z), it integrates the sensed amount of acceleration over time to figure the current velocity. Then it integrates the velocity to figure the current position.

Inertial guidance is difficult without computers. The desire to use inertial guidance in the Minuteman missile and Project Apollo drove early attempts to miniaturize computers.

Inertial guidance systems are now usually combined with satellite navigation systems through a digital filtering system. The inertial system provides short term data, while the satellite system corrects accumulated errors of the inertial system.

An inertial guidance system that will operate near the surface of the earth must incorporate Schuler tuning so that its platform will continue pointing towards the center of the earth as a vehicle moves from place to place.

Basic schemes

Gimballed gyrostabilized platforms

Some systems place the linear accelerometers on a gimbaled gyrostabilized platform. The gimbals are a set of three rings, each with a pair of bearings initially at right angles. They let the platform twist about any rotational axis (or, rather, they let the platform keep the same orientation while the vehicle rotates around it). There are two gyroscopes (usually) on the platform.

Two gyroscopes are used to cancel gyroscopic precession, the tendency of a gyroscope to twist at right angles to an input force. By mounting a pair of gyroscopes (of the same rotational inertia and spinning at the same speed) at right angles the precessions are cancelled, and the platform will resist twisting.

This system allows a vehicle's roll, pitch, and yaw angles to be measured directly at the bearings of the gimbals. Relatively simple electronic circuits can be used to add up the linear accelerations, because the directions of the linear accelerometers do not change.

The big disadvantage of this scheme is that it uses many expensive precision mechanical parts. It also has moving parts that can wear out or jam, and is vulnerable to gimbal lock. The primary guidance system of the Apollo spacecraft used a three-axis gyrostabilized platform, feeding data to the Apollo Guidance Computer. Maneuvers had to be carefully planned to avoid gimbal lock.

Fluid-suspended gyrostabilized platforms

Gimbal lock constrains maneuvring, and it would be beneficial to eliminate the slip rings and bearings of the gimbals. Therefore, some systems use fluid bearings or a flotation chamber to mount a gyrostabilized platform. These systems can have very high precisions (e.g. Advanced Inertial Reference Sphere). Like all gyrostabilized platforms, this system runs well with relatively slow, low-power computers.

The fluid bearings are pads with holes through which pressurized inert gas (such as Helium) or oil press against the spherical shell of the platform. The fluid bearings are very slippery, and the spherical platform can turn freely. There are usually four bearing pads, mounted in a tetrahedral arrangement to support the platform.

In premium systems, the angular sensors are usually specialized transformer coils made in a strip on a flexible printed circuit board. Several coil strips are mounted on great circles around the spherical shell of the gyrostabilized platform. Electronics outside the platform uses similar strip-shaped transformers to read the varying magnetic fields produced by the transformers wrapped around the spherical platform. Whenever a magnetic field changes shape, or moves, it will cut the wires of the coils on the external transformer strips. The cutting generates an electric current in the external strip-shaped coils, and electronics can measure that current to derive angles.

Cheap systems sometimes use bar codes to sense orientations, and use solar cells or a single transformer to power the platform. Some small missiles have powered the platform with light from a window or optic fibers to the motor. A research topic is to suspend the platform with pressure from exhaust gases. Data is returned to the outside world via the transformers, or sometimes LEDs communicating with external photodiodes.

Strapdown systems

Lightweight digital computers permit the system to eliminate the gimbals, creating "strapdown" systems, so called because their sensors are simply strapped to the vehicle. This reduces the cost, eliminates gimbal lock, removes the need for some calibrations, and increases the reliability by eliminating some of the moving parts. Angular rate sensors called "rate gyros" measure how the angular velocity of the vehicle changes.

A strapdown system has a dynamic measurement range several hundred times that required by a gimbaled system. That is, it must integrate the vehicle's attitude changes in pitch, roll and yaw, as well as gross movements. Gimballed systems could usually do well with update rates of 50 to 60 updates per second. However, strapdown systems normally update about 2000 times per second. The higher rate is needed to keep the maximum angular measurement within a practical range for real rate gyros: about 4 milliradians. Most rate gyros are now laser interferometers.

The data updating algorithms ("direction cosines" or "quaternions") involved are too complex to be accurately performed except by digital electronics. However, digital computers are now so inexpensive and fast that rate gyro systems can now be practically used and mass-produced. The Apollo lunar module used a strapdown system in its backup Abort Guidance System (AGS).

Strapdown systems are nowadays commonly used in commercial and tactical applications (arcraft, missiles, etc). However they are still not widespread in applications where superb accuracy is required (like submarine navigation or strategic ICBM guidance).

Motion-based alignment

The orientation of a gyroscope system can sometimes also be inferred simply from its position history (e.g., GPS). This is, in particular, the case with planes and cars, where the velocity vector usually implies the orientation of the vehicle body.

For example, Honeywell's Align in Motion[1] is an initialization process where the initialization occurs while the aircraft is moving, in the air or on the ground. This is accomplished using GPS and an inertial reasonableness test, thereby allowing commercial data integrity requirements to be met. This process has been FAA certified to recover pure INS performance equivalent to stationary align procedures for civilian flight times up to 18 hours. It avoids the need for gyroscope batteries on aircraft.

Vibrating gyros

Less-expensive navigation systems, intended for use in automobiles, may use a Vibrating structure gyroscope to detect changes in heading, and the odometer pickup to measure distance covered along the vehicle's track. This type of system is much less accurate than a higher-end INS, but it is adequate for the typical automobile application where GPS is the primary navigation system, and dead reckoning is only needed to fill gaps in GPS coverage when buildings or terrain block the satellite signals.

Hemispherical Resonator Gyros ("Brandy Snifter Gyros")

If a standing wave is induced in a globular resonant cavity (i.e. a brandy snifter), and then the snifter is tilted, the waves tend to continue oscillating in the same plane of movement - they don't fully tilt with the snifter. This trick is used to measure angles. Instead of brandy snifters, the system uses hollow globes machined from piezoelectric materials such as quartz. The electrodes to start and sense the waves are evaporated directly onto the quartz.

This system has almost no moving parts, and is very accurate. However it is still relatively expensive due to the cost of the precision ground and polished hollow quartz spheres.

Although successful systems were constructed, and an HRG's kinematics appear capable of greater accuracy, they never really caught on.[citation needed] Laser gyros were just more popular.[citation needed]

The classic system is the Delco 130Y Hemispherical Resonator Gyro, developed about 1986. See also [1] for a picture of an HRG resonator.

Quartz rate sensors

This system is usually integrated on a silicon chip. It has two mass-balanced quartz tuning forks, arranged "handle-to-handle" so forces cancel. Aluminum electrodes evaporated onto the forks and the underlying chip both drive and sense the motion. The system is both manufacturable and inexpensive. Since quartz is dimensionally stable, the system can be accurate.

As the forks are twisted about the axis of the handle, the vibration of the tines tends to continue in the same plane of motion. This motion has to be resisted by electrostatic forces from the electrodes under the tines. By measuring the difference in capacitance between the two tines of a fork, the system can determine the rate of angular motion.

Current state of the art non-military technology (2005) can build small solid state sensors that can measure human body movements. These devices have no moving parts, and weigh about 50 grams.

Solid state devices using the same physical principles are used to stabilize images taken with small cameras or camcorders. These can be extremely small (≈5 mm) and are built with MEMS (Microelectromechanical Systems) technologies.

MHD sensor

Sensors based on magnetohydrodynamic principles can be used to measure angular velocities and are described in "MHD sensor".

Laser gyros

Laser gyroscopes were supposed to eliminate the bearings in the gyroscopes, and thus the last bastion of precision machining and moving parts.

A laser gyro splits a beam of laser light into two beams in opposite directions through narrow tunnels in a closed optical circular path around the perimeter of a triangular block of temperature stable cervit glass block with reflecting mirrors placed in each corner. When the gyro is rotating at some angular rate, the distance traveled by each beam becomes different - the shorter path being opposite to the rotation. The phase-shift between the two beams can be measured by an interferometer, and is proportional to the rate of rotation (Sagnac effect).

In practice, at low rotation rates the output frequency can drop to zero after the result of "Back scattering" causing the beams to synchronise and lock together. This is known as a "lock-in, or laser-lock." The result is that there is no change in the interference pattern, and therefore no measurement change.

To unlock the counter-rotating light beams, laser gyros either have independent light paths for the two directions (usually in fiber optic gyros), or the laser gyro is mounted on a piezo-electric dither motor that rapidly vibrates the laser ring back and forth about its input axis through the lock-in region to decouple the light waves.

The shaker is the most accurate, because both light beams use exactly the same path. Thus laser gyros retain moving parts, but they do not move as far.

Pendular accelerometers

Principle of open loop accelerometer.  Acceleration in the upward direction causes the mass to deflect downward.
Principle of open loop accelerometer. Acceleration in the upward direction causes the mass to deflect downward.

The basic, open-loop accelerometer consists of a mass attached to a spring. The mass is constrained to move only in-line with the spring. Acceleration causes deflection of the mass and the offset distance is measured. The acceleration is derived from the values of deflection distance, mass, and the spring constant. The system must also be damped to avoid oscillation. A closed-loop accelerometer achieves higher performance by using a feedback loop to cancel the deflection, thus keeping the mass nearly stationary. Whenever the mass deflects, the feedback loop causes an electric coil to apply an equally negative force on the mass, cancelling the motion. Acceleration is derived from the amount of negative force applied. Because the mass barely moves, the non-linearities of the spring and damping system are greatly reduced. In addition, this accelerometer provides for increased bandwidth past the natural frequency of the sensing element.

Both types of accelerometers have been manufactured as integrated micromachinery on silicon chips.

See also

References

  1. ^ Doug Weed, et al.: GPS Align in Motion of Civilian Strapdown INS. Honeywell Commercial Aviation Products.

External links