Flight management system
From Wikipedia, the free encyclopedia
A flight management system is a fundamental part of a modern aircraft in that it controls the navigation. The flight management system (FMS) is the avionics that holds the flight plan, and allows the pilot to modify as required in flight. The FMS uses various sensors to determine the aircraft's position. Given the position and the flight plan, the FMS guides the aircraft along the flight plan. The FMS is normally controlled through a small screen and a keyboard. The FMS sends the flight plan for display on the EFIS, Navigation Display (ND) or MultiFuction Display (MFD). The modern FMS was introduced on the Boeing 767, though earlier navigation computers did exist. Now, FMS exist on aircraft as small as the Cessna 172. In its evolution an FMS has many different sizes, capabilities and controls. However certain characteristics are common to all FMS.
Contents |
[edit] Navigation Database
All FMS contain a navigation database. The navigation database contains the elements from which the flight plan is constructed. These are defined via the ARINC 424 standard. The navigation database (NDB) is normally updated every 28 days, in order to ensure that its contents are current. Each FMS contains only a subset of the ARINC data, relevant to the capabilities of the FMS.
The NDB contains all of the information required for building a flight plan and information relevant to it. These include
- Waypoints,
- Airways (highways in the sky)
- Radio navigation aids including DME (Distance Measuring Equipment), VOR (VHF Omnidirectional Range) and NDB (Nondirectional beacons)
- Airports
- Runways
- Standard Instrument Departure (SID)
- Standard Terminal Arrival (STAR)
- Holding patterns
- And a variety of related and often installation specific information
[edit] Flight Plan
The flight plan is generally determined on the ground, before departure either by the pilot for smaller aircraft or a professional dispatcher for airliners. It is entered into the FMS either by typing it in, selecting it from a saved library of common routes or (Company Routes) or via a datalink with the airline dispatch center.
During preflight other information relevant to managing the flight plan is entered. This can include performance information such as gross weight, fuel weight and center of gravity. It will include altitudes including the initial cruise altitude. For aircraft that do not have a GPS, the initial position is also required.
The pilot uses the FMS to modify the flight plan, in flight for a variety of reasons. Significant engineering design minimizes the keystrokes in order to minimize pilot workload in flight and eliminate any confusing information (Hazardously Misleading Information). The FMS also sends the flight plan information for display on the Navigation Display (ND) of the flight deck instruments (EFIS). The flight plan generally appears as a magenta line, with other airports, radio aids and waypoints displayed.
Special flight plans, often for tactical requirements including search patterns, rendezvous’, in-flight refueling tanker orbits, calculated air release points (CARP) for accurate parachute jumps are just a few of the special flight plans some FMS can calculate.
[edit] Position Determination
Once in flight, a principal task of the FMS is to determine the aircraft's position and the accuracy of that position. Simple FMS use a single sensor, generally GPS in order to determine position. But modern FMS use as many sensors as they can in order to determine and validate exactly their position. Some FMS use a Kalman filter to integrate the positions from the various sensors into a single position. Common sensors include:
- Airline quality GPS receivers act as the primary sensor as they have the highest accuracy and integrity.
- Radio aids designed for aircraft navigation act as the second highest quality sensors. These include;
- Scanning DME (Distance measuring equipment) that check the distances from five different DME stations simultaneously in order to determine one position every 10 seconds or so.
- VOR’s (VHF Omni-directional Radio Range) that supply a bearing. With two VOR stations the aircraft position can be determined, but the accuracy is limited.
- Inertial Reference Systems (IRS) use ring laser gyros and accelerometers in order to calculate the aircraft position. They are highly accurate and independent of outside sources. Airliners use the weighted average of three independent IRS to determine the “triple mixed IRS” position.
The FMS constantly crosschecks the various sensors and determines a single aircraft position and accuracy. The accuracy is described as the Actual Navigation Performance (ANP) a circle that the aircraft can be anywhere within measured as the diameter in nautical miles. Modern airspace has a set Required Navigation Performance (RNP). The aircraft must have its ANP less than its RNP in order to operate in certain high-level airspace.
[edit] Guidance
Given the flight plan and the aircraft's position, the FMS calculates the course to follow. The pilot can follow this course manually (much like following a VOR radial), or the autopilot can be set to follow the course.
The FMS mode is normally called LNAV or lateral navigation for the lateral flight plan and VNAV or vertical navigation for the vertical flight plan. LNAV provides roll steering command to the autopilot and VNAV provides speed/altitude targets to the autopilot...
[edit] References
 |
This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (June 2009) |
- ARINC 702A, Advanced Flight Management Computer System
- Avionics, Element, Software and Functions Ch 20, Carl Spitzer, ISBN 0-8493-8438-9
- FMC User’s Guide B737, Ch 1, Bill Bulfer, Leading Edge Libraries
[edit] External links
Makers
|
||||||||||||||
