Flight management system

A flight management system (FMS) is a fundamental component of a modern airliner's avionics.

The FMS can be summarised as being a dual system consisting of the Flight Management Computer (FMC), CDU and a cross talk bus.

The navigation database (NDB) is normally updated every 28 days, in order to ensure that its contents are current.

The flight plan is generally determined on the ground, before departure either by the pilot for smaller aircraft or a professional dispatcher for airliners.

The flight plan generally appears as a magenta line, with other airports, radio aids and waypoints displayed.

Simple FMS use a single sensor, generally GPS in order to determine position.

But modern FMS use as many sensors as they can, such as VORs, in order to determine and validate their exact position.

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.

VNAV provides speed and pitch or altitude targets and LNAV provides roll steering command to the autopilot.

The FMS needs to have a comprehensive flight and engine model in order to have the data required to do this.

The function can create a forecast vertical path along the lateral flight plan using this information.

Performance optimization allows the FMS to determine the best or most economical speed to fly in level flight.

This is based on the cost index, which is entered to give a weighting between speed and fuel efficiency.

In this case, VNAV regulates the cruise speed or cost index to ensure the RTA is met.

The FMS calculates the TOD by “flying” the descent backwards from touchdown through the approach and up to cruise.

On Boeing aircraft, if the aircraft gets too far off the prescribed path, it will switch from VNAV PTH (which follows the calculated path) to VNAV SPD (which descends as fast as possible while maintaining a selected speed, similar to OP DES (open descent) on Airbuses.