Issue 39: July/August 1987

The aircraft of the 1990s will have a comprehensive electronic flight control system (EFCS) which, when the pilot moves his control (probably a sidestick) to create a trajectory order, computes that trajectory order with database-held aerodynamic data to stabilise the aircraft. The EFCS by reference to the database will not let the aircraft exceed permissible speeds, stall or become overstressed. With such control the aircraft is inherently safe, as it will keep to a logical track and altitude in the absence of pilot input or with pilot input that is unacceptable to the logic This article deals with one aspect of the avionics that will be incorporated in aircraft of the future and is already with us today-the flight management system (FMS). This facility, recently fitted in Airbus Industrie aircraft in service, is largely concerned with the third goal of aircraft technology- the optimization of a flight programme to provide cost effective flying.

Until recently, navigational management systems fitted in aircraft have been concerned only with lateral steering to conform to a track over the ground and information for this has been derived from an on board database combined with automatic sensor tuning and input of positional data from inertial and radio sources. In its onward march, microcomputer has not been content just to order flight in two dimensions but has now also gained control of the vertical axis to provide a true fight management system in three dimensions.

Smiths Industries started development of the FMS for Airbus in 1979 and it is significant that the standard for commercial flight management (ARINC 702) at that time specified a computer program size of 70k 16-bit words. The certification of lateral performance was received in 1983 and in April 1986 certification was achieved for the first stage performance and vertical guidance. The computer program size in the interim has risen to more than 250k words, while the navigation and performance database is also over 250k words.

Upwards of 200 sets of FMS equipment have now been produced, each consisting of two fight management computer (FMC) units and two control and display (CDU) units. The pilots use the flight deck pedestal-mounted CDUs to enter data into the system via the alphanumeric and function keypad and observe the text display providing flight plan information.

. The display provides 14 lines each of 24 sharply defined characters in two font sizes and has a flicker-free operation. A set of eight annunciators is included to give pilots warnings or information on flight phase. The CDU operating procedures have been optimised to give clarity of information with a minimum of keystrokes on the part of the pilot. This provides a very pilot-friendly interface.

The FMS receives inputs from inertial references, VHF on range (VOR) and distance measuring equipment (DME) receivers, engine parameters, autopilot glare shield controls and from the fight control computer. Data outputs from the system are sent to the autopilots, thrust control computer and to its CDU as well as the electronic flight instrument system (EFIS).

By performing routine calculations at a speed far beyond human capability and providing guidance computations for autopilot and auto-throttle, the pilot’s workload is substantially reduced. The aircraft’s position is determined by the FMS using the best sensor information available and data is stored on all significant navigational beacons relevant to the operator’s preferred routes.

The most economical path is established by the FMS by computing optimum speed and height for a particular flight mode. It also computes the fuel and route times to the destination and alternative airfields. The system guides the aircraft vertically from just after takeoff to just before landing, taking account of all altitude constraints necessitated by terrain and air traffic control along the route.

Performance predictions are computed for the selected flight plan, assuming a climb from takeoff to cruise altitude, a level cruise, an idle descent and a level decelerating approach to the glide slope intercept point.

Before takeoff the FMS computes fuel planning predictions and during flight the fuel management predictions, including fuel remaining on board at destination, are computed from the aircraft’s position for display on the CDU.

Pilots have the facility to interrogate any performance pages on the CDU or to modify the fight plan on the ground or in the air. Where no route is stored, pilots can select a key origin or destination from the menu and define explicitly the route by entering the waypoints. The flight plan can be modified on the ground or in the air by changing the destination, the terminal area procedure, or by changing any on-route fix or airway sector. Smiths Industries is now developing an enhancement for the FMS in A310-300 aircraft to supply a full interface with the trim-tank system to enable an optimum centre of gravity to be provided. Diversion times to specified airports will be included as well as ‘en route’ support for single-engine operation.

Minimum off-route altitudes (MORA) will be included in the navigational database. C ompensation factors will be included to correct deviations of actual aircraft performance from the ideal model of the computer storage.