The year: 1980. The mission: develop a new digital avionics system for pilots to plan and automatically fly a 3-dimensional flight trajectory optimized to provide the most efficient route possible.
Success would have a number of industry-changing benefits:
Crew workload would be safely reduced to the point where air transport crew size could be reduced from 3 to 2
Aircraft position would be automatically calculated and would be more accurate than past systems
Full flight trajectories would be flown automatically
The system would understand the unique aerodynamic characteristics of the airframe and the engines so that it could minimize fuel burn or minimize time, or strike a pilot-defined balance between the two.
It was a tall order, to be sure. The engineers at Sperry Flight Systems (a Honeywell legacy company) embarked upon this challenge with two initial customers; Boeing and Airbus. The Boeing target platforms were the 757 and the 767, while for Airbus it was the A310.
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The new system, dubbed the “Flight Management System,” or FMS, became a major undertaking. The hardware platform included a Honeywell-build digital processor card (the SDP-175), and this processor became the execution engine for the biggest software development program the company had ever undertaken. Because of the very high cost of solid-state non-volatile memory at that time, the engineers designed and developed a 4Mb hard disk drive, an industry first, and a noteworthy electromechanical challenge considering the shock and temperature requirements that this ruggedized disk needed to meet.
A new user interface was required to allow the entry and display of FMS alphanumeric data, and the solution was the Control and Display Unit (CDU); the first true computer terminal in an air transport aircraft. The CDU proved to be such a benefit that later versions were modified to allow other avionics systems to connect to the CDU and use its data entry and display capabilities, and the CDU was then renamed “Multifunction Control and Display Unit,” or MCDU. The MCDU went from a monochrome display to color on the A320 program.
Understanding new technology
As complicated as the FMS was, its capabilities can be summarized into four functions:
Flight Planning – defining where the airplane needs to fly
Navigation – figuring out where the airplane is, along with its velocity
Guidance – looking at where the airplane is supposed to be (flight plan), then using the current aircraft position and velocity to control the airplane to the flight plan
Performance – calculate the optimum trajectory for the aircraft to fly and provide predictions for all the waypoints in the flight plan
The FMS included two databases: a Navigation Database and a Performance Database. The Navigation Database contained detailed information for possible flight plan waypoints, airports, departure and arrival procedures, and radio navigation aids, or “navaids.” The Performance Database was actually two databases: the aerodynamic model of the airframe and the engine model that gave detailed performance characteristics of the propulsion system.
The FMS navigation function provided a major operational improvement to the crew. The function performed an automatic calculation of aircraft position by first using ground-based radio navaids to calculate a radio position, then mixing this position with the aircraft position calculated by the Inertial Reference System.
The mixing algorithm used each position solution to minimize the position error that is inherent in each individual position calculations, and thereby yielded an aircraft position solution that was more accurate than either radio or inertial position alone. In addition, it didn’t require any crew involvement. The FMS even automatically determined the best navaids to use and tuned them automatically.
The FMS performance function calculated optimum speeds and the optimum point to begin descending from cruise altitude. This optimized “top of descent” point allowed the aircraft to coast down with engines at idle thrust for as long as possible, thereby minimizing fuel burn. The function also calculated the optimum point to do a step climb to a higher cruise altitude in order to provide the greatest fuel savings.
Certification and continued evolution
The initial FMS programs certified in 1984. The FMS became a baseline system for all new air transport aircraft and was retrofitted on a number of platforms over time. The system also migrated to the business jet market. In the late 1980s, the need to move data between the FMS and the ground was satisfied with ACARS (Aircraft Communication Addressing and Reporting System). The airline operations center could now upload flight plans, along with wind and weather data, to the FMS.
In the 1990s, the global positioning system (GPS) prompted further modification to the navigation function, which provided even greater position accuracy. This accuracy could enable closer aircraft spacing in oceanic airspace, which would allow the flying of more efficient flight plans. The FMS used ACARS datalink via satellite to send position information to Air Traffic Control (ATC) and ATC would send clearances back to the aircraft. This capability, called FANS (Future Area Navigation System), is a baseline function on most long haul aircraft today.
The transition from MCDU textual flight planning to graphical flight planning over the last decade was a big one. Pilots make flight plan changes on the cockpit map display using a cursor control device. This has been a very well-received human factors improvement.
Aircraft can fly more safely and efficiently, thanks to the FMS. It will be an essential component of the next generation air traffic management system, currently being defined by industry and various government agencies around the world.