USA   F-111 Aardvark OZ
Digital Flight Control System

Last Updated 8 October, 2003

By Capt James Sunter, United States Air Force.

The F-111 DFCS (Digital Flight Control System) was a STARS funded shadow research project for the development of the F-111 DFCS OFP (Operational Flight Program) in Ada. A non-competitive study contract was awarded to the Fort Worth Division of General Dynamics in October 1988. General Dynamics is the prime contractor for the F-111 DFCS program and developed the JOVIAL DFCS OFP. The main objective of the Ada shadow project was to prove if mature Ada compilers and tool sets were available for MIL-STD-1750A targets. This was to be proven by implementing, in Ada, the F-111 digital flight control system, an embedded real-time system that was developed in JOVIAL (J73-MIL-STD-1589B).

The main issue to be addressed by the shadow project was whether Ada was mature enough to be used to develop an embedded real-time system. A decision to use the existing top-level design was made to verify that the Ada OFP matched the characteristics of the JOVIAL OFP. In order to compare the two systems at the end of the program, it was important that the Ada software be developed to meet the same requirements as documented in the JOVIAL Software Top-Level Design Document. The DFCS Software Detailed Design was developed independent of the programming language and the Ada shadow project team used it with very few changes. To ensure that the JOVIAL implementation would not influence the Ada software engineers, the engineers were not allowed to examine the JOVIAL code.

The Ada OFP is approximately 35% larger in Read-Only Memory and 17% slower in execution speed than the JOVIAL OFP. The is partly due to lack of maturity of the Ada tool set and partly due to critical decisions made by the shadow team to use some of the best features of the Ada language to obtain a more maintainable OFP. The over- riding concern with the JOVIAL team was to produce an OFP that was error-free and met all performance requirements. Since the requirements were evolving while they developed the software, this team tended to place an emphasis on optimizing memory and real-time resources. The Ada shadow team approached the development of the OFP with a different philosophy by placing a stronger emphasis on exploring and documenting the benefits and problems associated with using Ada.

The final conclusion of the Ada shadow team was that Ada compilers and tool sets are currently available to support software development of programs using MIL-STD-1750A Instruction Set Architecture. This was proven by the successful demonstration of the Ada OFP using the F-111 Flight Simulator, extensive testing using the JOVIAL program's existing CSCI test procedures and test cases, complete unit testing using a MIL-STD- 1750A debugger/emulator and evaluations of memory and real time data for the software. The demonstration used Air Force test pilots flying the F-111 Flight Simulator at General Dynamics using Ada and JOVIAL OFPs in back-to-back blind comparisons. From a pilot's perspective, this demonstration provided evidence that the two OFPs behaved identically.

The Ada shadow project afforded team members the opportunity to research and investigate diverse engineering practices employing the Ada language and to document these findings in detail for other Ada users.

Copies of the final report may be requested in writing from:

Attn: Capt. Jim Sunter
Wright-Patterson AFB, OH

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