Central Air Data Computer
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A Central Air Data Computer computes altitude, vertical speed, air speed, and mach number from sensor inputs such as pitot and static pressure and temperature. Early CADC systems were electromechanical computers, such as in the F-111. From 1968 to 1970, the first digital CADC was developed for the F-14. Following the success of the Swedish CK37 in the AJ37. In the 1980s, the Standard Central Air Data Computer was developed to retrofit U.S. Air Force and U.S. Navy aircraft.
The F-14 CADC was a ground-breaking integrated flight control system developed by Garrett AiResearch. It was used in early versions of the US Navy's F-14 Tomcat fighter. It is notable for its early use of AiResearch's custom-designed MOS-based LSI microprocessor chipset, the MP944.
The CADC was designed and built at Garrett AiResearch by a team led by Steve Geller and Ray Holt, and supported by the startup American Microsystems. Design work started in 1968 and was completed in June 1970, beating out a number of electromechanical systems that had also been designed for the F-14. Ray Holt's story of this design and development is presented in the book The Accidental Engineer.
The CADC consisted of an A-to-D converter, several quartz pressure sensors, and the MOS-based microprocessor. Inputs to the system included the primary flight controls, a number of switches, static and dynamic air pressure (for calculating stall points and aircraft speed) and a temperature gauge. The outputs controlled the primary flight controls, wing sweep, the F-14's leading edge "glove", and the flaps.
The MP944 ran at 375 KHz. It contained six chips used to build the CADC's microprocessor, all based on a 20-bit fixed-point-fraction two's complement number system. They were the Parallel Multiplier Unit (PMU) in a 28-pin DIP, the Parallel Divider Unit (PDU) (28-pin DIP), the Random Access Storage (RAS) (14-pin DIP), the Read-Only Memory (ROM) (14-pin DIP), the Special Logic Function (SLF) (28-pin DIP), and the Steering Logic Unit (SLU) (28-pin DIP). The complete microprocessor system of 28 circuits consists of 1 PMU, 1 PDU, 1 SLF, 3 RASs, 3 SLUs, and 19 ROMs.
In 1971, Holt wrote an article about the system for Computer Design magazine, but the Navy classified it, and finally released it in 1998. For this reason, the CADC and MP944 remain fairly obscure in spite of their historical importance.
- Dictionary of Military and Associated Terms. DIANE Publishing. Oct 1, 1987. p. 63. ISBN 9780941375108.
- F-111 Aardvark Pilot's Flight Operating Manual. United States Air Force. August 2007. p. 1–57. ISBN 9781430312123.
- "New Avionics Standardization Initiative - Standard Central Air Data Computer (SCADC)". Feedback. Wright-Patterson Air Force Base. II (1): 3.
- Standard Central Air Data Computer (PDF). GEC Avionics. 1985.
- Ray Holt's page on the CADC and the F-14 Archived 2011-05-30 at the Wayback Machine
- 1971 paper on the CADC (which was classified and never published)