Central Air Data Computer
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. In the 1980s, the Standard Central Air Data Computer (SCADC) was developed to retrofit USAF and USN aircraft.
The F-14 CADC was a ground breaking integrated flight control system developed by Garrett AiResearch and was used in the 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.
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 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), the Parallel Divider Unit (PDU), the Random Access Storage (RAS), the Read Only Memory (ROM), the Special Logic Function (SLF), and the Steering Logic Unit (SLU). The complete microprocessor system used 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.
In the early 1970s, minicomputers for scientific floating point calculation did use broad words but early microprocessors had architectures no wider than 8 bits. The 1971 Intel 4004 was only 4 bits, with clock rates up to 740 kHz; the 1972 Intel 8008 had a maximum clock rate of 500 kHz and used an 8-bit word. This made them far slower than the 1970 F-14 Tomcat's MP944, with its 375 kHz clock rate and 20-bit word size.
- 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. 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
- 1971 paper on the CADC (which was classified and never published)