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C-5 Air Data Computer

Technical Information

Catalogue No: C0158
Category: Air Data
Object Type: Signal/Data Processor
Object Name: C-5 Air Data Computer
Part No: 50-001-08
Serial No: 0001
Manufacturer: Elliott Bros
Division: Instrument Systems [ISD]
Platform(s): C-5A/B Galaxy 
Year of Manufacture: 1966
Dimensions:
Width (mm):
200 
Height (mm):
200 
Depth (mm):
560 
Weight (g):
16,000 
Location: Main Object Store
Inscription(s):

Computer Central Air Data, MC 0159A

Notes:

This is an Air Data Computer from 1967 designed for the C-5A The Lockheed C-5 Galaxy is a large military transport aircraft built by Lockheed. It provides the United States Air Force (USAF) with a heavy intercontinental-range strategic airlift capability and has been operated by the USAF since 1969.
An Air Data Computer computes altitude, vertical speed, air speed, and mach number from sensor inputs such as pitot and static pressure and temperature. The early systems were electromechanical computers, and Elliotts backround wasas much in electronics as in precision gear making. After WWII Elliotts was acquired by Leon Bagrit who already managed Swift and Swallow on the Rochester site. During the Second World War the manufacturing company, styled B & P Swift Limited turned to war work, and manufactured aircraft equipment for the Ministry of Aircraft Production. This included flap and undercarriage actuators, which called for good gear and screw-cutting facilities and they also had a commercial arm making weighing scales which required gear wheels. Elliotts gained a precision gear capability and this can be seen in the early instruments such as this Air Data Computer,



An order for modular air data computers for British-built Jaguars, worth more than £250,000, was placed with Elliott Flight Automation by Mintech in1970. The unit was the smallest and lightest for its performance yet developed. A pre-production unit was flown in the S.06 prototype on its first flight in1969. The air data computer in the Jaguar forms a single central source of corrected signals of height, indicated airspeed, true airspeed, Mach number and related information, giving 16 different outputs for flight instruments, head-up display, navigation and weapon aiming. It also provides a digitised output of height for automatic altitude reporting through the secondary radar transponder. Individual functional modules can be replaced without recalibrating The mechanical transmission between modules also reduces weight and improves reliability.

The Air Data Computer was made lighter and smaller similar to the range of air data computers which were already being produced for the Nimrod, the American Lockheed C-5A heavy logistics transport, BAC One-Eleven, HS.748s for the Royal Australian Air Force and for the engine intake control system of the Concorde.

In 1981 Instrument Systems Division began the design of a new generation of Air Data Computers designed for the combined USAF and USN Standard Central Air Data Computer (SCADC) programme. The system was designed to fulfil the retrofit requirements of 38 different aircraft types and to do so with only four configurations. The aircraft types include the following:-

A-4M and TA-4J Skyhawk, A-6E/F, KA-6D and EA-6A Intruder, EA-6B Prowler, TC-4C Gulfstream, A-7D/E/K and TA-7C Corsair II, C-2A Greyhound, E-2C Hawkeye, KC-135 Stratotanker, C-5A/B Galaxy, C-141A/B Starlifter, F-111A/E/D/F, FB-111A, EF-111A, F-4C/D/E/G/J/N/S, and RF-4B/C Phantom and the S-3A/B and US-3A Viking. In 1992 an order for a further 290 units was received, being the eighth option, bringing the total procured to 5552 units. 387 units of the SCADC system were at this time being supplied for the F-14 Tomcat.

The high commonality was achieved with a unique software re-configurable design. Output parameters, ranges and scalings are selected for each aircraft type by the software, which recognises the host from a code wired into the aircraft connector pins. The SCADC core hardware set provides over 80% of the hardware in every application and the remainder is addressed by one or two special-to-type modules. A multiplicity of analogue interfaces can be handled and the Mil-Std 1553 DataBus is also a feature. Production commenced in 1985 with first deliveries the following year and in 1989 these deliveries surpassed 4,000 units. In addition in 1988 ISD received the Queen’s Award for Technological Achievement for SCADC.

Air data systems provide accurate information on quantities such as pressure altitude, vertical speed, calibrated airspeed, true airspeed, Mach number, static air temperature and air density ratio. This information is essential for the pilot to fly the aircraft safely and is also required by a number of key avionic subsystems which enable the pilot to carry out the mission. It is thus one of the key avionic systems in its own right and forms part of the essential core of avionic sub systems required in all modern aircraft, civil or military.

The air data quantities; pressure, altitude, vertical speed, calibrated airspeed, true airspeed, Mach number etc. are derived from three basic measurements by sensors connected to probes which measure:

Total (or Pitot) pressure
Static pressure
Total (or indicated) air temperature

The total pressure, PT, is measured by means of an absolute pressure sensor (or transducer) connected to a Pitot tube facing the moving airstream. The Pitot pressure is a measure of ram air pressure (the air pressure created by vehicle motion or the air ramming into the tube). When airspeed increases, the ram air pressure is increased, which can be translated by the airspeed indicator.

The static pressure of the free airstream, PS, is measured by an absolute pressure transducer connected to a suitable orifice located where the surface pressure is nearly the same as the pressure of the surrounding atmosphere. The static pressure is obtained through a static port which most often is a flush-mounted hole on the fuselage of an aircraft located where it can access the air flow in a relatively undisturbed area. Some aircraft may have a single static port, while others may have more than one. When the aircraft climbs, static pressure will decrease.

High performance military aircraft generally have a combined Pitot/static probe which extends out in front of the aircraft so as to be as far away as practicable from aerodynamic interference effects and shock waves generated by the aircraft structure. A Pitot-static tube effectively integrates the static ports into the Pitot probe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of the probe, outside the direct airflow, to measure the static pressure. Some civil transport aircraft have Pitot probes with separate static pressure orifices located in the fuselage generally somewhere between the nose and the wing.

From the measurements of static pressure PT and total pressure PS it is possible to derive the Pressure Altitude, Vertical Speed, Calibrated Airspeed and Mach number. Measurement of the air temperature is made by means of a temperature sensor installed in a probe in the airstream and from this a function called Total Air Temperature can be calculated.

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