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Pitch Input Switching PCB

Technical Information

Catalogue No: C1594
Category: Flight Control
Object Type: Module/Sub-Assembly/Component
Object Name: Pitch Input Switching PCB
Part No: 7708-00029
Serial No: 075
Manufacturer: Marconi-Elliott Avionic Systems Ltd
Division: Flight Controls [FCD]
Platform(s): Lynx
Year of Manufacture: circa 1980
Width (mm):
Height (mm):
Depth (mm):
Weight (g):
Location: Rack RAA03 [Main Store]

Type No. 7708-00029
S/No. 075

6721-0000-294 (Reverse Side)


PCB from WG 13 (Lynx) Flight Control. The exact box it was used in has not currently been identified but it could be the Computer Acceleration Control or the Engaging Controller

Marconi-Elliott Avionics of Rochester designed and produced a modular AFCS, under subcontract to Westland Helicopters, for the utility, Royal Navy and French Navy  versions of the Lynx. A modular, or building-block, system is needed because the aircraft has to operate in several roles.  The auto-stabilisation system is common to all three variants, with a range of auto-pilot facilities being added to meet specific requirements. The basic version, as fitted to the utility Lynx, has holds for airspeed, heading and barometric altitude.  In the Royal Navy aircraft these have been supplemented by a radio altitude hold, and in the French naval variant automatic transition between cruise and hover) and sonar cable hold (operating in both angle and height) are fitted. The heart of the Marconi-Elliott AFCS is the autostabiliser system, which provides attitude stabilisation in pitch and roll. Demands are fed to actuators which in turn drive the main powered flying controls.  In the yaw axis the equipment operates as a rate-damping system to stabilise the helicopter during both manual and automatic flight. All autostabilisation channels are fully duplicated, with monitoring circuits to indicate any discrepancy between the two lanes.  The auto-stabiliser is active throughout the flight envelope, including operation under autopilot control. A unique feature is a simple control loop to provide additional pitch stability by sensing normal acceleration and applying a collective pitch demand to counter the pitch rate divergence and instability induced at high forward speeds with an aft centre of gravity. This system—collective acceleration control (CAC)—has also reduced pitch stick activity, and hence workload, particularly in turbu-lence and during manoeuvring.  The autopilot modes of barometric altitude, radio altitude, radio altitude acquire, heading hold and airspeed hold operate individually or together. For example, barometric altitude, heading and airspeed hold can be used at the same time. Transition and sonar hover require simultaneous control in pitch, roll and collective axes, and the heading-hold mode would normally also be engaged. All autopilot control signals are fed to limited-authority series-mounted actuators and also to full-authority paralleled actuators, which operate to prevent the series units from becoming saturated with control demands during manoeuvres under autopilot control.  All autopilot channels are duplicated in the collective axis, but are simplexed in pitch, roll and yaw.

The AFCS consists of the computer AFCS, computer acceleration control, pilot’s controller, test controller, sonar transition controller, parallel actuators, yaw-rate gyros and stick-position transmitters. Overall system weight varies between 30lb and 50lb, depending on the variant. Mean time between failure (MTBF) is quoted in excess of l.000hr and built-in test equipment facilitates first- and second-line fault diagnosis.

Design work on the Marconi-Elliott AFCS began in late 1969 and the first equipment, comprising the computer acceleration control, was aboard the prototype Lynx on its maiden flight.  Basic development work had been completed by March 1972, when transition-mode equipment was ready for flight-testing.  

The British Army ordered over 100 Westland Aerospatiale Lynx for a variety of roles, from tactical transport to armed escort, antitank warfare , reconnaissance and casualty evacuation. The Lynx AH.1 first entered service with the Army Air Corps (AAC) in 1979. The Mk34 AFCS is fitted to the Army variant.

Four Rotary Actuators form part of the Mk34 AFCS system which provides attitude stabilization in the pitch and roll axes. Stabilization demands are fed to limited authority series actuators which in turn drive the main powered flying controls. In the yaw axis the system operates as a yaw rate damping system to stabilise the aircraft during manual and automatic flight. The Rotary Actuators provide a rotary output of nominally ±60deg for autopilot functions. They are controlled directly from the pilot’s stick switch in pitch and roll for trim adjustments in those axes.

See FLIGHT International, 8 November 1973

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