Interlock Failure Monitor

Technical Information

Catalogue No: C1071
Category: Unknown
Object Type: Signal/Data Processor
Object Name: Interlock Failure Monitor
Part No: 8383-A-1
Serial No: 048/70
Manufacturer: Elliott Automation
Division: Unknown
Platform(s): BAC 1-11  Concorde  VC10 
Year of Manufacture: circa 1970
Dimensions:
Width (mm):
60 
Height (mm):
150 
Depth (mm):
545 
Weight (g):
Location: Archive Object Store
Inscription(s):

Elliott
Interlock Failure Monitor
Type No. 8383-A-1
Ref No.
Ser No. 048/70

Notes:

The Interlock Failure Monitor does not appear on the system description of the BAC1-11 2000 AFCS although the box is ‘BAC 1-11 AIRCRAFT ONLY’. The unit is described in the literature for the VC10 AFCS so was used in both systems. In 1964 Elliott Bros designed an ‘Interlock Monitor’ system to locate which element of the automatic landing system had failed in the event of an interlock opening and causing autopilot disconnect. In those days, autopilot engagement was permitted when a discrete number of series relay contacts had been closed by the discrete monitor circuits the nature of the design was that, if one relay opened, there would be subsequent operation of the other relays due to the failsafe method of combining the fault detecting relay contacts. The equipment added to the automatic landing system was basically an on aircraft of fault diagnosis. It comprised a small digital computer and a simple multi-lamp display panel and reset switch. The Interlock Monitor Computer scanned the series of relay interlocks and detected which one opened first. This therefore located the failure to the particular comparator, identified by a corresponding lamp number on the display panel. The equipment was first used in 1965 and became standard equipment for BOAC. Considerable problems were encountered due to the large number of interlocks which were in use over long periods in flight. The Monitor was eventually able to discriminate faults to an acceptable level of non-ambiguity only after considerable refinement of the in-line monitoring design. Tthis system was applied to the VC 10, the super BAC 1-11 and Concorde. This unit is marked 'BAC 1-11 AIRCRAFT ONLY'

The BAC 1-11 AFCS, like that in the VC10, was based on the well proven Bendix PB-20 Autopilot and was designated the Series 2000AFCS. New features over the PB-20 system include separate pitch and azimuth control computers, a modular Air Data Sensor and a range of units specifically designed for autoflare and autolanding.

Each unit in the BAC 1-11 AFCS is built to a common configuration with circuit modules arranged in stacks either side of the chassis. The stacks are connected by plugs to a mother board and are physically separated into ‘command’ and ‘monitor’ functions to preclude common failures. The computers are entirely solid state and there is a high degree of built-in-test. Self-monitoring techniques and multiple channel redundancy are used to give automatic failure survival in approach and cruise flight.

From the early days of the company it had been hoped to enter the civil aircraft flight control field, in order to reduce dependence on military projects. The opportunity to take this step came in the late 1950’s with the planning of the Vickers 'VC 10' for which Elliott Brothers secured an order to provide a complete automatic flight control system.

From the outset, the 'VC10' system was planned to make provision for fully automatic landing of the aircraft. For certification ever to be possible an extremely high standard of reliability was essential, and even in the case of failure of the equipment it was a requirement that the aircraft must not be subjected to violent manoeuvres. After a detailed study of possible alternatives, the solution chosen was to duplicate the whole of the major system, one half to be operative while the other was to be 'standing by', with a changeover mechanism of the utmost reliability to permit instant switching from one to another.

By 1960 the basic development was substantially complete and the requirements for automatic landing were being explored in detail with full 'autoland' capability available from January 1963. Successful development of the 'VC 10' system resulted in the opportunity to supply broadly similar equipment for the British Aircraft Corporation 'BAC 1-11', which has been produced in substantial numbers.

The automatic flight control system of the Standard and Super VC10 was designed to be capable of development to full blind landing. To meet this requirement the system had to be capable of failure survival and this includes associated services such as power supplies and flying controls. The method of autopilot failure survival chosen was to provide two monitored systems which are fail soft, i.e. there is negligible aircraft disturbance after a failure. Only one autopilot is used to fly the aircraft, and the two systems, including power supplies, are completely independent.

Each autopilot has a comparison monitor which detects faults and, in flight, will disconnect the system if these faults are likely to lead to dangerous conditions. For autoflare the system provides for automatic changeover to the second monitored autopilot system, in the event of fault in the first. Under these conditions the second autopilot is primed and ready to take over. If for any reason the monitoring system fails to prevent an autopilot runaway, the control movement is limited to a safe amount by the yielding of a torque-limiting spring.

Many of the needed components were already present in the autopilot fit on the Standard VC10s, to achieve the autoland capacity the system on the Super received some additional items. The system, supplied by Elliott Brothers (London) Ltd, was based largely on components of the well-proved Bendix PB-20 autopilot, made under licence by Elliott, and interchangeable with American built components as installed in Boeing 707s. However, the system as a whole i.e., the dual autopilot concept was novel, and designed entirely by Elliott.

A comprehensive description of the VC10 systems will be found at this VC10 website.
 
 

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