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Old 29th Aug 2015, 22:03
  #541 (permalink)  
Join Date: Oct 2010
Location: UK
Age: 50
Posts: 476
HIGH PRESSURE PUMP ISOLATION SWITCH (HPPIS). When the switch is set to ISOLATE, one HP pump is cut off from the servo system which continues to control only the other HP pump.

Extracts from an old AAIB report:

In May 1980 an RAF two seat Hunter T7 experienced a power reduction, and a hot relight was attempted. As this was being completed by one pilot, the other pilot had retarded the throttle slightly and operated the HPPIS as the first pilot ceased pressing the relight button. The engine speed increased to 6,000 RPM, but the EGT exceeded full scale deflection and the engine began to vibrate. The instructor took control and advanced the throttle slightly, whereupon there was a slight increase in engine speed, followed by a 'muffled bang' and the engine speed then decreased rapidly. Both pilots then ejected safely, but the aircraft was destroyed in the ground impact and post crash fire. The power reduction was attributed to an unidentified failure in the fuel system.

In December 1981, another RAF Hunter T7 experienced an engine malfunction while preparing to land. At about 1000 feet agl and after selecting flap, gear down and airbrake in, and as the airspeed decreased through 250 kt, the instructor advanced the throttle slightly. Both pilots then noticed a slight 'rumbling sound' and that the indicated engine speed was lower than expected for the throttle setting. The instructor moved the throttle further forward, but the engine did not respond. He then operated the HPPIS and the engine speed increased as expected before reducing to low RPM. At about that time, witnesses on the ground saw a plume of flame from the aircraft's jetpipe. Both pilots then ejected safely. Subsequent examination of the wreckage found that the engine turbines had experienced severe in-flight overheat damage. A fault was later found in the BVCU diaphragm which had caused the compressor to stall as the throttle was opened. It was subsequently established that the HPPIS had been operated while the throttle had been set to a position corresponding to about 7,000 RPM.

In addition to these accidents, records kept on a computerised database between 1980 and 1992 showed 22 cases involving the Avon Mk 122 engine where engine speed had dropped and subsequent engineering investigation had not established a clear cause. Anecdotal evidence indicated that Avon Mk 122 engines had suffered from unexplained power reductions from time to time during RAF service, but in most cases the aircraft had returned safely and the subsequent RAF engineering investigations, including related engine ground runs, had failed to identify associated causes or to reproduce the symptoms.
Doesn't sound right to me. If the pilot lost thrust going up or over the top then they would just roll out as others have said. If the power was lost going down then the throttle would be at idle anyway - again as others have said. Sounds like journo-bollo to me!
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