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Please shut up about the Mull of Kintyre Chinook crash

RAF Chinook fleet is actually a rare MoD success story

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Analysis New information is said to have emerged in the case of the 1994 RAF Chinook helicopter crash on the Mull of Kintyre. Internal MoD documents, casting doubt on the safety of the engine-control software in the wrecked Chinook, have been leaked to the media.

According to the BBC and venerable IT mag Computer Weekly - which has investigated the Mull of Kintyre crash with almost unbelievable thoroughness (vast pdf here) - today's unpublished documents indicate that the UK military flight-test centre at Boscombe Down had serious concerns about the reliability and safety of the Full Authority Digital Engine Control (FADEC) system used to run the RAF Chinook Mark 2's engines.

The FADEC system, which interposes software between the pilot and direct control of the engine's fuel flow - much as many modern "fly-by-wire" systems eliminate mechanical connection between cockpit controls and aeroplane elevators, ailerons etc - was just being introduced when Chinook ZD576 crashed in poor visibility in June 1994, killing all 29 people on board.

An initial RAF investigation concluded that the pilots had made an error and flown the helicopter into the ground, a mistake which is all too easy to make when flying on instruments at low level in poor visibility. However the pilots' families and campaigners on their behalf have always said that a problem with the FADEC software might instead have caused an engine on the helicopter to go out of control and cause a crash.

Various commentators and analysts examining the Mull of Kintyre crash have suggested that the RAF's decision to go for full digital engine control was unwise, that such systems are somehow inherently dangerous or unusual, and that some kind of analogue electromechanical backup or override should have been retained.

That viewpoint, however, has pretty much been consigned to the dustbin of history. New helicopters are nowadays routinely supplied with FADEC-controlled engines, and most older ones have now been retrofitted with the technology. There are nowadays more than 1,000 Chinooks flying, almost all FADEC equipped. Most other choppers also use such kit nowadays.

Full-authority digital control isn't used just for fun. Helicopter engines are, by the nature of their task, often worked very hard - it's very easy, with a manually controlled turbine, to push needles past red lines and cause serious damage. Pilots don't need the distraction of trying to manually control their engines while at the same time performing difficult feats like landings, or instrument flight. FADEC is actually safer than direct pilot control, and indeed the Chinook Mark 2 and its much larger companion CH47-D fleet in the US have had a good safety record since the Kintyre crash. FADEC is also widely used on aeroplane engines nowadays.

But in 1994 FADEC was fairly new in actual flight service, certainly new to the RAF Chinook fleet. It has long been well-known that test pilots and engineers at Boscombe Down had concerns about the original system fitted to the Mark 2 Chinooks. The software - the worrisome bit - was actually written by British programmers at the company known at the time as Hawker Siddeley, since absorbed into the BAE Systems empire, working for US engine firm Textron.

Several subsequent investigations have said that the FADEC could have caused the crash, though if it did so the resulting sudden power loss or overpower before the crash must have been transient - the engines showed no sign of fuel loss or damaging over-revving at the time of impact, and the crash investigators said "available evidence of instrument indications and control settings in the aircraft suggested normal operation of both engines". Again, if an engine had merely powered down briefly, expert pilots like Flight Lieutenants Cook and Tapper - both selected for operations with special forces - would have been expected to manage a safe landing, or at least a Mayday call. Even in a sudden power runaway, at least a partial call would be likely from pilots of this sort, but none was received.

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