The Air AcCidents Investigation Branch (AAIB) report into the crash of Percival Provost G-AWVF just north of Waddington on July 8, 2009, was published on October 15. The aeroplane suffered a catastrophic engine failure and inflight fire, before crashing and claiming the life of hugely experienced air display pilot John Fairey.
The inquiry initially focused on a fatigue crack of the No 6 “master” connecting-rod gudgeon-pin, but following discussions with specialists from the Historic Aircraft Association (HAA), it recognised the likelihood of a high-load event such as a hydraulic lock initiating the crack.
The AAIB reports that the aircraft’s low utilisation during the previous 45 years probably contributed to corrosion. It recommends calendar time limits between overhauls on such engines, but the HAA favours increased inspection rather than costly overhauls. The most important recommendation alerts aircrew to the correct technique for clearing “hydraulic lock”.
All radial engines are susceptible to hydraulic lock as a result of oil draining into the combustion chambers of downward-facing cylinders or lower intake pipes. If this is not cleared, the oil, being incompressible, can result in critical loads and component failures.
The report makes it clear that the pilots who flew the Provost followed established procedures for “pulling through” the propeller manually before flight. However, the AAIB indicates that many pilots will, if undue resistance is encountered, turn the propeller backwards in order to “clear” any hydraulic lock.
While not covered in British pilots’ notes, wartime American engine manuals highlight the serious danger of this procedure, which merely drains liquid from the cylinder into the intake manifold, with the potential of a lock recurring on the subsequent start. The AAIB and HAA are therefore advocating the American procedure of the removal of the relevant spark plugs to allow the oil fully to drain from the cylinder and manifolds.