IMPORTANT LESSONS LEARNED FROM PROVOST ACCIDENT REPORT
The recently released AAIB report into the accident to Percival Provost G-AWVF in July 2009 (see link below) has highlighted the necessity of proper pre-start procedure for radial engines. The aeroplane suffered a catastrophic engine failure and in-flight fire, before crashing and claiming the life of pilot, the late Vice-President of the HAA, John Fairey DL.
The report initially focussed on a fatigue crack of the no.6 ‘master’ connecting rod gudgeon pin and on the presence of corrosion pits on the inner surface of the pin. However following discussions with the HAA Engineering Working Group, the AAIB final report has recognised the likelihood of a high-load event such as a hydraulic lock initiating the crack in the pin.
The AAIB report states that the aircraft’s low utilisation during the previous 45 years probably contributed to presence of the corrosion and has suggested that the Civil Aviation Authority consider implementing calendar time limits between overhauls for Alvis Leonides series engines, and other historic aircraft engines that do not have manufacturer-recommended calendar limits.
We will be working with the CAA to determine whether this is sensible and have informed the CAA that the HAA favours increased inspection rather than costly overhauls which may not provide any degree of added safety.
The AAIB report also advocates the wearing of parachutes in ex-military aircraft. This is merely reaffirming the recommendation for PTF ex-military aircraft on the UK register operating under CAP 632. The publication states that “Parachutes SHOULD be worn”.
The most important recommendation from the AAIB report alerts aircrew to the correct technique for clearing a “hydraulic lock” on an aero engine.
As the AAIB report states, 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 a cylinder being blown out, a bent or fractured connecting rod, or damage to the gudgeon pin.
The AAIB report also quotes the universal practice that to avoid hydraulic lock during engine start, the propeller should be turned through a few revolutions by hand in the direction of rotation (with the ignition switches off). If any excessive resistance is felt while pulling the propeller through a compression stroke, then liquid is present in one of the cylinders, and the propeller should not be pulled through any further.
While it is clear that the pilots who flew this aeroplane followed established procedures for ‘pulling through’ the propeller manually before flight, the AAIB states that anecdotal evidence indicates that many pilots today will, if undue resistance is encountered, turn the propeller backwards in order to clear any hydraulic lock.
Wartime American engine manuals highlight the serious danger of this procedure injecting liquid from the cylinder into the intake manifold, with the possibility of a lock and internal damage occurring on the subsequent start.
As the AAIB report states: When the propeller is turned backwards, the piston which has encountered the first valve to open is the intake valve. As the propeller continues to be rotated backwards the piston will help to force any liquid out through the intake port.
As the propeller continues to rotate, the exhaust valve will open and some liquid might also drain into the exhaust port. Oil in the exhaust port is safe and will either drain out through drain holes in the exhaust, or result in smoke being produced during engine start.
However, oil in the intake port is not safe, as it will not drain away and is likely to be sucked back into the cylinder during engine start, potentially causing damage as a consequence of hydraulic lock.
In the event of a hydraulic lock, the AAIB and HAA advocate the removal of the relevant spark plugs to allow the oil to drain from the cylinder. The AAIB has accordingly recommended that the Civil Aviation Authority notify operators of piston radial engines of the correct technique for clearing a hydraulic lock. It should also be noted that this advice may also be appropriate to those operators of aircraft with inverted in-line engines too.
http://www.aaib.gov.uk/publications/bulletins/october_2010/hunting_percival_p56_provost_t1__g_awvf.cfm