TBO and beyond
In December’s Airmail, correspondent Ed Lennox, who says he looks after a PA-28, advocates ground running for 0.1 on the Hobbs before entering the runway to warm the engine oil. I am a member of a PA-28 group based at Tollerton, and the Pilot Operating Handbook and the Lycoming manual are both quite specific on this point: ‘Engine is warm enough for takeoff when the throttle can be opened without the engine faltering’.
I imagine that best practice will vary from one aircraft type to another, but unless there is some obvious reason to act differently, surely the POH and engine manufacturer are likely to be the best guide. Peter Riley by email I run an EASA 145 maintenance facility in Germany and found the December letter on TBO interesting. Engine TBO always has been (and probably always will be) one of my major bugbears. The engine manufacturer recommends overhaul at whatever flying hours (often between 1,600 and 2,400) and normally twelve years. The private owner, unless he/she does a great deal of flying, will obviously reach the twelve years long before the flying hour limit. If the aircraft is operated commercially these limits are compulsory, but for the vast majority of GA aircraft operation beyond these recommendations is permitted via European/ national regulations.
All well and good — many of our customers do, indeed, have engines which have exceeded their TBO. I think, however, that ‘if anything happens’ (i.e. failure of aged engine components) there might be a liability problem. The guy signing the aircraft off (Certifying Staff in our case) is assuming responsibility for the serviceability of the parts/ components/aircraft. On the engine, there is not really much he/she can physically check without stripping the engine. Sometimes, relatively simple tasks can cause hidden problems for the future: e.g. a con rod banging on the crankcase housing on cylinder removal can flaw the con rod, causing a fatigue crack in the future (without wishing to be a scaremonger...) Who knows what might have been done to your engine in the past twenty years, especially with an incomplete or dubious history?
As mentioned in the letter, corrosion/lack of use can be a problem, especially on the pre-roller tappet Lycomings. The oil-additive LW16702 should help to protect the camshaft (on top of the engine). If, however, the engine has not been operated for a couple of months the oil will have dripped off and the cam lobe/tappet faces will be running dry until fresh oil has circulated. We measure the valve lift regularly (and thus check for cam lobe wear) once the engine has reached twelve years. This is not an engineering requirement but we have, this year, had two new customers requiring engine overhauls on their first visits to us on low hours/high calendar life engines, due to worn cams.
Theoretically, if the camshaft has worn this could be picked up by a thorough examination of the oil filter, but that is not always the case.
Talking of oil, these engines (mostly air-cooled) have to deal with a vast range of thermal expansion/contraction. The different properties of the various dissimilar metals used necessitate quite large clearances (compared to modern liquid-cooled engines), so some wear is inevitable. This is one of the various reasons for frequent oil and filter changes.
There are many considerations to operating beyond TBO. I appreciate the ‘never change a running system’ point of view (especially when looking at overhaul costs), but don’t think it should be taken too lightly. Colin Russell by email