TBO and be­yond

Pilot - - PILOT -

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

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