Aircraft Engine Wear
Posted: Thu Nov 25, 2021 2:06 pm
Aircraft engines are hideously expensive. When you go to buy a private aircraft,
how do you know if the engine is ok? And, how can you take care of your aircraft
engine, so that it is reliable and strong, and doesn't prematurely fail on you?
People look at hours since major, and that's probably the least important factor,
and in fact a very low number is generally very bad.
Years since major overhaul is probably a more interesting metric. In commercial
use, for TBO calculations engine manufacturers assign hours per month regardless
if they are flown or not, so by 12 years (or so) your engine is actually "beyond TBO"
regardless of the hours on the tach.
Note that TBO has no meaning for a private aircraft, regardless of what anyone
tells you. I'm interested in years since major overhaul. If it's 20 or 30 years, it's
core value only. If it has 5 years since major, it would be nice to see 500 hours
on it. An engine flown 100 hours per year is generally in pretty good shape.
If it's 10 years since major with 10 or 20 hours SMOH, it's time to take it apart
and major it again. It's probably going to be a mess inside, from corrosion and
deposits gumming everything up (see AvBlend).
So. What is wears out an aircraft engine?
1) internal corrosion. This is the killer of especially Lycomings, who's cam lobes
and lifters and cylinder walls are terribly vulnerable to corrosion from disuse. You
probably don't fly every day, and if you live someplace warm, humid and salty, your
engine isn't going to survive very long.
People like to jabber on about borescopes and bullshit, but I have found you can
learn a lot about an engine by popping a valve cover.
If it looks like that Lycoming 540 that sat for a year, it's garbage inside and will
soon be making metal. Lycoming went to roller lifters which should help their
cam lobes, but remember you have to change them at overhaul, so that just got
more expensive.
2) excessive high temperature. The hotter you run metal, the shorter it's life. You
want to keep your CHTs under 400F and your oil temp under 200F. If you are
seeing higher numbers, you are shortening your engine's life. But remember to
keep the CHT above 300F as well, to keep the lead gaseous out the exhaust
port. Colder is not always better, ok?
3) excessive low temperature. Cold starts. A problem for you, not me. People
struggle with a start because the fuel won't evaporate, but that's the least of their
problems. For winter ops, you need multi-viscosity oil - NOT straight grade - so
it flows when it's cold. But even that has it's limits. As an engine gets colder,
the differential contraction of the dissimilar metals means that your clearances
go to zero. You can hang on the prop in the Arctic. This will destroy your bearings
after start. You must preheat when clearances are too tight. Similarly, after start,
you must sit and idle like a radial engine at minimum RPM to allow the cylinder
to heat up. If you hammer the RPM after start, the aluminum piston/pins will
expand faster than the steel cylinder, and will scuff and make metal in your oil.
Every engine is different, but my recipe for a cold start - which for me is 60F -
for a Lycoming is six seconds of prime at full throttle and rich mixture, after I
see six GPH on the flow meter. Then mags off and pull it through six cylinders,
so there is fuel in every cylinder and absolutely no chance of hydraulic locking
due to incompressible fluid in the cylinders, which will bend a connecting rod.
Then, I leisurely strap on my parachute and climb in and fasten all the fucking
buckles. No hurry, and I move slower in the cold, because 100LL takes a long
time to evaporate - the colder, the slower it evaporates - and liquid 100LL does
not burn - only after it slowly evaporates. So, take your time. Mogas in the
winter is awesome, if you can run it (I can't).
If I do this, I get a perfect start every time I crank, and RPM immediately down
below 1000 RPM, check the oil pressure climbing, mixture back an inch and
alternator field on to charge the battery.
If you start only on the left magneto (eg Citabria, Decathlon, etc) don't forget
to turn the right mag on after start! If you don't have toggles or rotary switch
for your mags, and you have a fancy Bendix twisty combined mag/starter switch,
it actually will do this for you, if you need it (right mag grounded while cranking).
I shouldn't mention this, but I know a Citabria owner who would use ether
(you know that starter stuff in a spray can) in the cold because 100LL wouldn't
evaporate worth shit. Horrified me, but it worked. He would get a start without
preheat, but I don't expect his bearings would last very long. You preheat for
engine clearance, not to fucking evaporate gasoline. No one understands this,
especially in the age of multi-grade oil, which has a pour point of -40F. Or -40C,
your choice. I only remember that temperature because that's what it was in
Cold Lake, the day I was born.
Anyways. Fly your private aircraft as often as you can. Be aware of the dangers
of heat, humidity and salt in the air. Run straight grade in the summer, and
multi-grade in the winter (if you have real winter). Keep the CHT below 400F
and above 300F. Keep the oil temp below 200F and if you are running straight
grade (W100 or W120) I strongly recommend at least oil temp 90F before takeoff.
Preheat in the winter. Not only your engine, but your cockpit, for many reasons.
And I simply cannot recommend strongly enough: Camguard and a Battery Tender.
Like Top Gate, I know I'm kind of a broken record, but Camguard with cheap oil
is the best oil that money can buy. And a Battery Tender ($30 on Amazon) keeps
your battery strong, extends it's life, and stops it from freezing and cracking and
peeing destructive battery acid all over your aircraft - I've seen that many times
before.
If you can't be bothered to put a Battery Tender on - or you don't have power
(I have seen solar power for them) please take your battery out of your aircraft
in the winter and take it home with you. It will do less damage and live longer
and maybe you could even charge it there. Remember that whenever a lead
acid battery drops below 12.8V it starts to sulfate the plates and it's life is
shortened. Ever wonder why some people can get 10 years out of a battery,
and other people are replacing them every year?
Remember that there is nothing special about airplanes. The same applies
to cars, motorcycles, boats, etc. I have more motorcycles than any sane person
would ever need, and they sit with a row of Battery Tenders on power bars on
the wall, all plugged in all the time with solid green lights.
Whenever you shut it down, plug it in. You may not plan it, but it may be months
before that engine runs again.
Pessimists live longer. Sorry about that, but it's true.
-- EDIT --
Next subject: hydraulic locking. You may think it's only something that radial engines
suffer from (engine oil in bottom cylinders) but that's not true. There is a back of an
envelope near me which will soon be scribbled on.
how do you know if the engine is ok? And, how can you take care of your aircraft
engine, so that it is reliable and strong, and doesn't prematurely fail on you?
People look at hours since major, and that's probably the least important factor,
and in fact a very low number is generally very bad.
Years since major overhaul is probably a more interesting metric. In commercial
use, for TBO calculations engine manufacturers assign hours per month regardless
if they are flown or not, so by 12 years (or so) your engine is actually "beyond TBO"
regardless of the hours on the tach.
Note that TBO has no meaning for a private aircraft, regardless of what anyone
tells you. I'm interested in years since major overhaul. If it's 20 or 30 years, it's
core value only. If it has 5 years since major, it would be nice to see 500 hours
on it. An engine flown 100 hours per year is generally in pretty good shape.
If it's 10 years since major with 10 or 20 hours SMOH, it's time to take it apart
and major it again. It's probably going to be a mess inside, from corrosion and
deposits gumming everything up (see AvBlend).
So. What is wears out an aircraft engine?
1) internal corrosion. This is the killer of especially Lycomings, who's cam lobes
and lifters and cylinder walls are terribly vulnerable to corrosion from disuse. You
probably don't fly every day, and if you live someplace warm, humid and salty, your
engine isn't going to survive very long.
People like to jabber on about borescopes and bullshit, but I have found you can
learn a lot about an engine by popping a valve cover.
If it looks like that Lycoming 540 that sat for a year, it's garbage inside and will
soon be making metal. Lycoming went to roller lifters which should help their
cam lobes, but remember you have to change them at overhaul, so that just got
more expensive.
2) excessive high temperature. The hotter you run metal, the shorter it's life. You
want to keep your CHTs under 400F and your oil temp under 200F. If you are
seeing higher numbers, you are shortening your engine's life. But remember to
keep the CHT above 300F as well, to keep the lead gaseous out the exhaust
port. Colder is not always better, ok?
3) excessive low temperature. Cold starts. A problem for you, not me. People
struggle with a start because the fuel won't evaporate, but that's the least of their
problems. For winter ops, you need multi-viscosity oil - NOT straight grade - so
it flows when it's cold. But even that has it's limits. As an engine gets colder,
the differential contraction of the dissimilar metals means that your clearances
go to zero. You can hang on the prop in the Arctic. This will destroy your bearings
after start. You must preheat when clearances are too tight. Similarly, after start,
you must sit and idle like a radial engine at minimum RPM to allow the cylinder
to heat up. If you hammer the RPM after start, the aluminum piston/pins will
expand faster than the steel cylinder, and will scuff and make metal in your oil.
Every engine is different, but my recipe for a cold start - which for me is 60F -
for a Lycoming is six seconds of prime at full throttle and rich mixture, after I
see six GPH on the flow meter. Then mags off and pull it through six cylinders,
so there is fuel in every cylinder and absolutely no chance of hydraulic locking
due to incompressible fluid in the cylinders, which will bend a connecting rod.
Then, I leisurely strap on my parachute and climb in and fasten all the fucking
buckles. No hurry, and I move slower in the cold, because 100LL takes a long
time to evaporate - the colder, the slower it evaporates - and liquid 100LL does
not burn - only after it slowly evaporates. So, take your time. Mogas in the
winter is awesome, if you can run it (I can't).
If I do this, I get a perfect start every time I crank, and RPM immediately down
below 1000 RPM, check the oil pressure climbing, mixture back an inch and
alternator field on to charge the battery.
If you start only on the left magneto (eg Citabria, Decathlon, etc) don't forget
to turn the right mag on after start! If you don't have toggles or rotary switch
for your mags, and you have a fancy Bendix twisty combined mag/starter switch,
it actually will do this for you, if you need it (right mag grounded while cranking).
I shouldn't mention this, but I know a Citabria owner who would use ether
(you know that starter stuff in a spray can) in the cold because 100LL wouldn't
evaporate worth shit. Horrified me, but it worked. He would get a start without
preheat, but I don't expect his bearings would last very long. You preheat for
engine clearance, not to fucking evaporate gasoline. No one understands this,
especially in the age of multi-grade oil, which has a pour point of -40F. Or -40C,
your choice. I only remember that temperature because that's what it was in
Cold Lake, the day I was born.
Anyways. Fly your private aircraft as often as you can. Be aware of the dangers
of heat, humidity and salt in the air. Run straight grade in the summer, and
multi-grade in the winter (if you have real winter). Keep the CHT below 400F
and above 300F. Keep the oil temp below 200F and if you are running straight
grade (W100 or W120) I strongly recommend at least oil temp 90F before takeoff.
Preheat in the winter. Not only your engine, but your cockpit, for many reasons.
And I simply cannot recommend strongly enough: Camguard and a Battery Tender.
Like Top Gate, I know I'm kind of a broken record, but Camguard with cheap oil
is the best oil that money can buy. And a Battery Tender ($30 on Amazon) keeps
your battery strong, extends it's life, and stops it from freezing and cracking and
peeing destructive battery acid all over your aircraft - I've seen that many times
before.
If you can't be bothered to put a Battery Tender on - or you don't have power
(I have seen solar power for them) please take your battery out of your aircraft
in the winter and take it home with you. It will do less damage and live longer
and maybe you could even charge it there. Remember that whenever a lead
acid battery drops below 12.8V it starts to sulfate the plates and it's life is
shortened. Ever wonder why some people can get 10 years out of a battery,
and other people are replacing them every year?
Remember that there is nothing special about airplanes. The same applies
to cars, motorcycles, boats, etc. I have more motorcycles than any sane person
would ever need, and they sit with a row of Battery Tenders on power bars on
the wall, all plugged in all the time with solid green lights.
Whenever you shut it down, plug it in. You may not plan it, but it may be months
before that engine runs again.
Pessimists live longer. Sorry about that, but it's true.
-- EDIT --
Next subject: hydraulic locking. You may think it's only something that radial engines
suffer from (engine oil in bottom cylinders) but that's not true. There is a back of an
envelope near me which will soon be scribbled on.