loss of compression on lycoming

[simonmollier]

Hello,

I maintain an Aircraft with 2 Lyco IO-360-A1B6, and at the 100h inspection, we found 3 cylinders at 20Psi/80 on RH engine, and 1 cyl at 26Psi/80 on LH engine. At last 100h inspection, all compressions on both engines was very good (above 70PSi).

Somebody have an idea about what could happened to loose compression so quickly ?

During run-up before maintenance, we didn't notice any variation of parameters, but the engines was more harder to start than currently.

Thanks to help me to understand !

[culver10]

Did you determine where the air was leaking from, intake, exhaust or rings??

[c170b53]

Carbon on seats or stems would be my first guess

[cgzro]

internal damage from a damaged cam lobe or lifter has rained metal on the piston skirts which killed the cylinder walls/ rings.

Seems unlikely 3 valves/seats would all leak at same time but a front lifter can affect many pistons.

Was there metal in the screen.

Anyway sounds bad:(

Hope Im wrong!

[Colonel Sanders]

Do the leakdown test again, and with the 80 psi
going into the cylinder, put your ear to the exhaust -
is it hissing? Put your finger over the crankcase
breather tube - is it blowing air out?

First you need to figure out where the leak is,
as mentioned above. Generally it's rings or
exhaust valve, but it could be intake (rare).

[simonmollier]

Leaks seem to come from différent ways for eqch piston : sometime exhaust, on one other from intake. We have removed 3 cylinders (2 on lh engine and 1 on rh). We noticed some scratches on pistons (only on a small area).
So I couldn't find a relationship that could damaged 4 cylinders on both engines. Is a unappropriate operation of engine or mistake during maintenance could damage in this manner, in 100h of flight ?

Thanks for your experience and knowledge.

[Colonel Sanders]

Ok, I understand the leak is (mostly) exhaust. And that you have removed the cylinders.

What does the exhaust valve look like? What does the seat look like? If they are in good shape, time to look at sticking valves as mentioned above.

While it is all apart, be sure to clean the carbon from the valve stems, and the lead from the valve guides as per the Lyc SL.

Are the plugs fouled? Has someone been operating it for extended periods of time at low power settings with an excessively rich mixture? How many hours since major on the engines?

[simonmollier]

There are a lot of lead deposits on bottom sparks plugs (more.than previous inspections). Pilots currently run a long time(about 15/20 min) at 1100rpm before take off.
Engines TSN : 1500h.

[simonmollier]

There are a lot of lead deposits on bottom sparks plugs (more.than previous inspections). Pilots currently run a long time(about 15/20 min) at 1100rpm before take off.
Engines TSN : 1500h.

[simonmollier]

On cylinder leaking from intake. It seems to be rare, why ?

[Colonel Sanders]

Ok. You need to teach your pilots about leaning
the mixture. Esp on the ground. When you must
idle, do it at 1100 to 1200 RPM with the mixture
leaned for max RPM to avoid buildups on plugs
and valves as you describe.

If you drop the valves out and clean the stems
and the guides, carefully inspect the seat and
valve face, and put it back together again it
will probably be just fine.

But please teach your pilots to LEAN THE MIXTURE
to avoid doing this again in the future.

I LEAN THE MIXTURE on the ground, immediately
after start and after landing. During the landing
rollout, actually.

I LEAN THE MIXTURE in the descent to keep the
engine warm and avoid fouling.

I LEAN THE MIXTURE in cruise at any altitude to
reduce my fuel burn. Even if you don't care what
fuel costs, some day you might want a little extra
when you're landing.

I LEAN THE MIXTURE in climb above 3000 feet.
Watch the CHT.

I suspect the instructors at your FTU need to
find a clue, because they don't have one.

PS See the Lyc operator's guide for more
information on how to correctly operate a
Lycoming.

[Colonel Sanders]

cylinder leaking from intake. It seems to be rare, why?

It happens, but much more frequently the exhaust
valve show signs of distress, because it lives a much
harder, hotter life than the intake valve, which is cooled
by the fuel/air mixture.

Rings and cylinder walls wear, and rings eventually break.
Which rather surprisingly is not a big deal, although the oil
consumption will skyrocket.

Any leakage at the exhaust must be taken very seriously
because of the possibility of a crack in the valve face. A
lot of the time it's just carbon/lead/sticking, but if an exhaust
valve fails due to a crack in the face or excessive necking
at the stem, the engine is going to run really really roughly.

This is not the case with broken rings.

[Colonel Sanders]

I might humbly suggest that you learn the "rope trick"

http://www.lycoming.com/support/publica ... I1425a.pdf

This is a method of cleaning the carbon and lead from
exhaust valve stems and guides, without removing the
cylinders.

The Lyc SI is missing a bunch of stuff - like dental floss
and AvBlend - but it's a good place to start.

[culver10]

1500 smoh, how many years ago was that??

[Big Pistons Forever]

There are a lot of lead deposits on bottom sparks plugs (more.than previous inspections). Pilots currently run a long time(about 15/20 min) at 1100rpm before take off.
Engines TSN : 1500h.

I second the Colonels advice about aggressively leaning on the ground.

Let me guess it is a Seneca 1 used by a flight school and the reason the pilots take 20 minutes is because of some stupidly long FTU checklist. Here is a news flash for all you wannabe MEIFR CPL,s. If you need 20 miniutes to flash up and then get into the air in a simple light twin, the CP at your first job will fire your ass


Simon I suggest you sit down with the pilots and explain to them the cost of how they are operating the aircraft.

[iflyforpie]

I remember getting reamed out because I would get the ATIS before firing up, and then run up both engines at the same time instead of doing a 182 run up on each engine. I was costing my instructor valuable multi time!

Yes, lean lean lean. You will never hurt an engine by leaning it until it nearly dies on the ground. I even takeoff and climb lean and I've had nary a burned exhaust valve.

[Colonel Sanders]

I even takeoff and climb lean

Yes - at a high density altitude airport this is an absolute must. I flew a C185 just south of Leadville, Co and I just couldn't believe how much I had to lean the mixture for takeoff!

At sea level, it's really not required if the temps aren't ridiculously hot.

[simonmollier]

Ok until now, i believed that leaning would increase cht and was very bad for engine life !
So, anyway. What kind of mistake on operation could damaged 2 or 3 cylinders on each engine so quickly ? Or maybe an incidious trouble...

Sorry but it just ununderstandable for me.

An other question : is the use of mogas for this engine could deteriorate it quickly ?

Thanks very much for your explanations.