Cracked Cases? What Causes

ppragman

ppragman

FLIPY FLAPS!
I'm figuring wear and tear of the daily grind will do it, however, what specifically causes crank cases to crack on the bigger lycomings (IO540s)
 
Fatigue is the big one. Realize that the way that aluminum is, no matter how lightly you load it, it will fatigue over time. Contrast this with steel (like your crankshaft). A properly designed and loaded steel part will never fatigue. As a result, aluminum parts like crank cases and cylinder heads eventually fatigue and become no good, no matter whether they are taken care of properly or not. That's why welding up a cracked cylinder head is mostly pointless. The metal has already fatigued and will crack again and again.

Another big one is improper torque on crankcase through bolts. When those bolts are improperly torqued, it allows more flex in the crankcase, and therefore quicker fatiguing and cracking.
 
ppragman said:
what specifically causes crank cases to crack on the bigger lycomings (IO540s)
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Bad castings from the foundry (for all engines).

Fatigue over time, as the case thermal cycles, will create stress on any flaws in the casting. This stress will gradually grow into a crack. That crack will grow slowly until it can't carry the load anymore and fails.

Nothing you do from the pilot seat has any effect on all of this. The case is under a realitvely constant load, and heats up and down VERY slowly.
 
USMCmech said:
Bad castings from the foundry (for all engines).

Fatigue over time, as the case thermal cycles, will create stress on any flaws in the casting. This stress will gradually grow into a crack. That crack will grow slowly until it can't carry the load anymore and fails.

Nothing you do from the pilot seat has any effect on all of this. The case is under a realitvely constant load, and heats up and down VERY slowly.
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I appreciate that, thanks.

I was curious as there has been a rash of cracked cases around here, and preferably (as I am still not so familiar with the big lycomings - I'm a continental man myself) would be interested in knowing if there was anything I was doing that could cause it. I haven't been flying that particular airplane lately, but it'd be good to know if more experienced user out there had some insight.

My usual sequences for flight:

Break ground, and positive rate --> Prop back to about 2550 (Lycoming recommends this in their operational tips booklet).
1000' AGL --> Pump off, Power to 25x25 (per the company on the power setting) unless its really warm then I might leave the power up there for much of the climb.
Cruise --> 75% power (usually around 24 squared), 18gph (per company)
Descend --> Power at about 75% all the way down till I have to slow down.
In Range --> Pump on, and then about 1 to 2" MP per NM until I'm slowed down, and try to keep 15" or so until final at least.
Flare --> Power to idle.

This is how I operate, and is somewhat similar to how I operated the IO520F. Just curious as to whether or not I was cracking cases.
 
ppragman said:
This is how I operate, and is somewhat similar to how I operated the IO520F. Just curious as to whether or not I was cracking cases.
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Nothing you can do from the cockpit will have any effect on the case.

Any time you can look at it, make sure you look at stressed areas for cracks, but that's about it. Possibly operating in the cold environment can make the aluminum more brittle and prone to cracking, but you can't do anything about that.


Now, the cylinder heads are another story.
 
I've personally taken out some cylinder heads that looked like they endured the fires of tartarus. Student pilots, not too far off I guess.
 
No worries.
It can’t be something that you have done unless you (or your prop governor) somehow momentary (3seconds) overspeed your IO540 over 10% of its rated RPM which works out to be over 2800RPM (your redline should be 2575)and you would freaking know it happened..............maybe then it might crack.

Crankcase cracking is very common in GA and both Lycoming and Continental put out articles about it. BTW, most crankcase cracks are acceptable by manufactures.what kills it is the oil leak pushed out by crankcase pressure that nothing short of a weld job will fix.

Since the manufactures moved away from the old narrow-deck crankcase (Bulletproof) designs to a lighter and more efficient crankcase designs (prone to nasty looking smiley cracks) they have been slowly improving their casting techniques.


minitour said:
.....lean of peak operation and running "over squared".

-mini
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This equates into one pissed off engine............not happy.
 
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