April 2005-
If properly maintained and operated, today’s Continental and Lycoming engines are exceedingly reliable. Having one actually stop is a long shot that most of us pilots don’t like to talk or even think about. If these engines were the least bit prone to stopping, we’d have boats not airplanes.
In just shy of 10,000 hours, including thousands of engine test flights, ferry flights bringing “sick” ones in for repair and flying junk airplanes for an air taxi operator, I have had more than a normal share of engine problems.
I’ve had four complete power losses on takeoff. Only one was an actual engine failure and that was in a Baron. The single-engine stoppages were carburetor ice in a 182, fuel tank feed blockage in a T210 and my apparent failure to use the boost pump during takeoff in a carbureted Cherokee Six that can lead to inadequate fuel pressure to operate the engine.
We read the accident reports and it looks like there are an awful lot of failures. Those reports just say “lost power” or “engine quit” but seldom answer the question as to why.
Truth be known, most of them are due to what I call “wheelhouse trouble.” That is because there was no fuel in the tanks or no fuel in the tank selected. The classic was an accident involving an acquaintance who was repositioning a Bonanza less than 20 miles to buy cheap fuel. All six tanks were dry!
For several reasons, I won’t try to analyze the published statistics on engine failures. One, it has been done over and over again and two, I think the results of the analysis are misleading.
Typically the same basic engine will have drastically different statistics in different airplanes. These are the kind of numbers that caused the drowning of the statistician wading across the creek that had an average depth of one foot.
Catastrophic failure of various components in the engine power train will cause it to stop NOW without warning. You might think that breaking the crankshaft would be the quickest way to stop the engine now. My (mechanical, not piloting) experiences with broken crankshafts show this not to be true.
Most of the broken cranks I’ve seen came from engines that were still running. The fracture of the crank is typically on an angle that, because of the crank being held in place by the crankcase, effectively “keys” the broken pieces so the engine still runs. The actual stoppage of many of these engines was caused by resultant failure of other components such as connecting rods.
These failures were caused by lack of oil flow as a result of blockage by metal contaminants created by the primary failure. The warning signs involved in these failures gave adequate time for precautionary rather than forced landings. The warning signs are decreasing oil pressure, increasing oil temperature and the perception that the engine was “not running well.”
Camshaft breakage on the other hand will cause instantaneous and total loss of power. We think of cam failure as a long slow deterioration of lobe profile that almost never results in a serious power loss but if the cam stops turning, it’s “GAME OVER” now.
The good news is camshafts rarely break. I have never seen a broken Lycoming camshaft but can’t say the same for Continental.
The camshaft will also stop if the gear train making it turn stops. Gear failure is another “no warning” deal, but is very rare.
Probably the most common final cause of failures that stop engines totally is the classic “throwing a rod.” I have seen what appeared to be spontaneous failure of the beam of both Lycoming and Continental connecting rods. That is relatively uncommon now that we’ve purged “the system” of most of the “bad” Continental 360 connecting rods through compliance with Service Bulletins.
Rod bolts have been known to break but again this is rare. Cause can be a defective bolt or improper installation. Again, no warning.
The more common failure of connecting rods is caused by thermal failure of the rod bearing due to lack of lubrication. The lack of lubrication results from blockage of oil passages by metal from another failure such as aluminum from piston pin plugs on Lycomings.
With inadequate oil pressure the bearing contacts the crankshaft and heats until it melts. Once it melts, the clearance between the rod and the crankshaft open up, causing a hammering effect. This is the knocking you hear when this happens. The hammering causes the rod cap or bolts to fail.
Once the rod comes off the crankshaft it usually hits what’s left of it smashing it through the crankcase. Amazingly, while this causes serious power loss, the engine will usually run until the oil is lost through the hole and the main bearings melt. Then the engine seizes and doesn’t turn anymore.
This sounds like a long involved process but I’d say five minutes will do it. The warning is increasing oil temperature, lowering pressure and the knocking. This can also result from a simple bearing failure. I have seen a couple of these in the last year or two, whereas before that I don’t remember one.
We have an IO-360-A3B6D from a Mooney 201 in the shop that had this happen. The owner/pilot was fortunate enough to notice the degradation of oil pressure and temperature rise so when the knocking started he had already diverted toward the nearest runway.
The cost, over and above the overhaul is that of replacing the smoked crankshaft and connecting rod plus flushing the prop, governor and cooler. Still better than replacing a whole engine and possibly dealing with a bent airplane.
By the way, all of the above can result from any total loss of oil. That usually only happens when a hose fails. For Lycoming owners heed the fact that flex hoses should be replaced every five years unless they are the new Teflon type.
I had two separate seven-year-old fuel hoses fail on my Twin Comanche a couple of years ago. Those hoses carried less than 30 PSI. Oil hoses are exposed to more than triple that amount.
Most of our fleet’s engines have two separate magnetos, so immediate engine stoppage due to loss of ignition is unlikely. Loss of one mag can, however, lead to an engine failure. Operation on one magneto causes an EGT increase of about seventy-five degrees.
If you unknowingly lose one magneto while leaned to 75 degrees rich of peak you are now at peak EGT. This can cause detonation. Detonation can erode the piston or burn a hole in it, which then blows out all the oil.
The dreaded “Siamese” magneto has over its 30 years evolved into a fairly reliable product as long as it doesn’t fall off. It is VERY important that installation is done correctly. Some have fallen off!
Loss of fuel flow, or in certain circumstances too much fuel, can cause immediate and total loss of power. Once a carburetor is supplying the proper amount of fuel to a running engine it is highly unlikely, provided the supply to it continues, that it will stop the engine due to lack of fuel.
It is possible under certain circumstances that a power loss or stoppage may occur due to excessive fuel being supplied. This can happen as a result of a sticking float in the carburetor. The power loss will be smooth. The EGT will go off the bottom of the scale.
Given the time, this can be overcome by finding the position on the mixture control that will regulate the fuel down to a level on which the engine can run.
Fortunately, stuck floats seem to manifest themselves during idle operation on the ground. Excessive fuel pressure can cause the same problem by overcoming the operation of the carburetor float and literally blowing fuel into the induction plenum.
Again, this will cause smooth, but most likely, complete loss of power. The pressure gauge will show very high or off scale pressure. The same operation into the idle cutoff area of the mixture control will restore operation of the engine.
Cause for this is a blocked engine crankcase breather. The pump is vented to the crankcase. Loss of breather function can raise crankcase pressure to the point where it affects the pressure regulation of the pump.
Of course failure of the pump to maintain the minimum ½ PSI can cause fuel starvation. That’s why we take off and land with the auxiliary pump on and turn it on immediately any time the engine quits.
The pump on the Bendix (Lycoming) fuel injection system is much the same. It is not part of the fuel regulating system which will operate anywhere from about 14 to more than 30 PSI.
Provided the fuel pressure is in that range, the servo is unlikely to cause fuel starvation or enough excess fuel to cause a power loss unless a diaphragm ruptures. That is very unusual. Power loss due to this would be smooth and show a drastic drop in EGT. Fuel flow can be regulated by use of the mixture control given the time to find the spot where it will run.
Servos can and do once in a while “stick’ internally. This causes the fuel flow to remain at the level it was when a power reduction is made, either leveling off and reducing power or reducing power for descent or landing. Power loss again is smooth and followed by a lowering of EGT.
It can usually be unstuck by returning the throttle to its previous or a higher setting one or more times. It can also be overcome by use of the mixture control although if it loosens after being overcome by the mixture control power, it will be lost by inadequate fuel.
Continental systems are an entirely different ball of wax. The fuel pump is part of the regulating system. If the pump fails or pressure falls to a level causing power loss, boost pump operation is in order, but remember, once the boost is on you have lost that portion of the fuel regulation controlled by the speed of the engine. Close monitoring of the mixture and adjustment of the mixture control will keep that under control.
Too much fuel from a Continental system can also result from an engine-driven pump regulator problem. This is highly unusual and can be controlled by mixture.
It all boils down to this: if the engine quits cold with no warning indications on the gauges, in the seat of your pants or in your ears and unless your dual mag fell off, it is almost undoubtedly fuel related. It’s time to quickly—while still flying the airplane—go through the POH procedures you’ve committed to memory.
When gauges, your posterior or ears tell you all is not right with the engine, it is now up to you to decide to continue, turn back or divert to where you can avail yourself of mechanical expertise. You must decide if you have the experience and mechanical expertise to assess the risk and continue to your destination.
Charles Melot is an A&P who has been involved in the engine business since 1972. President and founder of Zephyr Aircraft Engines, he’s a 10,000 hour pilot with commercial, multi-engine and seaplane ratings.
