I’ve read this in an article from the US, from the 1970s. It sounds like it was a well known thing back then.
I have also heard many times that failures occur mostly when power settings are changed.
What would be the physical/mechanical explanation for that?
Old Instructor’s wife tale. Never have seen any evidence that supports this.
The only reason I could think of, although extremely rare might be the following:
Some piston engines have a feature that enriches the mixture at max power.
If such engine is badly calibrated (too low fuel flow at max power or not efficient cooling for example) then during takeoff it may be already at critically high temperatures (CHT).
When the pilot reduces power – the mixture becomes leaner and CHT goes up and at proper conditions some cylinder may fail…
Statistically very rare and the failure as always requires a chain of some events.
Peter wrote:
I have also heard many times that failures occur mostly when power settings are changed.
I would think nothing wears and rattles (well designed) complex machinery more than starts and stops (vibrations, thermal changes, lubrication etc). At least that is the case for big industrial stuff. Power changes may have a similar effect perhaps, but nowhere near starts and stops.
I think it makes sense.
For example if you have any induction leak, you are likely to have a problem with manifold pressure reduction where as everything will be fine at wide open throttle. Yes you would probably see this during run up and not take off anyway but i am just speaking of theoretical cases where power reduction could cause an issue.
Another scenario could be failure of the fuel servo ball valve. If it gets stuck during take off with full fuel flow, reduction of manifold pressure would cause engine to run extremely rich and even stall. This actually happened to me mid flight and became a serious issue during landing. During run-up everything was fine.
I am sure there are other scenarios which can turn into a problem..
Generally I am not a fan of changing any configuration during the critical phases of take off. If everything runs fine, don’t touch it until a safe altitude. Safe attitude is not 500 or 1000agl where people tend to reduce power for noise abatement. The engines are designed to run continuously at max power.
Simply not true!
If you look at European accident reports of the last 10 years, it is hard to find any case of engine failure that happened at first power reduction (i.e. typically shortly after takeoff).
From the limited data we have (as mechanical engine failures are very rare events), the opposite seems to be true: Engine failures happen moist often after some time of cruise flight – presumably partly due due thermal stress of a hot engine (but this “explanation” is as sound as the “explanations” for the obviously wrong assertion that engine failures mostly happen at first power reduction)
There are several scenarios which result in EFATO (mis fuelling, component damage which becomes apparent due to the counterweights on power reduction, etc) that is why we practice EFATO on both single and multi.
It is self evident statistically based on exposure time that more engine failures occur in the cruise phase than EFATO.
Ham fisted power reductions can cause damage due to the counterweights in these large volume low compression engines.
It reminds me more our friend’s adventure in St Johns while crossing the Atlantic. It occurred at power reduction for descent.
I think it’s important to eliminate coincidence.
In the one serious EFATO that I’ve had, it happened about when most people with CS props reduce the power (about 500 feet, we hadn’t touched the power), but that’s just when it happened to occur – the fuel pump would have died anyway whether the power was reduced or not (and incidentally, in many low wing planes, people are taught to turn the electric fuel pump off about the point where they would reduce power to 25" MP). Similarly, mis-fuelling incidents – enough Jet-A is mixed in with the avgas to stop the engine just about when many with CS props would reduce the MP to 25 inches. But again, pure coincidence. There are many scenarios where you can have the bad thing that’s going to happen already cued up before you even start the engine, and it’s just coincidentally going to happen about 30 seconds to a minute after takeoff.
I don’t recall reading a single NTSB report or AAIB report where the cause was something directly to do with the power reduction, if power reduction was mentioned at all, it was coincidental with the failure not the cause of it.
Malibuflyer wrote:
From the limited data we have (as mechanical engine failures are very rare events), the opposite seems to be true: Engine failures happen moist often after some time of cruise flight – presumably partly due due thermal stress of a hot engine (but this “explanation” is as sound as the “explanations” for the obviously wrong assertion that engine failures mostly happen at first power reduction)
Could the simple reason for this be that an engine spends a lot more time in cruise than the 5 seconds during “first power reduction”.
Lycoming Flyer Operations page 65 tries to put this wifes tale to death:
