The end of the avgas piston twin?

JasonC wrote:

I assume the loss of one piston engine increases risk to the other due to the higher power setting needed to fly OEI.

As I wrote, it’s only for amusement. E.g. one would think that a real engine would have a “memory”. You expect the classic “bathtub” – increased failure rate at the beginning (errors in assembly, faulty parts, etc.), then period of calm and increase in failure rate as the engine is getting old (parts are worn out, corrosion shows it teeth, etc.). You would need actual numbers for more detailed analysis. I wanted to show how redundancy works with respect to MTBF/ MTTF. It’s trivial to calculate your own number with whatever degraded value you want (just replace that 1000 with say 700). And excuse the use of period as a separator, I forgot I’m writing in English and you guys have it the other way around.

RobertL18C wrote:

Fuel mismanagement/contamination/icing is possibly the main cause of double engine failures on twins?

I think it’s a big factor even in singles. I think the only benefit a twin can bring in this is removal of the need to alternate tanks (since you have two consumers; well, assuming both engines are running).

JasonC wrote:

I assume the loss of one piston engine increases risk to the other due to the higher power setting needed to fly OEI.

and correlation between failure modes (one engine failed due to a reason, and there is a reasonable chance that that reason affects the other engine too (bad maintenance being only one of them))

Martin wrote:

You expect the classic “bathtub” – increased failure rate at the beginning (errors in assembly, faulty parts, etc.), then period of calm and increase in failure rate as the engine is getting old (parts are worn out, corrosion shows it teeth, etc.).

But you don’t get the bathtub with aero engines due to regular inspection and repairs.

Noe – I think reasonable chances puts it too strongly. Obvioulsy fuel icing could manifest itself in both engines around the same time, but most other causes of engine failure, other than pilot induced, would seem unlikely to happen at the same time or close to the same time. Fortunately I have only had one engine failure on a twin which was caused by a bolt becoming detached. The bolt was subsequently wired on all aircraft but both engines were the same at the time. However the chances of the bolt detaching itself in both engines at the same time and the bolt ending up in the same place at the same time must have been infinitesmally remote. Even running the second engine at or near full power seems unlikely to upset the apple cart. After all in many twins in the cruise this will not be necessary anyway. Moreover in most twins the engine will be excercised at full power on every departure. Moreover if the aircraft is used for any degree of training then one or other engine will frequently be “failed”. I guess there could be an argument that on types used for training where the same engine is usually failed there is some increase in the risk of failure of that engine in the cruise, which, in some types, I guess may not be a bad thing if an engine has got to fail.

Sorry, to clarify, I didn’t mean that the chance that the 2nd engine stops is reasonable, just that given you have a failure on one engine, the chance of the other engine failing is increased (not necessarily by a huge amount).
Put it this way: If I had an engine failure in a twin, putting all assymetry flying aside, I’d be more concerned that the 2nd engine failed that if I were flying normally on a single engine plane.

Noe wrote:

If I had an engine failure in a twin, putting all assymetry flying aside, I’d be more concerned that the 2nd engine failed that if I were flying normally on a single engine plane.

Why?

Because at that point I wouldn’t know the cause of the failure of the 1st engine, and think whatever caused it might also affect the other engine. (For instance poorly done maintenance)

Airborne_Again wrote:

But you don’t get the bathtub with aero engines due to regular inspection and repairs.

It should still be true for the engine itself. Even though scheduled overhaul cuts it off at some point (it stands to reason it should be before the rate increases too much; don’t ask me what too much is). I don’t want to consider what preventative maintenance (e.g. top or turbo overhaul) does to the curve – on one hand, you take a detailed look at a potentially worn component (in case of an overhaul), on the other hand any work carries the risk of causing a failure. Point was that the assumptions don’t hold in the real world.

Noe wrote:

Because at that point I wouldn’t know the cause of the failure of the 1st engine, and think whatever caused it might also affect the other engine. (For instance poorly done maintenance)

OK, but that was my point. Even if it were poor maintenance the chances of the second engine failing within the same flight window seems remote, not withstanding the engineers managing to create a similiar problem on both engines, which also seems unlikely. In reality you would also want to land as soon as possible so even on a longer trip it is likely that you would not be running on one engine for more than an hour, and probably a lot less, so the second engine would have to fail in pretty short order of the first. If you wish in the case I had exactly the same fault existed in both engines, but two things had to happen at the same time to cause the failure, and then those two things would need to happen in the same flight window when neither had happened in the previous 300 odd hours.

Of course the first thing that enters your mind is are there any reasons that caused the failure of the first engine that might result in the failure of the second, but as I intimated earlier, there arent many scenarios where that would be likely.