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Flying over fog

Peter wrote:

But not (what I thought was) fog.

Having lived in Milan for a year or so, I can assure you that the fog in the ‘pianura padana’ can linger for weeks and cover huge areas. Not pleasant at all. North of the Alps it tends not be as persistent but can also hang around for a couple of days.

Fuji_Abound wrote:

If you selected two different populations, one, let us say where the engines had been blue printed, only the best engineers employed to service and check the engines, and the engines always operated within recommened parameters are you suggesting this populartion would have the same mtbf as a radomn selection?

Well, MTBF is an engineering parameter used in engineering. The premise IS the bathtub curve. Max MTBF is valid only along the flat bottom of that curve. This is “assured” through:

  1. Certification (all components are of equal quality engine vs engine, no components have “child diseases”, everything is working in harmony etc)
  2. The engine is operated according to the handbook
  3. TBO is followed, maintenance is followed, maintenance is done properly and so on

Obviously this is an idealized setting in a way, but it’s not that difficult to achieve. The problem is more that there are no good value for the MTBF for these engines. I have only seen guesstimates, or perhaps numbers made from NTSB reports, which are still only guesstimates because the operating time is unknown. It’s reasonable to assume it’s above 2k h and below 100k h somewhere, but closer to 2k than 100k IMO. According to this, the failure rate of a PT6 turboprop is 12 in a million flight hours. The MTBF then becomes 1/F = 83k hours. I refuse to believe that a Lycoming/Continental/Rotax is more reliable than a PT6, so more than 100k h is pure nonsense IMO.

Also, only a small fraction of failures, even compete failures where the engine stop 100%, cause fatal accidents. Most pilots do after all fly (mostly, lets say 99% of the time) in a manner so that engine failures are survivable. This is IMO the essence of the discussion. What is the risk of flying in a way so that an engine failure is not survivable? The accident statistics won’t really help you all that much, it doesn’t include those numbers. The only thing you “know” is that the MTBF is somewhere between 2k and 100k hours.

As I have shown, even if you assume 100k hours (which must be above the actual number IMO), the risk of flying in a way so you rely 100% on the engine, is way above most other stuff you can possibly do. That is, do on an hour per hour basis.

JasonC wrote:

The average does not describe the shape of the curve or imply a particular outcome for an individual member.

Irrelevant. Max MTBF is what you get when everything is working properly, and the only failures to occur are random failures. You cannot do maintenance to prevent random occurrences. An engine is not a fail safe device. I have said nothing about the average. I have only assumed an MTBF for a well working (top condition) engine. If the engine is not well maintained, you can be sure the MTBF is much closer to 2k than 100k, it could be much lower than 2k in fact.

What you mean is perhaps about probabilities, exposure and risk? Base jumping is highly risky, but you stand no risk if you don’t do base jumping. You stand no risk of dying due to engine failure if you fly so that an engine failure is survivable. But what happens if you don’t?

How we deal with risk is for a large part based on our personality. Some omit risks, others take calculated risks (many pilots) and others have not a big problem with taking risks. In the end, you can decide to stay home or fly and run some form of risk. How much you feel comfortable with depends for a large part on your personality.

EHRD, Netherlands

It should also depend a lot on what you are flying

Administrator
Shoreham EGKA, United Kingdom

AeroPlus wrote:

How much you feel comfortable with depends for a large part on your personality.

Exactly. But also age. For instance, the MTBF of a 70 year old male is what? a couple of weeks? I mean seriously At a certain age, you stand a much higher risk of your heart stopping, or some other acute and deadly thing happens than the risk of the engine stopping. The risk of flying is small compared with the risk of being so old. I think we unconsciously feel this fact. The risk of dying at any minute increases for each day, so the problem becomes more about having a great time while alive, than reducing the risk associated with that time. The risk of living (the risk dying at any time due to no particular reason other than age) becomes so large it will eventually overshadow everything else.

Accidental death is one thing. We can control that to some extent. But age, we cannot do anything about.

I can’t really follow the numbers, @LeSving.

The number of engine failures (other than fuel starvation) in a Lycoming or Continental is around 1 per 10,000 hours (see here, for example). But that is across the entire “bathtub curve”, with real-life maintained and operated engines, not a theoretical MTBF in the flat bit of the curve,

So a “top condition” engine will have a failure rate quite a bit lower than 1 in 10k, and of course a badly maintained one a higher one.

Biggin Hill

AeroPlus wrote:

Some omit risks, others take calculated risks (many pilots) and others have not a big problem with taking risks

Excatly. I can, probably, put myself in the second category. The question that makes the difference for me in this topic is that there is no plan B. I was taught from the very beginning to have always a plan B, and flying half an hour or more with no escape in case of failure is what makes me unconfortable. Even in mountains you can try to save your day when the engine quits, but over fog…

LECU - Madrid, Spain

172driver wrote:

Having lived in Milan for a year or so, I can assure you that the fog in the ‘pianura padana’ can linger for weeks and cover huge areas.

And in Spain, you’ve flown here, we have two big plateau where fog can be present also for several days in a row.

LECU - Madrid, Spain

Cobalt wrote:

But that is across the entire “bathtub curve”, with real-life maintained and operated engines, not a theoretical MTBF in the flat bit of the curve,

There are good arguments that a well-maintained engine doesn’t have a bathtub curve. Statistics show that the failure rate is highest when the engine is new or newly overhauled. After about 500 h the failure rate as stabilised and is essentially constant until the engine is overhauled again – even if this is done well past the official TBO.

ESKC (Uppsala/Sundbro), Sweden

So a “top condition” engine will have a failure rate quite a bit lower than 1 in 10k, and of course a badly maintained one a higher one.

Could be right. It’s roughly 10 times the rate of a PT6 turbine (also across the board by the looks of it). They are certified though. It means the MTBF is around 10k, and the PT6 has 88k h.

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