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Flying over fog

What’s you view on flying a long time over a large fog layer?
In the rare event of an engine failure, it is clear for me that you will have little opportunity to make it safe to the ground. On the other hand, you are also not vary safe flying over rugged mountains, extense forests, etc and I do it. But everytime I found myself with a long fog layer ahead I’m not comfortable going on with the flight.

LECU - Madrid, Spain

Statistically, it’s negligible; but after addressing all significant risks, one could mitigate this one by flying aircraft with a low stall/approach speed – or a parachute.

Glenswinton, SW Scotland, United Kingdom

Or twin engine

LDZA LDVA, Croatia

It is unusual to get a fog layer that has no breaks at all. There are (almost) invariably hill tops and other features that you could see enough of to make a reasonable go at a forced landing. I guess that you could fly from one bit of exposed ground to another.

But to me it seems no worse than night.

If you don’t want to be in a position where an engine failure in the cruise exposes you to risk, you should be looking at a twin, and never letting your fuel state get lower than about an hour.

EGKB Biggin Hill

For me over the sea October to April, mountains, fog and at night are all uncomfortable, however statistically remote engine failure. It is one of the reasons it is comforting flying a twin, even though I know that the odds are so small that it is difficult to make a rational case.

Fuji_Abound wrote:

even though I know that the odds are so small that it is difficult to make a rational case.

Compared with what? The MTBF of a piston aero engine is somewhere between 10 and 50 k hours (as far as I can remember). Let’s say 25k. So, one hour over fog has a probability of (roughly) 1/25000 = 0.0004 of failing (stopping), or 0.04 % or simple one in 25 thousand. It’s like playing Russian roulette with a whole lot of empty rounds, still a very countable lot. Remember, it is only one hour of your entire life, and that hour gives you a 0.04% chance of dying.

The probability of dying in a car accident (as a passenger or driver) is roughly 1/600 for you entire life. Lets say you drive a car 200h per year for 50 years. This means 10k hours with a 1/600 chance in total, or a chance of 1/6000000 per hour, or 1 in 6 million hours. Flying over fog in a SEP for one hour is 240 times more risky than driving a car for one hour In other words, you have to drive a car for 240 hours to get the same total risk as flying over fog in a SEP for one hour.

Negligible risk? maybe, the acceptance of risk is highly subjective. But it’s definitive not difficult to make a rational case when you start comparing with other things you do. The same risk in one hour as 240 hours in a car (more than a year’s driving), is, well, something

IMO the actual risk isn’t the issue though. The main thing is a risk factor you have zero control over. You cannot do a thing to increase the MTBF of an engine. It’s set in stone, cold hard statistics. The only thing you can do, is to always have a plan B in case the engine should stop. That plan B is now thrown out the window.

Last Edited by LeSving at 29 Oct 19:49

LeSving wrote:

Compared with what? The MTBF of a piston aero engine is somewhere between 10 and 50 k hours (as far as I can remember). Let’s say 25k. So, one hour over fog has a probability of (roughly) 1/25000 = 0.0004 of failing (stopping), or 0.04 % or simple one in 25 thousand.

Sorry, that’s simply nonsense, as it implies flying only over fog. You would have to find a statistic how many hours are spent flying over fog as a percentage of the entire life time of an engine and then apply your calculation to that. I venture to say it’s close to zero.

172driver wrote:

Sorry, that’s simply nonsense, as it implies flying only over fog.
It doesn’t. LeSving says that every hour you fly, the chance of engine failure during that hour is 0,04%. If you take (for the sake of argument) the chance of surviving an emergency landing in fog to be 10% and the chance of surviving an emergency landing otherwise is 90%, then it follows that one hour over fog is as risky as nine hours over clear weather.

It is very unusual to fly over fog. I can only recall having done so for about 30 minutes during my 800+ hours of flying, so for me the additional risk has been negligible. I would have no hesitation to do so again as long as it stays a very unusual occurrence.

ESKC (Uppsala/Sundbro), Sweden

The problem with this type of statistical analysis is also that it deals with a mixed population ranging from high hours poorly maintained engines, to engines operated by pilots with poor skiils. The stats would be very different for well maintained engines operated by skilled pilots. There is still a risk, but, as ever, there are various ways of mitigating a risk including by eliminating as many factors that contribute to increasing the level of risk.

Fuji_Abound wrote:

There is still a risk, but, as ever, there are various ways of mitigating a risk including by eliminating as many factors that contribute to increasing the level of risk

The MTBF is valid for the straight bottom of the “bath tub curve”. Failures are purely random. There is nothing you can do to improve the failure rate. Even if the MTBF should be 100k hours (more closer to turbine category airplane). One hour over fog would still be about 60 times more risky than one hour in a car.

But as AA say. It’s not often you fly over fog (with patches large enough to prevent you from gliding out of it). I have never experienced that, other than fog at sea, which is easy to stay out of. In a life time the total risk is close to negligible. Still, the risk is very real minute by minute, and it is higher than most other things you can possibly do. Except traffic accidents (like passenger or driver in a car), the only thing more lethal (risky) is being unintentionally poisoned, by whatever substance (drug addicts have a much larger chance of course). Flying over fog in a SEP also beats that, and the only cause is random failure of an otherwise perfectly maintained engine.

But again, it happens seldom, so in total it makes little difference.

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