Dual Engine Failure

Originally posted by cas
a pilot once told me that a single engine aircraft losing an engine is going down……..
in a twin with an engine out the second engine is only there to take you to the crash site??????????????????

Not strictly true, some modern twins have a reasonably acceptable single engine performance but not many and those that do usually require the following conditions to achieve it.

1) a clean aircraft i.e. flaps & gear up, cowl flaps closed

2) being above Vse when the engine fails, a lot of them can’t accelerate on one engine so the only way to get abe Vse, especially on the approach woould be to put the nose down which you wouldn’t want to do if you had just realised that you were too low

3) a pilot who is current on single engine flight in that type and is ready for the engine failure when it happens (if he or she’s flying a twin they should be, just as a single engine pilot should always be prepared for an engine failure)

a pilot once told me that a single engine aircraft losing an engine is going down……..
in a twin with an engine out the second engine is only there to take you to the crash site??????????????????

“Garry, I suggest you reread the above and take note of this, as Moggy says the risk of engine failure is doubled it has to be as you have twice as many engines to quit on you and the bit I really think Moggy “

The Risk of one engine failure is doubled… the risk of “engine failure” is halved.

Not all twin engine aircraft are poor flyers on one engine. The Ka-32 helo can climb at maxiumum payload (and about half fuel) with one engine shut down.

The saying “twin engine safety” applies to where a second engine is added for safety reasons. Obviously if two low power engines are used for fuel economy instead of twin engine safety then of course you will have problems with individual engine failure.

Originally posted by Moggy C
Oh, you really have opened a can of worms there Ewen.

Presuming you meant the risk of any one engine failing is doubled you are correct.

However the consequences are by no means halved.

On the few general aviation light twins with a respectable engine-out performance, and with a pilot who is very current on his engine failure routine and assuming that it doesn’t happen below the blue line the consequences are likely to be much reduced in severity compared to a single. ie a controlled landing on a runway of choice as opposed to a forced landing in the best available field for a single.

But if ANY of the above conditions are not met then the consequences are far, far worse. An off-airfield landing in a faster, heavier aircraft that does not have the bulk of an engine directly in front of the pilot, but rather has a great lump on each wing to drag the lightly protected cockpit through any handy trees etc, is not something to contemplate with any degree of sanguinity.

Moggy
Lapsed twin driver

Garry, I suggest you reread the above and take note of this, as Moggy says the risk of engine failure is doubled it has to be as you have twice as many engines to quit on you and the bit I really think Moggy should have underlined ist that if the pilot is defficient in any one of his required conditions a crash is nigh on inevitable and one thing he didn’t say is that even with some of the most modern twins performance is so poor with an engine out that there’s a damn good chance the live engine will just fly you to the scene of the crash

Originally posted by GarryB
No, 12.5% isn’t the failure rate of each engine, it is the likelyhood of both engines failing, and that was using your calculation, not mine.

The calculation of that resulted in a 25% chance that both engines would fail is the answer, there is no need to multiply that result against the individual engine failure rate.

A few years ago a woman here in the US who has a weekly column and who is on record as having the highest IQ in the world or something was given the question “are multi-engine aircraft more safe?” We’ll she made the ignorant assumption that all the engines must be working in order to fly so rather than multiply the failure rate she added them together and declared a single engine aircraft safer than a 4 engine 747. She received letters from irrate pilots trying to explain the err of her ways but she was sure she was correct. Just goes to show that IQ doesn’t mean anything. 😉

MoggyC, sorry but nothing is making you participate. I rather enjoy bouncing thoughts/ideas back and forth with GarryB. If a thread doesn’t suit you, move on, don’t tell those involved to stop just because you aren’t happy with it.

Aw come on chaps!

This must be the most pointless and boring thread in the history of the internet.

Stop it.

Moggy

“I’m just confused by your post where you show individual failure rate for one engine being 12.5%. The individual engine failure rate doesn’t change with 2 engines, it’s still 50%.”

No, 12.5% isn’t the failure rate of each engine, it is the likelyhood of both engines failing, and that was using your calculation, not mine.

The real problem was that although you were right and I was wrong, you chose the wrong example… by choosing 50% as the failure rate for each of 2 engine you actually chose the only situation where my method would have been right. (and that was due to the fact that as long as only two engines are used and their failure rate is 50% then halving the failure rate of each engine will give the correct failure rate for two… ie half of 50% is 25%… but your method of multiplying the values is accurate but in this case when you multiply anything by 50% you are halving it too, so the results are the same…

I’m just confused by your post where you show individual failure rate for one engine being 12.5%. The individual engine failure rate doesn’t change with 2 engines, it’s still 50%.

Didn’t you notice the results you were getting?

If the failure rate is 50% for each engine then the failure rate for two engines is half… half of 50% is 25%. ie 50% of 50% is 25%.

If the aircraft can’t fly with one engine and is too far from a safe landing area then whether the second engine fails or not has nothing to do with statistics… either the pilot will turn it off or it will failwhen it impacts with the ground/water. Other factors like impurities in the fuel or lack of fuel due to twisted lines or empty fuel tanks mean that twin engine safety can improve safety but not always. Often it is used because the required level of power is not available from one engine or for the purposes of reducing fuel burn by potentially shutting down one engine during cruise flight.

ummmm

RANDOM

thanks

Originally posted by GarryB
The calsulation is correct but your conclusion is wrong. With a 50% failure rate the chance that with two engines both will fail is halved… and half of 50% is 25%. If the chance of both engines failing was reduced by 1/4 then the result would need to be 1/4 of the failure rate for one engine, which is given as .5 … 1/4 of .5 is 12.5% or .125.

:confused: :confused: :confused:

Multiplying the odds together gives you the odds for both failures occuring at the same time. I don’t know where you get process of mulitplying the result against each engines failure rate. What kind of statistic rules are you using? I think you’re over complicating a simple process.

I did a quick search and came up with this website that gives these simple examples of the probability calculation:

http://www.mathgoodies.com/lessons/vol6/independent_events.html

Perhaps you can quote a source for your process.

MoggyC: This is just one of those stats I’ve seen run in publication editorials that states that multiengine aircraft are more likely to crash as they have more engine and therefor more likely to lose an engine. They always see to use false stats to show their point so I’ve spent some time looking into this one.

Is this the Statistics Monthly forum? Have I missed something?

:confused:

Moggy

“Say you have a coin and a dice. What are the chances that the coin will land on heads?

1 in 2″

Actually no, due to the varying weights for the two different sides of a coin it is often not a 50/50 thing but actually slightly in favour of the heavier side. Of course the way it is flipped has far more influence on which will be “Up”.

“The same can be said for 2 engines to fail. If the had a 50% failure rate what would the chances be that both will fail?

1 1 1
_ x _ = _
2 2 4

….1 in 4 or 25%.”

The calsulation is correct but your conclusion is wrong. With a 50% failure rate the chance that with two engines both will fail is halved… and half of 50% is 25%. If the chance of both engines failing was reduced by 1/4 then the result would need to be 1/4 of the failure rate for one engine, which is given as .5 … 1/4 of .5 is 12.5% or .125.

Just being pedantic again. 🙂

The Rules of Statisical Probablity show that mutually exclusive events (not dependant on the other) are multiplied.

Say you have a coin and a dice. What are the chances that the coin will land on heads?

1 in 2

What are the chances that the dice while land on 3?

1 in 6

What are the chances that tossing both will have the coin land on heads and the dice land on 3?

1 1 1
_ x _ = _
2 6 12

…one in 12.

The same can be said for 2 engines to fail. If the had a 50% failure rate what would the chances be that both will fail?

1 1 1
_ x _ = _
2 2 4

….1 in 4 or 25%.

How about the pedantic angle…

“”What are the chances of having 2 engine failures during one flight?”

The risk with two engines is doubled not halved, although the consequences of a disaster should be halved.”

The risk of having two engine failures during one flight with a single engined aircraft would be the risk of the single engine failing twice, which of course assuming it can be restarted after it fails the first time.

Originally posted by Moggy C
… and the marginal engine-out performance of most GA light twins.

That’s putting it politely. I remember being told in my multi-engine training that all the remaining engine will do for you in an emergency is fly to you the scene of the crash.

I don’t think there are too manyaircraft that fall into that catagory thankfully and those that do are used mainly for training as they have docile SE handling. Even though, Light-Twins are my least favorite method of transportation. Give me at least a turbine powered twin and then you have enough power and reliability.

As for the statistics, there are 2 questions one can have with a multi-engine aircraft.

1. The chances of a 1 engine failure? That doesn’t change with additional engines as each engine has the same chance of failing.

2. The chances of a 2 engine failure? When combining 2 engine stats you multiply the failure percentage. So if one engine fails 50% of the time, the chances that both with fail at the same time is 25%.

I think this is what everyone is saying here, I just wanted to spell it out.

Originally posted by GarryB
The risk of crashing due to engine failure is halved with two engines

No it isn’t. You can’t make any prediction about the risk of crashing due to engine failure. Most engine failures do not result in crash, merely a forced landing.

All you can talk about is the risk of having an engine failure. What happens afterwards is up to the pilot

as the likelyhood of two engines failing at the same time are statistically lower than either engine failing on its own.

Correct – but the fact that a twin has one engine still going does not guarantee that the aircraft will not come to grief. Many do due to poor piloting skills and the marginal engine-out performance of most GA light twins.

Moggy

The risk of crashing due to engine failure is halved with two engines as the likelyhood of two engines failing at the same time are statistically lower than either engine failing on its own.

As I mentioned the advantage is only statistically lower as whatever made one engine fail might also effect both engines (ie Contaminated fuel would effect both, or one engine exploding and taking the wing off will mean a crash whether the other engine is working or not etc etc.)

Having a second engine would not effect the likelyhood of the first engine failing or not, therefore the likelyhood of engine failure remains the same.

(Much like if a couple have a 1/3 chance, genetically, of having a boy, they have two girls already, what is the chance of them having a boy when they have their third child? Obviously it is 1/3.

Originally posted by EwenT
The risk with two engines is doubled not halved, although the consequences of a disaster should be halved.:confused:

Oh, you really have opened a can of worms there Ewen.

Presuming you meant the risk of any one engine failing is doubled you are correct.

However the consequences are by no means halved.

On the few general aviation light twins with a respectable engine-out performance, and with a pilot who is very current on his engine failure routine and assuming that it doesn’t happen below the blue line the consequences are likely to be much reduced in severity compared to a single. ie a controlled landing on a runway of choice as opposed to a forced landing in the best available field for a single.

But if ANY of the above conditions are not met then the consequences are far, far worse. An off-airfield landing in a faster, heavier aircraft that does not have the bulk of an engine directly in front of the pilot, but rather has a great lump on each wing to drag the lightly protected cockpit through any handy trees etc, is not something to contemplate with any degree of sanguinity.

Moggy
Lapsed twin driver

Garry

“What are the chances of having 2 engine failures during one flight?”

The risk with two engines is doubled not halved, although the consequences of a disaster should be halved.:confused: