Explosive Bolts for Aircraft Engines?

[Martin-123]

TL;DR Summary

Obviously not, but why not?

Hi All. The idea has obviously been considered and rejected, so I'm just asking for someone to educate me on why having the ability to (manually) dump the engines of an airliner using explosive bolts, in the event of an engine fire or explosion, is not a good idea.
Surely this would considerably extend the range of the plane for the emergency landing that follows and, in the event of ditching on water, considerably reduce the deceleration and stress on the airframe on initial contact as well as considerably improve the buoyancy of the plane when it comes to a halt. The engines could also be fitted with parachutes so as to minimise ground casualties.
Many thanks for any answers.

 

[Baluncore]

Welcome to PF.

Because;
1. Engines are safe, once they have exploded. Too late then to dump.
2. Good engines would be embarrassingly dumped, accidentally.
3. Valuable evidence of the engine failure mechanism would probably be lost.
4. The system would increase the weight and fuel consumption of all aircraft.

 

[Martin-123]

Welcome to PF.

Because;
1. Engines are safe, once they have exploded. Too late then to dump.
2. Good engines would be embarrassingly dumped, accidentally.
3. Valuable evidence of the engine failure mechanism would probably be lost.
4. The system would increase the weight and fuel consumption of all aircraft.

Thank you Baluncore.
1. Failed engines contribute considerable drag and weight in the air and on water.
2. Double control mechanisms and pilot training?
3. Parachutes and floatation devices?
4. True. Also design, build & maintenance costs.

Tia.

 

[Baluncore]

1. Failed engines contribute considerable drag and weight in the air and on water.

Balance and symmetry are more important during an emergency landing.

Twin engine aircraft are specified to fly on one engine. Range extension by lowering drag is not required.
https://en.wikipedia.org/wiki/ETOPS

 

[berkeman]

I can see it now...

Captain: "This is your Captain speaking. We're flying through a storm right now, but other than a bit of a bumpy ride, we don't expect any problems."

<<Lightning hits the plane, causing all engines to be blown off...>>

Captain: "Hi again folks, we have a change of plans..."

 

[Baluncore]

<<Lightning hits the plane, causing all engines to be blown off...>>

I imagine they will be stuck up there, until someone can deliver and fit a couple of spare engines to get them home.

 

[Vanadium 50]

How many accidents would this have prevented? Can you point to five where it would have? Or even might have?

How many near misses have there been where the wrong engine was shut down - this would mean the wrong engine would be blown off instead - turning a near miss into a crash.

It seems that you have some homework to do.

 

[Flyboy]

Speaking from an aviation background, there is literally no reason to use this. If it’s severe enough damage to warrant losing the engine, odds are it’s gonna leave the plane just fine on its own.

Explosive bolts, or any other pyrotechnic separator, are life limited, have very strict handling requirements, are weaker than a standard bolt even without the pyrotechnic charge included, are a MASSIVE failure point…

If it was even remotely practical, they probably would have at least tested it in the past. The fact that they haven’t is quite telling.

 

[Lnewqban]

The idea has obviously been considered and rejected, so I'm just asking for someone to educate me on why having the ability to (manually) dump the engines of an airliner using explosive bolts, in the event of an engine fire or explosion, is not a good idea.

Welcome, @Martin-123 !

It is not a bad idea, but it has many practical obstacles.
Besides the structural one, such an engine has many connections to the wing/fuselage of an airliner.

Only one of those practical obstacles is to achieve the disconnection of all those many wires, tubes, cowls, ducts and hoses at once.

If only one of those disconnections fails, we could have a dangling heavy engine and a very difficult to control rest of the airplane.

 

[mfb]

Large rockets have a "quick disconnect" system between launch tower and the rocket, supplying the rocket with propellant, electric power and a data connection. It separates at takeoff. You don't want to cut the connection earlier because scrubs (aborted launch attempts) can happen. It's a complicated setup, and that is not load-bearing.

There is essentially no situation where dropping an engine would have a significant advantage, and such a system would add a lot of weight, cost and complexity to an aircraft.

 

[Martin-123]

Thank you
Lnewqban, mfb and other responders.

I have to admit to being slightly disappointed. It's such a cool idea, but if the advantages aren't there, then they aren't there.

... I have another engine question for my next post...

Many thanks to all.

 

[Arjan82]

An then there is of course the added risk that something goes wrong in the releasing process and the engine tears off a part of your wing. That's not very helpful...

 

[Vanadium 50]

Captain: "Hi again folks, we have a change of plans..."

That's not how they do it.

Captain: I have good news and bad news. The good news is free drinks for everybody!

 

[russ_watters]

You don't get credit for a joke that took you a month to think of, @Vanadium 50

Anyway, something else nobody mentioned: Even if it didn't throw off the balance and make the plane uncontrollable, it (counterintuitively) would not change the glide distance. The speed increases but the angle stays the same because the angle/slope is whatever the most efficient performance angle of attack is for the wing. More lift comes from more speed, which means you get to the ground faster, but in the same distance.

Glide ratio is not affected by weight because, while a heavier glider sinks faster, it does so at a greater airspeed. The glider descends faster, but covers the same horizontal distance (at a higher speed) as a lighter glider with the same glide ratio and starting altitude.

https://www.faa.gov/sites/faa.gov/f...manuals/aviation/glider_handbook/gfh_ch05.pdf

 

[Baluncore]

The speed increases but the angle stays the same because the angle/slope is whatever the most efficient performance angle of attack is for the wing.

What you write is true for weight, but not for drag.

The engines have mass, thrust, and drag. While an engine is operating, it is compensating for its drag. Engines also interfere with the clean airflow passing the wing. Dropping an engine will reduce the aircraft drag, and may improve the performance of the wing.

The parallel with gliders does not hold, because gliders do not have engine drag that can be shed. They have speed brakes that can be activated. Speed brakes or spoilers increase a glider’s drag and sink rate, just as do the inoperable engines on a commercial aircraft.

I am not arguing that the facility to drop a failed engine from a commercial flight should be provided.

 

[russ_watters]

What you write is true for weight, but not for drag.

The engines have mass, thrust, and drag. While an engine is operating, it is compensating for its drag. Engines also interfere with the clean airflow passing the wing. Dropping an engine will reduce the aircraft drag, and may improve the performance of the wing.

Granted, though I doubt the significance of either factor is very high:

I wonder what the drag impact of a freewheeling engine is (vs no engine). I doubt that it is very significant as a fraction of the total airplane's drag, as the friction in the engine is very low.

I also doubt that the impact on the wing is very significant since it is hanging below the wing. But I don't know how significant it is.

I'll add that I did an actual power-off landing last year and noticed the difference in drag between idle and actual power off is significant. But on a piston/propeller plane it's much more significant than it would be on a jet.

 

[Baluncore]

I wonder what the drag impact of a freewheeling engine is (vs no engine).

The engine drag will be determined in part by the bypass ratio. Any air that enters the higher compression - combustion path, to exit via the cold turbine, will be inefficient, resulting in significant drag.
Bypass air, that windmills the first compressor, will offer some drag, but not as much as a locked first compressor shaft.

I also doubt that the impact on the wing is very significant since it is hanging below the wing. But I don't know how significant it is.

A quick estimate. 2/3 of the lift comes from the upper surface pressure reduction, while 1/3 is from lower surface pressure increase. (Which is why the engine is below the wing). Assume 10% of the under-wing airflow is obstructed by the engine nacelle, and we get 10% / 3 = 3.3% loss of lift.

Engines above the wing would cost 3.3% more to run than engines below the wing, and it would preclude the option to drop the engines in an emergency.

 

[russ_watters]

A quick estimate. 2/3 of the lift comes from the upper surface pressure reduction, while 1/3 is from lower surface pressure increase. (Which is why the engine is below the wing). Assume 10% of the under-wing airflow is obstructed by the engine nacelle, and we get 10% / 3 = 3.3% loss of lift.

The 10% feels a bit high to me, but regardless 3.3% is a pretty small number.