Why not just drop an engine on fire?

Question

I am interested in the case of Qantas Flight 32 and similar incidents, where there was a fuel leak after a number two engine explosion due to the disintegration of the turbine disc. It was a very dangerous situation since the fuel leaks were very close to the failed, then burning, engine. And to make the matter worse, fuel dumping system was down due to hydraulic failure.

Although the crew managed to shut it down, the engine took considerable amount of time for fire to be extinguished completely.

I wonder in those kind of situations, could adding eventually a "drop-engine-on-fire" feature make the situation less dangerous?

You may consider the following:

• Modern airplanes could fly with one engine out.

• Dropping the engine must be in water to not cause any harm to the people below.

• Usually, the engines are mounted on the wing only at three points.

• Even if the main evidence of the investigation (that is, the burning engine) will be lost and maybe never found, I think that saving lives is a bit more important.

EDIT :
In response to comments, I think that every feature (even if its seems silly) is subject to pro-con studies. For example, Boing 747 had fuse pins that are designed to break cleanly in case of excessive loads/vibrations.

Answer 1

The current position of regulators like the FAA is that dropping things off of airplanes in flight is a bad thing. Dropping extremely large and heavy things like engines would be extremely bad. This poses an unacceptable risk to people and property on the ground (or even in the water). Most aircraft with engine fires do not kill anyone on board, so why risk killing people on the ground?

The preferred method of dealing with engine fires is similar to that of fire protection for buildings; take steps to prevent them, and provide methods for putting them out should they start. We don't go blowing up buildings that are on fire just because it might be quicker than putting it out.

So instead manufacturers are required to provide many features to prevent fires. Areas around the engines are separated into zones by firewalls and seals, required to prevent spread of fire. Any flammable fluid is required to be contained and drained, and separated from ignition sources.

If a fire does start, the engines have fire suppression systems installed. The system has to be tested to show that it provides the required suppression to a fire, and the aircraft generally carries enough suppressing agent for at least 2 uses. Even if this doesn't work, the isolation features built in should prevent the fire from spreading, and possibly allow it to burn itself out of fuel or oxygen. In most cases an aircraft is close enough to an airport to land before this becomes an issue.

These measures have been shown to be adequate. When was the last time someone died because of an engine fire on an airplane? Fires are extremely rare, and fires that can't be dealt with by the existing measures are even more rare.

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There are many more reasons why dropping engines is a bad idea.

Some designs allow an engine to shear away from the wing in the event of a crash, but this is not designed to work in flight. Aircraft that drop items like weapons in flight require extensive design and testing to make sure that the object will drop away from the aircraft properly. An aircraft in an emergency like an engine fire could be making extreme maneuvers that would make this even more difficult.

Objects like weapons that are designed to be dropped regularly still malfunction. How can you make an engine that's been installed for decades separate reliably? If you are relying on this feature for safety, there will be added cost for designing, installing, and maintaining the system. Those costs could be reduced if you don't rely on the feature as much. But then if the current measures are enough, why accept that extra expense?

Investigators generally take interest in engine fires. If you drop the engine, there goes the best evidence for figuring out how the fire started, and what could have prevented or mitigated it. Something large like an engine would be easier to find than smaller parts, but there is still the risk of loss or damage hindering the investigation. If the problem is bad enough that airplanes are dropping engines, it would be better to fix the problem than address the symptoms.

If you constrain pilots to only dropping an engine over water, this complicates things further. There are already rules about where airliners can dump fuel, but dumping fuel is generally a less stressful situation. The pilots chose to stay in the air and dump fuel, so any emergencies are generally less urgent and they have time and attention for where they could dump fuel. If there's an engine fire that requires jettison of an engine, things are going to be more hectic in the cockpit, and it may not be the best time to be considering where this would be allowed, taking things like trajectory into account. And going back to the cost issue, if this would be limited to only areas over water, you couldn't rely on this option to always be available, so is it worth the cost?

Airliners can fly with an engine shut down, but missing an engine is a different matter altogether. The aircraft is required to be tested and certified in many different conditions with an engine shut down. How can we test this with a missing engine? Do we drop an extremely valuable engine every time we test? Do we just make the flight with one engine, which is still in testing itself? Do we not test, and accept that after going to all the trouble of allowing jettison of an engine, the aircraft may not be controllable in some situations?

Answer 2

The fact that the modern aircraft could fly with one engine out does not mean that the offending engine should be dropped. It is better to have it in the aircraft rather than risk the engine taking off with a chunk of aircraft. After all, in case of El Al Flight 1862,

... engine no.3 and its pylon separated from the aircraft and damaged part of the leading edge of the right wing. The no.3 engine then struck engine no.4, causing this engine and its pylon to depart the wing.

Uncontained engine failures are rare and in case of most engine failures, the fire, if there's any is extinguished quite quickly. Also, in most cases, dropping of the engines is not going to wait until a (suitable) water body is found. In case of failures during takeoffs/landings, like the American Airlines Flight 383 or the British Airways Flight 2276, the pilot will have one more thing to worry about.

Then there's always the possibility of uncommanded detonation and failure to detonate when commanded- this will add further checks to the maintenance schedule. Once the engine has been jettisoned, the fuel and other lines have to closed. All this adds complexity to the system, which may well be not worth the cost.

Finally, the engine weighs quite a lot and dropping it would affect the aircraft's weight and balance. This could be a problem if the aircraft is maneuvering, requiring the pilot to take this sudden change into account.

Answer 3

A major issue is that it's not really the engine itself which is on fire, in fact they are supposed to be on fire in a sense. In many cases an engine fire really means that you have lost control of the fuel/hydraulic fluid flow to the engine. For example if a turbine blade detaches, penetrates the engine casing and cuts a fuel or hydraulic line.

Indeed in the example in the question the damage was caused by the original engine failure and resulting fragmentation rather than a fire in the engine itself as such.

Even if you drop the engine you've still got fuel spewing out and burning so unless you can stop the fuel leak at source you haven't really solved the underlying problem and if you can cut off the fuel dropping the engine doesn't achieve much.

If you've got a leaking tap cutting the tap off with a hacksaw isn't a solution.

You are then also in a situation where you have created a whole new risk of accidentally dropping the engine. There are also lots of fuel, sensor, hydraulic, compressed air and other connections to a jet engine and unless you can guarantee separating all of these cleanly you run a very real risk of doing even more damage or even worse ending up with an engine half hanging off.

There is also the fact that while aircraft are deigned to fly with one or more engine failures actually dropping and engine is going to have a fairly dramatic effect on the trim of the aircraft just in terms of the missing mass.

Answer 4

The first reason I can think of is that an aircraft engine on an intercontinental jetliner weighs about 6000-10,000 lbs depending on the type and simply jettisoning the thing would significantly affect the weight and balance of the craft, making things worse. An object that size simply released from the jet presents other hazards such as tumbling and then striking the airframe upon release, causing catastrophic damage and possibly departure from controlled flight.

Answer 5

Any time the question is "why don't we have...", the answer is almost certainly in the trade-off between weight, maintenance cost, fuel cost, & utility. If it weighs a lot, needs maintenance, burns fuel, and will only be useful once a decade, then it just is not productive.

Putting an engine release system on a plane will be heavy, meaning fewer passengers, or less fuel or cargo on board. All that extra weight has to be flown around, requiring fuel. The release mechanism would need regular maintenance checks, which is another cost. And it would only be useful very, very infrequently (on most planes, it would never be used).

So it is just impossible to justify adding new equipment for a once-in-a-lifetime scenario, which can be better managed with good maintenance, good training, and good planning.

This same line of reasoning goes for "why don't more airplanes have rocket thrusters?", "why don't planes have anti-missile systems?", "why don't planes have airbags?", "Why don't airplanes have parachutes?" and many other random items.

Answer 6

I think the "uncontrolled bomb" answer is probably sufficient on its own but there are also technical reasons.

In order to be able to drop the engine you need to build the engine with a rapid release system that will safely, reliably and near-instantly disconnect the engine and all the wires and fuel lines connected to the engine. What's more it needs to be able to do this while it is on fire following an emergency failure that caused the fire. It is not obvious that this is actually possible, but even assuming it is you are adding a new complex system to the engine assembly that is, itself, capable of failure and where failure has potentially disastrous consequences.

Thus its possible, likely even, that adding an "engine eject" module to a plane is likely to increase the overall risk to the plane rather than decreasing it.