Why don't aeroplanes automatically put out engine fires?

Question

When an engine fire is detected the fire handle lights up and the pilot pulls it to extinguish the fire.

Why doesn't it do that automatically? Why does the pilot have to pull the handle?

Answer 1

Shutting off the computer is difficult from a technical implementation standpoint. You're handing over a lot of power to the computers and sensors letting it shut down an engine, where these usually do not have access and control. This is certainly not always a good idea.

• Pulling the fire handle is a pretty aggressive move: It will typically disable hydraulics, electrical power, and bleed air coming from that engine. If you're not prepared for it, it can end badly.

• The engine will often be partially functional, depending on the type of failure. If you get dual engine fire indications, you'd probably wait with shutting them down, if doing that at all. This sort of behaviour would have to be coded into the flight computers.

• I don't think there are any catastrophic consequences of not switching the engine within 8 seconds rather than maybe 2 seconds. It does not make a great difference. To set off the detectors, it must already have spread a little. I don't think you could do it much faster with the pilot staying updated on what's going on.

• Needless to say, computers sometimes get things wrong. There is a human level to check what's going on before proceeding.

• There are times when you don't want the computer doing it's own thing. Airbus states that:

  If an engine fire occurs during takeoff or go-around, the Pilot Flying shall first establish and stabilise the aircraft on a safe climb path and then proceed with the ENG FIRE procedure.

Footnote: It could certainly be done, but with the pilot-computer relationship on current flightdecks it's not a good idea.

Answer 2

There are a few reasons why this might not be a good idea:

• An engine fire in a typical jet aircraft does not necessarily need to be shut down immediately. Most of the jets that I have flown include a concept along the lines of "Shut down the engine if you are not in a thrust critical situation." There is a saying that you never want to accomplish anything fast in a jet airplane (other than put on your oxygen mask if the cabin depressurizes). It is possible that shutting an engine down could put you in a worse situation than letting it burn a little longer while you climb above an obstacle.
• There are times when there is a false indication of a fire. This could be caused by a faulty sensor or a hot bleed air leak on the engine. In this case, you don't always need to shut the engine down. Some aircraft include reducing the power to idle first to see if the fire indication goes away.
• We don't want a faulty computer to shut down a perfectly good engine. Many fire handles are mechanically connected to the fuel shutoff valves making it so that it is nearly impossible to "accidentally" activate this feature. This design also allows you to shut down the engine even if there is no electrical power whatsoever.

Answer 3

You think about the danger of a fire. The engine is still fed with fuel and the fire maybe damages other parts of the aircraft or leads to an explosion. For sure these danger is real, but on the other hand we are talking about losing an engine, half the thrust of modern airliners. There are two reasons, I can think of, why this is not done automatically.

• Even though the engine is on fire it is still producing thrust. Not as much as a good engine, with an extremely high probability to fail completely and for sure a bunch of other risks, but still thrust you maybe need to clear obstacles, or to escape any other danger. Think about an engine fire occurring during take off (past $V_1$), on initial go-around, or while escaping a downburst, situations you really don't need a loss of thrust. Also on takeoff roll prior reaching $V_{MCG}$ (minimum control speed on ground) you first like to have your directional control and the possibility to safely decelerate the aircraft prior to securing the engine.

• For sure you are aware that it is a hard decision to shut down an engine and also my first point will remember you of some of the dangers doing it, but in some situations (e.g. fire) it have to be done to prevent further damage. This decision should not be done by computer because this would just be an extra chance that something goes wrong. This system would need a lot of logics, not only 'over temperature -> disconnect the engine, shoot a bottle'. Why should this be done by a computer, which possibly extinguishes a good engine during climb out, only because of a faulty indication, or a computer error if there are two well trained pilots capable to deal with the situation?

You already mentioned the 'flow' how to extinguish an engine fire on most modern airliners. Identify the problem, close the throttle (cut off the fuel) pull the fire handle, see what happens, eventually shoot a fire bottle, or even the second. That's really no hard work but a bunch of factors which needs to be taken into account. In my eyes this is still a job which should be in the hand of the flight crew.

Answer 4

The other answers here do a good job of explaining why the pilot should be able to decide when and if to put out an engine fire. This affects other design considerations as well. The potential for fire on an airplane is taken very seriously.

The pilot may decide to not put out a fire right away, but the fire suppression systems could also fail. To address this risk, airplanes are designed to minimize as much as possible the danger posed by a fire. This is described in AC 20-135:

5. a. The primary objectives of fireproof and fire resistant materials and components are to contain and isolate a fire and prevent other sources of fuel or air from feeding the existing fire, and to ensure that components of the engine control system will function effectively to permit a safe shutdown of the engine or APU and safe feathering of the propeller.

Components in fire zones near the engine or APU are required to be fireproof or fire resistant, which means maintaining functionality in the following conditions:

• Fireproof: 2000°F flame (+/-150°F) for 15 minutes minimum
• Fire resistant: 2000°F flame (+/-150°F) for 5 minutes minimum

Engine controls are generally required to be fire resistant, allowing the engine to be controlled or shut down and a fire extinguished even up to 5 minutes after a fire starts. Firewalls are generally required to be fireproof, containing the fire long enough to provide the pilot a chance to land the plane, or hopefully for the fire to burn out on its own, before spreading to other areas in the plane. This also allows for the fact that the fire detection system could take some amount of time to detect a fire, depending on the location and severity.

The AC goes into more detail about specific requirements and testing procedures. Of course the FAA has a section of regulations on fire protection for airliners. The goal is to give the crew as much opportunity as possible to ensure the safety of everyone on board as is reasonably possible.

Answer 5

Automation is not always the best idea. There was an incident at an airshow where the pilot was doing a low, slow flyby in a commercial jet (I believe it was one of the newer Airbus models), and he applied full power to climb away, and the computer decided that it was an incorrect control movement. Needless to say, the aircraft crashed.