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Airplane engines catch fire from time to time (and are falsely reported as being on fire even more often). In a modern airliner, the "fire handle" shuts down the engine, closes the fuel pipe, and discharges the fire extinguisher.

Occasionally it's desirable to leave the burning engine running because you need the added thrust. For example during WWII B-29 engines had an unfortunate habit of catching fire on takeoff. Standard procedure was to leave the burning engine running until you got enough altitude to safely discard the bomb load and/or bail out.

Is it possible to activate the fire extinguisher without shutting down the engine?

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    $\begingroup$ Remember that a correctly functioning jet engine is just a carefully controlled fire. So by suggesting fire extinguishing without shutting down the engine, you're proposing to somehow extinguish the "bad fire" while somehow leaving the "good fire" burning (and pumping large quantities of fuel into it). That does sound quite difficult. $\endgroup$ – David Richerby Mar 28 '18 at 8:36
  • $\begingroup$ I think you should narrow down (or atleast specify) the type of engine you are referring to. The B-29 from your example had piston engines while most modern aircraft are equipped with turboprop or jet engines, which process fuel in a different way. $\endgroup$ – MadMarky Mar 28 '18 at 12:07
  • $\begingroup$ The engine is already operating outside of its normal regime. You don't have time or means to determine the exact means of failure. Your think the choices are "shut down the engine" or "assume the engine is otherwise operating normally, other than this fire"? Why do you think that this is a reasonable approach? The engine isn't operating normally. You cannot presume anything about its continued operation (such as not throwing blades through the fuselage) - the engine isn't operating normally $\endgroup$ – Damien_The_Unbeliever Mar 28 '18 at 18:03
  • $\begingroup$ @Damien_The_Unbeliever, "I'm on landing approach. The left-hand engine is missing its cowling, is leaking fuel, and has been shut down. The right-hand engine just started trailing heavy smoke. It sure would be nice if I could activate the right-hand fire extinguisher without needing to perform a dead-stick landing." $\endgroup$ – Mark Mar 28 '18 at 21:27
  • $\begingroup$ @DavidRicherby, many of the engine fires reported on Aviation Herald are of the form "slow leak causes flammable fluid to build up in the engine machinery spaces and eventually ignite". It's quite possible to extinguish that sort of fire while leaving the engine core running. $\endgroup$ – Mark Mar 28 '18 at 21:36
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In most modern jets (e.g. Boeing 757/767) when you pull the fire handle the engine shuts down (fuel, hydraulic, etc., are cutoff). But pulling the fire handle only "arms" the extinguishing system. You have to rotate the handle (left or right) to discharge a bottle. (there are other bottle configurations based on the airplane type, but this is the general scheme)

Usually, after pulling the fire handle (before rotating it left or right) you wait momentarily to determine if the overheat or fire warning lights have extinguished. If not, you rotate the handle to discharge the extinguisher.

So, if you are asking about modern jets, the answer would be no, you can't discharge the extinguishing agent without shutting down the engine first.

I am not aware of a modern design that would allow the engine to continue operating after activating the fire extinguishing system.

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I wanted to add to 757toga's answer.

In addition to being required to "pull" the fire handle before you can discharge the fire suppressant, which cuts off the fuel, hydraulics, electrical generation and pneumatic flow to and from the engine. Even aircraft that do not have fire suppression systems (such as the B-52), require the fire handle to be pulled if you have a fire indication, which shuts off the engine.

To the broader part of your question about an aircraft needing the extra thrust. All multi-engine aircraft are certified to be able to complete a takeoff minus one engine from V1 (decision airspeed). V1 is the cross over speed at which the aircraft could safely reject a takeoff without overrunning the runway or can safely continue the takeoff minus one engine. Therefore, there is no time that an aircraft HAS to have the extra thrust from one failed engine.

An interesting corollary from this requirement, is that the most overpowered aircraft will be twin jets (100% extra power at V1), while an aircraft like the B-52 with 8 engines has much lower power margins (14% extra power at V1).

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  • $\begingroup$ You're assuming that nothing can cause a problem with more than one engine at a time. Shortly after asking this question, I came across this incident, where a maintenance error left both engines in questionable condition. $\endgroup$ – Mark Mar 28 '18 at 7:27
  • $\begingroup$ @Mark Full engine failure is such a serious condition that your loss of power is becoming almost marginal. Remember that this means loss of hydraulics, electricity for anything but the big six, often loss of quality radio contact and navigational aids. Of course, during takeoff you need the instruments less and the thrust more, but still, you're going to have many more problems than just the thrust. Therefore, all procedures (maintenance and certification especially) are made in a way that this should not happen. $\endgroup$ – yo' Mar 28 '18 at 9:51
  • $\begingroup$ @Mark Dual engine failure is a different animal that aircraft are not really certified for. Thus, pilot's generally have to do the best they can. In the Incident linked, only one engine was on fire and it was shut down once the fire started. The AAIB did note that the fire may have been prevented if the crew had shut down the engine when the fuel leak was first noticed. The crew made the decision to leave the engine on due to the condition of the LH engine, until a fire actually started then they shut it down. A Fuel Feed engine fire is a serious and immediate threat to the airframe, though. $\endgroup$ – OSUZorba Mar 28 '18 at 14:07
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    $\begingroup$ @Mark Another interesting note from the link incident, the suppression system depends on the nacelle being intact to contain the suppressant. So in this case, being able to deploy the suppressant without shutting off the engine would not have actually suppressed the fire. $\endgroup$ – OSUZorba Mar 28 '18 at 14:11
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The very nature of an engine fire suggests that either fuel or lubricating oil is going where it shouldn't be going. Cutting off the source of fuel and the source of ignition is essential in extinguishing a fire. An extinguishing bottle might put out the fire, but if the engine is still running, then it's getting fuel and producing heat... it will probably just reignite.

The B29 engine fire situation was more a matter of desperation than anything. My great uncle was a B29 pilot, killed when the plane crashed on takeoff due to engine failure. The early B29's were prone to engine fires when the engine was under very heavy load, such as during takeoff. If they were fully loaded with fuel and bombs, they couldn't climb or even remain airborne for long on three engines... this was wartime and certain risks were justified to continue the war effort. Modern airliners can climb after losing an engine, and the B29 could lose engines and remain in the air later in flight after it had burned off a good deal of it's fuel load, or better still, disposed of its bombs.

If a B29 engine caught fire on takeoff, it was kept running to try to gain enough altitude to bail out, only because the alternative was definitely to crash. Typically, though, either the engine failed and the plane went down, or the fire weakened the wing and it came off.

The problem was finally solved when they replaced the carburetor on the R3350 engine with fuel injection, but they lost quite a few crews in the process.

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My father was a crew chief on the UH-1H model Hueys and I asked a similar question once. I will give his answer, followed by my understanding. Disclaimer My dad had a habit of giving partly wrong answers on occasion.

His answer: By throttling down to less than the ignition intake.

My understanding: Reducing fuel intake to less than what is necessary for ignition does not quite shut down all engines, some use batteries to keep the engine going and the fuel simply adds thrust. In either case, extinguishing a fire will reduce the thrust available, however. No way around that.

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  • $\begingroup$ Internal combustion aircraft engines can't be kept going by battery power (unless you count the starter on the APU, which, as the name suggests, is only for starting, not running.) Also, engines require a massive amount of energy. The aircraft do not have batteries capable of supplying that much current and, even if they did, they'd deplete them in short order. The energy density of batteries is very small compared to that of gasoline or jet fuel. Also, any amount of fuel can catch fire, and, if it couldn't, it wouldn't be able to provide thrust. $\endgroup$ – reirab Mar 28 '18 at 15:34
  • $\begingroup$ @reirab I consider myself corrected, then. As I mentioned, my father wasn't always the best source of information... "Look it up" was one of his favorite phrases. $\endgroup$ – wolf Mar 28 '18 at 18:44

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