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EASA just announced an Emergency Airworthiness Directive grounding all A320neo planes with a certain type of engine. Reason is, that they have apparently the tendency to shut down in flight more often than acceptable.

I was wondering, why are IFSDs so critical? Can't a jet engine be just restarted? If not, is it something with the inherent design of jet engines? Or something that most engines are just designed for?

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    $\begingroup$ If you read your question again, you see the cause of such action is not "IFSD" but "IFSD more often than acceptable". So if you question the necessity of this action, you are effectively arguing the definition of "acceptable". When that's the case, you need to provide the current definition and your exception to make this question answerable. $\endgroup$ – user3528438 Feb 11 '18 at 0:29
  • $\begingroup$ IMO unexpected IFSD can have pretty severe consequences so I guess the tolerance of its occurrence should be relatively low, lower than say a phone or computer shut down unexpectedly. In this case the occurrence is "several occurrence" out of less than 100 airplanes of this model, I would say that's dangerously high for almost any product. $\endgroup$ – user3528438 Feb 11 '18 at 0:38
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    $\begingroup$ It's not enough to make an answer of, but I'll point out that having an IFSD may not be a big deal in certain cases, but to take off knowing that an IFSD is more likely in your particular model of aircraft is crazy. $\endgroup$ – nexus_2006 Feb 11 '18 at 2:36
  • $\begingroup$ This is a comment as it probably doesn't apply to engines now, but prior to my retirement in 1999, if at altitude and you pulled the power too fast on Pratt and Whitney JT9D engines of the time, they would occasionally quit. The procedure was to tell the flight engineer to restart it once the aircraft was low enough that that was possible. $\endgroup$ – Terry Feb 11 '18 at 2:39
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    $\begingroup$ @DavidRicherby Actually... the aerodynamic surfaces are the only thing holding the plane in the air. The engines help the aerodynamic surfaces do their job. ;-) $\endgroup$ – Shawn Feb 12 '18 at 16:14
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Having an engine shutdown during the flight is an engine failure. An engine failure is one of the procedures where every pilot is trained to deal with because it is easy to simulate. Engine failure is an emergency, meaning that the pilots must land the aircraft as soon as possible.

Can't a jet engine be just restarted?

Maybe, maybe not. It depends on the diagnostics in the cockpit. You certainly do not want to restart a damaged engine. All jet engines are capable of being shut down and restarted in flight, provided that nothing is broken and proper procedures are followed.

Now, of course, if everything was working then the engine would not have shut down in the first place. It is an event which pilots are prepared for, but it should not have occurred. When it does, that means something is wrong. The decision of whether to attempt a restart is evaluated by the flight crew based on engine data, distance to nearest airport, weather, and possibly a discussion with ground support. In short, it is a risk management exercise.

Why are in flight shutdowns so critical?

With one engine shut down, your chances of making it to the destination are virtually none. Aircraft performance is calculated assuming that all engines are working. With one engine shut down, the amount of thrust is reduced and the aircraft must descend to a lower altitude. At lower altitude, fuel consumption is higher, therefore you will not have enough fuel to reach your destination and you need to land somewhere closer.

Furthermore, you are now running on your last good engine. If that engine fails as well, the airplane will become a glider. ETOPS is designed so that an aircraft may limp to the nearest airport in no more than X minutes, and the presumption of ETOPS is that the engine failure rate is low enough. In other words, the aircraft failed to meet its certification standards.

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    $\begingroup$ While an engine failure on a two-engine aircraft calls for a landing as soon as safely possible, the failure of an engine on a four-engine aircraft, at least 747s up to my retirement in 1999, did not require a landing. Indeed, 3-engine ferry flights are permissible, where you take off with an engine out. They are practiced in the sim, and I did three as I remember for real. $\endgroup$ – Terry Feb 11 '18 at 1:18
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    $\begingroup$ @Terry You can continue OEI on a triple too, and they are two-engine ferried as well. 14 CFR 121.565 refers in the US. $\endgroup$ – user71659 Feb 11 '18 at 3:01
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    $\begingroup$ @user71659 I figured that was the case, but didn't want to mention it as I wasn't certain, and especially with dementia increasing the rate of my 'senior moments'. LOL $\endgroup$ – Terry Feb 11 '18 at 4:18
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    $\begingroup$ Also when the cause is unknown, it cannot be excluded both engines will experience the same failure at the same time . E.g. the cause is an erratic failure of a sensor in particular atmosphere conditions, both engines can experiment the sensor failure at the same time. $\endgroup$ – mins Feb 11 '18 at 9:18
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    $\begingroup$ As pointed out already, this answer (particularly the second half of it) may well be applicable to two-engine aircraft such as the A320neo, but for three- or four-engine aircraft the loss of a single engine wouldn't necessarily be immediately critical. Particularly, if the engine shutdown happens late in flight, the destination airport might be the most suitable airport to aim for, while letting ATC know that you can't tolerate any delay because you're going to, at least, be tapping into your reserve fuel. You may want to emphasize that. $\endgroup$ – a CVn Feb 11 '18 at 12:31
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@Kevin is quite correct but to add to it the airworthiness issuance was issued because of the risk of a dual engine failure

...issued an emergency airworthiness directive (AD) warning of a potential “dual engine” inflight shutdown on A320neo family aircraft powered by Pratt & Whitney PW1100G geared turbofan (GTF) engines.

@Kevin covers the single engine failure shutdown well, but in this case a dual engine failure could potentially be catastrophic if there was no useable landing site within gliding distance. Although evidence has shown that dual engine failures can end safely like this one and this one

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