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The title really asks it all: When can a pilot disobey ATC commands?.

Of course, the pilot has the final decision, but when would the pilot really disobey someone in ATC with much more knowledge of planes and other potential hazards near by?

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marked as duplicate by fooot, Federico, CGCampbell, mins, Mark Jan 29 at 22:42

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Others will chime in with much better and documented answers, but ultimately, the pilot has final authority. If ATC tells it to crash into a cliff or another aircraft (we'll suppose that would be unintentional, of course), he/she has every right (to put it mildly) not to do so. – jcaron Jan 29 at 13:12
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Another less dramatic illustration would be an ATC instruction for a VFR flight to climb or turn into a cloud. This happens all the time as a radar scope doesn't show clouds, so the VFR pilot would have to reply "unable" and hopefully give a reason, after which ATC will issue new instructions (probably climb/turn "when able"). – Ron Beyer Jan 29 at 14:48
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There are two examples on YouTube, at least one of which has also been discussed here. Maybe someone can dig out the links. In one case, a pilot was issued a clearance he deemed risky, and asked for a different one, only to be pressured by the controller to take that clearance or leave it. The pilot then threatened to declare an emergency and do what he deemed necessary for the safety of the flight, which would have led to an investigation, not to mention screwing up the whole sequence, after which the controller backed down and issued the clearance the pilot had originally asked for. – Jörg W Mittag Jan 29 at 16:14
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The other case happened at Geneva, I believe. An airplane got lost while taxiing, and despite telling the controller numerous times that they were lost and that someone had just taken off directly in front of them (i.e. they believed to be on or near an active runway), the controller not only insisted that they were wrong and they couldn't possibly be anywhere near where they said they were, but issued a takeoff clearance for an airplane on the very runway the lost plane was just about to cross. The pilot replied something like "Until you figure out where that plane is, we are going nowhere." – Jörg W Mittag Jan 29 at 16:17
    
@JörgWMittag It sounds like you're thinking of this one? That was in the US, but something similar could easily have happened somewhere else too. – Pondlife Jan 29 at 16:33

In case of emergencies, yes. The pilot in command has the final authority and responsibility for safety of the flight and can disobey ATC commands in case of emergencies. The reasons for that and the results will decide the consequences.

From 14 CFR §91.123 Compliance with ATC clearances and instructions:

(a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory.

(b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.

From 14 CFR §121.557 Emergencies: Domestic and flag operations:

(a) In an emergency situation that requires immediate decision and action the pilot in command may take any action that he considers necessary under the circumstances. In such a case he may deviate from prescribed operations procedures and methods, weather minimums, and this chapter, to the extent required in the interests of safety.

Emphasis mine. Obviously, it would be prudent for the the pilot (and required under FAR 91.123(c), as @Pondlife points out) to report the deviation to the ATC as soon as possible.

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Note that it isn't just prudent to inform ATC about what you're doing, it's required by 91.123(c) – Pondlife Jan 29 at 13:38
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Interesting to read that "only" persons are bound to ATC instructions. What about the Autopilot? Is it a "person" in a transitive way? – hiergiltdiestfu Jan 29 at 14:11
    
@hiergiltdiestfu in what way are you expecting the autopilot to deviate from ATC commands? It only does what the pilots have programmed into the FMC - if deviation from that is necessary, the pilots switch off the AP (either hit the switch or pull on a control long enough for it to automatically disengage). – FreeMan Jan 29 at 14:28
    
@FreeMan Well, I don't know.. I was mainly thinking about the whole Google self-driving car debate about who is responsible if the car crashes -- the occupant, the engineer who programmed the AI, the manufacturer of a sensor to blame? The regulation talking explicitly about persons seems to open up a loop hole with respect to (semi-)autonomous systems. – hiergiltdiestfu Jan 29 at 14:31
    
It would be a shame (and waste!) if we spent thousands of hours training pilots how to handle all these obscure scenarios only to have them yield to under-informed ATC controllers when an emergency actually occurs. – corsiKa Jan 29 at 16:18

A pilot can break any rule in the book for the safety of the flight or those on the ground. Ultimately the safety of the flight rests with the pilot in command of the aircraft and therefore not just the right but the duty to diverge from ATC instructions if the situation warrants. Examples of this might be:

  • A cabin depressurization or engine failure requiring an immediate descent
  • A change in course or altitude to avoid a collision with an aircraft or terrain
  • Passenger illness
  • Unlawful interference
  • When intercepted by law enforcement or the military
  • If complying with an instruction is hazardous, for instance putting the aircraft outside its flight envelope, into a possible collision
  • Fuel emergencies
  • Mechanical problems

In cases such as these pilots will take immediate action and inform ATC afterwards if they can.

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"Fuel emergencies" -- so they could land without clearance when running out of fuel and ATC not caring about it? – hiergiltdiestfu Jan 29 at 14:13
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@hiergiltdiestfu, when you run out of fuel, you will land. The only questions are A) where and B) under how much control. No, ATC won't "not care", but they understand you have to get on the ground now. – FreeMan Jan 29 at 14:24
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If you are in an airplane that's running out of fuel then you can land without clearance, however you shouldn't have to. If you declare an emergency ATC will help you and make sure the way is clear. – GdD Jan 29 at 14:24
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@hiergiltdiestfu To add to what Freeman says, if you are running out of fuel and you need to get on the ground now, you will be declaring an emergency. ATC will then do whatever you need to get the aircraft safely down. Afterwards you will probably get a phone number to call and a very stern talking to from an individual at your nearest FSDO. – Ron Beyer Jan 29 at 21:28
    
I see. Thanks to all who were so nice as to show me the true nature of fuel emergencies. I kind of forgot that this is nothing that comes as a surprise, but develops gradually, such as that you'll have time enough to communicate beforehand – hiergiltdiestfu Jan 29 at 21:33

To add one more case to what's been mentioned, if ATC tells you to climb to avoid a conflict and TCAS tells you to descend, pilots are required to follow the TCAS command rather than ATC's.

The reason behind this is that TCAS in the two aircraft can typically coordinate "behind the scenes" which aircraft will do what in order to avoid a collision, but there's no guarantee that both aircraft are hearing the same plan from ATC. Maybe separate controllers both issue a "climb" command, or one aircraft is on the wrong frequency, or whatever. But the TCAS-to-TCAS communication is sufficiently reliable that this is now a required (although extremely rare) case of disobeying ATC.

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This was (one of the) factors in the Überlingen crash. – Jörg W Mittag Jan 29 at 16:17
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It's not the only reason. En-route RADARs have a scan period of 10 seconds. That means controllers have a stale picture. When TCAS goes off, then it's too late (and dangerous) for ATC to interfere. When a pilot reports "TCAS RA", ATC can't say anything to the planes, (traffic information included) until pilot reports "clear of conflict". – Stelios Adamantidis Feb 4 at 11:12

Others have talked about emergencies, which allows a pilot to take any action he sees fit. This is correct, and you could even argue that it's not really disobeying, as in the moment the emergency was declared, all clearances sort of went out the window.

There's one important part missing and that's the option for a pilot to not accept a clearance or an amendment. A clearance is not an instruction as much as it's a negotiation, and once established, is more or less valid until both pilot and controller can agree on something different. If they can't agree, by either the pilot saying "unable" for whatever reason, or in the case where radio communication is lost and the pilot didn't actually hear anything, he is expected to carry on as previously cleared.

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I know pilots can refuse a LAHSO clearance. – TomMcW Jan 29 at 14:50

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