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After reading this question about being cleared to go faster than 250kts in airspace that otherwise has a speed limit, I began to wonder how far can Air Traffic Controllers (ATCs) can bend the rules on an aircraft-to-aircraft basis?

Is there a limit to what exceptions ATC can make for someone? I'm particularly interested for the answer in the USA, but I'm sure information about other countries would also be enlightening.

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    $\begingroup$ If you are using power in terms of authority, this question discusses it thoroughly and in detail. $\endgroup$ – Farhan Jan 22 '15 at 16:02
  • $\begingroup$ On the other hand, if you're using power to mean power, it will vary a lot by the equipment under their control at a given field, but will usually probably be measured in kilowatts. :) Pilots, on the other hand, may have megawatts (GE90-115b comes in around 75 MW, IIRC.) $\endgroup$ – reirab Jan 22 '15 at 22:38
  • $\begingroup$ Related: aviation.stackexchange.com/questions/30134/… $\endgroup$ – J. Hougaard Oct 19 '16 at 18:34
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As an active area radar controller, I completely agree with @rbp answer. To better explain relationship between power and responsibility, I will try to describe what a controller would usually do: If I need an aircraft, for example, to reduce speed... I know approx. what would be the minimum convenient speed for a particular type to ask, so I'd take this into account prior asking. But if a pilot tells me that he's unable to comply with the request, I'll find another solution i.e. radar vectoring to loose few miles or in extreme situation, level change. In short, a controller will never force a pilot to action if pilot answers that he can't do that. A controller must plan in advance that pilot may not be able to fulfil his request and have alternative solution. If you, as a controller, put yourself in a situation where you have no alternative, that is usually too late. There are usually only few things where I can't make exception:

  1. Minimum separation between aircraft
  2. Danger/Prohibited zone
  3. Terrain

Even then, in extreme case, I've never met: Imagine a situation where you have an aircraft which can't change level due to traffic above and below and has a crossing traffic. Then only remaining action I have available is radar vectoring, but what if the pilot advises me that his unable to turn due to weather, (the described situation is usually the controller fault because he should have predicted that earlier). Then controller probably would give the pilot traffic information (tell the pilot what to expect) and advise the minimum distance but it will not force the aircraft to CB.

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    $\begingroup$ Hello Sinisa Drpa, welcome to Aviation.SE! $\endgroup$ – DeltaLima Jan 24 '15 at 21:46
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As far as I know, most regs have some sort of out that says "unless directed by ATC," and ATC has two unbreakable rules which limit much of what they are allowed to do:

  1. thou shall not vector airplanes into obstacles, terrain, unflyable weather or atmospheric conditions, restricted (in the lay sense of the term) airspace, or other aircraft
  2. thou shall not deviate from aircraft separation

Other than that, when it comes to clearances, ATC has a fair amount of latitude to give you what they want, or what you ask for.

The balance of responsbility between ATC and the pilot in command, is covered by 14 CFR 91.123:

§ 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. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC.

(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.

(c) Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible.

(d) Each pilot in command who (though not deviating from a rule of this subpart) is given priority by ATC in an emergency, shall submit a detailed report of that emergency within 48 hours to the manager of that ATC facility, if requested by ATC.

(e) Unless otherwise authorized by ATC, no person operating an aircraft may operate that aircraft according to any clearance or instruction that has been issued to the pilot of another aircraft for radar air traffic control purposes.

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  • $\begingroup$ From what I've learned reading here (I am not a pilot), basically any instruction ATC gives can be overridden by the pilot, though, who has the ultimate responsibility for the safety of his plane. Is that correct? $\endgroup$ – FreeMan Jan 22 '15 at 17:17
  • $\begingroup$ @FreeMan i've updated my answer with the answer to your question $\endgroup$ – rbp Jan 22 '15 at 22:19
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The ATC guys can't just do whatever they want. They have to operate within guidelines - they will have an ATC handbook/manual etc, much like a pilot has a flight operations manual, which has been merticulously developed and approved by the authorities. There are references in this manual as to when ATC can cancel speed restrictions, or deviate from the other usual procedures. In most cases it will read something like "Speed restrictions under 10,000 feet may be cancelled in Class C airspace only, and if the controller is satisfied that no risk of LOSA (loss of separation assurance) exists".

So in practical terms, ATC do have the ability to deviate from most of the usual procedures, but always within existing guidelines. Of course, everything goes out of the window in an emergency, however.

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The speed limit of 250kts below 10,000 feet is mostly there to protect smaller aircraft and rotocraft.
For example, you might have a parachute jump plane flying around, and you don't want a Citation business jet bearing down on them (or the jumpers) at 400 knots: At that speed you don't have much time to react: You look away for two seconds to drink your coffee and BAM!

There was a famous accident a couple of years ago in which a glider pilot was flying around Reno and a private jet flew right into him (NTSB file LAX06FA277B) . That is exactly the kind of accident speed limits are supposed to mitigate.

If a pilot exceeds a speed limit, it really doesn't matter what ATC said, it is the pilot who is responsible. If the FAA determines he did something reckless it makes no difference that some controller told him it was ok.

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  • $\begingroup$ Thanks. But this answer is probably better suited for the question linked in my question, not to my question itself. $\endgroup$ – Keegan Jan 22 '15 at 17:55
  • $\begingroup$ @SpongeBob What exactly is your question? If it is "authority" of an ATC to delete a below-10k speed limit in the US, personally I would say "none" because like I said, if the pilot exceeds the limit and crashes into somebody and the FAA determines he used poor judgement in going over the speed limit, what ATC said will make absolutely no difference. $\endgroup$ – Tyler Durden Jan 22 '15 at 18:03
  • $\begingroup$ I'm asking about their authority to alter ANY rule in the FAR, not just the speed limit. $\endgroup$ – Keegan Jan 22 '15 at 18:06
  • $\begingroup$ @SpongeBob That is pretty broad. The FAR is like 1000 pages long. Just as a general principle my essential point would probably be valid, which is that an ATC instruction cannot immunize a pilot from a FAR violation. $\endgroup$ – Tyler Durden Jan 22 '15 at 18:56
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    $\begingroup$ The actual incident which lead to the 250-knot speed limit was a crash between a DC-8 and a Super Constellation. The CAB report on the incident is available from the FAA's website $\endgroup$ – voretaq7 Jan 22 '15 at 20:50

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