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.
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:
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.
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:
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.
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.
I concur with casey's answer on the linked question. There are two basic facts:
And then a complication:
Really what it comes down to, on both sides, is do what your career can handle. ATC shouldn't predicate separation on pilots agreeing to break a rule in the FARs, and pilots shouldn't break a rule just because ATC said so (unless they're certain that the rule is one of the "unless ATC says you can" rules). But if nothing bad happens the likelihood of anyone getting in trouble for it is minimal. Conversely if something bad does happen "ATC said I could" or "I didn't know pilots couldn't do that" might not carry a lot of weight in court. It's a judgement call.
I have seen anecdotal radio exchanges with pilots reporting obviously fabricated tailwinds when called out on the 250KT rule, or ATC snarkily commenting that two planes at the same altitude must have two different tailwinds.
The real bottom line is: the pilot-in-command is the absolute final authority as to the safe and legal operation of the aircraft, and "unable" is a perfectly valid response to any instruction so long as you can back it up with a good justification.
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.
