While in a flight, a pilot wants to sleep. Given that the aircraft has an autopilot facility, are they allowed to sleep for a while (e.g., half an hour)?
What would happen if it's not allowed, but they still did it? Would air traffic controllers know?
Airline flight crews generally consist of at least 2 pilots. One pilot is designated as pilot flying, the other is the pilot not flying, observing instruments and flight parameters. If required, pilots can use Flight Crew in-Seat Rest:
Flight Crew in-Seat Rest
This is the process whereby pilots may take short periods of sleep (naps), while temporarily relieved of operational duties in accordance with carefully prescribed ‘controlled rest’ procedures, when part of a two-man operating crew of an in-flight aeroplane.
This is not communicated to ATC and only allowed with two pilots present on the flight-deck. It is prohibited to have both pilots participate in in-Seat Rest at the same time.
See also these related questions:
How do pilots rest on long-haul flights?
What if pilot feels too tired before his flight?
The correct answer is that the answer to this question, like many in aviation regulations, varies greatly by country. As SentryRaven's answer mentions, some countries allow Flight Crew In-seat Rest. However, this is not a universally-allowed practice. In particular, sleeping in the flight deck is never allowed in the U.S. One (but not both) crew sleeping on the flight deck is allowed in Europe, though.
What hasn't seemed to have been addressed in the other answers yet is what I would expect (or at least hope) is the much more common situation of pilots sleeping on board: augmented flight crews. An "augmented flight crew" is when more pilots are assigned to a flight than the minimum number required to operate the aircraft (which is generally 2 on today's airliners.) This is typically used on long-haul flights, where airlines can have 3 or 4 pilots on board. At least in the context of the U.S., two pilots must remain awake and in the flight deck, but the other 1 or 2 pilots may sleep. This way the crew can rotate flight deck duty and sleep time.
According to 14 CFR 91.1061, 3-pilot crews may operate flights with scheduled flight times up to 12 hours and 4-pilot crews may operate flights with scheduled flight times up to 16 hours. In the case of either a 3-person or 4-person crew, at least two of them must be qualified to fly as pilot-in-command on the aircraft, while the other(s) must be qualified to fly as second-in-command. This ensures that someone qualified to fly as pilot-in-command remains on flight deck duty at all times. In order to use an augmented flight crew, adequate sleeping facilities must be installed on the aircraft for the pilot(s) who are not in the flight deck.
The answers to this question give more detail about the crew rest quarters that are common on airliners used on long-haul flights. These exist both for flight crew and for cabin crew, though, from what I understand, these are generally separate from each other.
No the pilot is not allowed to sleep on the job. Not unless another qualified human relieves him of his duties as pilot.
An emergency can happen at any time in flight and the pilot must be ready to deal with it.
A pilot sleeping would result in him not responding to air traffic control but that can have other causes like a failed radio.
Airline pilots have strict work hours they need to keep to ensure they are well rested before starting a flight. If he feels he is too tired he is obligated to refuse flying and let a better rested pilot take over.
In most cases, only if he is not on-duty.
Most transoceanic flights have flight times exceeding the amount of time a pilot is allowed by most aviation regulators to be at the yoke in one stretch. In the U.S., the FAA mandates a maximum of 8 hours in one continuous block, which can be extended by a few hours if the pilot is then allowed to rest for a longer-than-normal time afterward. Qantas Flights 7 and 8 between Dallas and Sydney are in the air for sixteen and a half hours in one continuous flight.
In these situations, the aircraft has multiple flight crews, and one crew will relieve the other in mid-flight. While not on duty, pilots may do almost anything the confines of the plane allow, including retiring to a special pilots' quarters, usually tucked in an out-of-the-way place on the plane, not obvious to passengers. These quarters, while not exactly palatial, are still of a level that even first class passengers would envy:
(source: core77.com)
The cabin crew also have an hours limit that is exceeded by most long-hauls, and they also have their own space to get away from their jobs, though it's slightly more spartan:
While on-duty, the FAA regs used to allow for "in-seat rest", allowing the "pilot not flying" to rest his eyes for a while if nothing interesting was going on. However, an incident with Northwest Airlines Flight 188 in 2009 was the final nail in the coffin for any allowance for sleeping in the cockpit. ATC lost all communication with this flight for almost two hours while it was heading to Minneapolis. By the time contact was re-established, the plane was 150 miles past the airport and still at cruising altitude. In the interim, F-16s were readied to intercept (but never launched) as the plane had been feared hijacked. While the pilots insist they were using laptops to go over airline schedules (a breach of Northwest's parent Delta's cockpit policies, not to mention basic flight fundamentals), other theories suggest they simply fell asleep. Unfortunately, the CVR only had the last 30 minutes of the flight in its data buffer, while the critical time to determine what they'd been doing before radio contact was re-established would have been at least an hour prior to landing.
British Airways and Qantas as well as a few other airlines do allow pilots on duty to sleep in the cockpit, as long as one crew member is awake and at the controls. However, they cannot do so while flying in U.S.-controlled airspace.
