Emergencies, Silence and Incidents

Doc 4444 ends where flights go wrong, and so does this module. This lesson collects the contingency procedures the syllabus tests: what an aircraft in emergency squawks and what it is owed, the choreography around an emergency descent, how the system handles an aircraft that has gone silent, the vocabulary for aircraft that are lost or unknown, the rules for dumping fuel, the temporary callsign change that untangles two similar-sounding flights, and, when aircraft pass too close, the AIRPROX report that turns a bad moment into safety data.

The emergency codes and the emergency's rights

The transponder lesson taught the codes; here Doc 4444 assigns them their exact meanings. An aircraft threatened by grave and imminent danger and requiring immediate assistance operates the transponder on Mode A Code 7700 (the general emergency signal), while an aircraft that is the subject of unlawful interference indicates it specifically with Mode A Code 7500 (and may still use 7700 when the danger is grave and imminent). Where the display does not automatically flag those codes, a controller suspecting unlawful interference verifies it by setting the decoder to 7500 and then to 7700.

What the emergency buys is the syllabus's next question. An aircraft known or believed to be in a state of emergency (including one subjected to unlawful interference) shall be given priority over other aircraft. Around a flight under unlawful interference the ATS units keep working even into silence: they attend promptly to the aircraft's requests and anticipated needs, take action to expedite the conduct of all phases of the flight, transmit safety-relevant information without expecting a reply, monitor and plot the flight and coordinate its transfers without requiring responses, keep every concerned unit informed, and notify the operator, the rescue coordination centre and the State's designated authority. Frequency and code changes, meanwhile, are avoided if possible for any aircraft in emergency and made only when an improvement in service can be expected.

Emergency descent

When pressurisation fails, an aircraft falls out of the flight levels at thousands of feet per minute, and the procedures split three ways. The ATS unit recognising an emergency descent takes immediate action to safeguard all aircraft concerned, which may include:

Broadcasting an emergency message.
Issuing traffic information and instructions to the aircraft affected.
Advising the minimum flight altitude and the altimeter setting for the area.
Informing the other ATS units affected.

The descending crew navigate as they judge best, advise the ATS unit of the emergency descent (adding intentions if able), set 7700, turn on the exterior lights, and watch for traffic visually and on ACAS.

The tested part is the third role — the aircraft that hears the broadcast. Unless specifically instructed to clear the area, or threatened by immediate danger, that pilot shall continue according to the current clearance, maintain a listening watch on the frequency in use for further instructions, and watch for conflicting traffic both visually and by reference to ACAS. Older training material answered this question with "leave the area immediately"; the amended text says the opposite: hold your clearance and listen, because a sky full of aircraft improvising avoidance is more dangerous than the one aircraft coming down through it.

Two-way silence

When two-way communication with a controlled flight fails, the controller's first question is whether the failure is total. Action shall be taken to ascertain whether the aircraft can still receive: the unit requests it to execute a specified manoeuvre that can be observed on the ATS surveillance system, or to transmit a specified signal (a squawk change, an IDENT) as acknowledgement. A receiver-only aircraft can be controlled almost normally: it hears the instructions and answers with its transponder and its track.

If the aircraft acknowledges nothing, separation is maintained on the assumption that it will fly the lost-communications procedures taught with the rules of the air (VMC: continue visually and land at the nearest suitable aerodrome; IMC: the 20-minute or 7-minute wait, the flight-plan route, the descent at the expected approach time). Meanwhile the ground keeps talking: information describing the action taken, and any instructions justified by an emergency, are transmitted blind on the frequencies available on which the aircraft is believed to be listening, including the voice frequencies of available radio navigation or approach aids: an ILS or VOR with a voice channel becomes one more way to reach a crew whose radios half-work. Traffic near the aircraft's presumed position is told about it, every unit along the route is informed, and the units serving the flight-plan alternates are warned and asked to try to make contact.

Strayed and unidentified

Two definitions organise the response to an aircraft in the wrong place. A strayed aircraft is one that has deviated significantly from its intended track, or reports that it is lost. An unidentified aircraft is one that has been observed or reported operating in a given area, but whose identity has not been established. The two describe different sides of the same event: to its own controller an aircraft is strayed; to the defence radar watching an unknown return cross a border, it is unidentified; and the same aircraft may be both at once, which is why the procedures make the units on each side exchange information until navigation assistance is given or identity established.

Fuel dumping

An aircraft that takes off heavy and must land at once may be above its maximum landing mass, and the reason for fuel dumping is exactly that: in an emergency or other urgent situation, to reduce to maximum landing mass in order to effect a safe landing. Within controlled airspace the crew advise ATC, and unit and crew coordinate three things: the route, if possible clear of cities and towns, preferably over water and away from reported or expected thunderstorms; the level, which should be not less than 1,800 m (6,000 ft), high enough for the fuel to vaporise before it reaches the ground; and the duration of the dumping. Other known traffic is kept at least 10 NM horizontally away, but not behind the dumping aircraft; traffic behind, within 15 minutes' flying time or 93 km (50 NM), is separated vertically instead: 1,000 ft if above the dumping aircraft, and a full 3,000 ft if below it, where the fuel mist descends.

Changing your callsign

Two similar callsigns on one frequency invite the wrong crew to take the right clearance, so ATC holds a small, sharp power: a unit may instruct an aircraft to change its type of RTF callsign, in the interests of safety, when similarity between two or more callsigns makes confusion likely. The change is temporary, applicable only within the airspace where the confusion is likely to occur; the aircraft to be changed is identified by its position or level so the wrong one does not answer; and the unit that ordered the change ensures the aircraft reverts to its flight-plan callsign on transfer to the next unit, unless the change was coordinated with that unit. The unit also advises the crew when to revert. The phraseology is the rule in miniature: "change your callsign to…", and later, "revert to flight plan callsign".

AIRPROX and the incident report

When the system's layers fail far enough that aircraft pass too close, the event gets a name and a form. AIRPROX is the code word used in an air traffic incident report to designate aircraft proximity: the pilot's word for "we were too close". The air traffic incident report itself is submitted, normally to the ATS unit concerned, for incidents specifically related to the provision of air traffic services: aircraft proximity, or other serious difficulty hazarding aircraft caused by faulty procedures, non-compliance with procedures, or the failure of ground facilities. Its task is investigation, not blame: the report feeds an inquiry that, for an aircraft proximity, determines and classifies the degree of risk: risk of collision, safety not assured, no risk of collision, or risk not determined, so that the procedure, the airspace or the facility that let it happen can be fixed.