Strayed aircraft. An aircraft which has deviated significantly from its intended track or which reports that it is lost.
Unidentified aircraft. An aircraft which has been observed or reported to be operating in a given area but whose identity has not been established.
Source: ICAO Doc 4444: PANS-ATM
The terms "strayed" and "unidentified" are not mutually exclusive. An aircraft may be considered, at the same time, as "strayed" by one ATS unit and as "unidentified" by another one. For example, following a navigation system failure (or, in the case of a VFR flight, loss of positional orientation), an aicraft may, while in the airspace of one unit, stray and ultimately enter the airspace of another unit. The former unit will consider the aircraft to be strayed, and the latter would treat is as unidentified, at least until contact has been established. While different in nature, the two situations share two risk scenarios:
- both situations can be a sign that the aircraft is subject to unlawful interference. If the controller has a reason to believe that this is the case, they should notify the appropriate military authorities in accordance with local instructions.
- both situations may prompt military interception (based on the first risk).
Therefore, the presence of a strayed or unidentified aircraft is normally considered and abnormal situation and is dealt with accordingly. The focus of the air traffic controller remains on ensuring the safety of that flight and of the other traffic. To this end, the following actions are taken, as necessary:
- Establishing two-way communication with the aircraft, using all means available (including emergency frequency 121.5 MHz, other ATS units, other aircraft, etc.). Depending on the circumstances, the controller may become aware of a strayed aircraft in their airspace by a radio call from that aircraft.
- After communication has been successfully established, the controller gathers additional information (from the pilot or from external sources) to determine the aircraft position, level, direction of flight, intentions, required assistance (if any), etc.
- After establishing communicaion and determining the aircraft position and other relevant data, the controller would advise the pilot of their position and the corrective action to be taken. If so requested, the controller would provide further guidance (e.g. appropriate headings, safe level information, etc.).
- Providing separation between the strayed/unidentified aircraft and the other aircraft in the sector. Note that depending on the circumstances, separation based on surveillance systems may not be applicable. If this is the case, procedural separation (e.g. vertical) will need to be applied.
- Informing other ATS units into whose area the aircraft may have strayed or may stray. This may include details regarding the corrective action (about to be) taken.
- Informing appropriate military authorities about the aircraft and providing them with pertinent information (e.g. flight details, reasons for straying, expected actions, etc.).
Note that the actions described above are in line with the ASSIST (Acknowledge, Separate, Silence, Inform, Support, Time) principle that normally outlines the controllers' response to an unusual or emergency situation.
While some unidentified aircraft are strayed as well, this is not always the case. Reasons for an aicraft not being identified include (the list is not exhaustive):
- An aircraft without a transponder or having experienced a transponder failure. Such an aircraft would only appear on a controller's situation display after being seen by a primary radar that generally covers a smaller area. Note that if there are no primary radars available, the aircraft will not be seen on the controller's screen at all. In this case, the information about its presence may have been received by another aircraft, a neighbouring ATS unit, a military agency or another external source.
- An aircraft is discovered by radars after climbing above certain altitude. Even if the transponder is operational, it may be using a non-discrete code (e.g. 2000) which could make identification more difficult.
- An aircraft enters the aispace of an ATS unit without making contact and without the controller receiving any information from the upstream ATS unit. This may happen for a variety of reasons, e.g. the upstream controller not forwarding appropriate information (e.g. an estimate) or the flight being under control of a military agency.
When a controller is informed that an unknown aircraft has entered their airspace, they would make every effort to establish contact and identify it, as specified in the local instructions. Example actions include:
- trying to contact the aircraft on the available frequencies (including the emergency frequency 121.5 MHz).
- inquiring other ATS units (within the FIR or in neighbouring FIRs) about the aircraft and requesting their assistance in establishing contact.
- inquiring the appropriate military authorities.
- attempting to obtain information from other aircraft within the area where the unidentified aircraft is observed or has been reported to be.
After the aircraft has been identified, the controller would inform the military authorities in accordance with local instructions.