Interception and Visual Signals

An aircraft that wanders off its route, cannot be identified or does not answer the radio creates a problem for air traffic services and, in some airspace, for air defence. Most such cases end on the radio: a lost pilot is given a bearing or a radar position, a flight plan is found for the unidentified target, and the aircraft continues. A few end with a military aircraft alongside. Interception is the last step in that chain, and because the two crews may have no frequency or language in common, it relies on a small set of visual signals that every pilot is expected to know.

The same need for silent communication exists on the ground. At an aerodrome, an aircraft without a working radio can still learn the landing direction, the circuit direction and any restriction from the ground signals laid out in the signal area. Both codes are set by ICAO Annex 2 and, in Europe, by SERA; the FAA publishes interception procedures in its AIM and relies more on the segmented circle at airports without a tower.

Strayed and unidentified aircraft

Annex 11 distinguishes two cases:

• A strayed aircraft is one that has deviated significantly from its intended track or that reports that it is lost.
• An unidentified aircraft is one that has been observed or reported to be operating in a given area but whose identity has not been established.

The same aircraft may be strayed for one ATS unit and unidentified for another. For a strayed aircraft, ATS works to establish its position, using radar, direction finding (see VHF direction finding and Q-code bearings) or other aircraft, and then to give it navigational help. For an unidentified one, ATS tries to establish its identity, by radio and from flight plans, and informs the military units concerned where the circumstances require it. Some airspace makes identification a condition of entry: an aircraft flying into, within or across the contiguous US air defense identification zone (ADIZ) must, with few exceptions, file an IFR or DVFR flight plan and carry a Mode C or Mode S transponder.

The pilot's part is to ask for help early, while there is still fuel and daylight to use it. The frequency in use is the first choice; on 121.5 MHz, direction-finding stations can also help to locate an aircraft that is lost (see distress and urgency communications).

Interception procedure

Interception of civil aircraft is governed by Annex 2 and, in European airspace, by SERA.11015. Interception is used to identify an aircraft, to guide it back to its track or away from restricted airspace, or to make it land. An intercepted aircraft must:

1. follow the instructions given by the intercepting aircraft, interpreting and responding to its visual signals;
2. notify the appropriate ATS unit, if possible;
3. attempt to establish radio contact with the intercepting aircraft or the intercept control unit, by a general call on the emergency frequency 121.5 MHz giving its identity and the nature of the flight, and, if no contact is made and it is practicable, repeat the call on 243.0 MHz;
4. select transponder code 7700, unless otherwise instructed by the ATS unit.

If radio instructions from an ATS unit, or any other source, conflict with the interceptor's visual signals, the intercepted aircraft complies with the interceptor while requesting clarification. The intercepting crew may be acting on information the ATS unit does not have, and apparent non-compliance can have grave consequences. For the same reason, turning away from an interceptor, or selecting 7500 or 7600, would give it a false picture: 7500 means unlawful interference and 7600 radio failure.

When radio contact is made but no common language exists, ICAO provides a short list of phrases, each spelled for pronunciation. The intercepted aircraft accepts with WILCO, meaning understood, will comply, and declines with CAN NOT, pronounced KANN NOTT, meaning unable to comply. Ordinary ATC words such as UNABLE are not part of the set.

Interception visual signals

The interception visual signals are the same by day and by night. Three are made by the intercepting aircraft and answered by the intercepted aircraft:

If the intercepted aircraft is a helicopter, the interceptor takes position to its right and turns to the right. Weather or terrain may also require the positions and the direction of turn to be reversed. The Series 2 break-away is deliberately made away from the intercepted aircraft's track, so that it can never be mistaken for a turn to follow.

Three signals are started by the intercepted aircraft:

The interceptor acknowledges "cannot comply" and "in distress" with the Series 2 break-away. Raising the gear in Series 4 reverses the "I will land" of Series 3, and the height band keeps the aircraft clearly visible without looking like an approach.

The signal area

A signal area is an area on an aerodrome used for the display of ground signals, typically a square edged in white and placed where pilots can see it from the air. It serves aircraft without radio, aircraft whose radio has failed, and aerodromes where no ATS unit is on watch. The tower's light signals, which do the same job for individual aircraft at a controlled aerodrome, are covered in radio communication failure.

Landing direction indicators

Several devices show the direction in which to land and take off:

• The landing T is a horizontal T displayed in the signal area. Aircraft land and take off in a direction parallel with the shaft of the T, towards the crossarm, like a small aeroplane flying towards its own wings.
• A right-hand arrow in the signal area shows that turns in the circuit are to be made to the right; without it, or other instructions, the circuit is left-hand (see aerodrome traffic and circuits).
• The tetrahedron, found at some US airports, is a landing direction indicator whose small, pointed end points in the direction of landing. A small end pointing towards about 300° means landing on runway 30.
• The segmented circle, used at US airports, is a ring of segments that gives a central, visible location for the airport's visual indicators, such as the wind cone or tetrahedron. Traffic pattern indicators set beside it show where turns are to the right.
• The windsock, or wind direction indicator, streams downwind, so its open end points into the wind.

Landing direction indicators may be set by hand, and a tetrahedron may be tied down, so they can lag behind a change of wind. The FAA advises cross-checking them with the windsock before relying on them.

Ground signals for restrictions

Other panels in the signal area restrict what may be done:

The dumb-bell signal matters most at grass aerodromes, where aircraft would otherwise be free to taxi or land anywhere on the manoeuvring area. The landing prohibited signal, a red square with yellow diagonals, is one of the few ground signals that forbids rather than informs; a pilot who sees it does not land there. The complete list of signals, including those for aerodrome traffic by light and pyrotechnic, is in Appendix 1 to SERA and to Annex 2.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.