Rule 40
Before an aircraft is attacked in the air, all feasible precautions must be taken to verify that it constitutes a military objective. Verification ought to use the best means available under the prevailing circumstances, having regard to the immediacy of any potential threat. Factors relevant to verification may include:
[Commentary]
(a) Visual identification.
[Commentary]
(b) Responses to oral warnings over radio.
[Commentary]
(c) Infra-red signature.
[Commentary]
(d) Radar signature.
[Commentary]
(e) Electronic signature.
[Commentary]
(f) Identification modes and codes.
[Commentary]
(g) Number and formation of aircraft.
[Commentary]
(h) Altitude, speed, track, profile and other flight characteristics.
[Commentary]
(i) Pre-flight and in-flight air traffic control information regarding possible flights.
[Commentary]
- The feasibility of taking precautions often depends on the degree of threat presented by an unidentified aircraft, or by other factors. A potential threat can be more or less immediate. A potentially hostile aircraft that is approaching rapidly represents a greater threat than one traveling in another direction. Considerations of force protection suggest that greater precautions ought to be taken to verify that the aircraft constitutes a military objective when more time is reasonably available before a decision to engage it.
- An attacker must bear in mind that verification can be complicated by camouflage, stealth or deception designed to conceal the presence or military status of an enemy aircraft. Certain forms of deception would, however, amount to unlawful perfidy, see Section Q.
- Rule 40 applies to any type of aircraft, provided that it constitutes a military objective. As to civilian airliners, they are entitled to particular care in terms of precautions, see Section J (I) and J (III).
- The factors listed are provided by way of illustration, and their specific relevance depends on the factual background. There may be other factors relevant to verification, such as information based upon intelligence gathering. As well, it ought to be borne in mind that medical aircraft have specific means of identification, such as a flashing blue light and radio message (see Commentary on Rule 76 (b)).
Visual identification means that an aircraft is identified as a military objective by the use of eyesight, including the use of binoculars or similar sight-enhancing devices. This is typically achieved through interception of the aircraft (see the chapeau of the Commentary on Section U).
An unidentified aircraft ought to be raised on radio and asked to identify itself and state its intentions. The “response” can be either explicit or implicit, in which case it may be demonstrated by conduct such as change of course or any other manoeuvres. The response, or absence thereof, may strengthen or weaken conclusions as to the aircraft’s nature.
Infra-red signature means the appearance of the aircraft to infra-red sensors. It depends on several factors, including the temperature of the aircraft and the waveband of the detecting sensor.
Radar signature means (i) the detailed waveform of a radar echo from the aircraft; and (ii) the detailed characteristics of a radar transmission, i.e. an indication of what kind of radar the aircraft is using. The radar echo can give indications about the size, shape and movements of an aircraft, including moving parts such as the rotor of a helicopter.
Electronic signature means the detailed characteristics of the electronic emissions from the aircraft, which may reveal, e.g., the type of radio communication equipment used by the aircraft. The second definition of radar signature, as set forth in Rule 40 (d), may also be regarded as a form of electronic signature.
Identification modes and codes refer to systems whereby an aircraft that is detected on radar is interrogated by an electronic signal (also called secondary surveillance radar (SSR)) and gives an automatic response by using a transponder. The response ought to be used to identify the aircraft to both an intercepting aircraft and to airspace managers, whether military or civilian. Civilian systems are designed to assist the manage-ment of air traffic, while the purpose of military systems (also called Identification, Friend or Foe (IFF)) is to avoid the direction of attacks against friendly forces (“blue-on-blue”) or civilian aircraft. The fact that an aircraft identifies itself as either a friendly military aircraft or as a civilian aircraft is not necessarily conclusive evidence of its character.
Civilian aircraft will usually fly alone, while military aircraft may fly in formation depending on their mission. However, civilian aircraft are sometimes “escorted” by military aircraft in which case they may become a military objective (see paragraph 3 of the Commentary on Rule 27 (d)).
- Military aircraft on attack missions can make manoeuvres that civilian aircraft are not likely to do, such as approaching the target at low altitude in order to avoid or postpone detection by radar and thereafter making a sharp climb before launching weapons that require a minimum altitude.
- A steady course by an aircraft originating from one exclusively civilian airport and heading towards another, may suggest that it is a civilian aircraft.
- In some areas, civilian air traffic (including civilian airliners) may be frequent, whereas in other areas such traffic may be light. These factors must be taken into account when determining the nature of an aircraft.
- Information received from air traffic control services as to whether scheduled or non-scheduled flights can be expected around a particular location at a particular time ought to be taken into account, although the possibility that an aircraft is off schedule or off course ought to be considered.
