Civil aircraft may be subject to interception by military aircraft, particularly if they have failed to maintain two way communications on the designated ATC frequency. Interception is especially likely if they have then entered national airspace without explicit or implied permission. Unauthorised - and usually inadvertent - entry into Prohibited or Restricted Airspace or into an active Danger Area may also result in interception (or other signalling methods). The probable close approach by intercepting aircraft may trigger a Airborne Collision Avoidance System (ACAS) RA; however, if the circumstances are appreciated when an RA occurs, the normal response is inadvisable for two reasons:
The intercepting aircraft may be expected to approach very close to the intercepted aircraft in order to ascertain its status: an unexpected abrupt manoeuvre may result in collision which would have been avoided if level flight had been maintained;
An abrupt manoeuvre by the intercepted aircraft may be misinterpreted by the intercepting aircraft as suspicious or even hostile.
For further information, see the article "Military Interception Signalling" which provides guidance to flight crews on what to expect when intercepted, the signals used by intercepting aircraft and how to respond appropriately.