Aeronautical Information Manual (AIM), page 413
12/10/15 AIM c. Helicopter Intercept phases (See FIG 5-6-2) 3. Post Intercept Phase. Visual signaling devices may be used in an attempt to 1. Approach Phase. communicate with the intercepted aircraft. Visual Aircraft intercepted by helicopter may be approached signaling devices may include, but are not limited to, from any direction, although the helicopter should LED scrolling signboards or blue flashing lights. If close for identification and signaling from behind. compliance is not attained through the use of radios Generally, the helicopter will approach off the left or signaling devices, standard ICAO intercept signals side of the intercepted aircraft. Safe separation (Table 5-6-1) may be employed. In order to maintain between the helicopter and the unidentified aircraft safe aircraft separation, it is incumbent upon the pilot will be maintained at all times. of the intercepted aircraft not to fall into a trail position (directly behind the helicopter) if instructed 2. Identification Phase. to follow the helicopter. This is because the helicopter The helicopter will initiate a controlled closure pilot may lose visual contact with the intercepted toward the aircraft of interest, holding at a distance no aircraft. closer than deemed necessary to establish positive NOTE- identification and gather the necessary information. Intercepted aircraft must not follow directly behind the The intercepted pilot should expect the interceptor helicopter thereby allowing the helicopter pilot to maintain helicopter to take a position off his left wing slightly visual contact with the intercepted aircraft and ensuring forward of abeam. safe separation is maintained.FIG 5-6-2 Helicopter Intercept Procedures National Security and Interception Procedures 5-6-5
Page 413 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures