This is a section from an Air Canada crew training document. It describes some of the signals used during an interception when radio contact cannot be established between interceptor and interceptee. These signals are used to communicate mechanical problems with the intercepted aircraft, and fuel status. It is referred to as the HEFOE system for Hydralics, Electrics, Fuel, Oxygen, Engine.

"Flight crews should be familiar with the military HEFOE signalling system. These standard hand signals permit the silent communication of aircraft emergency system status to an intercepting aircraft. The intercepting aircraft will initiate this communication by formatting on the intercepted aircraft and placing a clenched fist to his/her canopy. The intercepted aircraft responds in turn with a clenched fist followed by the display of one to five fingers. one finger for hydraulics, two for electrics, three for fuel, four for oxygen and five for engine. In the case of fuel problem, the intercepting aircraft can request your status by tipping an extended thumb back towards his/her facemask. Your response would be one clenched fist for each hour of fuel remaining and one finger for each 10 minutes."