NORDO
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NORDO, short for "No Radio", is a North American aviation term for aircraft flying without a radio. While sometimes used to denote small general aviation aircraft that are not equipped with one, the term is more commonly applied to aircraft that have experienced a radio failure in midair. This may constitute an emergency, as determined by the pilot. NORDO aircraft declaring an emergency are given priority over other aircraft (providing a more serious emergency does not occur on another aircraft).
If the radio failure occurs in VFR conditions, the pilot is expected to continue under VFR and land when feasible. In IFR conditions, the pilot is expected to follow the last instructions given by ATC, or follow their filed flight plan. Aircraft equipped with a transponder should indicate a NORDO situation by setting the appropriate transponder code: 7600.
Air Traffic Control may re-establish communications with NORDO aircraft by using emergency frequencies, voice features of NAVAIDs, or aviation light signals. In the event of one-way communications (i.e. aircraft can receive only), the controller may request the aircraft make identifying turns, flash their navigational lights, rock their wings, etc, to acknowledge clearances or instructions.
References
Aeronautical Information Manual Chapter 6, Section 4: Two-way Radio Communications Failure
FAAO 7110.65 Air Traffic Control Chapter 3, Section 2: Visual Signals