The term originates from the 5-character uppercase abbreviated notation "NORDO" displayed on controllers' radar scopes when an aircraft transmits the "radio failure" code on its transponder.
An alternate explanation is that "NO RDO" was the standard note made on maintenance and equipment sheets used in military aviation, starting in the 1930s, as a code to identify planes which needed radio repairs or were not equipped with radios.
[citation needed] The term has made its way into standard aviation jargon, used as an adjective or a noun to describe an aircraft without a radio, even among pilots and others who are not air traffic controllers.
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, transmit codes or IDENT signals on the transponder, rock their wings, etc., to acknowledge clearances or instructions.