What is the first action to take during a radio failure in controlled airspace?

In the event of a radio failure while operating in controlled airspace, the first action a pilot should take is to squawk 7600 on the transponder. This code specifically indicates to air traffic control that the aircraft is experiencing a radio communication failure. Squawking 7600 alerts ATC to the situation, allowing them to take appropriate measures to ensure safety, such as providing traffic advisories and either continuing to communicate visually or through another means.

Once this action is taken, the pilot can then proceed to follow established procedures for a communication failure, which might include attempting to re-establish contact with ATC or following published routes for situations involving loss of radio communication. However, signaling the radio failure by using the correct transponder code is the immediate priority, as it informs ATC of the situation first.