I'm going to go more in-depth about radio failures during IFR conditions.
If you have ever read transcripts of or heard flight clearences, you'd always see/hear (example only) "United 2868, you are cleared as filed via the Gunnis2 departure... fly the runway heading until 3,000' then turn left heading 240. Expect flight level 130, one-zero minutes after departure..."
By giving the pilot that clearence, the controller knows what to expect if that pilot's radio fails. Especially when he told the pilot to "expect flight level 130, one-zero minutes after departure." If the radio fails that pilot knows to climb to 13,000' 10 minutes after departure.
How does the controller know when the aircraft's radio has failed?? Simple really.
First, he gets no response. Second, the pilot squawks 7600 on his transponder. This puts a flashing four letter word under the respective blip that says "RDOF".
If the radio fails while en route, then the pilot still squawks 7600 and flys his assigned flight plan. Then as he approaches the airport there are certain special published flight routes that he must fly to make his approach and landings. With the combination of these special flight routes and the 7600 squawk code, the controller can resequence aircraft accordingly to give you priority.
Each Jeppesen approach/depature plate has a little box in the lower right hand corner with instructors on how to fly that approach/departure if your radio has failed.
I hope this has answered your questions.
- Neil Harrison