A student pilot flying a Cessna 172 on a solo cross-country was tuning in the destination airport’s weather when the engine gave a loud pop and began losing rpm, putting an off-airport landing in play.
Flight instructors frequently have their students simulate forced-landing scenarios, only to break off the exercise once it’s clear whether the forced landing would have succeeded.
“I scanned all my instruments and didn't notice anything critical other than a loss of rpm,” the student recounted. “I decided not to add power as the engine was running very rough and I didn't want to exacerbate any problems.”
After trimming for best glide speed, troubleshooting, turning toward a forced-landing spot, and setting the transponder to emergency code 7700, the student pilot decided that adding power was now worth a try.
It worked, so the pilot diverted toward a nearby airport.
“I then re-squawked 1200,” the student said in a filing with the Aviation Safety Reporting System. Upon landing, mechanics diagnosed the engine problem as a damaged exhaust system.
The ASRS report does not state whether the flight was in an area with radar coverage. If it was, the question arises: How would air traffic control have interpreted the switched-back transponder code?
When a flight squawks 7700, equipment in air traffic control radar facilities “normally triggers an alarm or special indicator at all control positions,” notes the Aeronautical Information Manual (page 6-2-1).
A former controller we asked to evaluate the scenario said ATC would not automatically conclude that a return to 1200 meant that the emergency was over.
“It can be very difficult for ATC to know exactly how to react,” the former controller said in an email. “It could be a mistake, the pilot may be lost, or may have lost an engine, ATC is stuck broadcasting in the blind over emergency/ATC frequencies to this unknown aircraft.”
Without direct radio communication—the one broadcast the pilot reported making was on the airport’s common traffic advisory frequency—ATC would probably monitor the aircraft’s track and based on “educated guessing” contact the most likely airport in hopes of finding the pilot, he said.
Because shutting down a 7700 squawk won’t shut down the emergency response, a better plan (if you can’t contact ATC by radio) is to stay on 7700, focus on landing at the nearest airport, then advise ATC of your safe arrival by whatever means are available, he said.