What Happens When a Transponder Is Turned Off?

The controller's data block. Secondary radar sees the aircraft only through its transponder reply. Switch it off and the tag showing callsign and altitude disappears from the screen.

ADS-B. Position broadcasting runs through the same unit. Ground and satellite ADS-B both go silent.

Collision avoidance. TCAS works by interrogating other aircraft's transponders. An aircraft with its transponder off is invisible to every other aircraft's collision avoidance system, and its own system cannot interrogate anyone. This is the most immediately dangerous consequence.

Primary radar. It reflects energy off the airframe and needs no cooperation. Where primary coverage exists, the aircraft still appears — as an unidentified return with no altitude and no callsign. This is how MH370's turn back across the Malay Peninsula is known: Malaysian military primary radar continued tracking an unidentified target for about an hour after the transponder stopped.

Primary radar coverage is limited and concentrated near populated airspace and military installations. It does not exist over oceans.

The aircraft itself. Radios still work, the crew can still speak, and every other system is unaffected. Switching off a transponder does not degrade the aircraft's ability to fly.

This is the part usually left out. There are legitimate reasons to turn a transponder off or change its mode.

A faulty transponder generating spurious replies can disrupt air traffic control and trigger false TCAS alerts on other aircraft, and controllers may ask a crew to switch it off. Certain electrical fault procedures involve shedding non-essential systems. Military operations use different modes. And on the ground at some airports, transponders are switched to standby to avoid cluttering surveillance.

A control that can be turned off in flight is a design decision with a rationale, and it is periodically revisited. The counter-argument is that a crew must be able to isolate a system that is malfunctioning or on fire, and a transponder that cannot be de-energised is a fire risk that cannot be managed.

Rather than removing the switch, regulators required a parallel path. Autonomous distress tracking transmits the aircraft's position when its state becomes abnormal, by a route that does not depend on the transponder and is not controllable from the flight deck in normal operation.

The reasoning is that the problem was never that a transponder could be switched off. It was that when one was, nothing else was reporting.