Is there a reason that commercial jets do not have a “dead man’s switch”?

Very intriguing question. I do wonder what it actually solves, though. Does it only solve the mystery of whether the very rare instance of incapacitation of the flight crew occurred? The Cockpit Voice Recorder does that if it is recovered. In the rare event that the CVR is unrecoverable, the dead man’s switch would only answer if pilot input occurred prior to a crash. How would it save lives? The pilot should already be in periodic verbal communication with Air Traffic Control.

On the other hand, a global monitoring system of the aircraft would make more sense. Each commercial aircraft already has transponders. In the US, each aircraft has ADS-b. This gives a nearly real-time position of each aircraft that can be tracked and archived. This would give searchers a better idea of where to search for survivors. Incorporating a 406 MHz PLB-style beacon into the Flight Data Recorder and the CVR would be even more advantageous in solving the mystery of what happened. Even an automated system that alerts ATC of when contact is lost with the transponder or ADS-b, or when the CVR or FDR 406 beacons were activated, or when the aircraft deviated too far off its filed flight plan, would be better than a dead man’s switch.

After all, in today’s era of modern telemetry and aviation security, would scrambling fighters be of any benefit to the passengers and crew on board while the aircraft is in flight? They would have no way of intervening to prevent a crash.

It’s only benefit would be to alert ATC of a terrorist highjacking in the event that the target were close to the airport. And, the overtaking of the cockpit happened in the last phases of flight. Now, the security of modern cockpits and heightened alertness makes the notification of ATC of a more likely.