Plane tracking
If these had not gone silent, air traffic controllers would have seen two of the planes heading for Manhattan; instead they lost precious time trying to figure out where the aircraft were.
At the time it was generally assumed that transponders would be re-engineered to prevent them being turned off illegally but nothing was done and 13 years later along came MH370.
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Hide AdSwitches were first installed to avoid grounded planes sending signals that disrupted airport radar, but modern ground-scanning radars do not get confused by transponders on taxiways.
In fact, private planes had dealt with the old radar problem by using automated transponders that turn on when the planes become airborne and turn off when they slow to taxi speed.
(Dr) John Cameron
Howard Place
St Andrews