How does ATC locate a stealth aircraft if its transponder fails? Stealth aircraft are built to have near-zero radar visibility, so that, in a warzone, it'll be harder for the enemy to see it on their radars to shoot it down.

However, air traffic control also use radars.

Normally, this wouldn't be a problem, since the aircraft is carrying a transponder that continually broadcasts positional and movement data (so-called "secondary" radar returns), not just to ATC, but also to other nearby aircraft (so that they can avoid running into it).

If an aircraft's transponder fails, though, then ATC is reduced to tracking it via "primary" radar returns (literally bouncing radio waves off of it and seeing where they're reflecting from)... but stealth aircraft are specifically built so that they can't be picked up by most primary radars (at least without a lot of trouble).

So how does ATC - and, for that matter, all the other planes in the air who would rather not run into it - know the position of a stealth aircraft if its transponder fails?  Do they have the flight crew of the stealth aircraft continually read off map position, height, speed, direction, etc. data and try to use this to steer other planes away from it?
It depends on the aircraft.  One example was on the F-117, which had removable radar reflectors on the sides of the aircraft, allowing the jet to be detected by primary and secondary ATC radars.



As to more modern stealth aircraft, it’s possible they use similar systems to ensure ATC tracking in peacetime.