I've got another potentially dumb aircraft question for you: Why don't commercial airliners just have transponders that aren't capable of being turned off? Is there really any reason that a commercial airliner that's used for everyday passenger service should have its transponder off?
Yes! Great question actually and I’m surprised more people aren’t talking about it.
There are at least two reasons I can think of that transponder’s have an on/off switch. First, turning one’s transponder off and on is one of the many methods of radar identification (albeit not a very commonly used one). For example if I’m trying to locate an aircraft on my radar screen among many other targets, I can instruct him to turn his transponder off, or “squawk standby,” then tell him to turn it back on. If I see a target switch from a secondary to a primary and then back again I’ve successfully identified the aircraft and I can now provide radar service.
The more common reason however that you would need to turn a transponder off is in the event of a malfunction. Let’s say an aircraft is plowing along at 24,000 ft when its transponder craps out and starts to transmit an erroneous altitude readout of, say, 34,000 ft. In addition to setting off bells and whistles on the controller’s radar scope, it could also set off false traffic alerts to other airplanes flying over him actually at 34,000 ft as their TCAS systems would interpret from erroneous transponder signal that they were about to hit another aircraft at their same altitude. In this scenario the controller would instruct that aircraft to “stop altitude squawk” and then use altitude reports for the purposes of vertical separation. That’s actually the procedure when an aircraft’s transponder is off by as little as 300 ft or more, as the minimum IFR vertical separation is 1,000 ft).