Why can transponders be turned off?

TV news says the hijackers were smart enough to know that it would be beneficial to them to turn off the aircraft transponders. Now did they just damage them to the point of not working or is there really a simple on/off switch somewhere? If that is the case, why?

I would think that planes that are on the ground turn off their transponders routinely. I mean, surely it would be kind of confusing to be getting signals from all the planes that are not in the air.

I have found some stuff about turning them on, but not turning them off. :slight_smile: For instance:

The faceplates on most of our transponders are labeled as follows: Off, Stby, On, and Alt. Regardless of whether you have mode “C” Altitude reporting or not, you should be in the “ALT” position unless the controller says different. The ground station is looking for mode “C” framing pulses even if the altitude data is present. I guess “standby” is OK on the ground, but I usually forget to turn it on until the controller says, “recycle transponder”. He knows that I’ve forgotten to turn it on, and it’s his polite way of saying, “wake up, dipstick!” If your encoder for mode C takes a dive and starts putting out wrong information the ATC controller will have to “stop squawking altitude”. At that point turn the selector knob to the “ON” position. Be prepared to get a nasty letter from your local feds if you encoder is off by more than 300Ft. Another thing to remember is if you left the switch in the “off” position and take off in IFR conditions, then ATC yells that they can’t see you, I’ve got bad news. Some transponders take up to three minutes to warm up. Believe me that can be the longest three minutes of your life.


One thing that people seem to forget is the tragedy that just happened was not obvious to anyone and just because a particular piece of equipment was used (or turned off) for the terrorist’s benefit does not mean that it needs to be redesigned. There are several scenerios where NOT being able to turn a transponder off would be detrimental. Here are a few that I can think off.

  1. Ground based terrorists are intercepting the transponder signals from aircraft to down them with missles. You would want to be able to turn them off then wouldn’t you?

  2. A short in the transponder could cause a fire or smoke in the cockpit. It would certainly need to be switched off in this case.

  3. A complete electrical failure, except for the reserve battery failure, would mean that some equipm,ent may need to be turned off to conserve remaininmg power (more likely in small planes but it does happen).

Continuing from what Shagnasty started, also keep in mind that, for a multitude of reasons, an electrical device does need to be turned off for service, maintenence or replacement.

The “transponder”, as i understand them, are actually small radar emitters in their own right. As such, there’s a radiation danger to those close by on the ground, such as the guys in the baggage carts and the dude with the big flashlights guiding the plane to the jetway.

Further, an electrical device, even one of such importance, is subject to the occasional voltage spike or short circuit, just like any other electronic doodad. As such, they almost certainly have fuses, relays or circuit breakers- yank those, and it has the same effect as turning it off at the switch.

The first report I’d heard was that they had, in fact, yanked “circuit breakers” to disable the transponders.

OK, disclaimer time - I’m a pilot that flies small planes so I speak from that experience and have no direct experience with the big jets.

That said - from the pilot’s viewpoint the transponder is one of many boxes in the control panel. You can dial in a four-digit code (either “1200” for visual flight rules or a code assigned by air traffic). There is an on/off switch easily accesible.

When the box is on and it gets hit by air traffic radar it sends information back to the originating radar giving registration, altitude, and so forth. When you are starting the plane you are supposed to have it off to protect the device from stray voltage spikes. You then put it on stand-by to let it warm up. Just before you take off you put it to “ALT”, which means it’s transmitting information.

I have had two occassions to turn it off. In the first instance it started malfunctioning. Air traffic control asked me to turn it off because it was too distracting to have the bad information on their radar screen (my plane, with a maximum altitude of 12,000 feet above sea level, was claiming to be at 35,000 feet).

On the other occassion I have an alternator failure, leaving me with only battery power to operate instruments. In such a case standard operating procedure is to turn everything off that you can to conserve power. This is even more critical at night, and on planes that require power to operate things like flaps.

So, although initially puzzling, there are reasons to have an on/off switch on the transponder. There are also other ways to turn it (and other instruments) off or isolate an electrical circuit in the event of fire.

A transponder is not a radar emitter. As I recall, it uses conventional frequencies to simply broadcast a signal when it detects a radar signal hitting it.

The airport radar system can then tell where you are easier, because it gets a signal every time the radar hits that small piece of sky. The signal it sends back is a 4-digit encoded number, which can be set by the pilot in the cockpit. This number is used for a variety of purposes - first, every aircraft not under positive control will ‘squawk’ 1200. If an airplane is under positive control (either VFR flight following, or IFR), then the area control center will tell you to ‘squawk’ a special number that allows them to further identify you. Thus, if their radar paints two aircraft very close to each other, the two different squawks will let them identify each one.

A normal transponder also has an ‘ident’ button, which sends the signal back even if radar isn’t painting it. A controller who wants to positively identify you will sometimes ask you to ‘squawk ident’. This will show up on the screen beside your radar target, so they know who is who if they lose track.

Finally, a standard transponder is also used to alert the ground to emergency situations. There are three special 4-digit numbers that indicate an emergency, a hijacking, or a communications failure. Obviously, I won’t say what they are on a public forum, but it’s not hard to find out if you’re really interested. It’s not a big secret.

When you squawk one of the emergency codes, an alarm will immediately go off in the area control center, and the radar symbol for your aircraft will begin to flash, or will change color. It’s not recommended that you do this accidentally, although it happens quite often, especially during flight training. I’ve actually been in a control center when an emergency squawk went off, and you can’t miss it. (In this case, the controller actually called an aircraft and asked him to squawk his emergency code, just so I could hear it).

A transponder can also send back data. a “Mode C” transponder uses an altitude encoder in the aircraft to transmit altitude data back to the control center that painted it with radar. Most aircraft now have Mode C in them, because you can no longer fly in controlled airspace without it without special permission.

‘Mode S’ transponders can send a whole stream of data, including text, flight data, and accurate position information from the plane’s onboard flight management system. Mode S transponders are expensive, and generally only found in airliners and high-end business aircraft. Mode S, if I recall, is also bidirectional and can receive digital information like weather alerts and clearances.

Every transponder can be turned on and off. For one thing, you don’t want transponders on while the aircraft is taxiing, because it clutters radar displays and confuses operators. So the takeoff checklist for every transponder-equipped aircraft has you turn the transponder from ‘standby’ to ‘on’. Also, as others said it’s important for every piece of avionics in the flight deck to be able to be turned off in case it becomes a fire hazard or to reduce battery consumption in case of a generator failure.

I’m not sure if this is off-subject or not.
What real difference would it have made anyway? If at the time of the takeover the terrorists had radioed their intentions would there been enough time to change the outcome? A lot of time probably would have elapsed before any effective action could have been taken.
But I don’r know.

Well, for one thing it would have alerted the ground that the airliner would have been hijacked, which would have given the military more time to scramble an intercept, and also give people in Washington more time to evacuate buildings.

By turning the transponder off, the hijackers made the plane more difficult to track (not impossible). So the initial assumption by air traffic control might have been that they had had a transponder failure, or perhaps were in some sort of distress due to a systems failure, but the immediate assumption would not have been hijacking because in this country hijackings are rare. It bought the hijackers a little more time.

This thread is a great reminder of how difficult it is as an “outsider” to figure out what corrections are needed. What is strikingly simple to “outsiders” is really a part of a more complex system that only “insiders” may realize. There just won’t be any easy fixes.


What would have been the likely chronology of the response had the air traffic controllers known early on about the takeovers?
Each plane impacted the WTC approx. 45 minutes after take-off. The first sign of trouble came about 15 minutes later, as far as I can tell. And nobody even knew about the targets until just minutes before they were hit, if then.
So, if the terrorists had announced their intentions at the time of the take-over (approx. 30 min. warning), would the ATC’s, and the system, have had time to take any effective action? And what would those actions have been? Evectuation, of course. Nearly impossible though. And the US surely wouldn’t have shot the planes down, even had they had time ot make the decision and carry it out.

"And the US surely wouldn’t have shot the planes down, even had they had time ot make the decision and carry it out. "

Well, after the attacks, I think we may have a different answer. But even during, the attacks I wonder if the US military, with enough warning to get fighters into position, wouldn’t have shot down the second plane headed for the WTC or the third plane headed to the Pentagon. Certainly a fourth plane would have been a target.

I don’t know, Rillian. Probably the one headed for the Pentagon, and surely one headed for the White House. Especially if the pres, were there.
There were innocents aboard all these flights. I don’t think that even the military would take this decision lightly at all.
And given time. In this case they simply didn’t have the time to consider a course of action. If they had warning and time, they wouldn’t have needed transponder signals to track the planes.

Sam Stone said:

Why wouldn’t you just up and say what the codes are? It’s not at all obvious to me why it matters that this is a public forum. Every pilot either knows these codes or has them on his/her checklist or something, right?

Sorry if it sounds like I’m going off half-cocked here, but I’ve seen this kind of statement a few times in the past couple of days, and it’s starting to bug me. I mean, it’s part of the public record already, why not spread the beans on the floor a bit?


Of course they wouldn’t have taken it lightly, but they would’ve done it. Interceptor jets were alerted and in flight, but they were too late to do anything.

I mentioned – or rather, didn’t mention – the codes in another thread. (I don’t remember which one.) I can see your point. Anyone interested in finding out the codes could find them very easily. But why make it easier for someone who may read this forum and decide to use the information criminally?

Maybe I’m carrying procedures over from when I worked with classified data. You may have the level of clearance necessary, but unless you have a “need to know” you can’t see it. Heck, there’s even a normal English word that I’m nervous about saying because… well, just because.

So basically, it doesn’t matter to anyone who doesn’t need them, what the codes are; and there is a very, very small possibility that they could be used by the bad guys if they happened to read them here. Someone else may post them, but I won’t.

If you want to know what the magic codes are - look 'em up. They’re not on any checklists I’ve ever seen, but the information is something you can look up in easily accesible information sources. But I agree with Johnny - no need to make this any easier for the bad guys.

In addition to the electronic indicators of hijack, airlines have their own procedures. They keep them confidential.

Everything in an airplane has a circuit breaker, for safety reasons (to protect against a short circuit, fire, death, et cetera). So even if the transponder didn’t have an on/off switch it would be a simple matter to pull the breaker.

When landing at a large airport I turn off my transponder shortly after I land, because responding transponders that are very close to the interregator can mask more distant responses. When I flew into Oshkosh I was required to have have my transponder off within 50 miles. There is so much traffic around Oshkosh during the airshow that it would be impossible to see any transponder returns because the frequncy would be so saturated.

I’m not so sure. But we’re getting way off-topic here, barbitu8.
There would be many factors to consider. Flaming debris over heavily populated areas, for example?
Maybe another topic? Be my guest. :slight_smile: