I was speaking to someone about the MH370 incident, and they suggested that a hijacker could turn off the device that sends the plane's current position, resulting in the appearance of a catastrophic midair failure.

I assume that such functionality could not be turned off, but I could be wrong. Is there any way that this could be done? Could an aircraft's internal tracking systems be "turned off" midair, and also not be located by external tracking like radar?

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    $\begingroup$ It's not your question, but even if what you propose is possible then it seems unlikely for such a large aircraft, if it were commandeered and rendered untraceable, to land at an airfield capable of accommodating it without drawing attention. Also, anyone commandeering such a high profile aircraft would likely want to draw public attention, an indispensable element of today's attacks. Nobody's (yet) claimed resp. for attacking the aircraft in question which, when coupled with reports it attempted to RTB, lends credence to either a catastrophic mech. failure or mech. failure & pilot error. $\endgroup$
    – cfx
    Commented Mar 11, 2014 at 4:44
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    $\begingroup$ @cfx Yep. Another issue is that the moment the plane lands, or even gets close to landing, mobile phones would likely get some signal and people would be sending messages. $\endgroup$
    – andrewb
    Commented Mar 11, 2014 at 9:18
  • $\begingroup$ The transponder can be turned off at any time by anyone with access to the cockpit. You can also change the code on a transponder to any value. For example, you could change your code to match that of another aircraft so there was no way to tell the difference between them. You can also put the transponder on standby, so that it is on, but not transmitting. $\endgroup$ Commented Mar 11, 2014 at 16:54
  • $\begingroup$ why a Pilot can't be involved in doing all this turning off or routing to somewhere else? if not a pilot crew person can get entry into cockpit allowing others (hijackers etc) to take control or force pilots to do exactly they want. cheers Aman $\endgroup$
    – user1385
    Commented Mar 11, 2014 at 22:26
  • $\begingroup$ Related: aviation.stackexchange.com/q/2278/193 $\endgroup$
    – asheeshr
    Commented Mar 15, 2014 at 2:47

6 Answers 6


The device you are talking about is called a transponder. This is a device that listens for a signal (an interrogation) and responds with information about the airplane including an ATC assigned code, altitude information and additional aircraft info for certain equipment.

There are 2 ways ATC watches airplanes:

The transponder is interrogated by the Secondary radar and the response is listened for. Even with no transponder, the aircraft can still be tracked by primary radar. This is just a normal radar that is good at seeing airplane sized objects and not so great at seeing weather and can occasionally spot a large flock of birds. The primary radar is what provides the "blip" on radar and tracks the airplane. The datablock ATC has on the airplane comes from the secondary, so what ATC will see is a blip with no information. Turning off the transponder is as simple as setting it to OFF or pulling a circuit breaker and yes, it can be selected OFF in flight, it is just a switch.

The problem with radar is that it only works so far from a radar transmitter and the further away you are the higher you must be to be seen by the radar. Over the ocean away from land, you are going to be on the fringe of radar or out of radar contact completely and thus unable to be tracked directly by ATC (at this point, non-radar procedures such as position reporting and ETAs can be used to track positions).

The flight in question was apparently visible by a military radar before it disappeared from the scopes (it allegedly observed what appeared to be a turn back toward its origin).

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    $\begingroup$ @andrewb There are other ways that an airplane transmits it's location, both to ATC and to their company headquarters for flight tracking purposes. Many (especially newer) aircraft do have GPS that automatically transmits the position, however the communication system can be turned off if you know how.... $\endgroup$
    – Lnafziger
    Commented Mar 11, 2014 at 12:01
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    $\begingroup$ @DavidMulder The main problem in an airplane is that it needs to be designed for when things go wrong. What happens if there is an electrical short which causes a fire? What about a malfunction that causes electrical interference with other systems? They both happen. There needs to be a way to turn it off in cases like these in order to protect the aircraft in a "worst case scenario" for the electronic component. $\endgroup$
    – Lnafziger
    Commented Mar 11, 2014 at 12:22
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    $\begingroup$ @DavidMulder I'm with Lnafziger on this one; any thing powered needs a fuse (all accessible behind the captain), adding a battery just adds the risk of it leaking/exploding due to overcurrent cause be faulty wire somewhere else and damaging other things $\endgroup$ Commented Mar 11, 2014 at 12:30
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    $\begingroup$ @DavidMulder Yes, interference can just "magically manifest itself during a flight" when a component on a circuit board fails. Electrical wiring or pretty much any component on a circuit board can fail and create a hazard. That's why almost everything is protected with independent systems so that you don't lose the entire electrical system just because the sat phone shorted out. I'm not talking about the software side of things, just basic electrical hardware protection design. I'm a pilot who happens to be an electrical engineer. Let me assure you that it isn't as simple as you think. $\endgroup$
    – Lnafziger
    Commented Mar 11, 2014 at 12:44
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    $\begingroup$ @DavidMulder This conversation has gone far enough as a comment thread and is not what comments are designed for. If you have an interest in more information about this topic, please ask your own question where we can offer full answers (with formatting, pictures, and hyperlinks) to better address your question/concerns and get different feedback from different people. You are also welcome to drop by Aviation Chat if you just want to talk about it with other people in the industry. I understand your point of view, but there are technical reasons why things are the way that they are on airplanes. $\endgroup$
    – Lnafziger
    Commented Mar 11, 2014 at 15:18

With the proper knowledge of the aircraft systems almost anything can be turned off. That's what we do at the end of almost every flight day. Most of the communications systems can actually be turned off fairly easily, but the average person would not know how to do it.

In airplanes like the 777, there are multiple ways that they communicate but all of them can be turned off (transponders, ACARS reports, CPDLC, ADS-B, radios, satellite phones, etc.). Some are automatic and some are manual, and not all aircraft have all of them. Most of them have a switch or can be controlled through the Flight Management System, but the ones that don't will have a circuit breaker designed to protect the system which may be pulled in order to deactivate the system.


As a B-737 pilot, I can tell you that ALL electrical components are wired into the electrical system to a specific electrical BUS. It would not be hard to determine which BUS(S) run power to the associated component and therefore knowing which circuit breaker(s) to pull. The ACARS that transmits our FOQA information can be disconnected on the BBJ I fly. I assume it can be done in the B-777.

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    $\begingroup$ Not only that, I assume that you can just turn off the satellite and VHF data communication directly from your FMS. (At least I can on my airplane!) $\endgroup$
    – Lnafziger
    Commented Mar 12, 2014 at 1:28
  • $\begingroup$ Yes. I provided the information in my answer: aviation.stackexchange.com/a/7787/22550 $\endgroup$
    – summerrain
    Commented Dec 22, 2018 at 1:36

With today's locked cockpit policy it is extremely unlikely that hijackers would be able to turn off the transponder without pilots being able to set it to 7500, the code for unlawful interference, for at least a short while. Because while the transponder can be turned off, the hijackers would have to get into the cockpit first.

The cockpit door can't be opened from cabin side. Even if the stronger code is entered that will open the door (for cases when one pilot is incapacitated while the other is in the lavatory) the pilots still have some time to cancel the unlock request. Therefore the hijackers would have to threaten with something to get the pilots unlock, but that would give the pilots time to provide some indication of the hijack to the ATC.

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    $\begingroup$ Setting the transponder to 7500 when out of radar range won't help. $\endgroup$ Commented Mar 11, 2014 at 15:07
  • $\begingroup$ Also clever social manipulation combined with poor pilot practices could get you in there. There's also the possibility that a pilot could be in on it. $\endgroup$
    – andrewb
    Commented Mar 11, 2014 at 22:11
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    $\begingroup$ Had another quick look at the news and it turns out the pilot has a history of letting people into the cockpit! theguardian.com/world/2014/mar/11/… $\endgroup$
    – andrewb
    Commented Mar 11, 2014 at 22:13
  • $\begingroup$ @andrewb: Interesting... $\endgroup$
    – Jan Hudec
    Commented Mar 11, 2014 at 22:17
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    $\begingroup$ « With today's locked cockpit policy »? I succeeded to enter a 747 cockpit last year during the flight (end of cruise and begin of descent). Just needed to smile to the purser and say "I'm PPL". But it will depend of course on the airline, and the current crew. $\endgroup$
    – Fox
    Commented Jun 22, 2014 at 18:36

Another Boeing 777 on 12th July 2014 suffered smoke in the cockpit and cascading loss of electrical systems, navigation displays and Comms in the cockpit. That was Continental Flight 201 which made an emergency landing on Midway island.

There are disputed claims that flight 201's transponder also failed during the emergency.

In the case of MH370 it is acknowledged that the ACARS suffered a power interruption prior to log-on requests. It seems possible that the transponder was affected by whatever power interruption the ACARS suffered therefore it is not an automatic assumption that the transponder and ACARS were turned off deliberately.

  • $\begingroup$ With Continental Flight 201, did they lose all transponder capabilities? $\endgroup$
    – andrewb
    Commented Jul 21, 2014 at 1:20
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    $\begingroup$ "radar data indicate the transponder worked until touchdown." - Aviation Herald $\endgroup$ Commented Mar 26, 2015 at 10:17

The plane's position is reported by ACARS and by the transponder.

MH370 lost ...

  • ... its Mode S transponder (shortly before 17:20:33 UTC)
  • ... ACARS (somewhere between 17:07:49 - 17:37:29 UTC)
  • ... and the SDU (somewhere between 17:07:49 - 17:37:29 UTC).
    The SDU was repowered, however, at 18:25:27 UTC, when it logged onto the Inmarsat satellite network again.

The first 2 items can be easily disabled in the cockpit.
The SDU can be disabled by either turning off the AES circuit breaker in the E&E bay or depowering the left main AC bus in the cockpit (which results in the collateral loss of various systems).

India has made it mandatory for aircraft flown by Indian operators to install a device reporting the aircraft's location every 15 minutes, thereby preventing the aircraft from disappearing:

India makes its aircraft disappearance proof post MH370 incident

« India has taken several steps to remove any possibility of the aircraft operated by its airlines disappearing without a trace in the aftermath of the shocking incident in March 2014 involving Malysia Airlines’ MH370. It has made it mandatory for aircraft flown by Indian operators having a seating capacity of 19 passengers or takeoff weight of 45 tonnes or more to install a devise that will send location of the aircraft at 15 minute intervals.

The government had ordered airline companies to compulsorily install an automated aircraft tracking system (ATS) in all aircraft falling in the above categories. The system will pass location information to ground stations even if the aircraft is flying over the oceans during long international flights throughout the duration of the flight. » (source)

  • $\begingroup$ Of course, this only makes the aircraft disappearance-proof if the ATS doesn't happen to fail. $\endgroup$
    – Vikki
    Commented Jul 17, 2019 at 23:32

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