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These days, when reading news about missing Malaysia Airlines Boeing 777-200, I keep coming across a scenario where pilot might have deliberately turned off the transponder which is used for the communication of flight with ATC.

When there is a possibility that any bad thing can happen when pilot turn off transponder, why would one give the ability of turning off the transponder to a pilot when he/she usually depends on instructions from ATC or flight control. Is there anyway that ATC can turn on transponder back from ground?

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    $\begingroup$ The simple answer to your question is "pilots are trusted", we are after all in complete and authoritative control over the aircraft you are flying in. If you didn't trust us with this authority, presumably you wouldn't board an airplane. $\endgroup$
    – casey
    Commented Mar 15, 2014 at 23:17
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    $\begingroup$ Actual transponder-related accidents have occurred before; it's just that the media attention this event has been getting is making people "freak out" about it (media has a tendency to make people suddenly upset about issues that aren't new). For example, the crash of Gol 1907 in 2006 was caused in part by an accidental deactivation of a transponder (the FAA later released a safety alert regarding accidental deactivation of EMB-135/140/145 transponders). $\endgroup$
    – Jason C
    Commented Mar 15, 2014 at 23:18
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    $\begingroup$ Transponders are a part of a decades old systems that works as designed. A possible "solution" would be something similar to LoJack/OnStar installed so it cannot be access from the inside of the plane. The trucking industry uses something like this as well. It could report "continuously" using independent GPS. And it could be on aircraft power with battery backup. $\endgroup$
    – rheitzman
    Commented Mar 20, 2014 at 21:52
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    $\begingroup$ @casey: meh, I trust the pilot but I still don't mind minor displays of distrust by the ground authorities such as breathalyzing you ;-) $\endgroup$
    – Steve Jessop
    Commented Jun 5, 2014 at 18:45
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    $\begingroup$ @casey: right, from my personal POV there's breathalyzing you and then there's spending 3 hours running every test they have when I've got a connection to make. Presumably even if it was a good idea to control the transponder from outside the plane (i.e. all the technical reasons below magically went away), bureaucracy would find a way to make that a PITA for someone. $\endgroup$
    – Steve Jessop
    Commented Jun 5, 2014 at 18:54

3 Answers 3

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Basically everything that consumes power on an aircraft can potentially cause interference, short-circuits, or otherwise jeopardize the safety of flight and therefore must be switchable. Sometimes the switch is in the form a button, otherwise by a fuse.

There are several particular reasons that the transponder can be turned off.

If the transponder malfunctions, it may cause interruptions to all ATC surveillance in an area. There have been occurrences in the past that due to a fault the transponder it was basically acting as a radar jammer.

In one particular incident it took a while before the aircraft that caused it was identified and after requesting the pilot to switch of the transponder, secondary surveillance was restored.

Another reason is that when the aircraft is at the gate, the transponder is switched of to reduce the amount of radio transmissions. 100 aircraft on the surface of a large airport can produce a massive Radio Frequency noise, which negatively affect radar systems. When taxiing, radar replies are useful for aircraft identification, hence the transponder is switched on at push-back or engine start.

For an example to see how such requirements are worded in aircraft certification regulations, see rbp's answer.

The focus here and in the media1 on the transponder in relation to hijacks is a red herring. If an aircraft is hijacked, there are way more important things to worry about than an active radar transponder.
If hijackers turn off the transponder it means that they have access to the cockpit already.

1This answer was written shortly after flight MH370 went missing while en-route from Kuala Lumpur to Beijing. Initial speculation focussed on a possible hijack scenario.

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    $\begingroup$ It's also worth noting that if the transponder is switched off you generally only lose two pieces of information: the flight identification (mode A & mode S data) and altitude (mode C data). Primary radar will still paint a target (albeit unidentified) as long as the aircraft is within radar range. $\endgroup$
    – voretaq7
    Commented Mar 15, 2014 at 22:55
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    $\begingroup$ The range of the primary is often limited. Long range primary consumes an awful lot of power and requires active cooling. Expensive gadget :-) $\endgroup$
    – DeltaLima ♦
    Commented Mar 15, 2014 at 23:15
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    $\begingroup$ +1 "The focus here and in the media on the transponder in relation to hijacks is a red herring". Media focus on any thing in any event is frequently a red herring. $\endgroup$
    – Jason C
    Commented Mar 15, 2014 at 23:23
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    $\begingroup$ @BobbyAlexander In general that is the case. If the aircraft is flying within the operational coverage area of a secondary radar, there will be an indication on the controller display when the secondary transponder stops replying. However, when the aircraft is just handed over to the adjacent control area or is flying out of the radar coverage, there is no indication since the aircraft's transponder is no longer a concern to the air traffic controller. $\endgroup$
    – DeltaLima ♦
    Commented Mar 18, 2014 at 16:22
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    $\begingroup$ @NPSF3000 yes really. The transponder is not a magic bullet. It is for Air Traffic Control surveillance of cooperating aircraft. If you need to find a crash site you use an ELT (via satellite), if you need to intercept something that does not cooperate, you have the military track it using primary radar. $\endgroup$
    – DeltaLima ♦
    Commented Mar 18, 2014 at 21:58
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Everything including the flight recorder has a power switch or circuit breaker. Electrical devices occasionally malfunction, and you don't want a sparky transponder setting the plane on fire when you can just turn it off and use the other one.

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    $\begingroup$ are you saying that the flight recorder (the black box) can be switched off as well? $\endgroup$
    – Martin Vegter
    Commented Mar 16, 2014 at 10:52
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    $\begingroup$ @MartinVegter Yes, as Paul says, everything has a circuit breaker and that can be used to deactivate it. $\endgroup$
    – Lnafziger
    Commented Mar 16, 2014 at 13:14
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    $\begingroup$ Given what we know now, would it be possible to simply build a large battery into the transponder housing? $\endgroup$
    – Jaydles
    Commented Mar 17, 2014 at 19:34
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    $\begingroup$ @Jaydles - like the ones in the Dreamliners that caught fire? ;) $\endgroup$
    – Zhaph - Ben Duguid
    Commented Mar 18, 2014 at 10:08
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For safety purpoes, in order for an aircraft to be certified under 14 CFR 23, it must meet the following requirement:

§23.1357 Circuit protective devices.

(a) Protective devices, such as fuses or circuit breakers, must be installed in all electrical circuits other than—

(1) Main circuits of starter motors used during starting only; and

(2) Circuits in which no hazard is presented by their omission.

(b) A protective device for a circuit essential to flight safety may not be used to protect any other circuit.

(c) Each resettable circuit protective device (“trip free” device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that—

(1) A manual operation is required to restore service after tripping; and

(2) If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.

(d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be so located and identified that it can be readily reset or replaced in flight.

(e) For fuses identified as replaceable in flight—

(1) There must be one spare of each rating or 50 percent spare fuses of each rating, whichever is greater; and

(2) The spare fuse(s) must be readily accessible to any required pilot.

For aircraft certified under 14 CFR 25, there is similar wording:

§ 25.1357 Circuit protective devices.

(a) Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.

(b) The protective and control devices in the generating system must be designed to de-energize and disconnect faulty power sources and power transmission equipment from their associated busses with sufficient rapidity to provide protection from hazardous over-voltage and other malfunctioning.

(c) Each resettable circuit protective device must be designed so that, when an overload or circuit fault exists, it will open the circuit irrespective of the position of the operating control.

(d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be located and identified so that it can be readily reset or replaced in flight.

(e) Each circuit for essential loads must have individual circuit protection. However, individual protection for each circuit in an essential load system (such as each position light circuit in a system) is not required.

(f) If fuses are used, there must be spare fuses for use in flight equal to at least 50 percent of the number of fuses of each rating required for complete circuit protection.

(g) Automatic reset circuit breakers may be used as integral protectors for electrical equipment (such as thermal cut-outs) if there is circuit protection to protect the cable to the equipment.

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