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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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This has been a question on my head since the day one the flight went missing. How odd we have advanced so much that we cannot perfect a simple transponder to be non faulty and need a manual breaker for just in case situations :( –  rao Mar 14 at 21:52
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. –  casey Mar 15 at 23:17
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)‌​. –  Jason C Mar 15 at 23:18
There have been others, too, that I don't recall off the top of my head, but, irritatingly, internet research into the subject is hampered at this time by saturation of the (arguably not-actually-related-to-transponders) Malaysian flight story. –  Jason C Mar 15 at 23:21
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. –  rheitzman Mar 20 at 21:52

2 Answers 2

up vote 45 down vote accepted

Basically everything that consumes power on a aircraft can potentially cause interference, short-circuts, 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 instances in the past that due to a fault in the transponder it was basically acting as a 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 pushback or engine start.

The focus here and in the media 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.
The fact that it is turned off means that hijackers already have access to the cockpit.

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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. –  voretaq7 Mar 15 at 22:55
@voretaq7 true, provided that you have primary radar available. –  DeltaLima Mar 15 at 22:56
that's usually a good assumption since secondary transceivers are often colocated with primary dishes (there are a few places I'm aware of where there's no primary radar & only the transponder returns are plotted though - it may be more common outside the US since radar installations are rather expensive...). –  voretaq7 Mar 15 at 23:05
The range of the primary is often limited. Long range primary consumes an awful lot of power and requires active cooling. Expensive gadget :-) –  DeltaLima Mar 15 at 23:15
+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. –  Jason C Mar 15 at 23:23

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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are you saying that the flight recorder (the black box) can be switched off as well? –  Martin Vegter Mar 16 at 10:52
@MartinVegter Yes, as Paul says, everything has a circuit breaker and that can be used to deactivate it. –  Lnafziger Mar 16 at 13:14
Given what we know now, would it be possible to simply build a large battery into the transponder housing? –  Jaydles Mar 17 at 19:34
@Jaydles - like the ones in the Dreamliners that caught fire? ;) –  Zhaph - Ben Duguid Mar 18 at 10:08

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