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Why do airplane transponders have an 'off switch?'


WASHINGTON (AP) -- Ever since Malaysia Airlines Flight 370 disappeared, a fascinated public has asked: Why can somebody in the cockpit shut off the transponder?

It turns out there are several legitimate reasons why a pilot might want to shut off this key form of communication that allows air traffic controllers to identify and track airplanes.

Authorities believe that Flight 370's transponder was intentionally shut off, delaying search and rescue efforts and helping to conceal the plane's location - a mystery unsolved more than 10 days after the Boeing 777 vanished.

It's rare for a pilot to turn off a transponder during flight, but occasionally there is cause.

- Sometimes a transponder malfunctions, giving out incorrect readings.

- The device could have an electrical short or catch on fire. Pilots would want to shut it down rather than risk a fire spreading to the rest of the cockpit or airplane.

- Pilots used to routinely turn off transponders on the ground at airports so as not to overwhelm air traffic controllers with so many signals in one location. That is increasingly less the case as pilots now use "moving map" displays that take the transponder data and show them the location of other planes on the ground, helping guide them around airports without mishaps.

"As long as there are pilots, they'll be able to switch off systems," said Andrew Thomas, editor-in-chief of the Journal of Transportation Security.

Airplanes have two transponders. There are two knobs in the cockpit - one on the right, the other on the left - that control one or the other. When one transponder is on, the other is normally in standby mode.

To turn off a transponder, a pilot turns a knob with multiple positions and selects the "off" setting. The second transponder doesn't automatically activate if the first one is shut down - a knob would also have to be turned. In this case, it appears one transponder was turned off, and the second not activated.

Ross Aimer, a retired United Airlines pilot and former 777 instructor, said it is possible that one pilot could reach up and turn off the transponder without the other pilot seeing it, say if one was looking away or distracted.

If the plane was in contact with an air traffic controller, the controller would alert the pilots that the transponder signal had been lost. But, Aimer - now head of Aero Consulting Experts - said, if they were not in contact with an air traffic controller, a pilot might miss it if the other shut down the transponder.

In the case of the missing Malaysian plane, even pilots are a bit puzzled by somebody turning off the transponder.

John Gadzinski, a Boeing 737 captain, said that among fellow pilots "there is a raised eyebrow like Spock on `Star Trek' - you just sit there and go, `why would anybody do that?'"

Join the discussion

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Michael Buccieri March 18 2014 at 10:23 AM

Why can't a transponder recognize the altitude and stop sending a signal while on the ground. Second with technology as great as it is, why isn't there a second, third, Fourth or even a fifth transponder which communicates with satellites when ground radar is out of range? And these can not be turned off. These would only talk to satellites not ground radar. This way it would not confuse ground radar. There can be multiple of these transponders that talk to satellites. Some would tell how well the engines are running, direction heading, altitude and so on. There should never be a way to turn these devices off in flight or on the ground. Even in the event of a disaster these devices would still communicate their location and would keep talking for approximately 30 days as do the transponders of today. Lastly, built as strong as possible to resist a blast, fire etc. This is just my suggestion. Techs try and work on this idea.

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1 reply
georgessc Michael Buccieri March 18 2014 at 11:43 AM

Airport surveillance radar has a maximun range of 60 miles. Beyond that, any military tracking radar could see it if they knew it was off course.

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tom March 18 2014 at 10:58 AM

My truck doors auto lock at 5 mph. After this, something will change with the transponder situation.

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2 replies
lochadro tom March 18 2014 at 11:20 AM

It always takes an event to move forward with aviation safety. I foresee changes in ELT's or the addition of another electronic fail-safe device that could be inhibited, if need exists, by the radar/computer systems of air traffic control.

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retpo96 tom March 18 2014 at 12:31 PM

We certainly hope so. Passengers and their family members expect to have an answer to the present mysteries of these events. Landing on Mars can be done!

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mgae March 18 2014 at 8:47 AM

sound like a big mess

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Bob March 18 2014 at 1:57 PM

We are lead to believe we have the ability to count the hair on the head of someone we are spying on from a satellite in space, but we can't find something as big as a 777?

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2 replies
hdgoose Bob March 18 2014 at 2:35 PM

Someone knows something they are not releasing to the media. They must know where this plane is. Only so many places it could be; and we can see that detail from satellites. If a plane is off course so far like that; it is assumed it is a terrorist attack and would be shot down. Also; it may be covered up like TWA flt 800. We may never hear the truth about what happend with this plane.

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1 reply
Jim hdgoose March 18 2014 at 3:22 PM

It would only be shot down if it were puting a populated area in eminent danger.

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Bob Bob March 19 2014 at 3:00 AM

Crazy, but I Love it!

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Lisa A. Santos March 18 2014 at 8:57 AM

There should be some other device for tracking the plane that no one can touch just like the black boxes! and a satellite system that covers remote areas of the oceans to be able to keep track of all vessels whether in the air or the water!

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shirefox March 18 2014 at 9:25 AM

If we are capable of finding galaxies, planets and stars in distant outerspace, or measuring the ocean depths for earthquakes, and we seem to have the means to home in on and track microchips implanted in wildlife...it stands to reason that it's long overdue to imbed a fresh microchip on every plane out of reach of any human intervention in the air.

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1 reply
riverbirch11 shirefox March 18 2014 at 9:58 AM

You are misinformed. A microchip is not a tracking device. It is merely a means of identification, the same as an external tag. The equipment that detects a chip must be a few inches from it.

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Mark March 18 2014 at 11:00 AM

how about equiping each plane with a lo jack. If they can find a car with it then they could find a plane. Seems very simple.

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2 replies
lochadro Mark March 18 2014 at 11:18 AM

It would seem logical that having the "black boxes" or ELT (emergency locator transmitter) be interrogateable might be one solution. Ask the plane where it is instead of depending on the acceleration/deceleration sensors initiating the ELT signal.

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komivesdd Mark March 18 2014 at 11:52 AM

The airline could not afford that on the plane's option list when they bought it!

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1 reply
retpo96 komivesdd March 18 2014 at 12:28 PM

I can tell you many many modifications are installed after production oof an aircraft.. As to the cost just bear in mind each time an airline tacks on more and more Items to your fare,things that were once included.

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airlaws March 18 2014 at 9:37 AM

In all probability they have on -off switches, circuit breakers and electrical busses that could be isolated. They are turned off and on as part of normal checklists -On before flying -Off after landing.
The capability to turn off in flight is part of abnormal checklists concerning -electrical fire and overheat and /or smoke in cockpit. Normally they are required to be left on while in air.

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1 reply
Snake airlaws March 18 2014 at 9:47 AM

Wow, did you read the article too!?

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deanydextreme March 18 2014 at 1:47 PM

Be a former Pilot I have seen may times when crossing an area that I went off radar and was not seen for 30 minutes are so to pop up on another radar to hear glade to have you back but what dose not make any since to me is that it just disappeared now I am not trying to be funny at all for I fear for all on board and my Prayers go out to them and there families but a number od years ago on a flight of mine we saw something when we said this the tower said say again and my Co-Pilot says we have something a 3 0clock but it a second it was gone, They put us threw so much crap over this and a month later I am getting ready to depart Chicago and something comes over the air port and it just sat there it shut down the hole air port it must have been major for the us sent out 2 fighters and this thing was seen by the fighter pilots. So my thing is this did it go down or did it just taking by something. I just really prays they find them.

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3 replies
perisichd March 18 2014 at 1:30 PM

Can anyone explain to me why the plane does'nt have a GPS where a satellite can locate it, how about todays phones, iphones, they all have GPS technology by satellite, how come the satellites can't track the GPSs, you can track a dog with a GPS anywhere in the world...Well?

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2 replies
nxston08 perisichd March 18 2014 at 2:15 PM

The answer is easy. None of your high tech toys will work once the electricity is shut off or the signals are jammed. In other words don't worship technology.

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juststeve35 perisichd March 18 2014 at 2:37 PM

You would not be able to track your dog if it decided to turn off it's GPS device.

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