The hijackers on 9/11 were able to turn off the transponders on the plane so they couldn't be tracked...more

Question

What's the reasoning behind having a switch to turn the transponder off?? And if there *is* a legit reason, did it have to be where anyone in the cockpit could access it? Couldn't it have been placed like in the cargo hold or something? Or somewhere that the plane would have to be on the ground to turn it off? Don't give me those "I don't know" answers for 2 points - if you're that desperate, you need professional help more than yahoo answers.

Dear Jerry,

You're still an idiot.

Tell me how this "sensationalized" movie gave me false information relative to this question, please.

Oh, wait, you mean they accurately depicted that things called transponders exist and they were turned off by the terrorists? Heavens to Betsy, how in the *world* did Hollywood get that detail right?

In the words of the French...Dumas! Dumas!!

Answer 1

That's a good question. Most of the answers you already have are more or less correct.

As a retired airline captain, I'd like to add my "two cents" worth.

First of all, the flight crew has to be able to turn off every single piece of electronic equipment from the cockpit in the event of an emergency such as an electrical fire. Just look at the disastrous results of Swissair 111 off the east coast of Canada - the plane had an internal fire within the in-flight entertainment system that the pilots could not switch off due to improper wiring.

Second of all, an aircraft can be tracked with the transponder off, but just not at most commercial airports. There are two types of radar - primary surveillance radar PSR and secondary surveillance radar SSR. PSR is what the military uses and it can detect any object that reflects the radar waves including airplanes with the transponder turned off. SSR is what air traffic controllers (ATC) use at commercial airports to track commercial aircraft as well as military aircraft squawking their IFF codes.

The transponder provides a lot more information to ATC than a 'blip' on PSR, such as the aircraft identification through the unique squawk code and its altitude.

There have been many instances of PSR being used to help aircraft navigate safely to an airport in case of transponder failure or a complete electrical failure. Fortunately, it never happened to me, but its been done.

With all the military airports in the northeast US, it should have been possible to easily track those 4 doomed airliners, but if you look a tthe reality of it all, nobody expected the consequences. Its easy with 20/20 hindsight to look back at it and realize the mistakes that were made.

Even if the aircraft were tracked on PSR, what were the ATC and military authorities supposed to do? Up until 9/11 pretty well every hijacked commercial plane landed safely at an airport other than its intended destination, so its not right just to shoot down a plane because the transponder signal is turned off!

Answer 2

When you are flying any plane that has a transponder, you have several thousands of codes you can set it for. If you 'squak' 1200 that tells the Air Traffic Controlers that you are flying VFR or Visual Flight Rules. If you are flying a plane and it's hijacked, you immediately turn the Transponder to 7700 and squak ID... this will alert the Air Traffic Controlers that your plane has just been hijacked.

By the hijackers on 9/11 turning off the transponder, they were not only harder to pick out on RADAR but they no longer were flashing the international sign the planes had been hijacked.

Answer 3

Well the simple answer is nobody ever thought about this, "What if?" The guys at Boeing working on millions of systems, never intended for someone to run a plane into a building, let alone 3 of them.......
The transponders have selector channels that the pilot may need to change. Also having a component like that far from the cockpit would increase maintence headaches. The real problem was the fact the hijackers had commerical flght training in the US. They knew the cockpit blindfolded. Most hijackers can't fly...... A simple locked door is a cheaper and easy to install solution.

Answer 4

Transponders will occasionally malfunction and transmit erroneous or incomplete data, at which point a crew will recycle the device -- switching it off, then on -- or swap to another unit. Typically at least two transponders are on board, and you can't run both simultaneously.

In any case, killing the transponder will not necessarily make a flight invisible. Coverage will lapse and vary, but radar can often portray a transponder-less plane as what's called a "primary target" -- a blip that provides the aircraft's position and speed, though not its altitude. The September 11th aircraft were tracked this way during portions of their wayward journeys.

Answer 5

In some airspace you aren't even required to have a transponder. Most bush pilots in Alaska fly without transponders (unless they transit Anchorage or Fairbanks). To them, that is weight that can better be used for cargo. Shoot some of them don't even have radios for the same reason. You only have to have a transponder if you are flying in Class A, B, or C airspace. Personally, I agree with you that in most cases there is no good reason not to have your transponder on. It's a safety thing unless - see below. In the military, there are some missions that you really don't want people to know exactly where you are. Also, as mentioned, flying formation, lead is the only aircraft that transmits mode C because ATC will get a warning that two aircraft are flying too close. Or if there are a lot of aircraft in the same vicinity, i.e. Osh Kosh, fighting wildfires, etc.

Answer 6

There is a switch in the cockpit because sometimes the airtraffic gets too busy and the controllers ask pilots to turn them off so they can get a clear screen for a plane having an emergency.There is more than just an on/off switch, also. There are circuit breakers which can cut the power to individual subsystems. There are several channels, also. There is one for altitude, one for the airplanes identification number (set by technicians), one for the pilot to put codes in, which is also used to send out an emergency code and a hijacking code.

Planes can still be tracked without a transponder. It just makes it easier to track them and identify them at long distances or when there is a lot of clutter, like storms or mountains, or if they are flying low. The flying low part only works on ground radars, though. Any airborne military radar could have tracked them IF THE GOVERNMENT HADN'T ORDERED ALL OF OUR PLANES OUT OF THE AREA ON EXERCISES THAT DAY!!!
Yes, there ARE conspiracies all around us. We just have to be willing to accept that our cozy little country isn't as innocent and 'free' as we are told on Fox News.

Answer 7

The transponder simply relays information such as your aircraft type and altitude, radar will still p/u a plane without a transponder it just wont relay as much information,its in the cockpit and you are assigned a number 1200 is for visual flight rules and there is a transpoder code for hijacking also, it is simply a way for atc air traffic control to have information it simply was a shock to atc because nobody ever does it but they still knew where the planes were with basic radar

Answer 8

Hijacking code is 7500. 7600 is lost coms. 7700 is emergency when not radar identified.

As far as I've seen, the only way to turn off the transponder is to turn off your avionics. I'm not 100% sure on what appears on the scope if you leave it on standby, but I think there is still a little green dot with no info for tracking purposes.

I could be wrong on that last part, though.

Answer 9

Just a small addendum: in Europe we use 7000 instead of 1200 as the conspicuity code.

The reason for using a conspicuity code is that the controller is able to determine the altitude of the aircraft using a Mode 'C' transponder, rather than just being able to see a blip or 'maggot' on his screen.

Most of the info provided in your other answers is correct.

Answer 10

First, there are two types of ground based radar, primary (PSR) and secondary (SSR). Most of the ATC radar coverage is SSR which can't see an aircraft without an operating transponder.

The transponder has to be accessible to the pilot so he or she can turn it off when the aircraft is on the ground to declutter the radar screen near the airport.