Question about Flight 11 Transponder

This question has very probably been asked and answered before, but I have not been able to dig it out of the mountains of data here.

When Flight 11 had been hijacked, supposedly Air Traffic Control out of Boston lost contact with the airplane. Supposedly, the hijackers "turned off the airplane's transponder".

Why would there ever be a reason or a means to turn off the transponder? I can think of no reason to do so. And, have there been subsequent actions taken to prevent this from ever happening again?

I'm no pilot or anything but im geussing that it would probably be something to do with when your sitting on the tarmac waiting to takeoff it helps the controllers see the aircraft that are in teh air more clearly.

It may also be incase it develops a fault, so you can turn it off to see if its part of the problem.

Transponders are about the size of a car radio - in lightplanes anyway.

They're shut off when the aircraft is not in operation - IE: taxiing or flying.

Transponders increase the radar signature of the aircraft which benefits everyone.

When flying VFR and without a flight plan - commonly done around small airports - you spin the knobs or pushbuttons to read 1200 which indicates uncontrolled VFR flight.
Uncontrolled in this case meaning not in control of a control tower somewhere.

Once you're under control of a tower they assign a specific number that you dial in to ID your aircraft.

A large airline has a fairly large radar signature on it's own and they don't become invisible to radar if the transponder is off.
Even so, the radar return is improved when the transponder is on.

Originally posted by Desert Dawg
Transponders are about the size of a car radio - in lightplanes anyway.

They're shut off when the aircraft is not in operation - IE: taxiing or flying.

Transponders increase the radar signature of the aircraft which benefits everyone.

When flying VFR and without a flight plan - commonly done around small airports - you spin the knobs or pushbuttons to read 1200 which indicates uncontrolled VFR flight.
Uncontrolled in this case meaning not in control of a control tower somewhere.

Once you're under control of a tower they assign a specific number that you dial in to ID your aircraft.

A large airline has a fairly large radar signature on it's own and they don't become invisible to radar if the transponder is off.
Even so, the radar return is improved when the transponder is on.

Thanks, DD. But regardless of their in-flight capabilities, shouldn't there be a rule to track all commercial flights at every moment? Just a ping -pong to keep track. Doesn't have to be hi-tech, but imo, is crucial.

A large airline has a fairly large radar signature on it's own and they don't become invisible to radar if the transponder is off.

Well then how is it that Boston lost track of them?

One of the first anomolies that caught my attention is that an airliner (nevermind several) went missing and weren't intercepted. Disappearing out of a 3 minute block of airspace in the IFR system was a pretty big deal even on and before September 10th, 2001.

A transponder has modes of OFF/STANDBY/ON/IDENT and a 4 digit "sqwak" code. 1200 for VFR ops, an assigned code for IFR, and codes 7500/7600/7700 are Hijacking/Radio Failure/Emergency.

Standby keeps the unit "warm" and ready to transpond instantly when standby or ident is selected. ATC oftentimes will request that an aircraft "sqwak 2214 and ident" which means you select the code and switch to ident. You now show up as a distinct return on radar if you're within coverage and ATC verifies that you're who they're seeing on the screen and talking to. Mode C encoding transponders also send altitude information. The standby position is also used when dialing in a sqwak code, therwise as you tuned through the numbers on the dial, you'd be sqwaking all codes you passed through.

Originally posted by DezertSkies
One of the first anomolies that caught my attention is that an airliner (nevermind several) went missing and weren't intercepted.

Intercepted by whom?

I'll bet you think that it only took a couple of minutes for them to intercept the Payne Stewart plane also.

Originally posted by DezertSkies
One of the first anomolies that caught my attention is that an airliner (nevermind several) went missing and weren't intercepted. Disappearing out of a 3 minute block of airspace in the IFR system was a pretty big deal even on and before September 10th, 2001.

A transponder has modes of OFF/STANDBY/ON/IDENT and a 4 digit "sqwak" code. 1200 for VFR ops, an assigned code for IFR, and codes 7500/7600/7700 are Hijacking/Radio Failure/Emergency.

You now show up as a distinct return on radar if you're within coverage and ATC verifies that you're who they're seeing on the screen and talking to. Mode C encoding transponders also send altitude information. The standby position is also used when dialing in a sqwak code, therwise as you tuned through the numbers on the dial, you'd be sqwaking all codes you passed through.

Thanks for the info, DZ. When you say "distinct return", do you mean that now the ATC's can now correlate the blip on their radar screen with a particular plane? I am assuming that otherwise, the plane would be an "unidentified blip", but one way or the other the blip should be on the screen, right?

What I don't understand is how Boston supposedly lost Flight 11 on their radar. Maybe they were out of Boston's radar range by that point, but shouldn't NY radar have picked them up, as part of the hand-off procedure?

I still don't understand why a pilot should ever have the capability to turn off a transponder. Esp. on a commercial flight. Private planes, maybe I could be convinced there would be a reason to be able to do so. Not sure about that either.

Sorry if these questions seem basic, but I really don't know the air control system.