airline transponders

[Tdog]

Any of you pilots out there know of any reason that airline transponders can be switched off, as those hijacked planes were? I used to fly, eons ago, before a wife, two daughters & a few horses. For the life of me I can't think of a reason to allow the transponder to be turned off from the cockpit. I'm sure that air traffic controllers must have noticed when they lost the info that a transponder provides, & therefore probably knew that the blip that they were following on the screen was American Airlines Flight XX. Still, I'll bet it contributed to some confusion.

 

[cp1969]

It has to be able to be switched off from the cockpit, as well as change codes as the codes change for every flight. There is a code pilots are supposed to switch to (I'm not going to say what it is here, even though it's been so widely publicized by journalists that it's no secret any more) if a hijack occurs.

The problem with this method is that if the hijackers are trained, they know the code, too, and can see if it's been set in the transponder.

Even though the transponder gets turned off, radar can still paint a target. The ground will have no info as to altitude unless they have radar that sweeps in a vertical azimuth such as precision radar approaches use. And the fact that the transponder got turned off is reason enough for ATC to query the aircraft to switch to another transponder. Once an IFR flight deviates from a designated flight path and no reasonable answer for such a deviation comes from the cockpit, ATC has to know something is wrong. In the case of the Boston airplanes, ATC had to know for 15-20 minutes that there was something wrong with the airplanes that departed Boston.

 

[GlueGuy]

A transponder sometimes needs to be "cycled". I have on more than one occasion been asked by ATC to "cycle" my transponder (switch it off, then on again). This is probably a carry-over from the "old days" when some transponders would get into a "funny state" and nothing short of power-on reset would clear the problem. I guess a lot of the old ones might still be around....

Oh yeah. Turning the transponder off helps a lot when you need to run stuff across the border. /w3tcompact/icons/wink.gif

The GlueGuy

 

[Kelvin]

You also need to be able to turn off the transponder while on the ground to keep ATC's radar screens from being saturated with both ground and airborne targets. Of course, some of the larger airports want aircraft to have transponders turned on while on the ground.

In my military flying, I certainly like the option of turning my transponder off when dictated by operational necessity.

I don't believe that ATC's "skin paint" mode of identifying aircraft is very effective. I've had ATC near the airport call out low-flying traffic that didn't have a transponder, but have never had traffic called out to me that was higher than 5,000 or 6,000 feet with transponder turned off.

You can rest assured that anti-hijacking procedures are going to be changed dramatically. From now on, anyone flying as a passenger on a hijacked airliner will have to assume that the hijackers are planning to kill everyone onboard and thousands on the ground if able; therefore, they will need to be disabled regardless of lives lost during the attempt. As a pilot, I will no longer plan to "appease" hijackers in an attempt to save the aircaft, crew and passengers.

Kelvin

 

[HR_Mutt]

<font color=blue>You can rest assured that anti-hijacking procedures are going to be changed dramatically. From now on, anyone flying as a passenger on a hijacked airliner will have to assume that the hijackers are planning to kill everyone onboard and thousands on the ground if able; therefore, they will need to be disabled regardless of lives lost during the attempt. As a pilot, I will no longer plan to "appease" hijackers in an attempt to save the aircaft, crew and passengers.

Kelvin</font color=blue>

Agreed!
Regards
Mutt

 

[Tdog]

Thanx for the answers, all. Transponders that reported altitude were just becoming available in general aviation when I had to give up my short lived pilot life.