Aviation buffs... A question

[Lost_Count]

What is the legitimate use of having the ability to turn off transponders on civilian transport aircraft?

I can imagine some on the military side but I'm coming up blank on the civilian side of the house. Thoughts?

[santamargarita]

1. Certain airspace requires an encoding altimeter and...

transponder. Under part 121, which I think is what you're referring to there is never any reason to turn off your transponders. If you show up on ATC radar as a primary target (transponder is off) they will tell you to "check your transponder". But most all commercial aircraft have 2 transponders.

[okaawhatever]

8. I read that the last communication w/the aircraft was when Malaysian ATC was handing the plane off

to Vietnam. The plane was apparently just entering Vietnamese airspace when it went off radar. Also, it had just reached 35k feet, as in the radar showed 34 and change, 35, gone. . Do either of those two things have anything to do with being unable to locate the aircraft? With the altitude I thought reaching 35 may have been when they turned on auto-pilot and maybe there was a malfunction. (or if there was a bomb it was attached to an altimeter and maybe detonated). I also thought that if it was a pilot suicide that maybe they used the time in "no man's land" between the two ATCs to go down.


My father was a pilot and passed away two years ago, this is the first incident where I didn't have his input on these issues. I never realized how many questions a person has when something like this happens.

[santamargarita]

9. I'm retired after 43 years. I've seen a lot of accidents...

and everyone I try and make a logical reason for the accident, I'm always wrong.
If the plane exploded at 350 and the transponder is gone, it could still show up on radar for a few seconds a "primary target (no flight #, altitude ect). Bomb, structural failure, departure, we will never know for sure until we find the plane and black boxes.
I'm sorry for your loss.

[Agschmid]

2. At cruise... There isn't one really.

Only reason would be an electrical issue, which required switching it/the fuse off.

[longship]

3. On the ground, all the transponders would drown the air traffic control.

Therefore, it is standard procedure to shut it off upon landing and turn it on just before take-off.

Hope this helps.

[santamargarita]

6. Most of the planes I flew in the last 10 years or so

737's or any aircraft with a glass cockpit the transponder automatically goes to standby after landing - we never touched it except to enter a code. Old stream gage cockpits, yes, you would turn it off or standby.

[longship]

7. Thanks. One thing less on the checklist, eh?

I figure that would be good.

[groundloop]

4. It's easier to hide from radar if my transpoder is off

OK, legitimate answers:

ANY piece of electrical equipment can malfunction and being able to turn it off can be a matter of safety (smoke in the cockpit ain't real cool - been there and done that).

Sometimes for who knows why, a transponder will hang up and either quit responding or else send out a continuous signal (which will mess with ATC's radar display), in which case ATC will ask you to recycle your transponder (turn it off then back on).

[Laffy Kat]

5. That was my thought, too: malfunction.

But on another point: Don't airliners have ELTs? What is their range? (I'm just a glider pilot.)

[Angleae]

10. So you can safely replace components or troubleshoot malfunctioning systems.

Without the danger of damaging components or being electrocuted.

PS: The only way to turn off the transponders is to pull the circuit breaker or remove components.