Topic: Question for pilots

[Bob Wessner]

Why do commercial aircraft allow the pilot to turn off the transponder? Under what circumstances would you 'not' want it to transmit?

[azuredesign]

Very good question. Only reason I can think of is if it's sending incorrect info and screwing up controller flight following. I have had this happen. In private aircraft there is usually only one, but I would not be surprised if heavy commercial craft have a redundant unit. I will ask a friend who is a real pilot.

[ekpent]

Why do commercial aircraft allow the pilot to turn off the transponder? Under what circumstances would you 'not' want it to transmit?

Got you snow blowin' done yet Bob ?  Over 6 inches on this side so far in Kazoo, what a kick in the gut.  Back to the show and where the heck is that plane ??

[Bob Wessner]

Why do commercial aircraft allow the pilot to turn off the transponder? Under what circumstances would you 'not' want it to transmit?

Got you snow blowin' done yet Bob ?  Over 6 inches on this side so far in Kazoo, what a kick in the gut.  Back to the show and where the heck is that plane ??

Still snowing here, so I haven't even started any clean-up. Getting really, really old. Should put us over the top here in A2 for the snowiest winter on record though, so now they can stop talking about how 'close' we are getting to that. 

[azuredesign]

My friend related that another reason for switching the transponder on and off, or recycling it, is for controller ID purposes in heavy traffic situations.

[Bob Wessner]

My friend related that another reason for switching the transponder on and off, or recycling it, is for controller ID purposes in heavy traffic situations.

Now that you mention this, I think I recall reading about situations where a traffic controller would request this to confirm aircraft identity. Even so, seems like it should default to automatically switch "back on" after a brief period even if the cabin crew forgot to do so. But then, not being a pilot, this might have some shortcomings I'm not aware of.

[TwoTired]

Why do commercial aircraft allow the pilot to turn off the transponder? Under what circumstances would you 'not' want it to transmit?

All electronics, apart from the "black box" are under pilot control.  So, anything besides the box can be turned off including the transponder.

The transponder broadcasts a code within a time period after being interrogated by ground radar, which aids ground control operations.  The code can be set by the pilot and the code can be piggybacked with a report of current altitude, if the device is mode C capable. They also have and ident button, which when pressed sends an ident code, that show up on radar screens as ident.  This differentiates it on controller's screens from others in the area that may be sqawking the same general code.
her is also a mode S type which also broadcasts aircraft registration number.

I think the most probable cause of the pilot turning it off is with an equipment failure of some kind; incorrect altitude reporting, or inability to select the correct code for broadcast.

Not many desirable reasons to be seen as only a blip on the screen, besides attempted stealth.

[Bob Wessner]

I think I get it now. So, the transponder ONLY responds to a request and it's a moot point if the aircraft is not within radar coverage?

[bytio]

Mode S can give controller information such as call sign, actual flight levels entered into aircraft auto systems so controller can see if  the pilots have set correct altitude to go to. Most airports get aircraft to  switch ponder on after push back, some smaller airfields ask for it not to be turned on until line up.

[TwoTired]

I think I get it now. So, the transponder ONLY responds to a request and it's a moot point if the aircraft is not within radar coverage?

Yes.

However, I believe all IFR aircraft are almost always given a sqawk code, which includes all over 18000' flights, and certainly anything within a control zone.  You can always request a "special clearance".  The type of control zone determines the equipment requirements.

FYI there are certain codes reserved for in flight emergencies, hijacking, etc.

http://en.wikipedia.org/wiki/Transponder_%28aviation%29

[Stev-o]

Here's my question:

With modern technology, why cant the info from the black box be remotely accessed?!

[bytio]

Non IFR aircraft can squawk a non allocated number ( in the UK and Europe it's 7000) this allows controllers to see them even if not speaking to them, and other aircraft with TCAS will see them and give pilot avoiding climb or descent if too close.

[Bob Wessner]

Assuming this is accurate, it's a pretty good explanation of function and a answer to my original question.

 http://mobile.nytimes.com/2014/03/13/world/asia/malaysia-plane-transponder.html?from=world