question for pilots

[pinzgauer]

'Typically, I am asked to "Ident" if they want to make sure if my location. They will give the squawk code, then tell me to Ident which makes the info on their screen stand out for my airplane.

My understanding is Ident does highlight the digital ID. Turning it off or recycling lets them see the passive reflection, which could be smaller than the digital depending on aircraft type.

Was used to help sort close VFR from IFR traffic at times and also very close IFR situations.

My info is dated, and came from visits and discussions with towers. I'd expect the resolution in many systems to have improved, so maybe this is less needed?

If you are flying heavies I'd expect the need to do this type of thing to be rare, though I would hear it on hf transatlantic comms from heavies some..

I'm just a VFR pilot, though most of what I flew when active did have transponders, just sometimes not encoding ones.

[pinzgauer]

Did some checking, and STOP SQUAWK is still a valid ATC command and means turn the transponsder off rather than standby. AIP GEN 3.4 5.15.6 (on page AIP GEN 3.4 - 68)

Multiple mil usages, as well as can be used in formation flying for all but the leader.

The usage I was mentioning is essentially to allow ATC to use primary radar: "ATC radar resolution can be limited especially at longer ranges, so it can be confusing for the ATC's IFF system to interrogate along an azimuth and receive several simultaneous signals." More common on things like transoceanic routes.

By the way, the STOP SQUAWK command is in the test pool and apparently catches folks.

[Pilot1]

Did some checking, and STOP SQUAWK is still a valid ATC command and means turn the transponsder off rather than standby. AIP GEN 3.4 5.15.6 (on page AIP GEN 3.4 - 68)

Multiple mil usages, as well as can be used in formation flying for all but the leader.

The usage I was mentioning is essentially to allow ATC to use primary radar: "ATC radar resolution can be limited especially at longer ranges, so it can be confusing for the ATC's IFF system to interrogate along an azimuth and receive several simultaneous signals." More common on things like transoceanic routes.

By the way, the STOP SQUAWK command is in the test pool and apparently catches folks.

Interesting. The only time I've been asked to cycle my Transponder is when ATC was not receiving my Mode C (altitude info). It only happened when my damn Narco Transponder went TU.

[tb-av]

Interesting. The only time I've been asked to cycle my Transponder is when ATC was not receiving my Mode C (altitude info). It only happened when my damn Narco Transponder went TU.

That is exactly what happens in the audio transcript... They have lost American 11 and someone at the tower instructs them to cycle the transponder and squawk xxxx

[pinzgauer]

That is exactly what happens in the audio transcript... They have lost American 11 and someone at the tower instructs them to cycle the transponder and squawk xxxx

Cycle transponder is normally to try to clear an error. They can also tell you to take it out of ALT mode if it's reporting different altitude, etc.

SQUAWK STANDBY can also be used, and it's a different command. Not unusual to hear in busy patterns to declutter screens and also to reduce interference/falsing with TCAS users (apparently causes issues, don't know the details)

At any rate, there are a bunch of valid reasons to have an off position in addition to standby. Standby the transmitter is kept "hot", just not transmitting. Used to be a bigger deal when microwave stuff used traveling wave tubes, etc. It could take minutes for a transponder to stabilize.

[Pilot1]

That is exactly what happens in the audio transcript... They have lost American 11 and someone at the tower instructs them to cycle the transponder and squawk xxxx

So they were thinking it was equipment malfunction at first. Of course, what else could it be, as nobody would anticipate that cluster. I was grounded for sixty days after just buying my Tiger. Thirty days as we all were, then another thirty as I was based within five miles of a nuke plant. My Transponder decided to fail when I was in NYC Class Bravo, and those guys handled me like the PROS they are. NYC controllers are some of the best in the country. I also had a COMM failure in their Class B, couldn't transmit, but I still could receive. I had a handheld ICOM back up, and an airliner relayed my transmissions for me, and it was a non event. Those controllers are superb.

[Korgs130]

I know that most GA aircraft can turn off their transponders, but there isn’t a switch position that completely powers off the transponders used on airliners. In the event of an electrical issue you’d have to pull the associated circuit breakers to remove power. “STBY” just prevents the transponder from transmitting squawk code, altitude and Traffic Collision Avoidance System (TCAS) Traffic Advisory (TA) and Resolution Advisory (RA) information. At the gate, airliners operate their transponder in STBY and switch to TA/RA prior to moving after push. This allows ground control to track aircraft with their squawk during taxi on the ground.

Military aircraft use similar transponders. In formation, air traffic control isn’t responsible for aircraft separation (within the formation) and handles the entire formation as if it were one aircraft. Typically, the lead aircraft carry’s the squawk code while the wingmen operate their transponders in STBY. If the formation splits up, each individual aircraft is then assigned a unique squawk code switches out of STBY to TA/RA.

Here’s the transponder control head from a 737:

52181686-3EB2-4A5C-967A-D6280459079F.jpg

For those curious about the transponder’s role in TCAS. Transponders communicate automatically between aircraft. For example, if there ARE two aircraft beak to beak at the same altitude the TCAS will first give a TA (aural “TRAFFIC, TRAFFIC” and yellow intruder on the TCAS display) alerting the the crew to the traffic and potential conflict. Once the TCAS determines that there is a conflict between the two aircraft, the transponders on both aircraft in will communicate coordinated corrective RAs. In this example one aircraft would get a climb RA (red intruder on the TCAS display, aural “CLIMB, CLIMB” and associated climb guidance on the flight director) and the other would get a descent RA (red intruder on the TCAS display, aural “DESCEND, DESCEND” and associated descent guidance on the flight director.)

The TCAS display below is in “TEST” and show a TA intruder to the left with an RA intruder to the the right. Other TCAS traffic is shown in white. The numbers associated with each aircraft is their altitude relative to yours in hundreds of feet. An arrow indicates if the traffic is climbing or descending.

284FE3B0-57FA-4A3C-8143-6A4D41AD0589.jpg

[qsy]

I was listening to the 9/11 recordings of the air traffic control. Still listening actually it's 2 hours.

But someone raised a question, on a commercial flight why are transponders able to be turned off?

There are operational reasons to stop the transponder from replying to interrogation, so that capability must be available to the pilot/flightcrew. All transponders have the STBY (standby)option which disables replies. Some have an OFF position, other installations would have to be powered down by disabling the appropriate circuit breaker.

From the standpoint of ATC, OFF or STBY will have the same effect on their radar display. The high-jackers could have just selected STBY to achieve the same results.