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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?
FlyingHunter
11-17-17, 20:32
Emergency checklist for possible electrical or other source of fire, - turn off all electronics including transponders following emergency aka May Day call, then switch to hand held comms.
The transponder is an actual radio that sends and receives information. Most corporate and commercial airplanes have at least 8 radios(2) VHF voice communications radios, ATC and Ground(2) VHF navigation radio receivers for receiving VOR and LOC signals. The LOC frequencies are paired with UHF frequencies for receiving glideslope signals. (1) ADF (automatic direction finding) receiver. (1) HF (high frequency) voice communication radio.(2) transponders to send whatever the "squawk" is when interrogated. (1) SELCAL (selective calling) receiver, which will sound an alert if it receives a signal sent by a ground station. (1) weather radar, and, yes, radar is a radio (radio detection and ranging) employing both a transmitter and a receiver.(1) radar altimeter for use in the final stages of the landing approach. .(1) GPWS (ground proximity warning system). I'm not sure whether it and the radar altimeter shared a box/circuitry/antenaes. (1) GPS receiver, and (1) ELT (emergency locator transmitter). And if you have a life raft, you have a location radio for that. The transponder is switched off while on the ground to reduce the amount of radio transmissions. 100 aircraft on the surface of a large airport can produce a massive Radio Frequency noise, which negatively affect radar systems and could make it hard for ATC to identify individual aircraft. When taxiing, radar replies are useful for aircraft identification, hence the transponder is switched on at push-back or engine start. Plus the K band transponders are pretty powerful, some when used with a amp can produce 200 watts and could interfere. Plus like FlyingHunter said, in the event of a emergency declaration, turn off ALL electronics!
There is an off position on the transponder... Not quite sure how to explain it any other way. Occasionally, you have to cycle your transponder (turn off and then back on, or turn it from standby to ALT) and having an off position helps. Most airliners have two transponders as well for logistical purposes. But really, the transponders are mainly for vertical separation at RVSM altitudes and Mode C areas, which is why they should be on ALT all the time.
I'm sure the current, turbine aircraft may be different, but in my little bugsmasher, (single engine piston) all electronics are off prior to start up to prevent surges. Aircraft radios, and navigational electronics can be finicky. Then after start up, you engage an Avionics master, or if you don't have one, turn everything on individually. The Transponder goes to Standby, then ALT (engages Mode C for altitude info) during the departure checklist. I always turn the Transponder, and other radios off individually prior to shutdown.
The transponder is switched of while on the ground to reduce the amount of radio transmissions. 100 aircraft on the surface of a large airport can produce a massive Radio Frequency noise, which negatively affect radar systems. When taxiing, radar replies are useful for aircraft identification, hence the transponder is switched on at push-back or engine start. Plus the K band transponders are pretty powerful, some when used with a amp can produce 200 watts.
Most large airports that would have that many aircraft usually have ground descrimination tech that can tell if a specific squawk is moving or not and height above ground so they can tell wahts in the air vs not, but I agree on everything else. STBY is a great tool, and most aircraft have it auto switch between STBY and ALT based on ground speed or squat switches or whatever.
Wow, I never would of thought we had many pilots here. That's cool! ;)
NYH1.
Turning a transponder off can be used by ATC to determine which blip an aircraft is. Then back on. Seems like you'd not have to do it, but it's not uncommon on long flights and crowded airways.
Selcall is normally just part of the HF radio, it's just a tone squelch. I've never seen it be a separate receiver, but who knows.
I have an HF manpack with selcall.
Thanks for all the answers guys. Makes a lot more sense now.
Turning a transponder off can be used by ATC to determine which blip an aircraft is. Then back on. Seems like you'd not have to do it, but it's not uncommon on long flights and crowded airways.
Selcall is normally just part of the HF radio, it's just a tone squelch. I've never seen it be a separate receiver, but who knows.
I have an HF manpack with selcall.
'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. You push the little green button on the Transponder.
'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.
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.
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.
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
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 xxxxCycle 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.
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.
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:
48748
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.
48749
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.
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