question for pilots

[tb-av]

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]

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.

[elephant]

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!

[nimdabew]

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.

[Pilot1]

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.

[nimdabew]

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.

[NYH1]

Wow, I never would of thought we had many pilots here. That's cool!

NYH1.

[pinzgauer]

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.

[tb-av]

Thanks for all the answers guys. Makes a lot more sense now.

[Pilot1]

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.