AIM on ground transponder use

[c.b.powell]

I subscribe to the FAA Safety Team emails, and today received an interesting one discussing ground transponder use. 4-1-20 of the AIM has been updated to read the following:

"Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems."

Apparently this new recommendation came about in the Feb 9 revision of the AIM. See http://www.faa.gov/air_traffic/publicat ... 01.html.15

a) This is news to me. I was trained to set transponder to STBY during ground movement and set to ALT just prior to takeoff.
b) Did anyone else hear of this change? I hate feeling like the last to know...

Chris

[Keith Smith]

I found out about it a few months ago through Peter Gray.

Now that I'm using the IPad for most of my documentation, the AIM appears to self-update. I saw the revisions section at the front and noticed the change there, too. In other words, I feel better-equipped to keep up with the changes moving forward.

[gavink42]

...This is news to me. I was trained to set transponder to STBY during ground movement and set to ALT just prior to takeoff...

Chris,

That used to be the guidance. But in practice it really doesn't hurt anything to always have your transponder on, when on the ground.

ASDE (Airport Surface Detection Equipment) usage is widespread and growing. So, I believe the reason for the changed recommendation was to get everyone into the habit of transponder in ALT mode at all times. Another plus would be that it would help reduce the instances of forgetting to turn it on just before takeoff.

If you're at an airport with radar: Chances are good that it also has an ASDE system. In this case, there will be a requirement listed to operate in ALT mode when on the ground.

If you're not at an airport with radar (or ASDE): It's likely that the nearest one will not be able to interrogate your transponder while you're still on the ground. So ATC won't even see your transponder.

For the most part, primary and secondary radars tend to have their beam angle set slightly above horizontal. Plus, they're mounted on a tower. Short and long range radars (ASR and ARSR) are only concerned with what's in the sky. So, the further away from the antenna, the greater the blind spot it has from the ground up to the lower limit of the beam.

An exception to this could be a radar on a mountain, which might have an additional beam pointed downwards to provide coverage in a valley.

Gavin

[Orest Skrypuch]

One notable exception to this is Mode S transponders like the GTX327/330. These can auto-switch to ALT from GND and back, on the takeoff roll and on landing. However, you do NOT need to force them to ALT on the ground for the purposes of this discussion, as when they are in GND mode (not standby) they will correctly respond to interrogation by ASDE-X.

* Orest

[arb65912]

Gentlemen,

I know, I can "Google" it but would you mind to share simple explanation on below.

1.Transponder modes, what is the difference between mode Charlie and altitude?

2. What are other modes in use?

3. What is ASDE-X versus ASDE?

Thank you.

Cheers, AJ

[gavink42]

Good morning, AJ!

Here are the transponder interrogation modes that civil aviation is concerned with:

• Mode 3/A: The basic squawk, giving ATC only your selected octal beacon code (whatever is set in your transponder)
  Mode C: Same as saying "ALT". Your transponder replies only with the aircraft's pressure altitude.
  Mode S: Mode 3/A and Mode C interrogate every aircraft that receives the proper signal. Mode S is a selective mode that allows addressing only the desired aircraft. It also provides a 2-way data link, which gives us TCAS, among other things.

ASDE is the generic term for Airport Surface Detection Equipment. The two systems in use are ASDE-3/A and ASDE-X. ASDE looks at the current and projected locations of aircraft and ground vehicles. Primary use is to help tower controllers detect and prevent runway conflicts.

Gavin

[arb65912]

Gavin,

Thank you very much.

Now, do not get mad ...lol How does stand by mode fits into these 3 below?

Mode 3/A: The basic squawk, giving ATC only your selected octal beacon code (whatever is set in your transponder)
Mode C: Same as saying "ALT". Your transponder replies only with the aircraft's pressure altitude.
Mode S: Mode 3/A and Mode C interrogate every aircraft that receives the proper signal. Mode S is a selective mode that allows addressing only the desired aircraft. It also provides a 2-way data link, which gives us TCAS, among other things.

Promise, not to ask more.

Cheers, AJ

[gavink42]

No problem at all, AJ!

ATC does not see your transponder at all when it is in standby.

Gavin

[arb65912]

Thank you, Gavin, all clear now.

Now that I'm using the IPad for most of my documentation, the AIM appears to self-update.

Keith, where exactly are you looking up AIM on Ipad? ForeFlight?

Cheers, AJ

[Keith Smith]

Yes, you can add it to the Documents section on Foreflight.