Transponder Mode During Taxi

[Scotsman58]

A local pilot brought this FAA Safety Alert to my attention:

http://www.faa.gov/other_visit/aviation ... o15006.pdf

If I'm reading this correctly, transponders must be in altitude reporting mode during ground movement at all airports.

Two questions:

(1) Is it possible that Garmin's "Ground" mode satisfies the FAA's requirement? Garmin states that in ground mode, the GTX 33 "does not allow Mode A and Mode C replies, but does permit acquisition squitter and replies to discretely-addressed Mode S interrogations." I will confess that I have no idea whether "acquisition squitter" or ability to reply to Mode S interrogations permits the FAA's ground-based airport systems mentioned in the safety alert to see us while we're taxiing.

(2) If the answer to (1) is "no," then I guess we need to be in "Alt" mode while we're taxiing. Is there any way to get the G1000 to do this automatically, or does this have to be a manual step that we add to our checklists (as the safety alert suggests)?

For background -- my DA40 has a non-WAAS G1000 with GTX 33 Mode S transponder, so as of the moment I have no ADS-B issue to add to the above inquiry (which is a separate, quite frustrating topic).

Thanks in advance for any light that can shed on this.

[carym]

At the class C airports where I have landed (and departed) that require this transponder reporting (so they can monitor ground movement), there has been no problem when the transponder was in ground mode. I don't think you have to worry about this with your setup, and I don't think you have to manually turn it to ALT mode.

[Rich]

I always just left my GTX327 in ALT mode. It actually would come alive based on ground speed (from the 530) of 30 knots. This is configurable, but I'm not sure exactly how its ground response differed from the actual ALT mode, but I do know it did.

With my ADS-B KT74 I also leave it in ALT mode all the time. However, it actually annunciates a "GND" when on the ground and changes at about 40 knots airspeed (via a "squat switch" emulator). Observing traffic through Foreflight while I'm in flight, I note that traffic on the ground displays in brown, so it's clearly a different mode.

[c2mlp]

I too have a GTX33, and I always let it switch modes automatically. The only time I have ever had a controller specifically ask for me to switch it to ALT mode (it was in ground mode) was when I was taxiing for departure at Dulles. You will see a note on IAD's airport diagram which states "ASDE-X Surveillance System in use. Operate transponders with Mode C on all twys and rwys." I was under the impression that "ground" mode would satisfy this requirement, which it evidently did not. I currently depart from a TRSA airport and also frequent Class D airports and have never had a controller request that I switch modes.

[CFIDave]

I've always wondered the same thing. Our modern GTX33 Mode S transponders (relatively rare compared to Mode C) in GND mode are supposed to reply to FAA airport surveillance systems for taxi. However, the FAA now has multiple ground surveillance systems, so there's a chance we may now need to set transponders to ALT (Altitude mode) when taxiing at Class B/C airports. It seems the FAA simply ignored or overlooked Mode S transponders in their new guidance.

I've flown into quite a few Class B/C airports, including Washington Dulles airport twice/year for the Smithsonian Udvar-Hazy Center "Be a Pilot Day" and the Dulles "Plane Pull" events. Each time I've simply let the transponder operate automatically so that it's in GND mode while taxiing. I've never been told to switch to ALT at Dulles.

I dislike the new FAA guidance that we're all supposed to put our transponders in ALT mode while on the ground at ALL airports (including un-towered) -- this has the unfortunate side-effect of causing active traffic systems (our TAS 600) to generate nuisance collision warnings for aircraft on the ground (e.g., sitting in the run-up area) while we're flying in on final approach. I'm guessing that the new FAA guidance was really directed at airlines; they often forget to consider the GA perspective.

[DavidS]

Another anecdote, for a while at KHEF they were also going on about keeping your transponders on while taxiing. I've never done anything special (i.e., left it on GND mode) and they've never said anything about it. (While at the same time asking others to change.)

The key thing here is they talk about ensuring that the transponder is in altitude reporting mode - not necessarily 'ALT' mode. GTX33(0) in GND mode will respond to any MODE S inquiry - which can contain altitude data if it was requested.

MODE S FEATURES

The GTX 33 transponder is equipped with selective addressing or Mode Select (Mode S), capability. Mode S
functions include the following features:

• Level-2 reply data link capability (used to
exchange information between aircraft and
various ATC facilities)
• Surveillance identifier capability
• Flight ID reporting
• Altitude reporting
• Airborne status determination
• Transponder capability reporting
• Mode S Enhanced Surveillance (EHS)
requirements
• Acquisition squitter

The SAFO is worded as it should be, but can be really confusing due to the many different variations of the technology. (I had to look this up just to be sure)

Now, if something says "Operate transponders with Mode C on all twys and rwys" as commented below, then forcing 'ALT' mode makes sense.

A short summary of what I've gathered:
GND: Mode S only (acquisition squitter, aka beaconing your 24 bit IACO address, and selective replies, aka "hey <24 bit IADO id> tell me <something>")
ON: Mode A + S
ALT: Mode A + C + S


At any rate, I'm not going to worry about it unless a controller asks...

[Rich]

This set me to digging further. It appears that, to comply with this directive a GTX 327 needs to be configured with the automatic "squat switch" (or similar) feature disabled, so turing it to "ALT" really means ALT mode.

With the 33 or 330, it may be required that at some airports (good luck figuring out which) you may also need to have it disabled. (Though, with the G1000-based 33 interface, maybe you have a temporary override capability.)

The Garmin manuals (especially for the 33/330) are a bit fuzzy and I'm reading the tea leaves,so I could be a bit off here. But it does seem to jive with what I've been seeing in this thread.

With the KT74, in GND mode it still will report altitude, which is what they're really after. FWIW, the STC for installing a KT74 in the DA40 required the installation of an airspeed-based squat switch emulator.

[RC7]

I once had an ASI from the FSDO tell me that I needed to be operating with Mode C while on the surface at ROA (which is a class C airport, but does not require Mode C on the surface). Shortly thereafter, I had a controller at ROA tell me to turn off Mode C while on the ground because it cluttered up their screen

Flying GA aircraft, I usually operate with the transponder in GND or STBY while on the ground, unless the airport specifically requires mode C, in which case I will manually put in in ALT mode.

At my day job, we operate in ALT mode at all times on the ground at all airports (even the uncontrolled ones). It comes on during the after start checklist and doesn't get turned off until the shutdown checklist.

[rwtucker]

Then, there is this from the AOPA daily eBrief:

FAA to Pilots: Keep Transponders On While Taxiing
by Robert P. Mark AIN Online
- June 2, 2015, 12:01 PM
Safety Alert for Operators 15006 was published by the FAA last week to ensure that pilots realize the need to keep their aircraft transponders turned on to the altitude-reporting mode even when operational on the ground in airport movement areas. The FAA uses runway safety systems, such as airport surface detection equipment model X (ASDE-X) and advanced surface movement guidance and control system (A-SMGCS), at many airports in the U.S. to determine aircraft and vehicle locations when operating on an airport surface.

Both of these systems use data from transponders to obtain accurate aircraft and vehicle locations to increase airport surface safety and efficiency. Nationwide, the agency said that airports with ASDE-X report an average of 20 non-compliance transponder events per day, even with explicit airport diagrams or ATIS notification, or both, directing pilots to operate with transponders on. To address these problems, aircraft operating on all airport movement areas at all airports—not just those that are ASDE-X equipped—must taxi with their transponders on in the altitude-reporting mode.

[startingovercfi]

yup. regardless of what you have been doing or been told in the past, the new procedure is Mode C on all the time. I don't think GND mode is appropriate anymore.

This applies to any airport. It kind of makes sense to standardize procedure so that we don't have confused or forgetful pilots.