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Mode S-ES, Selected altitude, and GTN vertical profile

I have a G500 TxI, GTN750 with the latest software allowing altitude constraints in the flight plan, and a GTX345 mode S with extended info and ADS-B out.

Yesterday I was flying from Wolverhampton to Exeter IFR, talking to Cardiff at FL090. Weather at Exeter was CAVOK so I was planning a visual approach, which I selected in the GTN, setting a target altitude of 1280 feet on a 3nm final. Cardiff asked me to descend to 4000 feet, which I dialled in my altitude selecter, and started the descent. Cardiff then asked me to check my selected altitude, because they were seeing ‘xxx feet’ (I didn’t catch what it was, but it was quite lower than 4000 feet). I confirmed I was at 4000 feet selected, and said I thought it was actually great that they were seeing this; he asked if I could turn it off to which I said no.

Next leg was Exeter to Fairoaks. To help me fly the visual approach at Fairoaks I have a user waypoint on short final which works great at 600 feet, and which I set as an altitude target before crossing the Solent zone. I entered the zone at FL080, and passing the SAM VOR was given a descent to 4000 feet. I dialled that in the altitude selector and started the descent. Solent Radar said “please check your selected altitude, I see 600 feet”. I confirmed I had 4000 feet dialled in. They were clearly nervous about that, having a departure just below me, and asked to confirm I would not descend below 4000 feet which I did.

The rest of the flight continued normally, only after landing did I think this may be linked to the vertical constraint in the GTN. I was also surprised that this is the first time I have this issue, I haven’t done any changes to either my avionics or procedures (I always have an altitude constraint dialled in to calculate Top of Descent).

Has anyone had this? Is this a new capability from UK ATC to see this data and integrate them in their threat assessment (I got the feeling this was generating alerts on their side) ?

EGTF, LFTF

Sounds like your transponder is configured for Enhanced Surveillance (separate function from ADS-B). This output which broadcasts your selected altitude along with a group of other DAPS. It isn’t required for your weight of aircraft and can be turned off in the configuration pages.

Avionics geek.
Somewhere remote in Devon, UK.

denopa wrote:

Next leg was Exeter to Fairoaks. To help me fly the visual approach at Fairoaks I have a user waypoint on short final which works great at 600 feet, and which I set as an altitude target before crossing the Solent zone. I entered the zone at FL080, and passing the SAM VOR was given a descent to 4000 feet. I dialled that in the altitude selector and started the descent. Solent Radar said “please check your selected altitude, I see 600 feet”. I confirmed I had 4000 feet dialled in. They were clearly nervous about that, having a departure just below me, and asked to confirm I would not descend below 4000 feet which I did.

I am surprised they see the altitude “selected” in your MFD (600ft in the GTN FPL?) but not the one selected in on your PFD (4kft in the ALT BUG), what PFD you have G500? G1000? also was it on autopilot? were you descending on VNAV or VS mode?

I know some UK ATC units do see PFD BUG(ALT, HDG, QNH…) and will warn about these but I am not sure about MFD stuffs (DTK, FPL, DIRECT/VNAV…)

When you select “VNAV Direct” to waypoint & altitude in GTN FPL, does your MFD bug the waypoint altitude in the PFD ALT BUG?

I recall you get a “small magenta select altitude (600ft)” bellow the a “big blue select altitude (4000ft)”, maybe ATC system does not have two colors or distinguish between the two and some (sloppy) programmer decided to only filter smallest one on decent (or big one on climb ), if you go down on VS, they will only see the 4kft blue ALT BUG

Last Edited by Ibra at 20 Sep 09:12
Paris/Essex, United Kingdom

What autopilot do you have, @denopa? Configured as you were, it would have followed the PFD bug, not the GTN altitude constraint, right? (If you have an autopilot with enroute VNAV stuff, I’m not actually sure which one it would follow.)

My understanding (and my own experience of “check selected altitude” by ELLX APP after they upgraded their radar) was that the transponder radiates the PFD selected bug, which in my plane at the time was also what the autopilot will follow.

Ibra wrote:

what PFD you have G500?

He wrote:

denopa wrote:

I have a G500 Tx
ELLX

I have the STEC3100 and it follows the “blue stuff” selected altitude, not the “magenta stuff” altitude constraint which comes from the GTN but is displayed on the G500TxI too.

Thanks Wigglylamp, if this continues to be a problem I’ll turn it off.

EGTF, LFTF

denopa wrote:

I have the STEC3100 and it follows the “blue stuff” selected altitude, not the “magenta stuff” altitude constraint which comes from the GTN but is displayed on the G500TxI too

Ah ok (just a vague guess) maybe their new upgraded radar gets both magenta+blue from PFD via TXP and seems to display the wrong one?
If you go VS→ALTS, do they see the blue bug altitude? someone can check this for you? but as you fly in France it’s done already

lionel wrote:

If you have an autopilot with enroute VNAV stuff, I’m not actually sure which one it would follow.

VNAV for en-route, you will always have VNAV active with ALTS armed and it stops at blue altitude bug (best to have a match with magenta altitude bug but I guess the logic allows for ATC level steps in descent?)

GP/V for approach, it always ignores the altitude blue bug

Last Edited by Ibra at 20 Sep 10:06
Paris/Essex, United Kingdom

@denopa, could it be recent software update in one of the box in your aircraft introducing this bug/feature ?

Have you flown under the control of London TC or London Control? They have Mode S-ES for already quite a while.

Last Edited by Xtophe at 20 Sep 11:56
Nympsfield, United Kingdom

No update since February, I’ve done many flights under London Control, never had a remark – but maybe they simply toggled their “GA – ignore” mode

EGTF, LFTF

Ibra wrote:

Ah ok (just a vague guess) maybe their new upgraded radar gets both magenta+blue from PFD via TXP and seems to display the wrong one?

It is actually possible for the transponder to transmit both the altitude selected by the pilot on the PFD (“MCP selected altitude”) and the VNAV target in the GTN (“FMS selected altitude”) as well an indication of which one is the target altitude currently used by the autopilot/flight director.

I find it hard to believe that the ATC radar system does this wrong as then every airliner with VNAV would have the same problem. It’s more likely that denopa’s installation does something wrong. As both the PFD, navigator and transponder are made by Garmin, they should be able to explain this.

Last Edited by Airborne_Again at 20 Sep 12:25
ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

As both the PFD, navigator and transponder are made by Garmin, they should be able to explain this.

Yes all aircraft installation is Garmin, if there is something fishy with install it would be from STEC3100 which is not made by Garmin but denopa mentioned it follows the blue bug, so AP/FD knows which one to fly and ATC should get “MCP selected altitude” on their screen…

Airborne_Again wrote:

I find it hard to believe that the ATC radar system does this wrong as then every airliner with VNAV would have the same problem

Yes I agree primary en-route ATC radar units (LTC & Scottish) will have these things well sorted on their radars as every airliner with VNAV would have same problem, but maybe they rarely bug VNAV with 600ft amsl for visual approach on magenta as their SOP in TMA is HDG/VLOC and VS/ALT/GPV

Secondary ATC units (e.g. Southend, Cardiff, Solent) are just starting to upgrade their kits and they could be running on different hardware with few bugs, for instance some had issues with long reg characters when ModeS ID it was introduced on display

denopa wrote:

I’ve done many flights under London Control, never had a remark

Did you load GTN Visual to EGTF or VNAV direct to EGTF at FL400 while talking to them ? naybe they (or radar system) does not care where the blue or magenta bugs sits as long as it’s bugged outside en-route airspace

Last Edited by Ibra at 20 Sep 13:08
Paris/Essex, United Kingdom
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