Dark side of visual approaches.

[Will.iam]

4 hours ago, PT20J said:

I asked Garmin aviation support the following:

During a RNAV LPV approach, if the altimeter setting is grossly incorrect so that the baro altitude is significantly different than the GPS altitude, how will the GTN be affected? Will any error messages be displayed?

I just received the following answer: 

Thank you for contacting Garmin International. If the barometer is not correct, you could receive warnings for low altitude or busting through minimums on an approach. Once passed a certain point, a missed approach warning could result on the screen. 

I guess I do need to add this to my list of things to test 

Yes please do. We know that our terrain warning stsrem will go off anywhere outside the FAF but past the FAF that protection shrinks away to agl at some point before the runway so that false warnings do not trigger. If you have a big enough error maybe it would trip but a 200-400ft error could be small enough not to trip the terrain warning bur big enough for the plane to hit an object. Or hill short of the runway. 

[PT20J]

On 1/22/2024 at 8:58 PM, Will.iam said:

Yes please do. We know that our terrain warning stsrem will go off anywhere outside the FAF but past the FAF that protection shrinks away to agl at some point before the runway so that false warnings do not trigger. If you have a big enough error maybe it would trip but a 200-400ft error could be small enough not to trip the terrain warning bur big enough for the plane to hit an object. Or hill short of the runway. 

I shot a LPV approach today with the altimeter set 0.15" higher than the reported altimeter setting causing the airplane to be about 150' MSL lower than indicated. I set the Baro minimums on the G3X to published DA which was 200' AGL. I intercepted the approach course about 3 miles outside the FAF. I intercepted the GP inside the FAF (because I was low) and as I tracked the GP, the descent lined up with the PAPI. The G3X annunciated MINIMUMS at about 50' AGL.

So, as I expected, the erroneous altimeter setting had no effect on the GP but only caused the intercept to be at a lower altitude and closer to the MAP.

In your original post you mentioned the airplane flying into Key West was an airliner with a FMS. Is it possible that they were shooting a baro LNAV/VNAV approach and the FMS was not using GPS.

[Ibra]

On 1/23/2024 at 1:28 AM, PT20J said:

Thank you for contacting Garmin International. If the barometer is not correct, you could receive warnings for low altitude or busting through minimums on an approach. Once passed a certain point, a missed approach warning could result on the screen. 

Is your Mooney GPS connected to BARO inputs? I recall PA46 G1000xi and SR22 (G5) with G1000Perspective had barometric data, it seems possible in integrated PFD+MFD?

I doubt standalone GTN does this, it’s you PFD that gets QNH? it's too "modular" 

My understanding GTN doesn’t have baro input or encoder altitude input? on GTN/GTX installs for ADSB with RS232, GTX gets altitude from encoder (GAE12) and position from GTN and you can’t send baro altitude to GTN, I doubt GTN is aware of QNH?

For sure, GTN can’t handle BARO L/VNAV only SBAS L/VNAV (we tried it in Thailand ), I would be surprised if it’s aware of QNH and I would be interested to know if the warning exist tough?

On other other avionics maybe, you have ALTimeter Setting Monitoring (ALTSM) function…

https://safetyfirst.airbus.com/use-the-correct-baro-setting-for-approach/

 

 

 

 

Edited January 26 by Ibra

[PT20J]

17 minutes ago, Ibra said:

Is your Mooney GPS connected to BARO inputs? I recall PA46 G1000 and SR22 (G5) with G1000 Perspective had barometric data, I doubt GTN does that? it’s you PFD that gets QNH?

My understanding GTN doesn’t have baro input or encoder altitude input? on GTN/GTX installs for ADSB with RS232, GTX gets altitude from encoder (GAE12) and position from GTN and you can’t send baro altitude to GTN, I doubt GTN is aware of QNH?

For sure, GTN can’t handle BARO L/VNAV only SBAS L/VNAV (we tried it in Thailand ), I would be surprised if it’s aware of QNH and I would be interested to know if the warning exist tough?

On other other avionics maybe, you have ALTimeter Setting Monitoring (ALTSM) function…

https://safetyfirst.airbus.com/use-the-correct-baro-setting-for-approach/

 

 

 

 

In my installation, the GTN gets corrected baro altitude from the G3X (or G5 if the G3X is inop). The GTN uses corrected baro altitude for GPS integrity checking, VNAV, automatic sequencing of altitude terminated legs, and Smart Glide.

[Ibra]

Yes it will be great to get a PIREP if G3X+GTN would flag bad QNH that bust DA on LPV

Yes barometric altitude is used in augmentation checks before faf and in vnav or vpath en-route, for the latter it’s different as it’s PFD/HSI that emulate difference between target altitude and bugged altitude and sets a descent profile on that like fake glideslope, however, it does not care about correct or wrong setting: VNAV does not flag mixing STD and QNH while in cruise 

Do you have VPATH guidance with G3X+GTN combo? I recall this works in G500 txi units 

Edited January 27 by Ibra

[PT20J]

@Will.iam this might explain what your airline crew experienced. It sounds pretty similar to your description.

 

 

[Fly Boomer]

7 hours ago, PT20J said:

this might explain what your airline crew experienced. It sounds pretty similar to your description.

I had to turn this off after counting about ten screw-ups.  Terrifying.

[Will.iam]

9 hours ago, PT20J said:

@Will.iam this might explain what your airline crew experienced. It sounds pretty similar to your description.

 

 

Yes this is the issue and they highlight it very well. I just went flying yesterday and shot an RNAV approach with my garmin 530W and the garmin did not change my vertical path when i changed the barometer. So it doesn’t have this issue and works like i assumed a GPS approach would work. Finding out that the airbus will adjust the vertical path based on barometric setting was an issue i had never thought of and just highlights the importance of triple checking the baro is the correct local setting for the airport you are landing at. But just like testing the failure redundancy fall back on 2 garmin gi275 will work as expected before you get into IMC, i would recommend you test an RNAV approach with the wrong altimeter setting just to make sure your system whether it’s a G1000, Avidyne, dynon etc behaves as you would expect it to and not assume it will work like previous systems you have flown with in the past. 

[Marc_B]

I read this as the reason that it's good to build a habit of "glidepath captured, altitude checks, setting missed approach altitude" callout/flow.  Seems like the FAF/GP capture is the last, best time prior to landing that you have a published altitude to compare what you're showing to what you should be showing.  Essentially when you're setting the barometer to local you're hoping to make sure your indicated MDA/DA is where you're actually at.  Your comparison of published to indicated is your verification that it's accurate.

Are there any avionics out there that automatically preload mins??  With G500 I have to manually set the mins which makes sense given the variables of speed categories and type of approach.

I'll have to check if there is any flag with improper QNH (I've never accidentally or intentionally set the baro way off)...you'd think that if the FAF is discrete point in space within GPS database that you should be able to automatically check to see if indicated altitude matches the database at a specific waypoint such as the FAF.  But my guess is that this would be way more complicated for certification.  Of course my meat computer does this pretty easily, but definitely not an automatic process!

[Ibra]

3 hours ago, Marc_B said:

Are there any avionics out there that automatically preload mins??

I gather it’s impossible as these are calculated by operator (airlines, commuters, private pilots) and one can’t distinguish between 2D/3D capabilities unless the glideslope is live and well checked (LOC/ILS mins? GPS or FMS LNAV, L/VNAV, LPV?)

One can check altitude with airport QNH against GPS WAAS altitude during vectoring or feeder airway leg (errors are under 50ft) and check any altitude discrepancy against distance at FAF or other fixes, these will flag scenarios like clogged static ports or wrong altimeter setting 

On something as robust as the glide on ILS or LPV, altimeter errors are of secondary: they won’t matter that much, if the needles are in the middle the aircraft will fly all way to touchdown point anyway (on ILS, one has to take care of false glideslope or irreliable signal, however, these gets flagged by other traffic and from ground speed to vertical speed check) 

Also, it’s really hard to mess up these days with extra awareness from Synthetic Vision, if HSI/GS is dead in middle while flight path vector is not on threshold something is wrong…I gather, someone flying privately under Part91 can legally look at an iPad with ForeFlight Synthetic-Vision and he is free to cross check QNH altitude versus WAAS altitude (the crew who flew 6ft from 27R at Charles De Gaulle don’t have that luxury, if they were equipped with WAAS, LPV, Synt-Vision…we won’t be having this discussion) 

Edited February 5 by Ibra