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Do RNAV LPV approaches use GPS or Baro Alt? (and temperature compensation on approaches)

I did stick my hand up to query this, but was assured by the instructor that his statement was correct.
I don’t know whether this applies to the source for the audio annunciation “minimumums”, however I will write to the training instructor.

I don’t know your instructor, but getting into a yes/no discussion might not be the best strategy to keep a good working relationship.

I’ve had numerous discussions with instructors on the past. Things like
- Do I need to be visual with the ground while flying VFR
- Is it allowed to use GPS as the primary means of navigation
- Can I fly without doing a W&B
- Is it allowed to fly without paper map
- etc etc

Ask 10 instructors and you get 10 different answers!
Some instructors you can’t convince they’re wrong even though you can prove it…

Last Edited by lenthamen at 02 Dec 13:07

An RNAV approach to LPV minimums may not be flown by using Baro VNAV equipment. If an approach has LNAV/VNAV line of minimum and the aircraft is equipped only either Baro VNAV or a WAAS GPS, that line of minimums may be used

IOW, if the equipment annunciates LPV, then the LPV minimums may be flown and a Baro VNAV system will never annunciate LPV. If the equipment annunciates L/VNAV, then either WAAS vertical navigation or Baro VNAV vertical navigation may be used, but the latter has temperature limitations unless the system is temperature compensating.

Last Edited by NCYankee at 02 Dec 19:26
KUZA, United States

From here

Peter wrote:

1000s of feet difference between a GPS and an altimeter cannot be explained by a QNH or some other atmospheric profile difference, unless you are on top of Everest.

Peter,

GPS altitude is unaffected by temperature or pressure as it is a geometric calculation, usually based on the WGS84 spheroid. Typical vertical accuracy is about 50 percent worse than lateral accuracy and is on the order of 15 meters. This value is usually corrected by a database to account for small differences between the the WGS84 spheroid and the local geoid which can introduce an error on the order of 100 feet, some places as much as 200 feet.

The Barometric altimeter compensates for non standard pressure, but not for temperature and can generate errors in excess of 1000 feet, particularly when the temperature is far from ISA. That is one reason why in mountainous areas, IFR altitudes require 2000 feet of clearance verses 1000 feet in non mountainous areas. The higher you are above the location where the altimeter setting is measured and the greater distance one is from the measurement point, the greater the error. So GPS altitude is used for terrain and obstacle while, while QNH is used for vertical separation for aircraft. Pressure altitude is used above the transition level and can easily add another 500 to 1000 feet difference with QNE and/or GPS altitude. At 7000 feet, a temperature difference of 20C will create a 500 foot error in the baro altimeter but will have no effect on the GPS altitude.

So difference between what the altimeter reads verses what the GPS altitude indicates can easily exceed 1000 feet at modest altitudes, one does not need to be on top of Mt Everest, the Zugspitze will do quite nicely.

KUZA, United States

nobbi wrote:

Remember that during winterflying the temperature effect on pressure altitude is 4% per 10° C ISA deviation. In the US it’s presently very cold. Could be ISA minus 20°C – so flying at 10.000ft would result in minus 800ft = 9200ft. GPS altitude would more or less show that difference.

Is that one of the things in the IR TK that has no relevance to real flying? ;-)

ESKC (Uppsala/Sundbro), Sweden

In “classical IFR”, as taught in Europe since for ever, and to a large degree still today, you don’t use GPS at all

And if flying with a GPS, you cannot fail to notice the difference between the GPS altitude and the altimeter, or the GPS and the FL even if the QNH was 1013 all the way. Seeing a GPS altitude 500+ ft above the altimeter is pretty common in the summer, with ISA+15 being fairly common. I have never seen 1000ft but I can see how it could be achieved. I did once see ISA-17 and I was at FL195 so that would have been a good place… I have all the GPS track logs so should have that one somewhere too.

Incidentally the plane was doing +300fpm at FL195 in the ISA-17 condition, and would have probably gone to FL230 had ATC let me (it was a post maintenance flight test, in Class G) and I think this aspect is much more relevant to flying. Normally it works against us, in a very noticeable way.

You also see the GPS versus altimeter difference on a flight on a southern heading. Usually the GPS altitude climbs as you go along. It’s quite funny… In one of my trip writeups (I can’t find it now) I have a graph of the GPS altitude and you can see the slope on it.

Administrator
Shoreham EGKA, United Kingdom

Sorry for the dumb question, but how can it not be dangerous to do an LPV approach if there is a 500+ ft delta between the GPS altitude and altimeter altitude reading? I can’t believe that happens. Or are we only talking here about deltas up in the higher flight levels?

Sure, the MAP/DH is still based on the altimeter, not the GPS, even if the GPS is providing the glidepath. But if the two aren’t better aligned than 500+, then I’d be worried about who is right ;-) …. or put another way, how can the GPS glidepath be accurate if the two altitudes disagree by so much? The glidepath guidance on the GPS would show intercept at an altitude/distance that doesn’t agree with the chart. How does the GPS deal with that? What if the GPS glidepath has one descending early so that obstacle clearance is not assured? Obviously not a problem if the GPS reading is higher than the altimeter, but certainly if lower.

I know this thread is about an ipad portable, not a panel-mount. Were your comments only related to portables, Peter?

Last Edited by chflyer at 03 Jan 23:03
LSZK, Switzerland

First, for vertical guidance a WAAS GPS is required, so the altitude is more accurate and integrity is monitored. So the following assumes a negligible error in the GPS altitude, so GPS altitude = true altitude.

I am also assuming the QNH is accurate and set correctly.

The delta between what the altimeter reads and the true/GPS altitude in that case depends on the temperature profile of the atmosphere below the aircraft. If it is, on average, colder than ISA, the altimeter overheads and you are actually lower than what it says, so the GPS altitude will be low.

If it is, on average, warmer than ISA, the altimeter underreads and you are actually higher, so the GPS altitude will read higher. Hence Peter’s climb when flying south…

The magnitude of this error depend on how thick that layer is, and how large the difference from ISA.

Hence, in winter ops, you add something to the MDA / DA to be safe. The one place where this is not necessary is where the vertical guidance comes form the GPS, which is he glide path between FAF and the decision point.

Last Edited by Cobalt at 03 Jan 23:17
Biggin Hill

Chflyer wrote:

The glidepath guidance on the GPS would show intercept at an altitude/distance that doesn’t agree with the chart.

Just like on a ILS, on a LPV approach you never intercept the vertical path at the chartered threshold distance (unless flying in ISA conditions).
That’s why the final segment of a precision approach start at waypoint called FAP (that waypoint is never at the same place and depends on atmospheric conditions) and not at a FAF (fixed waypoint on earth)
This is taught during IR course, just like GPS when I did my IR in 2012.
From 2020 PBN must be taught during an IR training course (per commission regulation)

Last Edited by Guillaume at 03 Jan 23:41

Cobalt wrote:

Hence, in winter ops, you add something to the MDA / DA to be safe. The one place where this is not necessary is where the vertical guidance comes form the GPS, which is he glide path between FAF and the decision point.

You do need this correction for a GPS approach as well. The GPS doesn’t tell you when you reach the (M)DA, the altimeter does.

Last Edited by Airborne_Again at 04 Jan 07:37
ESKC (Uppsala/Sundbro), Sweden

I was playing with two external GPS. Sources yesterday; a Garmin Glo and a SkyEcho. The GLO was giving me an accuracy of 6m and a correct altitude readout 223ft. Meanwhile the SkyEcho was stuck at 30m and regularly indicating 400ft altitude.

Funny old thing, gps.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom
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