Let me put this differently. You are at the IAP platform, say 2200ft, there are 900ft obstacles nearby, you quickly check the GPS altitude, which says 700ft.
You are
I never use QFE. One is required to read it back if given by ATC.
gallois wrote:
I thought they had just gone out of fashion with pilots and ATC because QNH was less cumbersome
Probably, but also kollsman window range will not let you fly to an airport higher than 3000ft elevation on QFE (but this should not be an issue on GlassCockpits?), surely, QFE was invented by the Brits not the Swiss
gallois wrote:
Most/many of the IAC charts for non towered airfields in France will tell you where to get the QNH. It is also another reason for the circle to land at untowered airfields. In case it is not clear when I write untowered I mean there might be no-one on site. It is not necessary to have a Flugleiter, AFIS, airport manager or any such personnel.
I’ve always found it amazing that the same guy records the ASOS at something like a thousand US airports and somehow manages to keep them all current 
It’s actually a standardized and automated voice. Somehow it’s reassuring to hear that familiar and robotically confident ASOS voice when inbound to a tiny unattended airport where you’ve never been before
Ibra wrote:
why not on GPS altitude?
Because you don’t know where obstacles are in reference to GPS altitude?
WAAS/SBAS GPS meets vertical accuracy of 4m 95% of time with vertical alert limit (VAL) of 35m, if you are on the 3D glide path why that is that not enough for a “GPS DA”? I don’t fly that close to obstacles even in sunny days but I would go with that rather than QNH from 2h ago when a weather fronts just run through some unattended airfield
Maybe NYCYankee knows more but max vertical error on US airport elevation is 150ft that was using data from 2000 to 2020
As Peter said if your GPS shows 700ft on altitude you just run away without looking behind, it’s very accurate for matching AMSL on top of the runways, why it will not be accurate on top of some antennas or hills around it?
In US, obtaining the altimeter setting QFE is mandatory, either the charted or a remote alternate may be used, but the DA and MDA are adjusted accordingly on the chart. Using the GPS altitude to establish the altimeter setting QFE is not permitted. In general, the GPS altitude will not match the Baro-altimeter indication with QFE entered into the Kohlmann because GPS altitude is not affected by temperature, whereas the baro-altimeter is. At 1500 feet above the airport elevation, the error can be as much as +/ 210 feet at ISA +/- 35C. At 2000 feet above the airport, it grows to +/- 280 feet, and at 3000 above the airport, it is +/- 420 feet. Even if the temperature at the airport is ISA +/- 15C, the error at 1500, 2000, and 3000 is 90 feet, 120 feet, and 170 feet respectively. The error is much reduced at lower altitudes and goes away at field elevation where the GPS altitude will match field elevation within the GPS vertical accuracy tolerance.
Also one would need to use an appropriate GPS altitude and not the raw height above the ellipsoid which is what a GPS altitude calculates or this could add more error. As an aside, terrain and obstacle databases as well as heights specified in the SBAS GPS FAS data block are all expressed in terms of heights above the ellipsoid and they don’t use MSL values. A GPS based terrain and obstacle database is much superior to one based on a barometric altimeter input as it is independent of temperature, in particular cold temperatures where a baro based altimeter is going to indicate a higher altitude than in fact you are at.
Indeed, my point is on airfield elevation with WAAS/SBAS correction GPS ALT = ALT on QNH, if you go 300ft above GPS DA is close to ALT QNH DA with some tiny corrections (we are talking peanuts), however, the glide path up to 2000ft FAP/FAF or IAP platform will not get matched (temperature corrections will matters for DME/ALT checkpoints or Baro-VNAV glide path) but on 3D ILS, LPV, APV SBAS (LNAV/VNAV + WAAS) there is nothing there to correct
Yes RNP approaches obstacles are surveyed in GPS altitude, well I bloody hope so
Good point about “GPS height” but I doubt anyone should ever fly on that…
If i needed the QNH I could just visit the AWOS website before beginning the approach. Problem solved.
NCYankee wrote:
In US, obtaining the altimeter setting QFE is mandatory,
Are you sure you mean QFE and not QNH? QNH is the same thing as “altimeter setting” in the US, although the unit is hPa rather than inches Hg. QFE is the pressure value that will make the altimeter show zero on the ground at the airport.
