Board index FlightGear Support Flying

A fast question about altitude on "mountain" Airport.  Topic is solved

Controlling your aircraft, using the autopilot etc.

A fast question about altitude on "mountain" Airport.

Postby Barambu2sa » Mon Nov 21, 2022 7:39 am

When I landing I usually set the altimeter basing on real metar.
The problem is on "mountain Airports" like LIMW (Aosta, Italy)
Metar said

Code: Select all
metar LIMW
(METAR automatically generated)
Airport-Id:      LIMW
Report time:      2022/11/21 6:20 UTC
Visibility:      >=10 km               >=6.2 US-miles
Wind:         from variable directions at 3.7 km/h      2 kt = 2.3 mph = 1 m/s
Temperature:      5 C               41 F
Dewpoint:      -5 C               23 F
Rel. Humidity:       48 %
Pressure:      1011 hPa            29.85 in. Hg
Sky condition:      clear skies


but on the runway the pressure is 947!

Image

in real world how to set the correct value? If I set 947 during the cruise i give
a kiss to a big mountain
Barambu2sa
 
Posts: 288
Joined: Tue Mar 25, 2008 5:49 pm

Re: A fast question about altitude on "mountain" Airport.  

Postby V12 » Mon Nov 21, 2022 9:43 am

If this screenshot is from Aosta rwy 09, then Your altimeter should read 1790-1800 feet with baro pressure 1011 hPa, not zero.
Fly high, fly fast - fly Concorde !
V12
 
Posts: 2757
Joined: Thu Jan 12, 2017 5:27 pm
Location: LZIB
Callsign: BAWV12

Re: A fast question about altitude on "mountain" Airport.

Postby SurferTim » Mon Nov 21, 2022 10:52 am

Lately I've noticed errors in the metar weather when there are slashes ( /// ) in the metar report. If before the altimeter setting, it isn't used, at least what I experience. I'm not certain what FG weather uses if no altimeter.

Saba (TNCS) uses TNCE (St Eustatius) weather, and it normally has slashes in the weather report, and the altimeter setting isn't used.
Fly low. Fly slow. Land on a dime. Twin Otter. https://github.com/SurferTim/dhc6p
My other aircraft is a Citation-X https://github.com/SurferTim/CitationX
PirateAir videos at https://www.youtube.com/user/SurferTim850
User avatar
SurferTim
 
Posts: 1708
Joined: Sun Dec 09, 2018 6:49 pm
Location: Miramar Beach, FL
Callsign: Pirate
Version: 2020.4.0
OS: Ubuntu 18.04

Re: A fast question about altitude on "mountain" Airport.

Postby Barambu2sa » Mon Nov 21, 2022 11:28 am

V12 wrote in Mon Nov 21, 2022 9:43 am:If this screenshot is from Aosta rwy 09, then Your altimeter should read 1790-1800 feet with baro pressure 1011 hPa, not zero.

So the answer is "use the metar".
Thanks
Barambu2sa
 
Posts: 288
Joined: Tue Mar 25, 2008 5:49 pm

Re: A fast question about altitude on "mountain" Airport.

Postby SurferTim » Mon Nov 21, 2022 11:46 am

Current metar for LIMW
LIMW 211050Z AUTO 09007KT 9999 NCD 08/M04 Q1008

My experience tells me the metar parser stops when it encounters a slash.
Just saying...

Edit: I may be mistaken, but I believe the slashes are an Aviation Weather Service replacement for a "word" that contains non-English (not standard ascii ) characters. Most slashes go to the Dutch, and next the French. I haven't flown much in eastern Europe, but I bet there is the same slashes.
Fly low. Fly slow. Land on a dime. Twin Otter. https://github.com/SurferTim/dhc6p
My other aircraft is a Citation-X https://github.com/SurferTim/CitationX
PirateAir videos at https://www.youtube.com/user/SurferTim850
User avatar
SurferTim
 
Posts: 1708
Joined: Sun Dec 09, 2018 6:49 pm
Location: Miramar Beach, FL
Callsign: Pirate
Version: 2020.4.0
OS: Ubuntu 18.04

Re: A fast question about altitude on "mountain" Airport.

Postby WoodSTokk » Mon Nov 21, 2022 2:38 pm

The slash that you are showing is normal. It means temperature 8°C / dew point -4°C.
AFAIK the METAR parser had a problem with two or more slashes in a row in the past.
But this should be fixed since more than one year ago.
So, V12 is correct. If you set the QNH on airodrom, you should read the field elevation.
If you want to read the altitude above ground, you should set the QFE, but this will normaly only be used by glider.

Historical Background:
The Q-codes are a very old standard dating back to the beginning of the radio communication.
The characters after the Q dont have a relation to that what it means.
But in this case, there are mnemonic sentence:

QNH -> Normal Height (altitude in respect to Meridian Sea Level "MSL")
QFE -> Field Elevation (altitude Above Ground Level "AGL")

You can find more about the Q-codes on Wikipedia.
In todays aviation, only QNH and QFE are used.
WoodSTokk
 
Posts: 1077
Joined: Tue Oct 17, 2017 3:30 pm
Location: Milky Way/Sol/Earth/Europe
Callsign: SX-W57
IRC name: WoodSTokk
Version: 2020.4.0
OS: Debian Bullseye

Re: A fast question about altitude on "mountain" Airport.

Postby SurferTim » Mon Nov 21, 2022 2:48 pm

Thanks WoodSTokk. I know what it is. The "2 or more slashes" is not fixed in 2020.4.0. TNCE weather has slashes all the time. The altimeter setting isn't processed by the metar parser. If I just leave the altimeter setting alone after leaving St Barth until I get to a weather station with no slashes in the report (TKPK), the runway elevations are correct.
Fly low. Fly slow. Land on a dime. Twin Otter. https://github.com/SurferTim/dhc6p
My other aircraft is a Citation-X https://github.com/SurferTim/CitationX
PirateAir videos at https://www.youtube.com/user/SurferTim850
User avatar
SurferTim
 
Posts: 1708
Joined: Sun Dec 09, 2018 6:49 pm
Location: Miramar Beach, FL
Callsign: Pirate
Version: 2020.4.0
OS: Ubuntu 18.04

Re: A fast question about altitude on "mountain" Airport.

Postby WoodSTokk » Mon Nov 21, 2022 2:56 pm

Can you confirm that even the single slash in the temperature stop parsing the metar?

EDIT:
I spawned now at LIMW with the following live METAR:
Code: Select all
2022/11/21 13:50 LIMW 211350Z AUTO 10005KT 060V130 9999 FEW130/// 10/M04 Q1003


I had absolutely no problem and FG works correct.
As you can see, the QNH comes after 3 slashes and it works.
I use FG 2020.4 compiled at 8. November.
Last edited by WoodSTokk on Mon Nov 21, 2022 3:08 pm, edited 2 times in total.
WoodSTokk
 
Posts: 1077
Joined: Tue Oct 17, 2017 3:30 pm
Location: Milky Way/Sol/Earth/Europe
Callsign: SX-W57
IRC name: WoodSTokk
Version: 2020.4.0
OS: Debian Bullseye

Re: A fast question about altitude on "mountain" Airport.

Postby SurferTim » Mon Nov 21, 2022 3:03 pm

On that I don't know. I guess maybe I'm a bit overly suspicious of slashes.

I didn't notice the "2 or more slashes" for a while. Usually airports close to each other have similar weather and the altimeter settings are very close. But since I like to fly in the crappy weather now, it has become more apparent. Fronts passing thru change the altimeter setting a bunch. St Maarten can be a lot different than St Eustatius during a storm.

Edit: I just experienced the slashes at Guadeloupe TFFR just the other day. Slashes in the weather, but I used the altimeter setting anyway. When I landed, I was below sea level.
Fly low. Fly slow. Land on a dime. Twin Otter. https://github.com/SurferTim/dhc6p
My other aircraft is a Citation-X https://github.com/SurferTim/CitationX
PirateAir videos at https://www.youtube.com/user/SurferTim850
User avatar
SurferTim
 
Posts: 1708
Joined: Sun Dec 09, 2018 6:49 pm
Location: Miramar Beach, FL
Callsign: Pirate
Version: 2020.4.0
OS: Ubuntu 18.04


Return to Flying

Who is online

Users browsing this forum: No registered users and 8 guests