QNH
07-18-2014, 09:41 AM
#1
New Hire
Thread Starter
Joined APC: Jul 2014
Posts: 8
QNH
Hello everyone
I understand that the QFE is the pressure at the airfield and the QNH is the actual pressure at airfield reduced to sea level using ISA. But I can't understand why at high elevation airports the QNH value is high (i.e QNH 1030). Any ideas?
I understand that the QFE is the pressure at the airfield and the QNH is the actual pressure at airfield reduced to sea level using ISA. But I can't understand why at high elevation airports the QNH value is high (i.e QNH 1030). Any ideas?
07-18-2014, 11:29 PM
#3
New Hire
Thread Starter
Joined APC: Jul 2014
Posts: 8
Originally Posted by Twin Wasp
You said it yourself. It is the airport pressure reduced to sea level.
07-19-2014, 08:18 AM
#4
Gets Weekends Off
Joined APC: Oct 2007
Position: Sr. VP of button pushing
Posts: 2,730
Corrected to sea level would be better way of saying it. The normal pressure lapse rate down low (say sea level to 10,000 feet) is about 100 for every .1 of an inch of Hg. So on a standard day the actual pressure in New Orleans is 29.92 and in Denver it'll be about 24.32. The Met people in Denver have a correction factor they add to the actual pressure to bring it up to 29.92. We could use the local pressure and everything would be QFE.
Believe it or not, the world does not revolve around pilots. The weather wizards use pressure to figure out where fronts are and what's going to happen. If everyone just used their local pressure uncorrected for altitude, drawing isobars on a weather map would be useless.
Believe it or not, the world does not revolve around pilots. The weather wizards use pressure to figure out where fronts are and what's going to happen. If everyone just used their local pressure uncorrected for altitude, drawing isobars on a weather map would be useless.
07-19-2014, 09:14 AM
#7
Gets Weekends Off
Joined APC: Mar 2011
Position: Left seat on a kite
Posts: 1,884
Originally Posted by PerfInit
If you set the altimeter to read "0" (zero) while parked at the gate, the pressure setting in the kholsman window = QFE
07-19-2014, 09:23 AM
#8
Gets Weekends Off
Joined APC: Aug 2007
Position: I pilot
Posts: 2,049
The RAF (UK) still like to use QFE, and QFE is also used in some parts of Eastern Europe and Asia. A friend of mind requested QNH somewhere in the 'stans but was given QFE, and luckily caught on before he bent some metal.
07-19-2014, 10:02 AM
#9
New Hire
Thread Starter
Joined APC: Jul 2014
Posts: 8
Originally Posted by Twin Wasp
The Met people in Denver have a correction factor they add to the actual pressure to bring it up to 29.92.
Thats my question.. Because at Addis Abeba airport for example the elev is 7700ft the QNH is always high as 1030 hPa or something
Last edited by FlyFly; 07-19-2014 at 10:23 AM.
07-19-2014, 02:33 PM
#10
Gets Weekends Off
Joined APC: May 2010
Position: Baja Vermont
Posts: 5,177
Flyfly,
There are many locations where a fairly permanent high pressure atmosphere exists, Addis near the Equator is likely one. Quito and La Paz, Bolivia is also under that high pressure influence. What you are seeing is an atmospheric effect, not a QNH effect.
GF
There are many locations where a fairly permanent high pressure atmosphere exists, Addis near the Equator is likely one. Quito and La Paz, Bolivia is also under that high pressure influence. What you are seeing is an atmospheric effect, not a QNH effect.
GF
Thread
Thread Starter
Forum
Replies
Last Post
