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When should I adjust the Altimeter?

Postby xen_steda » Sun Jan 26, 2020 6:47 pm

Hello, I would like to know what the proper (realistic) way to use the altimeter would be in regard to adjusting it/setting it to zero. I primarily use small planes like the Piper Super Cub and Warrior II.

Do I set the altimeter to zero if I will be landing at the same airfield that I take off from? Or do I leave it as is? For example the airfield I'm using says 100ft above sea level without adjusting anything on the altimeter.

Thanks for any attempt at clearing this up for me!
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Re: When should I adjust the Altimeter?

Postby Knüppelrührer » Sun Jan 26, 2020 7:21 pm

both very nice aircraft :)

this is the QNH/QFE-question.

it is common to dial in the field elevation (this is the same as using the QNH as heard from ATIS.). 100ft in your case.

gliderpilots often use elavation zero, so they can judge easier, if they can reach homebase with the present height (this is using QFE).

using first method seems to be preferable, especially when going cross-country. even more in moutains. and then you have the good habit already before going IFR.

have fun flying!
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Re: When should I adjust the Altimeter?

Postby xen_steda » Sun Jan 26, 2020 8:37 pm

Knüppelrührer wrote in Sun Jan 26, 2020 7:21 pm:both very nice aircraft :)

this is the QNH/QFE-question.

it is common to dial in the field elevation (this is the same as using the QNH as heard from ATIS.). 100ft in your case.

gliderpilots often use elavation zero, so they can judge easier, if they can reach homebase with the present height (this is using QFE).

using first method seems to be preferable, especially when going cross-country. even more in moutains. and then you have the good habit already before going IFR.

have fun flying!


Hello, thanks for the info, so if I understand correctly, if I'm at LIMW (a mountainous airport) or any other for that matter it is best practice to leave the altimeter at 2,800ft (for LIMW) when on the runway rather than setting it to zero right?
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Re: When should I adjust the Altimeter?  

Postby Knüppelrührer » Sun Jan 26, 2020 9:22 pm

if I'm at LIMW (a mountainous airport) or any other for that matter it is best practice to leave the altimeter at 2,800ft (for LIMW) when on the runway rather than setting it to zero right?


Affirmative.
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Re: When should I adjust the Altimeter?

Postby tdammers » Tue Jan 28, 2020 12:30 pm

At least under IFR, you are supposed to do the following:

- Before departure, set your altimeter to the local QNH
- When climbing through transition altitude, switch to standard setting (1013 hPa / 29.92 inHg)
- When descending through transition level, switch to local QNH

You can get local QNH's, and often also transition levels and altitudes, from ATIS. Transition levels and altitudes are mentioned on charts. QNH's are also a standard part of METAR. Approach controllers will sometimes give you the local QNH as a convenience together with your descent clearance.

As a double check, many pilots will set the altimeter to the local field altitude first, and then cross-check the QNH settings against the current METAR/ATIS.

So, say you're flying from EDLN to EHRD.

First you get your charts and ATIS for EDLN. This gives you a field elevation of 125 ft, so you turn the altimeter knob to show 125ft. Double check the QNH dial against the local QNH from ATIS (say 996 hPa); if it doesn't match, then either your ATIS is wrong, or the altimeter is faulty. Get your clearance, taxi, take off. As you pass the transition altitude (5000 ft at EDLN), switch the altimeter to standard QNH, 1013 hPa (or 29.92 inHg). From here on, all altitudes are given as "flight levels", rather than in feet. Then as you descend towards your destination, look up the transition level (FL060 for EHRD) and local QNH at your destination (let's say 998 hPa). As you descend through the transition level, dial your altimeter to local QNH, which should give you real sea-level altitudes. Unless you have a radio altimeter and you're going to fly an ILS approach (and maybe also even then, just to be sure), look up the field elevation for the destination in order to verify your descent profile through the approach.

When flying local VFR, you don't normally climb above transition altitude, so all you need to do is set your altimeter to local field elevation and cross-check QNH. For VFR cross-country, adjust QNH as you receive new ATIS information enroute, especially for your destination airfield.
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Re: When should I adjust the Altimeter?

Postby xen_steda » Wed Jan 29, 2020 2:56 am

tdammers wrote in Tue Jan 28, 2020 12:30 pm:
...When flying local VFR, you don't normally climb above transition altitude, so all you need to do is set your altimeter to local field elevation and cross-check QNH. For VFR cross-country, adjust QNH as you receive new ATIS information enroute, especially for your destination airfield.



tdammers, thanks for the detailed response, I really appreciate it.
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