View Full Version : US Altimeter Setting


The_green_penguin
10th Mar 2011, 13:13
Hi,

I understand that the US altimeter pressure setting is calibrated in inches of mercury. I am curious, however, whether the US have an equivalent to QFE that we have here in Europe. E.g. When I go and get the aerdrome ATIS, is the pressure setting going to give me height above the aerodrome, or is it a mean sea level pressure?

If it is height above aerdrome level, how do I then go about getting the enroute QNH (or equivalent)?

I am sure all of this stuff will be covered during my check out with instructors over there before I commence some solo hour building, however, I just wanted to be prepared.

Many thanks



crj705
10th Mar 2011, 13:22
QFE is not used in the US. If you reference FAR 91.121 it will answer all of your questions about altimeter settings in US airspace. Basically you will be using QNH your whole time below 18,000 ft.

Sec. 91.121

Altimeter settings.

(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating--
(1) Below 18,000 feet MSL, to--
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
(2) At or above 18,000 feet MSL, to 29.92'' Hg.

This information will be provided to you by ATC if you are IFR or using radar services. Otherwise you will have to tune in an ATIS or AWOS along your route.

The_green_penguin
10th Mar 2011, 13:42
So how are you supposed to know what circuit (traffic pattern) height is if there is no QFE? You have to mentally adjust QNH to take account of the aerodrome elevation, I take it?

CAT3C AUTOLAND
10th Mar 2011, 13:52
All you have to do mate is take the elevation and add it to circuit height, that will give you the altitude to fly the circuit, on the QNH.

The reason they dont have QFE is that some aerodromes are so high the altimeter wont wind that low!

The_green_penguin
10th Mar 2011, 14:10
I get it - thanks. I have also just spotted that traffic pattern altitude (TPA?) is usually stated in the Airport Facility Directory.

EK4457
10th Mar 2011, 14:15
You have to mentally adjust QNH to take account of the aerodrome elevation, I take it?

A bit confused I think - you make no adjustment to the QNH (altimeter setting) - that remains fixed unless told otherwise from a suitable source (ie ATC)

In theory you just add circuit height to aerodrome elevation to find the correct altitude to fly at.

In practise it's not even as difficult as that - you just get someone else to carry out this mathematical puzzle and fly at the published circuit altitude!

QFE is a quirk of the UK and is (as per usual) a result of old WW2 habits refusing to die.

IMHO there is absolutely no need for QFE.

I'm told that changing altimeter setting during the busy approach phase and having no idea what your clearance over the local terrain is can be useful to an inexperienced PPL. As an instrctor I find it causes far more problems than it solves. Forgetting to set QFE can cause havoc and be downright dangerous.

I'm sure I'll get shot down by far more experienced instructors but that's my tuppence.......

zondaracer
10th Mar 2011, 15:01
It is really easy. They will give you QNH, which you will hear on the radio or ATIS as just ¨Altimeter Setting¨, for example they will say ¨Altimeter setting two niner niner two¨, so you will set 29.92¨ in the window. we don´t really refer to the Q code in the US.

Then, if you are flying at a place like KAPA in Colorado, the elevation is 5885ft according to the Airport Facility Directory (A/FD). on the same line, you will see ¨TPA¨ and it will tell you the altitude that you must fly and also the pattern height, so in this case it is ¨TPA 6885 (1000)¨. 1000ft above the ground, 6885ft MSL. so if you set your altimeter correctly, on landing, your altimeter will read 5885.

http://aeronav.faa.gov/pdfs/sw_195_10MAR2011.pdf

Free FAA NACO pubs... Free Digital Products (http://aeronav.faa.gov/index.asp?xml=aeronav/applications)
you can also find good info at Airnav.com

Also, calculating pressure altitude is much easier using inches of mercury. since 29.92 is ISA, you subtract it from the local setting and multiply by 1000, and you get the correction factor.

so... (local setting-29.92) * 1000 + altitude = pressure altitude

clear as mud?

The_green_penguin
10th Mar 2011, 15:04
For airports where there is no published traffic pattern altitude, and no control tower - only UNICOM, what are you meant to do in that scenario? Just add say 1000ft circuit height to whatever the published elevation is? (is TPA as standard 1000 foot above the ground?)

Chilli Monster
10th Mar 2011, 15:10
Basically yes - add 1000ft to Aerodrome elevation and fly that. It really is that simple over there.

zondaracer
10th Mar 2011, 15:14
http://aeronav.faa.gov/pdfs/sw_13_10MAR2011.pdf

Scroll down to Gila Bend municipal. There is no ATIS/AWOS/ASOS, and it is uncontrolled. TPA is listed as 1589ft, and pattern height is 800ft so you fly at an altitude of 1589ft to be at TPA. you usually won´t have the altimeter setting so you just get one from the closest place or the closest NAVAID if it has weather info. Otherwise, you will be VFR and you can just eyeball it, aka try to use visual cues/reference points such as a wing strut or something to judge correct TPA and see if it matches the altimeter. it should be close or else your altimeter setting is way off or else you need to work on the visual references. nothing to it

zondaracer
10th Mar 2011, 15:16
where are you doing your time building? if you are going to florida, you have nothing to worry about since the highest point in the whole state is 385ft! hehe

The_green_penguin
10th Mar 2011, 15:29
Florida mostly, but plan to go up to West Virginia/Cincinnati and go to Appalachian Mountains so it is useful to know this stuff...

Thanks for everyone's advice...

zondaracer
10th Mar 2011, 18:09
No it isn't - TPA (ALTITUDE) is denoted as 1589.
Gila Bend is 789 feet ASL and they want you to fly at a TPA as indicated of 1589' and will use an Altimeter setting (probably got from Phoenix Approach or Prescott FSS - or whoever you have watching over you with Flight Following ) at which you fly at that TPA.

That´s what I mean to say. Pattern height is 800, TPA is 1589. I have flown many times to Gila Bend without talking to anybody, often times having to get an altimeter setting from nearby. Thanks for clarifying, I misspoke/mistyped. i fixed my previous post.

and like socal said, not a big deal, you just fly the altitude published.

Using QFE in the US is not practical since at many airports at medium to high altitudes, you wouldn´t be able to set the altimeter to 0.