Airport barometric pressure

[boldpilot]

Hi Guy's . Recently while flying into and out of Airports in South America in FS9 I noticed that while sitting on the runway the altimeter was showing ,depending on the airport, between 5000 and 13000 feet altitude with the Hg still at 92.92.
Would some kind person [a pilot] advise me if this is correct or in real life should the airport Hg pressure be adjusted so that the airport is at zero altitude .Should the airport pressure be advised by ATC before landing allowing for the altitude.
Is there an anomaly in FS9 regarding this. Thanks .boldpilot

[Timmo]

There are two pressure settings you should be aware of (there are more but two will do for now)

QNH and QFE

QNH refers to the actual pressure of the air at sea level- Setting this value on your altimeter will therfore give your height above sea level

QFE refers to the pressure of the air above another datum (i.e. an aerodrome)- Settings this value on your altimeter will give you your height above that datum (ie. height above the aerodrome)

Since sea level is a more useful datum/base level pilots usually fly with this set on their altimeter so that they know their height above sea level. All charts give heights above sea level (ie. Height of airfield, height of mountains etc) so its better to use this.

These pressure values change of course so part of a normal flight is to get ammended QNH values from ATC, weather reports etc so that your altimeter will always be showing you height above sea level.

[Timmo]

....if you know the height of the airfield you are sitting at, rotating the altimeter subscale so that it shows this height will give you the correct QNH.

Setting your altimeter so that it read zero at an airport would only be useful if you never strayed from that airport or if the pressure never changed. If you flew from a mountain airport to a coastal airport your altimeter would give a negative value....doh! haha

NOTE: In NZ we use the metric measure of pressure (Hpa) which has a 'standard' value of 1013.25. The 2992 value is the American/imperial way of expressing pressure in Hg (inches of Mercury?)

[boldpilot]

Thanks Timmo for your reply .However if you were flying your Cessna doing local circuits and bumps at an high altitude airfield would you set your altimeter Hg to local airfield pressure or Sea level, or alternativelly have two altimeters ,one set at sea level and one for local conditions .Is this possible and does it happen in any aircraft.

[Timmo]

As far as i know, for VFR/small a/c flying you always fly primarily with reference to QNH/Height above sea level but it is certainly possible/useful to set QFE on the secondary altimeter if one is fitted.

In saying that, if you are flying at your home aerodrome, its not too hard to just add 1000 ft to the aerodrome height to get the circuit height. Its also good practice for the future: you would hope that someday you would fly to other airfields eh?

[scon]

I know almost all the time (99.9999%) of the time GA aircraft in NZ will set QNH not QFE

[firefly]

Great reply Timmo, that covered it very well.

[G-HEVN]

QFE is only really useful when you are going to land at a relatively low altitude airfield (or for doing circuits). It's common in the UK, and when contacting a tower for joining instructions, you will commonly be given the QFE (bigger airports give QNH, but will still give QFE on request). When departing, if your intention is to leave the circuit, you will be given QNH rather than QFE.

QFE is useless beyond the immediate environs of the airfield that issued it.

At the high airports mentioned, it may be impossible to set a QFE, because the altimeter subscale simply will not go that far (try it, and see how close you can get it to zero), so you are forced to use QNH. Given a situation where you have to use QNH at some airfields, it's a reasonable attitude to say 'use QNH at all airfields'.

Instrument rated pilots will find all their approach etc charts in QNH, so it's a real pain having to switch to QFE for the landing and then switch back to QNH on a missed approach (not to mention an easy fail if you miss it on the check ride!), so increasingly, even in the UK, QFE is being used less and less.

Obviously, when using QNH, you need to know the field elevation. When using QFE, you know the field is at zero feet.

[Timmo]

Actually while we are on this- Is it possible to change the units as shown on the Altimeter in FS?

Ive changed my units/settings to use Hpa but the subscale still shows values in inches of mecury? From memory, ATC still gives QNH reports in Inches of Mecury too??

[G-HEVN]

In FSX, if you set metric or hybrid, you'll get pressures in millibars from both ATC and ATIS.

Regarding the altimeter, it depends entirely on the gauge, which can be programmed for either or both units. Unfortunately, the user can't change it. However, Reality XP make a set of drop-in replacement gauges (Flightline T) for FS9 only, that can be configured.

http://www.reality-xp.com/products/FLNT/index.htm