Altimeter settings

[Thunderbird8]

I am a bit confused about the basic rules for altimeter settings. I do all my flying in the UK at the moment:

1. Why does ATC say 'altimeter 2992' and what is the significance of this?

2. My altimeter when flying from Stansted to Heathrow said 700 feet lower than my 'actual' height (checked by using Shift-Z)

3. How do you set the altimeter to read 0 feet at aeordrome level? Why do this, is it even a good idea, is it best to do something else?

[jwenting]

1) it's the standard number for use over a specified altitude. It's an international standard. What that altitude is however depends on the country you're flying in (and can sometimes change within a country) however the automated ATC in MSFS uses the US number of 18000ft worldwide.
2) with real weather that's quite possible. Set your altimeter to the correct pressure.
3) it's not a good idea in general unless you're flying VFR and don't plan to land anywhere else.

The number 29.92 indicates 29.92" of mercury of air pressure, which is a semi-arbitrary number chosen decades ago as the international "standard pressure".
If your air pressure at surface level is higher than that you're in a high pressure area, if it's lower you're in a depression.
Aircraft callibrate their altimeters to this number over a specific altitude (see 1) and to the actual pressure as reported by ATIS (automated weather reports) when under it.

[Thunderbird8]

Sorry to be such a noob but how do I set the pressure?

Tanks for your response Jwenting, always good to read your posts.

[jwenting]

there's usually a hotspot either on the digits indicating the pressure, or on a knob on the side of the gauge.

Or if you're lazy just press "B" and have FS set it for you

[Tim_A]

1. 29.92 is the barometric pressure, given in inches of mercury. If you look on a standard wall barometer, you'll see that it equates to "fair". In the UK, we adopted SI units (hectopascals/millibars) some years ago, and 29.92 inches is the same as 1013mb. Anything above this value is regarded as "high" pressure, and anything below it as "low". In the days before weather forecasts were dumbed down, you'd often see synoptic charts with pressure values on them.

It's a "magic" number in that it represents the air pressure at sea level in what is called an "International Standard Atmosphere - ISA. You'll often see aircraft performance figures quoted as "at ISA", which means 15°C and 1013mb at sea level. It's also the setting used to make the altimeter read "flight levels", something that Jeroen hinted at.

If you have FSX, you can make the sim use the correct units for the UK - Go to the General settings tab, and change the value from "U.S." to "Hybrid".

Now on to what ATC was saying. "altimeter 2992" is giving you a value to set in the little window (called the Kohlsmann window) on the altimeter. To do this, you rotate the knob next to it. (You can also press the "B" key, which will always set the altimeter to the correct pressure) The value given is such that your altimeter will read your altitude above sea level.

In Europe, again, we don't say "altimeter", instead we use what are known as "Q codes", that go back to the early days of Radio Telephony and Morse Code. So instead of hearing "altimeter 2992", in the UK you would hear "QNH 1013". Effectively it means the same thing. And FSX can be set to do that.

There comes an altitude where it's more important to be at the same altitude with respect to other traffic than to know your exact height above the ground (or sea). This is known as the "transition altitude". In the US it's 18,000 ft, in Australia, 10,000ft, in the UK, 3,000ft. FS uses 18000ft regardless of where you are. Above this altitude, you set the barometer to 1013/2992, and you are now flying "flight levels". So the altimeter is no longer telling you 18000ft, it's telling you "flight level 180".

2. Could be because you set your altimeter before the weather engine had fully kicked in. Or possibly you didn't set your altimeter at all, and just left it to read whatever. Either manually twist the knob so the altimeter reads the airfield elevation, or press the B key before departure (this is one of the standard "before take off checklist" items).

3. Remember those Q codes? What you're asking is called "QFE", and is quite common in the UK and much of Europe, although little used elsewhere. There's no magic button to set it in FS, you simply have to wind the altimeter knob until it reads zero. The only time you would do it on the ground is if you are flying circuits. If you are planning to leave the traffic pattern, you would always set QNH. In real life, Most GA airfields will only give you QFE if you are joining the VFR circuit to land; bigger airports will give QNH and only QFE if you ask for it. FS ATC doesn't use QFE at all, but if you fly online with VATSIM, you can be given the QFE.

[Thunderbird8]

Many thanks Tim, very useful indeed.

[InsyleM]

I am a bit confused about the basic rules for altimeter settings. I do all my flying in the UK at the moment:

1. Why does ATC say 'altimeter 2992' and what is the significance of this?

2. My altimeter when flying from Stansted to Heathrow said 700 feet lower than my 'actual' height (checked by using Shift-Z)

3. How do you set the altimeter to read 0 feet at aeordrome level? Why do this, is it even a good idea, is it best to do something else?

2992 = 29.92" Hg of Atmospheric Pressure. Standard day/Sea Level conditions are:
29.92" Hg and 15C / 59F
Naturaly as weather changes, so does the barometric pressure. To correct for this you enter the current pressure into the altimiter. Somedays it could be 29.85 or 31.02. This would make your altimiter read the elevation of your airfield. If the airfiend is 700' above sea level, your altimiter should read 700. Your pressure altimiter gives you your altitude in MSL Mean Sea Level. Your Radar Altimiter will use radio beams to detect how high off the ground you are. Some systems say 0 to -4 on the ground (To account for nose up landings with struts fully extended getting a reading of 0) When flying above 18, 000' (FL180) It's standard that EVERYONE use 29.92 reguardless of weather. Yes, you may be 700' lower from what you're altimiter says, but, whats more important is that EVERYONE will be 700' lower. So the vertical spaceing between you and oncoomming traffic will be the same.

" = Inches
' = Feet
RADAR = Radio Detecting and Rangeing
FL = Flightlevel
Hg = Atomic Symbol for Mercury
F = Ferenhight
C = Celcieus

[lnuss]

...In Europe, again, we don't say "altimeter", instead we use what are known as "Q codes", that go back to the early days of Radio Telephony and Morse Code. So instead of hearing "altimeter 2992", in the UK you would hear "QNH 1013". Effectively it means the same thing. And FSX can be set to do that.

That mechanism would be rather awkward in the U.S. (and many other countries), due to the rather considerable range of altitudes of the terrain. I live at 5,230 feet, and airports around Denver (and much of the Rocky Mountain west, plus many areas in other parts of the country) are at similar altitudes, or even higher (Leadville, CO is almost 10,000 feet). If you are near sea level, try setting your altimeter to 5200 feet. I don't think it has the adjustment range to allow that. In any case, it would take quite a while to rotate the knob that far, even if it adjusted that far.

I don't think that system would work very well in the Alps, either. But it's obviously doable where all fields are not too far from sea level.

[dobar]

2992 is the American system of altimeter pressure. When I operate the radio at Beccles (EGSM), I have to give pressure in millibars, the UK's system. 2992 is the International Standard Pressure at Sea Level, which equates to 1013.2 mb.

[Vasco2305]

I am a bit confused about the basic rules for altimeter settings. I do all my flying in the UK at the moment:

1. Why does ATC say 'altimeter 2992' and what is the significance of this?

2. My altimeter when flying from Stansted to Heathrow said 700 feet lower than my 'actual' height (checked by using Shift-Z)

3. How do you set the altimeter to read 0 feet at aeordrome level? Why do this, is it even a good idea, is it best to do something else?

Theres a way to know the air pressure without a altimeter

When the temperture is high and NO Clouds, the Air pressure is High
When the temperature is LOW and A Lot of Clouds and Rain then the Altimeter is Low