About the altimeter setting
 

Hey guys I have come across with one question about altimeter setting in my textbook:

Altimeter setting is the value to which the scale of the pressure altitude is set so the altimeter indicates:

A- true altitude at field elevation
B- pressure altitude at field elevation
C- pressure altitude at sea level

I wonder why A should be the answer.......I know that B and C should be wrong as pressure altitude refers to the altimeter setting when it is set to 29.92
However why A should be the answer???
Assume when a local altimeter setting is set, the altimeter should read 0, instead of the true altitude at field elevation, right?

Thanks in advance!:):):)

Your choice - you can choose to set QFE (it reads zero) or QNH (it reads true altitude of the field). If the question doesn't specify which, then it is unanswerable.

It sounds like rather a daft textbook! Or perhaps American, where QFE is probably unknown, so 'altimeter', (when below 18000 ft) invariably means QNH.

If an altimeter subscale is set to the local QNH value at an aerodrome reference point, then it should indicate the true elevation of that point.

I lately saw some current EASA course material and was surprised to find they talk about QFE and QNH in text parts, but use unspecific "altimeter setting" in questionnaires. From the answers one can see that QNH is meant unless explicitly QFE is stated in the question. I find this rather bad.

... and today I was actually given (without having asked for it, because I never do) a regional pressure setting, which would be "none of the above".

A confusing and obscure question but I think the answer is probably 'A' because it includes the term "True". There's supposed to be a temperature correction in there somewhere.

If you keep getting QFE and QNH mixed up, just remember


QNH.... Nautical Height
QFE.... Freshold Elevation

Since "hinhin" has not chosen to give his location, the choice of 2992 suggests USA. Q codes don't mean nuffink to them furriners.

Russ.

QFE...Field Elevation

Aren't people missing the point? An altimeter is a pressure instrument and depending on what is set on the subscale, will read pressure heights relative to that setting. In the UK we are supposed to fly cross country (when below the transition altitude) with a regional QNH set. This is a value which should ensure we are always no lower, and usually a bit higher than the reading on the altimeter so we don't fly into things. The altimeter is reading a pressure altitude, not a true altitude.

In Class G airspace you can fly on any altimeter setting you want (unless complying with the semicircular rule). It makes a lot more sense to fly on the QNH of a nearby airfield for a number of reasons, one of them being that the base of controlled airspace (when an altitude) is with respect to the controlling aerodrome QNH. The threat of flying on regional QNH below controlled airspace is that you might infringe the airspace as by definition the regional QNH is the lowest pressure in an area.

fireflybob Point taken about controlled airspace although not all bases are set on airfield QNHs. I was thinking about safety issues when flying IMC, and I accept that we could probably abandon regional pressure settings without a problem. My main point is that the altimeter is a pressure instrument that measures altitudes (or height) assuming a 'standard atmosphere'. It does not, and cannot, measure true altitudes. GPS makes a reasonable attempt at doing this but has to make assumptions about the shape of the earth which will not always be right. I'm not sure whether B or C is the correct answer to the OP, but I don't think A is the right answer unless, very specifically, the altimeter is set to an airfield QNH. The altimeter should then read a true altitude when, and only when, the aircraft is on the ground at that airfield. At all other times it will give a pressure altitude which is usually going to be different from any true altitude.

A- is wrong as true altitude is pressure altitude corrected for temperature.

B- is wrong as the altimeter would indicate airport elevation.

C- correct

That would make the elevation of every airfield in the world zero feet wouldn't it?

CG

Or 1013 so it reads flight levels

No, only those at the same altitude as the field where you set it, assuming the barometer reads the same.

While you're reading this there's a good chance that a few GA aircraft in the UK are flying with incorrect altimeter settings - they departed the circuit and forgot to set QNH or they rejoined and are flying the pattern height at altitude instead of height because they haven't set QFE. Every time an altimeter sub-scale setting is changed there's also the chance it'll be mis-set.

The UK altimeter setting protocol and transition altitudes were formulated many decades ago. Regional settings were intended to provide an approximate QNH when it was not so easy to get reliable updates of local QNH. The climb performance of the average airliner meant that several minutes would elapse after take-off before TA was reached. Nowadays there are plenty of en route sources offering current QNH and many airliners, especially twins, can reach TA much more quickly. As another poster has mentioned, flying on regional settings might lead to airspace infringements.

Every time a low pressure weather system crosses the UK the CAA have to issue panicky notams reminding pilots of the vital importance of altimeter resetting at TA.

The Americans have got it right - just one setting for altimeters below transition altitude. I understand that - at long last - the UK CAA (and EASA?) are consulting on raising TA to 18,000 feet. Long overdue!

More on the topic of altimeter setting here.

Negative.

When on the ground at the airfield to which the QNH applies, the altimeter should be reading the true altitude above mean sea level.

At any point above the surface, then the altimeter will be reading a form of pressure altitude, based upon that reference.

So A is correct.

I think that BEagle is also correct - this sounds like an American textbook, where they don't use QFE, and call QNH "Aldimeeder", spelled "altimeter" and being in the dark ages, declare it in inches of mercury.

Anywhere else in the world (except possibly Canada, who have a bad habit of copying their neighbours without thinking about it properly) it would be QNH set in hPa or mb (which are the same thing).

G

3wheels said:

I said:

You said:

Too subtle obviously...

I was making the point that setting QFE does NOT read the field elevation, it reads zero!

Aw, c'mon guys...it's American...!

Substitute 'Altimeter' with "Al Timmida" and all will become 'clear'...

:cool: