Altimetry queries

[ac hoon]

Hi everyone,

There's a couple of altimetry questions that are driving me crazy. Please help!!

A pressure altimeter indicates:

a. actual altitude above MSL
b. actual altitude above the terrain below
c. the distance between two isobaric surfaces in the actual atmosphere
d. the distance between two isobaric surfaces in the standard atmosphere

I chose (d) since the altimeter works on the principle of pressure difference in ISA conditions. (or am I wrong?)
Answer was (c)


At the official level of a specific airport an aircraft altimeter, set at QNH for the airport, is expected to give correct indications of:

a. the elevation of the airport, but only in a standard temperature
b. the elevation of the airport, regardless of temperature

Again chose (a) for the same reason.
Answer is (b)


Basically these questions come down to my lack of understanding of what conditions the altimeter works under (ISA or not).

If is does work under ISA conditions, why would the altimeter always indicate the elevation of the airfield when QNH for the airfield is dialled up on the ground, if the temperature is hotter or colder than ISA?

Just have to get these questions out of my system.

[silverknapper]

Not that I have a clue but until someone who knows what they are talking about comes along I may as well chip in - it'll cause an argument if nothing else and my Met revision is boring me!

You talk of ISA conditions. Remeber that ISA is not only a temperature but also a pressure. When we pass the transition altitude and dial up 1013, that is when you have the mean altitude if it were a standard day. Until then you work with the air pressure that day. Were it a standard day then you wouldn't change anything on the altimeter at the TA.
QNH - remember Nautical Height (thanks Bristol) gives the height of the airfield on any day. As temp and pressure change you have to tell the altimeter this, or the height of the field would 'change' every day.
Guess what I'm saying not very well is that it works in normal conditions that you input, and above the transition laevel works on ISA.

SK

[FlyingForFun]

For the first question, I agree with you - answer is D, for the reasons you say.

For the second question, the answer is B. QFE is defined as the pressure at the highest point on the airfield. QNH is defined as the QFE, reduced to sea level pressure, assuming ISA changes in pressure with altitude. What this means is that, since your altimeter also assumes ISA changes in altitude, the altimeter will indicate the airfield elevation when on the airfield. However, if you fly to sea level with QNH set, your altimeter will not read exactly zero unless the temperature is as per ISA.

Compare QNH with (erm, help, can't remember the Q-code - I think it's) QFF (but I'm sure someone will correct me if this is wrong). This is also the QFE reduced to sea level pressure, but this time you reduce it to sea level using the actual temperature. If you set this value on your altimeter, then dug a big hole and went down to sea level, your altimeter would read zero. As you climb back to airfield elevation, the rate your altimeter increases will depend on the ambient temperature. So by the time you reach the airfield, unless the temperature is as per ISA, the altimeter won't read the correct elevation. Hence this setting is not very useful for us pilots - but it is useful for the met guys, and this is actually the pressure which they show on their pressure charts.

Hope that makes sense!

FFF
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[kitwe]

(Simple) Altimeters measure pressure above the datum set on the millibar scale. Altimeters are calibrated to ISA conditions which assume, among other things, that the sea level tempreature is +15 degrees C. If the actual surface temperature is not +15, then the altimeter will read incorrectly.

If the temperature is less than +15 (as happens in UK much of the year), then the altimeter will over-read. You can work out the error by a simple calculation: approximately 4 feet per thousand feet (indicated altitude) times the number of degrees difference from +15 (ISA). e.g. cruising at 2000 feet on QNH, with a surface temperature of zero degrees, produces an error of 4x2x15 =120 feet. Your altimeter over-reads by 120 feet, therefore you are really flying at 1880feet.

[FlyingForFun]

Kitwe,

Your example is only correct if the surface elevation is 0 feet. This is the same mistake which ac hoon made in answering the second question.

If the surface elevation is (to use the numbers from your own example) 2000', and you have the QNH correctly set, your altimeter will read 2000' when you are at 2000'. This is essential to flight safety - your altimeter must read the surface elevation when you are on the surface!

The rule of thumb you give for the altimeter error actually gives the error in the height, not the error in the altitude, if QNH is set.

FFF
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[flystudent]

For both of them just think how do I want it to work. Then you get the 2 correct answers

Would be useless if the varied with temperature, or just ISA standard as it hardly ever is ISA standard.



Regards

[ac hoon]

OK guys, thanks for the help.

So basically, the altimeter must indicate the aerodrome level when on the ground, and QNH is worked out by reducing QFE to MSL using ISA conditions. Which would give the answer of "regardless of temperature" to the second question.:ok:

[No. 2]

ac hoon - You seem to have a duplicate thread going and I responded to the other one! Just in case it's of any help, here is what I put:

Regarding your second question. QNH is calculated by measuring surface pressure and adjusting to sea level based on ISA temperature deviation, and not actual temperature deviation. This is so that when you dial QNH into the altimeter, because it reads relative to ISA conditions, you will get airfield elevation regardless of the actual temperature deviation. This is deliberate so that you can use QNH for instrument precision approaches.

My understanding of the first - it says 'isobaric' surfaces, meaning surfaces with equal pressure. If you had two columns of air at ISA conditions then both surfaces would be at equal height, but if there was a temperature difference then the 'hotter' one would be higher. I'll wait for someone else to give a proper explanation of this as I'm not sure myself if I fully understand the question.

No. 2

[kitwe]

FFF,

You are quite correct about the airfield QNH. My example referred to a departure from a sea-level airfield where the altimeter had been zeroed before take-off and the pressure setting had not been changed. The temperature error in this case would have been zero before departure but would be as I quoted in my example. If you set Airfield QNH on the ground (no matter what is its elevation), your altimeter should read airfield elevation. If, however, you climb above the airfield, your altimeter will only indicate correctly if the temperature is ISA. If the temperature is below ISA, the altimeter will over-read.

Regards

[FlyingForFun]

Ok, so it seems like we all agree on the second question - good! :ok:

But what about the first question? Anyone got any ideas why ac hoon's source suggests answer c? Should definitely be answer d as far as I can see, and nothing anyone has written here has suggested otherwise.

FFF
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[No. 2]

It's the mention of isobaric surface that confuses me. In ISA condtions there would be no altitude indidcated between two isobaric surfaces since they have the same pressure. Or am I reading too much into that bit:confused:

[ac hoon]

I think "the distance between two isobaric surfaces" just means the distance between two pressure surfaces.

I would like to think the answer they've given is wrong.

[No. 2]

In that case I would agree with you entirely. BTW-where did you get the answers from in the first place?

No. 2

[ac hoon]

I think "the distance between two isobaric surfaces" just means the distance between two pressure surfaces.

I would like to think the answer they've given is wrong.

btw sorry about posting everything twice, i'm a rank amateur at this. Doing the ATPL course thru Atlantic Flight Training, was one of the questions in the altimetry section.

[kitwe]

I tend to agree that 'd' is the correct answer to the first question. Like most, I am a bit confused by the term 'isobaric surfaces' (Must ask the met lecturer tomorrow!) but, assuming that the term means the distance between 2 pressure surfaces, the altimeter indicates height/altitude etc of (usually) one above the other, calibrated to ISA conditions. In other words, once you have set whatever datum pressure is in use, the altimeter makes all its calculations based on ISA pressure changes, regardless of the actual atmospheric conditions.

[batty]

If temperature has no effect on the altimeter readings why do you have to make a correction the the altimetre reading when flying into very cold places?:confused:

[No. 2]

batty - Temperature doesn't have an effect on the altimeter reading as you say. However, it does have an effect on the pressure distribution in the atmosphere. So when you fly into a cold region the pressure is lower that it would be in ISA conditions and you're altimeter thinks it's higher than it is, because it is reading relative to ISA conditions.

No. 2

[batty]

Ummm sounds like temp effects it to me then....
I do realise that the altimeter just uses pressure as a source