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.

[No. 2]

ac hoon -

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