Altimetry queries
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.