Calculating actual altitude?

[guest33]

Hey all, Can you help me with this question?

Given an indicated altitude of 10,000', and an actual OAT of -20 degrees C, you set your altimeter to local station setting of 29.62". If the station elevation is 2,500' what is your actual altitude?

How do I figure this out?

Thank You.

[Pugilistic Animus]

Hey you have to use a flight computer/ chart--but remember the altimeter setting is for MSL so the station elevation need to be known as you lose one inch for every 1000'----your E6B [electronic or manual] has windows to plug in your parameters--just carefully read the instructions!!! to get true altitude


PA

[Sir Richard]

Temperature error is about 4% per 10c above/below ISA of the air layer beneath the aircraft.

In your example the OAT is -20, ISA at 10000ft should be -5 so the air mass is ISA minus 15 (assuming linear increase, airfield temp -5, MSL -0)

Indicating 10000ft, actual height AMSL 9400ft so your height above the airfield would be 6900ft.

If the air mass is 15c warmer than ISA your actual height would be similarly higher (+600ft) but that is not a problem as the Cumulo Granite is further away !

Computer not required !

[BOAC]

WHOA! - what you say, Sir R, is correct if you are using a sea level datum pressure such as area QNH or 1013.2/29.92. In fact, since Station QNH is ALREADY corrected for ISA deviation (by the clever met observer and his/her abacus), the vertical airspace we need to consider for OUR error is HEIGHT above datum, ie 10,000-2,500=7,500ft, to which the 4%/10 on the -15 error should be applied. Thus the answer is 9,550' (You are, of course, safe!). Also, for guest33, be careful as the question asked for your actual altitude (not 'height'), so do not take the station elevation OFF the 9,550. NB also the other 'catch' for the unwary in these dreaded questions is when they try to 'trick' you by telling you the Station QNH is 29.92in. Same answer.

Of course, in practical cockpit terms most of us do it your way for simplicity - and to err on the safe side (Dead sea and AMS excluded)

[Sir Richard]

Thanks BOAC, I must learn to RTFQ !