Going through Bristol's first MET progress test and this popped up:
An aircraft with the airfield QNH correctly set and flying at an indicated altitude of 2000ft is approaching ground of 1500ft elevation. The OAT at 2000ft is minus 9ºC. At what true altitude will the aircraft overfly the ground?
a)2020 ft
b)1520 ft
c)1980 ft
d)1960 ft
Answer is c, and I know I hate MET but I get the answer as 340ft
My logic:
ISA @ 2000' = 15 - (2 x 2 )= 11 deg
ISA is 20 degrees different from OAT (11 + 9)
Therefore, as there is 4% height difference for every 10 deg temperature difference.... = 8%
8% off 2000= 160ft.
True alt = 2000-160= 1840ft.
Question asks height a/c will fly over ground> 1840-1500 = 340 ft.
I know it's late but what am I doing wrong?
HB