density altitude and True altitude
 

Hi there,

Could someone help me solve this question/calculation?
I am not sure I am doing it right, most probably I am not :)

QNH 981 station at MSL
temperature = minus 14 ISA,
what is your true altitude at FL090?


standard pressure = 1013
1013 - 981 = 32
32 times 30(some use 27) = 960

Pressure is lower so ground is closer.
FL090 = 90.000 ft
90.000 ft - 960 ft = 89.040 ft

So pressure altitude is 89.040 ft, correct??

Then for true altitude

we have minus 14 ISA, so:
120ft x -14 = -1.680 ft

89.040 ft - 1.680 ft = 87.360 ft

FL 090 IS 9000FT,NOT 90000.
9000-960=8040
TEC=4ft/1000/8C- 4X8X14 =448.
8040-448= 7592 FT.....

Hello Fly.b737
FL090 = 9000ft indicated, with 1013.25hpa in your altimeter window. Now remember the rule "from high to low(pressure) and from warm to cold(temperature) => look below"
So your pressure corrections & your temperature corrections have to be subtracted from 9000ft with a QNH 981 and an ISA -14°C atmosphere
giving 9000ft - 960ft - 448ft = 7592ft true altitude

Density altitude(used for performance calculations, wings & engines only "know" density altitudes) = pressure alt, corrected for non standard temperatures
Density alt. = press.alt 9000ft +120(delta ISA) = 9000 + 120(-14) = 9000 - 1680 = 7320ft
BM