Why “Fly to low, watch below ; High to cold, watch below” ?

Ref Ace the technical pilot interview 2/E

Ans:

Always remember :

aneroid barometer is working based on MSL pressure (i.e pressure height) !

1.  FL formula

Flight Level (Pressure Height) in feet = Height @ QNH + (1013 – QNH) * 30

Example:

Now say 300 feet @ 1025

PH = 300 + (1013 -1025)* 30 = -60

i.e when the area pressure 1023 , we fly at 300 feet above ground (with PH = -60 )

Now, we fly to an area with pressure 1013, then calculate again

PH = -60 = height + (1013 – 1013)* 30

-60 = height + 0

height = -60

That is, the actual height of the aircraft become -60 feet ! (hit the ground)

So, high to low, watch below

2.  feet, temp and altitude pressure equation

feet = 96 * (absolute temperature / altitude pressure)

Notes : this is just a formula found online, need to be checked for the correctness!

Example:

say 15C and 1013 mb, feet = 96*(15+273/1013) = 27.3 ft

when fly to 1 C and feet no change, then

27.3 = 96 *( 1+273 / altitude pressure)

0.2843 = (274/altitude pressure)

altitude pressure * 0.2843 = 274

altitude pressure = 963.77

That is from 15 C to 1 C area , same height (27.3 ft) , it experiences altitude pressure drop

So, hot to cold = high to low => Watch below~

Another more easy way, in fact, we can observe it from error rectification table:

https://skybrary.aero/articles/altimeter-temperature-error-correction

When To Apply Corrections for temperature error?

As a general rule, when the aerodrome temperature is 0°C or colder, the temperature error correction must be added to all published minimum IFR altitudes