ATPL Forum – Personal Section Practice Test

To determine the outside air temperature (OAT) at flight level 300 (FL300) in an air mass that is 15 degrees warmer than the International Standard Atmosphere (ISA) at that altitude, it is crucial to first understand the standard temperature profile.

At FL300, the standard temperature is approximately -30 degrees Celsius. This standard profile indicates that as altitude increases, temperature generally decreases in a linear fashion up to the tropopause. Given that the air mass is 15 degrees warmer than ISA at this altitude, we add 15 degrees to the standard temperature of -30 degrees Celsius.

-30 degrees + 15 degrees results in -15 degrees Celsius. Thus, in an air mass warmer than ISA by 15 degrees, at FL300, the likely OAT would indeed be -15 degrees Celsius.

The chosen option reflects the calculations based on the standard temperature lapse rate and the adjustment for the additional warmth in the air mass.