This question is far simpler than it looks. An airport at sea level has an approach with a DA of 3500 ft (way higher than usual due to the terrain), and must be temperature corrected when the OAT is below -5°C; the current OAT is -10ºC, calculate the corrected DA.

Cold temperature corrections exist because barometric altimeters are calibrated for ISA conditions, and when the air column is colder than ISA, the pressure drops faster with altitude, and the altimeter will over-read. This over-reading makes the pilot think they are higher than they actually are, so reduces terrain clearance.

This means that cold temperature corrections are additions to the altitudes we fly, so we know we are actually at the intended true altitude, even with an over-reading altimeter.

The cold temperature correction is made from ISA to the actual OAT, and can either be done using a table, or using a calculation as follows:

• To determine the true altitude/height, the following rule of thumb, called the ‘4 %-rule’, shall be used: the altitude/height changes by 4% for each 10°C temperature deviation from ISA. For simplification: Height correction for the temp = 4 × (ISA DEV) × Indicated alt/1000 = _____ ft
• ISA Deviation from 15ºC to -10ºC is ISA - 25°C
• The indicated altitude for the correction is from sea level to 3500 ft.
• Height correction = 4 x 25 x (3500/1000) = 350 ft

The corrected Decision Altitude is 3500 + 350 = 3850 ft