**Calculating Altimeter Error**

Assuming a standard lapse rate, an airplane’s altimeter will over or under read by 4 ft per 1°C deviation from ISA per 1,000 ft above the station reporting the altimeter setting.

i.e. Altimeter error = 4 ft per 1°C deviation from ISA for every 1,000 ft

The above formula will help to solve following questions:

” You are planning to fly across a mountain range. The chart recommends a minimum altitude of 12000 feet above mean sea level. The air mass you will be flying through is an average 10 deg C warmer than ISA. Your altimeter is set to 1023 hPa (QNH of a nearby airport at nearly sea level). What altitude will the altimeter show when you have reached the recommended minimum altitude? “

Answer: 11,520

Explanation:

Altimeter error = 4 ft per 1°C deviation from ISA for every 1,000 ft

(i.e. 0.004 ft per 1°C deviation from ISA for every 1 ft)

The airmass is 10 deg warmer than ISA so 4 x 10 = 40

The error is 40 feet per thousand feet and for 12000 feet the error will be 40 x 12 = 480 feet.

Now its a matter of adding or subtracting 480 feet from 12000 feet.

Since the airmass is warmer than ISA the true altitude will be greater than indicated altitude.

So if we plan to fly at the minimum (true) altitude of 12000 feet to clear the mountains the altimeter will show (indicate) less than true i.e. 12000 – 480 = 11,520 feet.