FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsHard
An airplane is flying at a pressure altitude of 5,000 feet. The pilot fails to update the altimeter setting as the airplane flies from an area of high pressure into an area of significantly lower atmospheric pressure. The current altimeter setting is 30.42 inHg, but the actual local altimeter setting has dropped to 30.12 inHg. Assuming the altimeter indication is unchanged, approximately how does the airplane's true altitude compare to the indicated altitude?
- ATrue altitude is about 300 feet higher than indicated
- BTrue altitude is about 30 feet lower than indicated
- CThere is no difference because pressure changes only affect airspeed indications
- DTrue altitude is about 300 feet lower than indicated
Show answer & explanationAnswer & explanation
Correct answer: D. True altitude is about 300 feet lower than indicated
Each 0.01 inHg of pressure change corresponds to approximately 10 feet of altitude, so 1.00 inHg equals approximately 1,000 feet. The pressure difference is 30.42 − 30.12 = 0.30 inHg, which equals about 300 feet. Since the airplane flew from high pressure to low pressure without resetting the altimeter, the true altitude is lower than indicated — remember the rule 'high to low, look out below.'
Why the other options are wrong
- A. This reverses the direction; flying into lower pressure without adjustment makes true altitude lower, not higher.
- B. This understates the error by a factor of ten.
- C. Pressure changes directly affect altimeter indications, not just airspeed.
Altimeter Setting Error
Flying from high to low pressure without resetting the altimeter causes the airplane's true altitude to be lower than the indicated altitude, following the rule 'high to low, look out below.'
- 1 inHg of pressure change ≈ 1,000 feet of altitude error
- High to low pressure without resetting = true altitude lower than indicated
- Low to high pressure without resetting = true altitude higher than indicated
Memory trick: High to low, look out below!