FAA Instrument Flight Instructor (FII)Instruments and SystemsEasy
A pilot departs an airport where the altimeter setting is 30.42 and flies toward a destination reporting 29.85 without ever resetting the altimeter. If the pilot maintains a constant indicated altitude, the aircraft's true altitude will be
- Alower than indicated
- Bhigher than indicated
- Cimpossible to determine without temperature data
- Dthe same as indicated
Show answer & explanationAnswer & explanation
Correct answer: A. lower than indicated
The mnemonic 'high to low, look out below' applies: flying from a region of higher pressure (30.42) to lower pressure (29.85) without updating the altimeter setting causes the altimeter to over-read, meaning the true altitude is actually lower than the indicated altitude.
Why the other options are wrong
- B. This would occur going from low pressure to high pressure, the opposite scenario.
- C. Temperature affects the error too, but pressure alone already establishes the direction of error described here.
- D. True altitude only equals indicated altitude when the setting is current and accurate.
Altimeter Pressure Error
Failing to update the altimeter setting when flying from high to low pressure causes the true altitude to be lower than indicated, creating a hazardous situation near terrain.
- 'High to Low, Look Out Below'
- 'High to Low or Hot to Cold, look out below' includes temperature too
- Always update altimeter setting when crossing pressure regions
- Reverse situation (low to high) causes true altitude higher than indicated
Memory trick: High to Low, Look Out Below!