A small UA relying on a barometric altimeter is set correctly at the departure point. During the flight, it moves from an area of high atmospheric pressure into an area of significantly lower pressure without the altimeter setting being updated. What is the effect on the aircraft's true altitude compared to its indicated altitude?
- AThe altimeter automatically compensates for pressure changes, so no error occurs
- BTrue altitude will be higher than the indicated altitude, increasing obstacle clearance
- CIndicated and true altitude will remain identical throughout the flight
- DTrue altitude will be lower than the indicated altitude, increasing the risk of colliding with obstacles
Show answer & explanationAnswer & explanation
Correct answer: D. True altitude will be lower than the indicated altitude, increasing the risk of colliding with obstacles
This follows the classic aviation memory rule 'high to low, look out below.' When flying from high to low pressure without resetting the altimeter, the instrument continues to read based on the old, higher pressure setting, causing the true altitude to actually be lower than what is indicated — reducing real obstacle clearance and increasing collision risk.
Why the other options are wrong
- A. Barometric altimeters do not automatically correct; they require manual setting updates from a known source.
- B. This reverses the actual relationship; moving to lower pressure makes true altitude lower, not higher.
- C. Without a setting update, indicated and true altitude will diverge, not remain equal.
Altimeter Pressure Errors
An altimeter reads true altitude only if its setting is updated to current local pressure; flying into lower pressure without updating causes the aircraft to actually be lower than indicated.
- 'High to low, look out below' — true altitude is lower than indicated
- 'Low to high, altimeter reads high' means true altitude is actually higher
- Cold temperatures also cause true altitude to be lower than indicated
Memory trick: 'High to low, look out below' — flying into lower pressure puts you lower than you think.