Definition
The difference between the altitude an altimeter displays and the aircraft's true pressure altitude. In RVSM operations, total altimetry system error must remain within tight limits (typically ±200 feet under standard conditions) so that aircraft flying 1,000 feet apart vertically remain safely separated.
Plain English
How far off the altimeter's reading is from the actual altitude. A small error is normal; in airspace where aircraft are stacked closely, that error has to stay very small.
Context Anchor
Seen in Reduced Vertical Separation Minimum (RVSM) operations, where aircraft at high altitudes are allowed to fly with less vertical space between them and must keep very accurate altitude information.
Derivation
From 'altimetry,' meaning the measurement of altitude (Latin altus, 'high,' plus the Greek -metria, 'measurement'), combined with 'error,' meaning a deviation from the true value. Together: how far off your altitude measurement is.
Why Pilots Care
RVSM requires total altimetry error to stay within strict limits; exceeding them forces descent to conventional separation airspace.
Grounding Statement
If the altimeter says the aircraft is at 35,000 feet but the aircraft is actually at a slightly different pressure altitude, that difference is altimetry error.
Intuition Check
Do not assume “error” means the pilot made a mistake. In this context, altimetry error can be a system or measurement error even when the pilot set and read the altimeter correctly.
Example Sentence 1
Before entering RVSM airspace, the operator must show that the aircraft's altimetry error stays within the required limits.
Example Sentence 2
A large temperature deviation can increase altimetry error by altering the relationship between pressure and true altitude.