Definition
The minimum levels of accuracy that an aircraft and its altitude-measuring and altitude-holding equipment must meet in order to maintain a precisely assigned altitude within tight tolerances, particularly for operations in Reduced Vertical Separation Minimum (RVSM) airspace where aircraft are separated by 1,000 feet vertically between FL290 and FL410.
Plain English
The rules that say how accurately an aircraft must be able to hold its assigned altitude. To fly in airspace where planes are stacked closer together vertically, the aircraft and its altitude instruments have to prove they can stay on altitude very precisely.
Context Anchor
Seen in RVSM discussions, especially when checking whether an aircraft is approved and equipped for high-altitude operations with reduced vertical spacing.
Derivation
Altitude comes from the Latin altus, meaning high. Keeping means holding or maintaining. In this term, the idea is not just being high, but being able to maintain a high-altitude assignment accurately enough to meet a required standard.
Why Pilots Care
Ensures the aircraft can operate safely with the reduced 1,000-foot vertical separation used in RVSM airspace.
Grounding Statement
In RVSM airspace, even a small altitude error matters because another aircraft may be legally flying only 1,000 feet above or below.
Intuition Check
Do not read this as a general measure of how carefully the pilot flies. Here, it means a specific approval standard for the aircraft and its altitude-holding systems.
Example Sentence 1
The operator could not dispatch the aircraft above FL290 because it had not yet been certified to the altitude-keeping performance standards required for RVSM.
Example Sentence 2
Maintenance confirmed the altimeter met altitude-keeping performance standards for continued RVSM operations.