Definition
Atmospheric conditions in which the actual sea-level pressure differs from the standard value of 29.92 inches of mercury (1013.2 hPa). When pressure is lower than standard, the altimeter indicates a higher altitude than the aircraft is actually flying; when pressure is higher than standard, it indicates a lower altitude. Pilots compensate by setting the current local altimeter setting in the Kollsman window.
Plain English
The real air pressure outside is different from the textbook value the altimeter assumes. Until the pilot updates the altimeter setting, the altitude shown on the instrument will not match the aircraft's true height above sea level.
Context Anchor
Seen in altimeter error discussions, especially when weather systems or cold air make actual pressure different from the standard reference used by the instrument.
Derivation
"Nonstandard" simply means "not matching the agreed standard." Aviation uses a fixed standard atmosphere (29.92 inHg at sea level, 15 °C) as a common reference. Any real-world pressure that differs from that fixed reference is called nonstandard.
Why Pilots Care
These conditions cause the altimeter to read higher than actual altitude in cold air, creating a risk of controlled flight into terrain if uncorrected.
Grounding Statement
An altimeter is estimating height from air pressure, so if the pressure pattern is unusual, the height estimate can be off.
Intuition Check
Do not read nonstandard as “bad” or “not allowed.” Here it means “different from the standard atmosphere used as a reference.”
Example Sentence 1
Because of nonstandard pressure conditions along the route, the pilot updated the altimeter setting each time ATC issued a new value.
Example Sentence 2
Nonstandard pressure conditions caused the altimeter to indicate 400 feet higher than the airplane's true altitude over the runway.