Definition
A flight plan remarks entry used in the ICAO flight plan format to indicate that an aircraft is not approved to operate in airspace where Reduced Vertical Separation Minimum (RVSM) applies. RVSM airspace, generally between FL290 and FL410, requires aircraft and crews to be specifically certified to maintain 1,000 feet of vertical separation. Filing STS/NONRVSM tells ATC that the aircraft cannot legally fly within this airspace and must be routed around or below it.
Plain English
It's a note added to a flight plan that tells air traffic control the aircraft is not approved to fly in the high-altitude airspace where planes are kept just 1,000 feet apart vertically. The controllers will then keep the aircraft below or outside that airspace.
Context Anchor
Seen in ICAO flight plan entries, especially when planning a flight that may involve high-altitude airspace where RVSM rules apply.
Derivation
STS stands for 'status' -- a standard ICAO flight plan field used to flag special handling. NONRVSM is simply 'non' (not) plus RVSM (Reduced Vertical Separation Minimum). Together it reads as a status flag meaning 'this aircraft is not RVSM-capable.'
Why Pilots Care
Without RVSM approval the aircraft is restricted to altitudes below FL290 or above FL410 in RVSM airspace, reducing available cruising levels and potentially increasing fuel burn or routing constraints.
Grounding Statement
If the airplane is not approved for RVSM, this code flags that limitation before ATC assigns altitudes in RVSM airspace.
Intuition Check
Do not read STS/NONRVSM as permission to enter RVSM airspace. It is a notice to ATC that the aircraft is not RVSM-approved and may need special handling.
Example Sentence 1
Because the older twin wasn't equipped for high-altitude precision altimetry, the dispatcher added STS/NONRVSM to the flight plan so ATC would route it below FL290.
Example Sentence 2
ATC assigned a non-RVSM altitude after noting the STS/NONRVSM status on the flight plan.