ICAO Annex 10, Volume II
5.2.1.5.8 The following words and phrases shall be used in radiotelephony communications as appropriate and shall have the meaning ascribed hereunder:
UNABLE “I cannot comply with your request, instruction, or clearance.”
Note.— UNABLE is normally followed by a reason
- RVSM stands for Reduced Vertical Separation Minima. RVSM relies on accurate instrumentation, navigation, as well as good height-keeping. If the pilot is unable to comply with an ATC requirement, he/she should report "UNABLE"
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