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While cruising level, severe turbulence affects the aircraft’s capability to maintain the height-keeping requirements for RVSM. How should the pilot inform ATC?
  • A
    “Not RVSM any more due to turbulence”
  • B
    “RVSM not approved due turbulence”
  • C
    “Unable RVSM due to turbulence”
  • D
    “Negative RVSM”

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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