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An aircraft is cruising at FL130. ATC issues a climb clearance to FL150. This new level is incompatible with the aircraft's performance. According to SERA, the crew will respond to ATC clearance using the word...
  • A
    REFUSED.
  • B
    IMPOSSIBLE.
  • C
    CANNOT.
  • D
    UNABLE.

Refer to figure.
Learning Objective 010.07.02.05.02: Describe what a PIC should do if an ATC clearance is not suitable.
Source: ICAO Doc 4444, Chapter 4, 4.5 Air traffic control clearances, 4.5.1 Scope and purpose.

Whilst there is a large section of SERA (The Standardised European Rules of the Air) about ATC clearances, SERA.8015, this one is simple to remember.

When ATC issues something that you cannot do, whether that be your aircraft, your current flight rules, or the priveleges of your license, etc., you should reply with "UNABLE". It is usually required to have a very short explanation for why you are "UNABLE", such as, "UNABLE as VFR only" if given an IFR clearance or one into cloud.

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