Cannot Comply with an ATC Clearance: Unable



When you cannot comply with an ATC clearance, the magic word to use is “unable”. Simply saying “unable”, and nothing else, might not get you completely off the hook with ATC. I’ll explain why, and what to do about in this month’s edition of IFR Flight Radio.

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From the Aeronautical Information Manual’s Pilot/Controller Glossary:

UNABLE− Indicates inability to comply with a specific instruction, request, or clearance.

§ 91.123 Compliance with ATC clearances and instructions.
(a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. (Ed. Note: More follows but is irrelevant to this discussion.)

§ 91.185 IFR operations: Two-way radio communications failure.

(c) IFR conditions. If the failure occurs in IFR conditions, or if paragraph (b) of this section cannot be complied with, each pilot shall continue the flight according to the following:

(1) Route.

(i) By the route assigned in the last ATC clearance received;
(ii) If being radar vectored, by the direct route from the point of radio failure to the fix, route, or airway specified in the vector clearance;
(iii) In the absence of an assigned route, by the route that ATC has advised may be expected in a further clearance; or
(iv) In the absence of an assigned route or a route that ATC has advised may be expected in a further clearance, by the route filed in the flight plan.




I have examples of how to apply AVEF in my book, Radio Mastery for IFR Pilots available at

Your Question of the Week:

If you have ever departed IFR from an uncontrolled airport, ATC probably gave you a Clearance Void Time. The controller said to you, for example, “Void if not off by 10:15, time now 10:01 and one half. Basically, this meant ATC had approved your takeoff any time before the Clearance Void Time on 10:15 Zulu.

Here’s your question. What is a “Release Time” and how might ATC use it for your flight?

I’ll have the answer to that question, along with more topics to help you work with ATC while flying IFR, in the next edition of the IFR Flight Radio Show.

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