It is an official term, in the pilot/controller glossary
http://www.faa.gov/atpubs/PCG/U.HTM



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



"Steven P. McNicoll" wrote in
message
k.net...
|
| "Jim Macklin" wrote
in message
| news:3n_Yf.8891$t22.4772@dukeread08...
|
| The PIC can always say UNABLE, a magic word to
controllers.
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|
| How so?
|
|