Sometimes when you call air traffic control–usually clearance delivery or ground control–for an IFR clearance based on a filed IFR flight plan, ATC responds, “Clearance on request.”
In ATC-speak, the statement means that the controller doesn’t have your clearance immediately at hand. But he or she is tapping a keyboard or calling on a landline to retrieve it from the air traffic control system. The controller will call you back when your clearance is ready. This situation can occur if you call more than 30 minutes before your proposed (estimated) time of departure, but there may be other reasons why your flight “strip” isn’t immediately available.
I confess that “clearance on request” never confused me, but a long thread at BeechTalk, a popular forum for Beechcraft pilots, showed that several folks found the response puzzling. They thought it meant something like, “Reply when you’re ready to copy, and I’ll read your clearance.”
The discussion got me thinking about the wording. A colleague, Jeff Van West of Pilot Workshops, agreed that it had never confused him, but as we talked, he said the phrase was like a Rubin vase, an image that can appear either as two people in profile or as a vase, depending on how you look it at. The longer you stare, the more ambiguous the scene.
To my surprise, the commonly used phrase doesn’t appear in the AIM or P/CG or in the ATC handbook, Air Traffic Control (JO7110.65Y).
So I wrote the FAA, asking that “clearance on request” be explained. Here’s the initial response:
This office has carefully reviewed the matter. Although the phrase “Clearance on request”, does not appear in FAA directives, the Aeronautical Information Manual, or other FAA standards/procedures, this is by design, as the phrase is self-explanatory.
“Clearance on request” is in response to the pilot’s request, “I’m looking for my clearance to xxx”, “Request for clearance on Nxxxxx”, “This is N12345, requesting clearance,” etc. Although not regulatory, the controller’s response, “Clearance On Request”, “On Request”, “I have your request”, “Standby”, “I’ll be right back with you”, etc., are all phrases that are self-explanatory, accomplishing the same task. The controller proceeds to process the pilot’s request as higher priority duties allow.
Accordingly, this office does not believe the phrase “Clearance on request” needs further explanation. If you wish to appeal this determination, please contact John Reagan, Manager, Terminal Standards and Procedures Team, AJV-P31
I called Reagan, who is a very responsive, understanding fellow. He said that his group will reconsider and probably change the ATC handbook to use a different phrase (e.g., “Standby”) that will reduce potential confusion.
He noted that when he was a new IFR pilot, he was initially confused by the phrase and that the wording probably has been passed along informally and adopted without careful vetting.
In the meantime, if you’re an instrument instructor, make sure your IFR students and IPC customers understand “clearance on request.”
BruceAir, LLC 
I often hear pilots requesting a clearance by saying, “NXXXXX, clearance on request” when all the pilot need say is “NXXXXX, IFR to CDC” (in foreign lands, the pilot usually is also expected to state his/her preferred cruise altitude). The controller knows then what the pilot wants. With a tiny bit of training, the pilot understands the controller’s response, “Clearance on request”.
Training is key, but it’s also important to use phrases that are unambiguous. I hope FAA updates its guidance to controllers and pilots to avoid confusion.