Narrative:

We were given a 2 min window of XX03-XX05 for IFR departure from yum-lax by yum approach control. Tower was closed. Arrived at runway early, but ATC said no early release. At XX59, first officer heard ATC release us for departure. I (captain) heard only a partial transmission as I was talking to the first officer at the time. We departed at XX01, checked in with yum departure, and were told we had not been released. We think perhaps we heard ATC telling other traffic we were going to be released and took that to be our clearance. The first officer did read back this clearance, however. There was no conflicting IFR or VFR traffic at takeoff. Learned that both pilots must be clear and verify clrncs.

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Original NASA ASRS Text

Title: FLC OF AN LTT DEPARTED WITHOUT IFR RELEASE DUE TO MISUNDERSTANDING THAT THEY HAD BEEN GIVEN THE ANTICIPATED RELEASE.

Narrative: WE WERE GIVEN A 2 MIN WINDOW OF XX03-XX05 FOR IFR DEP FROM YUM-LAX BY YUM APCH CTL. TWR WAS CLOSED. ARRIVED AT RWY EARLY, BUT ATC SAID NO EARLY RELEASE. AT XX59, FO HEARD ATC RELEASE US FOR DEP. I (CAPT) HEARD ONLY A PARTIAL XMISSION AS I WAS TALKING TO THE FO AT THE TIME. WE DEPARTED AT XX01, CHKED IN WITH YUM DEP, AND WERE TOLD WE HAD NOT BEEN RELEASED. WE THINK PERHAPS WE HEARD ATC TELLING OTHER TFC WE WERE GOING TO BE RELEASED AND TOOK THAT TO BE OUR CLRNC. THE FO DID READ BACK THIS CLRNC, HOWEVER. THERE WAS NO CONFLICTING IFR OR VFR TFC AT TKOF. LEARNED THAT BOTH PLTS MUST BE CLR AND VERIFY CLRNCS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.