Narrative:

Approaching den on the tomsn 1 STAR, descending to intercept the falcon (fqf) 306 degree radial and cross tomsn intersection at FL190 -- in and out of clouds. Immediately prior to intercepting the fqf 306 degree radial, we noticed we were bore-sighted on and rapidly overtaking a twin turboprop immediately beneath (perhaps 1500 ft below). While there was no conflict (TCASII or otherwise) it was, nonetheless, disconcerting to be turning, descending and attempting to make a crossing restr in a few mi, and have a commuter pop up directly ahead of and immediately below us. No mention was made of the aircraft by ATC. Immediately thereafter we leveled at FL190, crossed tomsn intersection and were advised to expect runway 7 at den (a runway on which I had never landed). While the first officer and so kept me apprised of the whereabouts of the turboprop (he crossed left to right beneath us), I busied myself with getting out the approach plate and briefing the approach to runway 7. I believed we were close in and would need to get down quickly from FL190 to complete the approach. Unfortunately, given the perceived immediacy of the situation, I missed manually engaging the altitude hold feature of the autoplt. The aircraft began a slow descent and by the time the copilot and so noticed the deviation, and I had returned the aircraft to FL190, we had lost approximately 400 ft. ATC queried us about the descent but mentioned nothing about a conflict with any other aircraft. There were no TCASII alerts, nor did the altitude alerter sound -- which would indicate a 500 ft deviation.

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Original NASA ASRS Text

Title: B727 DSNDS BELOW CLRED ALT IN DEN AIRSPACE.

Narrative: APCHING DEN ON THE TOMSN 1 STAR, DSNDING TO INTERCEPT THE FALCON (FQF) 306 DEG RADIAL AND CROSS TOMSN INTXN AT FL190 -- IN AND OUT OF CLOUDS. IMMEDIATELY PRIOR TO INTERCEPTING THE FQF 306 DEG RADIAL, WE NOTICED WE WERE BORE-SIGHTED ON AND RAPIDLY OVERTAKING A TWIN TURBOPROP IMMEDIATELY BENEATH (PERHAPS 1500 FT BELOW). WHILE THERE WAS NO CONFLICT (TCASII OR OTHERWISE) IT WAS, NONETHELESS, DISCONCERTING TO BE TURNING, DSNDING AND ATTEMPTING TO MAKE A XING RESTR IN A FEW MI, AND HAVE A COMMUTER POP UP DIRECTLY AHEAD OF AND IMMEDIATELY BELOW US. NO MENTION WAS MADE OF THE ACFT BY ATC. IMMEDIATELY THEREAFTER WE LEVELED AT FL190, CROSSED TOMSN INTXN AND WERE ADVISED TO EXPECT RWY 7 AT DEN (A RWY ON WHICH I HAD NEVER LANDED). WHILE THE FO AND SO KEPT ME APPRISED OF THE WHEREABOUTS OF THE TURBOPROP (HE CROSSED L TO R BENEATH US), I BUSIED MYSELF WITH GETTING OUT THE APCH PLATE AND BRIEFING THE APCH TO RWY 7. I BELIEVED WE WERE CLOSE IN AND WOULD NEED TO GET DOWN QUICKLY FROM FL190 TO COMPLETE THE APCH. UNFORTUNATELY, GIVEN THE PERCEIVED IMMEDIACY OF THE SIT, I MISSED MANUALLY ENGAGING THE ALT HOLD FEATURE OF THE AUTOPLT. THE ACFT BEGAN A SLOW DSCNT AND BY THE TIME THE COPLT AND SO NOTICED THE DEV, AND I HAD RETURNED THE ACFT TO FL190, WE HAD LOST APPROX 400 FT. ATC QUERIED US ABOUT THE DSCNT BUT MENTIONED NOTHING ABOUT A CONFLICT WITH ANY OTHER ACFT. THERE WERE NO TCASII ALERTS, NOR DID THE ALT ALERTER SOUND -- WHICH WOULD INDICATE A 500 FT DEV.

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.