Narrative:

My flight was rerouted north of the den control airspace. My aircraft weighed approximately 249000# at takeoff. After takeoff I ascertained from my F/east that we could climb to FL390. At FL390 the aircraft weighed approximately 242900#. The initial buffet speeds, high and low, were 257 and 218, respectively. The cruise speed M.80 and 247 KTS. The sat -50 and no turbulence. The power setting was 89.6% N1 with a fuel flow first officer 10100# per hour. The position of the flight at level off was approximately 20-30 mi southeast from sioux falls VOR. We were in a stabilized cruise by this time. A few mins later the F/east called my attention to an oil pressure indication problem on #1 engine. As he started to explain the problem he (F/east) noticed the aircraft was loosing altitude rapidly. I resumed my forward looking position, disconnected the autoplt, applied power and attempted to level off the aircraft. At this point the aircraft had reached an altitude of 38400' and descending. ATC at this time radioed us to check our altitude, and in attempting to level off, the aircraft entered into a mach buffet. My first officer informed ATC of the mach buffet we encountered and that we were descending to FL350. At this time ATC gave us clearance to FL350. Again, at the same time, I reduced power and reduced pitch input to remain out of the mach buffet area. I leveled off at FL350 and continued the flight normally. At FL390 we were in smooth air, no turbulence and in the clear with no indications of any high or low speed buffet when the descent had occurred.

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Original NASA ASRS Text

Title: ACFT ENCOUNTERS HIGH SPEED BUFFET AT FL390.

Narrative: MY FLT WAS REROUTED N OF THE DEN CTL AIRSPACE. MY ACFT WEIGHED APPROX 249000# AT TKOF. AFTER TKOF I ASCERTAINED FROM MY F/E THAT WE COULD CLB TO FL390. AT FL390 THE ACFT WEIGHED APPROX 242900#. THE INITIAL BUFFET SPDS, HIGH AND LOW, WERE 257 AND 218, RESPECTIVELY. THE CRUISE SPD M.80 AND 247 KTS. THE SAT -50 AND NO TURB. THE PWR SETTING WAS 89.6% N1 WITH A FUEL FLOW FO 10100# PER HR. THE POS OF THE FLT AT LEVEL OFF WAS APPROX 20-30 MI SE FROM SIOUX FALLS VOR. WE WERE IN A STABILIZED CRUISE BY THIS TIME. A FEW MINS LATER THE F/E CALLED MY ATTN TO AN OIL PRESSURE INDICATION PROB ON #1 ENG. AS HE STARTED TO EXPLAIN THE PROB HE (F/E) NOTICED THE ACFT WAS LOOSING ALT RAPIDLY. I RESUMED MY FORWARD LOOKING POS, DISCONNECTED THE AUTOPLT, APPLIED PWR AND ATTEMPTED TO LEVEL OFF THE ACFT. AT THIS POINT THE ACFT HAD REACHED AN ALT OF 38400' AND DSNDING. ATC AT THIS TIME RADIOED US TO CHK OUR ALT, AND IN ATTEMPTING TO LEVEL OFF, THE ACFT ENTERED INTO A MACH BUFFET. MY F/O INFORMED ATC OF THE MACH BUFFET WE ENCOUNTERED AND THAT WE WERE DSNDING TO FL350. AT THIS TIME ATC GAVE US CLRNC TO FL350. AGAIN, AT THE SAME TIME, I REDUCED PWR AND REDUCED PITCH INPUT TO REMAIN OUT OF THE MACH BUFFET AREA. I LEVELED OFF AT FL350 AND CONTINUED THE FLT NORMALLY. AT FL390 WE WERE IN SMOOTH AIR, NO TURB AND IN THE CLR WITH NO INDICATIONS OF ANY HIGH OR LOW SPD BUFFET WHEN THE DSCNT HAD OCCURRED.

Data retrieved from NASA's ASRS site as of August 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.