Narrative:

The following details an incident on jan/xa/00, while en route from fort meyers, fl, to detroit, mi. At the time of the incident, the aircraft had been operating at FL350 for approximately 10-15 mins and was approximately 100 mi north of atlanta, GA, on an ATC assigned vector to volunteer VOR (knoxville, tn). The sic was the PF, with the aircraft on autoplt and autothrottle. Time of the incident was not noted. Aircraft weight was approximately 136000 pounds. While in level flight at FL350, we became aware of a high frequency vibration, which was first thought to be caused by turbulence. The vibration was unusual, in that it was rapid 'staccato' in nature and seemed to grow in magnitude at a regular rate. The only time I had felt a similar vibration was during a mach buffet demonstration in a B737 simulator. I felt that the vibration might have been caused by mach buffet, control flutter, or a structural problem. The vibration was increasing in severity and I told the sic to reduce speed. The sic disconnected the autothrottles and made a power reduction which had no effect. The vibration continued to get worse and I ordered a second power reduction. At some time during this scenario, the sic disconnected the autoplt and hand flew the aircraft. He later stated that when he disconnected the autoplt, the aircraft was 'in trim' and still maintaining altitude. A short time later, the stall warning actuated. The sic advanced the power levers and I backed him up to firewall power. The stall warning persisted. I told the sic to lower the nose and we started a descent. At approximately FL340 the stall warning ceased, but when the sic attempted to level the aircraft, the stall warning sounded again. We continued the descent and the stall warning terminated. I advised ATC that we had left FL350 as a result of a stall warning and requested FL310. The cockpit was very noisy and I was unable to hear the controller's readback. I made the advisory/request again and was issued FL310. We continued the descent to FL310 without further incident or query from ATC. At no time were there any other indications that the aircraft was in a stall or approach to stall situation, other than the actuation of the stall warning system. The sic stated that the aircraft responded normally to both pitch and roll commands at all times during the incident. However, at FL310, we observed rapid changes in mach number with no changes in power or indicated airspeed. At one time, the mach changed from .754 to .770 in less than 4 seconds. In another excursion, the mach was seen to move from .754 to .776 in about 10 seconds. Based on the sic's description of control response, the aircraft's stability and in-trim condition at the time of the stall warning event, and the noted malfunctions of the mach speed indicating system, we believed the stall warning to be a false indication. After reviewing the QRH, chapter 13 (airspeed/mach indications unreliable), I determined that we could safely continue the flight to the destination using indicated airspeed only. Callback conversation with reporter revealed the following information: crew was flying an MD82 aircraft. A new pilot, with approximately 100 hours in type, described mach buffet. This came on so intensely that he was unsure what it was. Power reduction did not seem to help. The MD82 has an aural 'stall' voice warning system. This is what activated along with a stall warning annunciator light. At no time was the stick shaker system activated. They encountered a secondary stall warning after descending only 1000 ft. Maintenance checked the entire airframe for possible sources of vibration, and nothing was found. Both mach meters varied in unison. The mach, air data system were checked in detail, and nothing was found.

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Original NASA ASRS Text

Title: MD82 CREW HAD AIRFRAME VIBRATION, AURAL STALL WARNING ACTIVATION, AND MACH INDICATION FLUCTUATION.

Narrative: THE FOLLOWING DETAILS AN INCIDENT ON JAN/XA/00, WHILE ENRTE FROM FORT MEYERS, FL, TO DETROIT, MI. AT THE TIME OF THE INCIDENT, THE ACFT HAD BEEN OPERATING AT FL350 FOR APPROX 10-15 MINS AND WAS APPROX 100 MI N OF ATLANTA, GA, ON AN ATC ASSIGNED VECTOR TO VOLUNTEER VOR (KNOXVILLE, TN). THE SIC WAS THE PF, WITH THE ACFT ON AUTOPLT AND AUTOTHROTTLE. TIME OF THE INCIDENT WAS NOT NOTED. ACFT WT WAS APPROX 136000 LBS. WHILE IN LEVEL FLT AT FL350, WE BECAME AWARE OF A HIGH FREQ VIBRATION, WHICH WAS FIRST THOUGHT TO BE CAUSED BY TURB. THE VIBRATION WAS UNUSUAL, IN THAT IT WAS RAPID 'STACCATO' IN NATURE AND SEEMED TO GROW IN MAGNITUDE AT A REGULAR RATE. THE ONLY TIME I HAD FELT A SIMILAR VIBRATION WAS DURING A MACH BUFFET DEMONSTRATION IN A B737 SIMULATOR. I FELT THAT THE VIBRATION MIGHT HAVE BEEN CAUSED BY MACH BUFFET, CTL FLUTTER, OR A STRUCTURAL PROB. THE VIBRATION WAS INCREASING IN SEVERITY AND I TOLD THE SIC TO REDUCE SPD. THE SIC DISCONNECTED THE AUTOTHROTTLES AND MADE A PWR REDUCTION WHICH HAD NO EFFECT. THE VIBRATION CONTINUED TO GET WORSE AND I ORDERED A SECOND PWR REDUCTION. AT SOME TIME DURING THIS SCENARIO, THE SIC DISCONNECTED THE AUTOPLT AND HAND FLEW THE ACFT. HE LATER STATED THAT WHEN HE DISCONNECTED THE AUTOPLT, THE ACFT WAS 'IN TRIM' AND STILL MAINTAINING ALT. A SHORT TIME LATER, THE STALL WARNING ACTUATED. THE SIC ADVANCED THE PWR LEVERS AND I BACKED HIM UP TO FIREWALL PWR. THE STALL WARNING PERSISTED. I TOLD THE SIC TO LOWER THE NOSE AND WE STARTED A DSCNT. AT APPROX FL340 THE STALL WARNING CEASED, BUT WHEN THE SIC ATTEMPTED TO LEVEL THE ACFT, THE STALL WARNING SOUNDED AGAIN. WE CONTINUED THE DSCNT AND THE STALL WARNING TERMINATED. I ADVISED ATC THAT WE HAD LEFT FL350 AS A RESULT OF A STALL WARNING AND REQUESTED FL310. THE COCKPIT WAS VERY NOISY AND I WAS UNABLE TO HEAR THE CTLR'S READBACK. I MADE THE ADVISORY/REQUEST AGAIN AND WAS ISSUED FL310. WE CONTINUED THE DSCNT TO FL310 WITHOUT FURTHER INCIDENT OR QUERY FROM ATC. AT NO TIME WERE THERE ANY OTHER INDICATIONS THAT THE ACFT WAS IN A STALL OR APCH TO STALL SIT, OTHER THAN THE ACTUATION OF THE STALL WARNING SYS. THE SIC STATED THAT THE ACFT RESPONDED NORMALLY TO BOTH PITCH AND ROLL COMMANDS AT ALL TIMES DURING THE INCIDENT. HOWEVER, AT FL310, WE OBSERVED RAPID CHANGES IN MACH NUMBER WITH NO CHANGES IN PWR OR INDICATED AIRSPD. AT ONE TIME, THE MACH CHANGED FROM .754 TO .770 IN LESS THAN 4 SECONDS. IN ANOTHER EXCURSION, THE MACH WAS SEEN TO MOVE FROM .754 TO .776 IN ABOUT 10 SECONDS. BASED ON THE SIC'S DESCRIPTION OF CTL RESPONSE, THE ACFT'S STABILITY AND IN-TRIM CONDITION AT THE TIME OF THE STALL WARNING EVENT, AND THE NOTED MALFUNCTIONS OF THE MACH SPD INDICATING SYS, WE BELIEVED THE STALL WARNING TO BE A FALSE INDICATION. AFTER REVIEWING THE QRH, CHAPTER 13 (AIRSPD/MACH INDICATIONS UNRELIABLE), I DETERMINED THAT WE COULD SAFELY CONTINUE THE FLT TO THE DEST USING INDICATED AIRSPD ONLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CREW WAS FLYING AN MD82 ACFT. A NEW PLT, WITH APPROX 100 HRS IN TYPE, DESCRIBED MACH BUFFET. THIS CAME ON SO INTENSELY THAT HE WAS UNSURE WHAT IT WAS. PWR REDUCTION DID NOT SEEM TO HELP. THE MD82 HAS AN AURAL 'STALL' VOICE WARNING SYS. THIS IS WHAT ACTIVATED ALONG WITH A STALL WARNING ANNUNCIATOR LIGHT. AT NO TIME WAS THE STICK SHAKER SYS ACTIVATED. THEY ENCOUNTERED A SECONDARY STALL WARNING AFTER DSNDING ONLY 1000 FT. MAINT CHKED THE ENTIRE AIRFRAME FOR POSSIBLE SOURCES OF VIBRATION, AND NOTHING WAS FOUND. BOTH MACH METERS VARIED IN UNISON. THE MACH, AIR DATA SYS WERE CHKED IN DETAIL, AND NOTHING WAS FOUND.

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.