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An air carrier large transport first officer report on encountering mountain wave clear air turbulence on descent into slc. The aircraft almost went into overspd on the barber pole.
| Attributes | |
| ACN | 425898 |
| Time | |
| Date | 199901 |
| Day | Mon |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | atc facility : slc |
| State Reference | UT |
| Altitude | msl bound lower : 31000 msl bound upper : 39000 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Operator | common carrier : air carrier |
| Make Model Name | Commercial Fixed Wing |
| Navigation In Use | Other |
| Flight Phase | descent other |
| Route In Use | enroute airway : zlc |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : instrument pilot : commercial |
| Experience | flight time last 90 days : 240 flight time total : 9000 flight time type : 220 |
| ASRS Report | 425898 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Events | |
| Anomaly | inflight encounter other non adherence other other anomaly other anomaly |
| Independent Detector | other flight crewa |
| Consequence | Other |
| Supplementary | |
| Air Traffic Incident | other |
Narrative:
En route descent to final destination, center directed descent to FL310. Started VNAV (1000 FPM) descent from FL390. Center then requested expedite through FL330. Captain reduced throttles to increase descent rate with VNAV. Cruise mach was approximately .81. During descent, experienced moderate mountain wave which increased IAS to barber pole (maximum IAS pointer). Captain at this point disconnected the autoflt system. The airplane's pitch decreased and we encountered some positive G's (approximately 1.5 - 2G) after autoflt disconnect. I felt that the mountain wave effect, along with some choppy air, aggravated the situation following the captain's decision to disconnect the autoplt and decrease the pitch attitude in order to slow below the maximum IAS pointer. The airspeed exceedance clacker never sounded and there was absolutely no discomfort throughout the maneuver. However, the flight attendants in the tail section were affected by the positive G's and said they were sitting on the floor after this occurrence. Mountain wave occurrences are subtle and occur over the western united states with great frequency. The VNAV speed selected with autoflt is unable to keep up with this severe mountain wave. Maybe a faster computer with better trend updates could hold the selected cruise speeds better.
Original NASA ASRS Text
Title: AN ACR LGT FO RPT ON ENCOUNTERING MOUNTAIN WAVE CLR AIR TURB ON DSCNT INTO SLC. THE ACFT ALMOST WENT INTO OVERSPD ON THE BARBER POLE.
Narrative: ENRTE DSCNT TO FINAL DEST, CTR DIRECTED DSCNT TO FL310. STARTED VNAV (1000 FPM) DSCNT FROM FL390. CTR THEN REQUESTED EXPEDITE THROUGH FL330. CAPT REDUCED THROTTLES TO INCREASE DSCNT RATE WITH VNAV. CRUISE MACH WAS APPROX .81. DURING DSCNT, EXPERIENCED MODERATE MOUNTAIN WAVE WHICH INCREASED IAS TO BARBER POLE (MAX IAS POINTER). CAPT AT THIS POINT DISCONNECTED THE AUTOFLT SYS. THE AIRPLANE'S PITCH DECREASED AND WE ENCOUNTERED SOME POSITIVE G'S (APPROX 1.5 - 2G) AFTER AUTOFLT DISCONNECT. I FELT THAT THE MOUNTAIN WAVE EFFECT, ALONG WITH SOME CHOPPY AIR, AGGRAVATED THE SIT FOLLOWING THE CAPT'S DECISION TO DISCONNECT THE AUTOPLT AND DECREASE THE PITCH ATTITUDE IN ORDER TO SLOW BELOW THE MAX IAS POINTER. THE AIRSPD EXCEEDANCE CLACKER NEVER SOUNDED AND THERE WAS ABSOLUTELY NO DISCOMFORT THROUGHOUT THE MANEUVER. HOWEVER, THE FLT ATTENDANTS IN THE TAIL SECTION WERE AFFECTED BY THE POSITIVE G'S AND SAID THEY WERE SITTING ON THE FLOOR AFTER THIS OCCURRENCE. MOUNTAIN WAVE OCCURRENCES ARE SUBTLE AND OCCUR OVER THE WESTERN UNITED STATES WITH GREAT FREQUENCY. THE VNAV SPD SELECTED WITH AUTOFLT IS UNABLE TO KEEP UP WITH THIS SEVERE MOUNTAIN WAVE. MAYBE A FASTER COMPUTER WITH BETTER TREND UPDATES COULD HOLD THE SELECTED CRUISE SPDS BETTER.
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.