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Clear air turbulence associated with mountain wave activity creates an altitude deviation altitude excursion.
| Attributes | |
| ACN | 204756 |
| Time | |
| Date | 199203 |
| Day | Wed |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | atc facility : onm |
| State Reference | NM |
| Altitude | msl bound lower : 34700 msl bound upper : 35450 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | artcc : zab tower : lgb |
| Operator | common carrier : air carrier |
| Make Model Name | Medium Large Transport, Low Wing, 2 Turbojet Eng |
| Navigation In Use | Other Other |
| Flight Phase | cruise other |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : atp pilot : flight engineer |
| Experience | flight time last 90 days : 220 flight time total : 6500 flight time type : 600 |
| ASRS Report | 204756 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Qualification | pilot : atp |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance |
| Independent Detector | other flight crewa |
| Resolutory Action | flight crew : returned to intended course or assigned course flight crew : overcame equipment problem |
| Consequence | Other |
| Supplementary | |
| Primary Problem | Aircraft |
| Air Traffic Incident | Pilot Deviation other |
Narrative:
Autoplt on in 'perf' mode, cruise conditions. Aircraft started a slight descent to about 300 ft below assigned altitude, whereupon captain selected 'vertical speed' mode and a 500 FPM climb. But aircraft started to climb at 2000 FPM and went right through selected altitude of FL350 to about 450 ft high, whereupon captain disconnected autoplt and returned to FL350. No conflict. I'm still not sure if this was due to mountain wave activity or autoplt malfunction or both. Captain assumed mountain wave and instructed me to report it to center. This particular autoplt, when used in the 'perf crz' mode (which is SOP) consistently deviates from selected altitude by + or - 100 to 200 ft. This makes it at times difficult to determine if autoplt is functioning 'normally' or malfunctioning until it is too late. Still, if we had been more aggressive in disconnecting autoplt sooner and flying proper altitude, we might have diminished the altitude excursion.
Original NASA ASRS Text
Title: CLR AIR TURB ASSOCIATED WITH MOUNTAIN WAVE ACTIVITY CREATES AN ALTDEV ALT EXCURSION.
Narrative: AUTOPLT ON IN 'PERF' MODE, CRUISE CONDITIONS. ACFT STARTED A SLIGHT DSCNT TO ABOUT 300 FT BELOW ASSIGNED ALT, WHEREUPON CAPT SELECTED 'VERT SPD' MODE AND A 500 FPM CLB. BUT ACFT STARTED TO CLB AT 2000 FPM AND WENT RIGHT THROUGH SELECTED ALT OF FL350 TO ABOUT 450 FT HIGH, WHEREUPON CAPT DISCONNECTED AUTOPLT AND RETURNED TO FL350. NO CONFLICT. I'M STILL NOT SURE IF THIS WAS DUE TO MOUNTAIN WAVE ACTIVITY OR AUTOPLT MALFUNCTION OR BOTH. CAPT ASSUMED MOUNTAIN WAVE AND INSTRUCTED ME TO RPT IT TO CTR. THIS PARTICULAR AUTOPLT, WHEN USED IN THE 'PERF CRZ' MODE (WHICH IS SOP) CONSISTENTLY DEVIATES FROM SELECTED ALT BY + OR - 100 TO 200 FT. THIS MAKES IT AT TIMES DIFFICULT TO DETERMINE IF AUTOPLT IS FUNCTIONING 'NORMALLY' OR MALFUNCTIONING UNTIL IT IS TOO LATE. STILL, IF WE HAD BEEN MORE AGGRESSIVE IN DISCONNECTING AUTOPLT SOONER AND FLYING PROPER ALT, WE MIGHT HAVE DIMINISHED THE ALT EXCURSION.
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.