 |
37000 Feet | Browse and search NASA's Aviation Safety Reporting System |
|
B737-800 crew complained the autoflt system exceed the maximum aircraft speed 'barber pole' when descending in VNAV.
| Attributes | |
| ACN | 512700 |
| Time | |
| Date | 200105 |
| Day | Tue |
| Place | |
| Locale Reference | navaid : abb.vortac |
| State Reference | IN |
| Altitude | msl single value : 18000 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | artcc : zla.artcc |
| Operator | common carrier : air carrier |
| Make Model Name | B737-800 |
| Flight Phase | descent : intermediate altitude |
| Route In Use | arrival star : mosey 5 |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : atp |
| Experience | flight time last 90 days : 70 flight time total : 5000 flight time type : 70 |
| ASRS Report | 512700 |
| Events | |
| Anomaly | other anomaly other anomaly other |
| Resolutory Action | other |
| Supplementary | |
| Problem Areas | Flight Crew Human Performance Aircraft |
| Primary Problem | Aircraft |
Narrative:
On the mosey 5 arrival into cvg, ATC gave us clearance to cross the nabb VOR at 13000 ft MSL. We were at FL220 when the clearance was received. The descent clearance was given a little late and VNAV with autoplt used for the descent. The autoplt in the descent tried to maintain VNAV path to make the crossing at nabb and due to a design flaw, when too high, adjusts the nose until the aircraft speed exceeds VNAV set speed by 10 KTS. In this case, the speed increased and as designed the VNAV kicked off at +10 KTS above VNAV speed sending us slightly into the barber pole by 2-3 KTS (with clacker warning). The captain (PF) put out the spoilers and aircraft slowed below barber pole in less than 5 seconds. This logic flaw in VNAV descent performance needs to be corrected to prevent this occurrence. Despite careful monitoring, it is not too difficult for VNAV, if trying to descend back onto the path from above, to ride close to, and if kicks off, bump through the barber pole. A slightly slower speed (more cushion from barber pole) and conservative top of descent point would correct this.
Original NASA ASRS Text
Title: B737-800 CREW COMPLAINED THE AUTOFLT SYS EXCEED THE MAX ACFT SPD 'BARBER POLE' WHEN DSNDING IN VNAV.
Narrative: ON THE MOSEY 5 ARR INTO CVG, ATC GAVE US CLRNC TO CROSS THE NABB VOR AT 13000 FT MSL. WE WERE AT FL220 WHEN THE CLRNC WAS RECEIVED. THE DSCNT CLRNC WAS GIVEN A LITTLE LATE AND VNAV WITH AUTOPLT USED FOR THE DSCNT. THE AUTOPLT IN THE DSCNT TRIED TO MAINTAIN VNAV PATH TO MAKE THE XING AT NABB AND DUE TO A DESIGN FLAW, WHEN TOO HIGH, ADJUSTS THE NOSE UNTIL THE ACFT SPD EXCEEDS VNAV SET SPD BY 10 KTS. IN THIS CASE, THE SPD INCREASED AND AS DESIGNED THE VNAV KICKED OFF AT +10 KTS ABOVE VNAV SPD SENDING US SLIGHTLY INTO THE BARBER POLE BY 2-3 KTS (WITH CLACKER WARNING). THE CAPT (PF) PUT OUT THE SPOILERS AND ACFT SLOWED BELOW BARBER POLE IN LESS THAN 5 SECONDS. THIS LOGIC FLAW IN VNAV DSCNT PERFORMANCE NEEDS TO BE CORRECTED TO PREVENT THIS OCCURRENCE. DESPITE CAREFUL MONITORING, IT IS NOT TOO DIFFICULT FOR VNAV, IF TRYING TO DSND BACK ONTO THE PATH FROM ABOVE, TO RIDE CLOSE TO, AND IF KICKS OFF, BUMP THROUGH THE BARBER POLE. A SLIGHTLY SLOWER SPD (MORE CUSHION FROM BARBER POLE) AND CONSERVATIVE TOP OF DSCNT POINT WOULD CORRECT THIS.
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.