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B747 captain reported that while descending on a STAR in VNAV mode; the VNAV abruptly reset 6;000 feet lower. A stabilized approach and uneventful landing were made despite the high; in-close position. An MEL on the right auxiliary pitot static probe heater may have been a factor as the center air data computer was affected.
| Attributes | |
| ACN | 1303594 |
| Time | |
| Date | 201510 |
| Local Time Of Day | 0001-0600 |
| Place | |
| Locale Reference | ZZZ.Airport |
| State Reference | US |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | B747 Undifferentiated or Other Model |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Initial Approach |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Pitot/Static Ice System |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe |
Narrative:
On approach; using STAR arrival. Flight was normal; descending via the STAR. We were on vnav path all the time; until vnav jumped around 6000 ft showing us above the path. At this time we saw the runway and extended flaps 20; gear down; speed brakes up; slow down to normal speed; reported runway in sight; and were trying to make stable approach until we reach 500 ft and; if unable; to go around. We made s-turns and descent in normal parameters. Landing was made well within landing parameters.we had [an] MEL on the right auxiliary pitot static probe heater inoperative which affected the center air data computer and possibly could cause unreliable ais or altitude which in our situation contributed to unreliable vnav path condition.
Original NASA ASRS Text
Title: B747 Captain reported that while descending on a STAR in VNAV mode; the VNAV abruptly reset 6;000 feet lower. A stabilized approach and uneventful landing were made despite the high; in-close position. An MEL on the Right Auxiliary Pitot Static Probe Heater may have been a factor as the Center ADC was affected.
Narrative: On approach; using STAR arrival. Flight was normal; descending via the STAR. We were on Vnav Path all the time; until Vnav jumped around 6000 ft showing us above the path. At this time we saw the runway and extended flaps 20; gear down; speed brakes up; slow down to normal speed; reported runway in sight; and were trying to make stable approach until we reach 500 ft and; if unable; to go around. We made S-turns and descent in normal parameters. Landing was made well within landing parameters.We had [an] MEL on the Right Auxiliary Pitot Static probe heater inoperative which affected the Center ADC and possibly could cause unreliable AIS or altitude which in our situation contributed to unreliable Vnav Path condition.
Data retrieved from NASA's ASRS site 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.