SriLankan A333 near Learmonth on Mar 21st 2021, autopilot disconnected, altitude deviation
Last Update: November 1, 2022 / 14:33:44 GMT/Zulu time
Incident Facts
Date of incident
Mar 21, 2021
Classification
Incident
Airline
Srilankan Airlines
Flight number
UL-606
Departure
Colombo, Sri Lanka
Destination
Sydney, Australia
Aircraft Registration
4R-ALR
Aircraft Type
Airbus A330-300
ICAO Type Designator
A333
The aircraft departed for the return flight about 2 hours after landing.
Sri Lanka's AIB rated the occurrence an incident and opened an investigation.
On Nov 1st 2022 Sri Lanka's AIB released their final report concluding the probable causes of the incident were:
- The disconnection of the AP on both occasions was caused by a force applied to the F/O’s side stick. A warning was triggered as this was not the recommended procedure to disconnect the AP.
- Poor situational awareness and non-adherence to SOPs in handing over and taking over of controls culminated in the AP disconnection on two occasions and which resulted in loss of altitude.
The AIB summarized the sequence of events:
The first segment was operated by the Captain and the Cruise Pilot. The second segment where the subject incident occurred was operated by the Cruise Pilot and the First Officer. The third segment was operated by the Captain and the Cruise Pilot.
After approximately 3 hrs, the operating crew of the first segment had changed seats for rest purposes. For the second segment of the fight, the F/O (Cruise pilot) took over the left seat as the PM, while the relieving F/O seated on the right hand seat and acted as the PF. The PIC had gone to the cabin to take his rest.
At FL 390, at about 0310 UTC, the aircraft had experienced Autopilot disconnections followed by an altitude loss of about 1540ft at a rate of descent of 5700ft/min over Jakarta FIR.
The flight crew had not reported this incident to the Flight Safety Section of the Airline. The incident was detected by the Flight Safety Section during the routing FDA monitoring. Subsequently MORs were filed by the operating Flight Crew, and submitted to the CAASL through the Airline on 2nd April 2021.
In more detail the AIB described the autopilot disconnects:
The aircraft was in cruise at FL390. Auto Pilot 2 and both FDs were engaged in ALT CRZ/NAV mode. Auto Thrust was in active mode and Mach was managed at M 0.81. At about 03:10 UTC, AP 2 had disconnected with the master warning sounding. The aircraft, started to descend below the selected FCU altitude. A few seconds later AP 1 was engaged by the CP from the left seat. The aircraft was still in a descent. Subsequently, AP 1 also disengaged and the master warning wastriggered. Few seconds later AP 2 engaged by the F/O (PF). The aircraft lost altitude and descended approx. 1540ft at a rate of 5700ft/min on the VSI (Vertical Speed Indicator). Then AP2 was re-engaged by the cruise pilot in open climb mode and the aircraft commenced climb to FL 390.
Incident Facts
Date of incident
Mar 21, 2021
Classification
Incident
Airline
Srilankan Airlines
Flight number
UL-606
Departure
Colombo, Sri Lanka
Destination
Sydney, Australia
Aircraft Registration
4R-ALR
Aircraft Type
Airbus A330-300
ICAO Type Designator
A333
This article is published under license from Avherald.com. © of text by Avherald.com.
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