Singapore A333 near Hong Kong on May 23rd 2015, temporary loss of power on both engines

Last Update: August 2, 2018 / 15:51:09 GMT/Zulu time

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Incident Facts

Date of incident
May 23, 2015

Classification
Incident

Airline
Singapore Airlines

Flight number
SQ-836

Departure
Singapore, Singapore

Destination
Shanghai Pudong, China

Aircraft Registration
9V-SSF

Aircraft Type
Airbus A330-300

ICAO Type Designator
A333

A Singapore Airlines Airbus A330-300, registration 9V-SSF performing flight SQ-836 from Singapore (Singapore) to Shanghai Pudong (China) with 182 passengers and 12 crew, was enroute at FL390 about 140nm southsoutheast of Hong Kong (China) when both engines (Trent 772) of the aircraft lost power. While descending the aircraft the crew worked the related checklists and managed to restore normal operation of both engines at about FL260. The aircraft climbed back to 9500 meters (about FL312) and continued to Shanghai for a safe landing about 100 minutes later.

The aircraft remained on the ground in Shanghai for about 4 hours, then departed for the return flight SQ-825 and reached Singapore with a delay of 2 hours.

The airline reported: "Singapore Airlines flight SQ836, operated by an Airbus A330-300, was bound for Shanghai from Singapore on 23 May 2015 when it encountered bad weather at 39,000ft about three and a half hours after departure. Both engines experienced a temporary loss of power and the pilots followed operational procedures to restore normal operation of the engines. The flight continued to Shanghai and touched down uneventfully at 10:56pm local time." The aircraft underwent thorough examination and tests with no anomalies detected. The occurrence has been reported to the Authorities of Singapore and is being discussed with Rolls Royce and Airbus.

Singapore's Air Accident Investigation Bureau (SAAIB) confirmed an engine incident over international waters on a Singapore Airlines Flight enrotue from Singapore to Shanghai. The SAAIB stated: "the AAIB will be the authority for investigating this incident. The AAIB is in the midst of gathering information and flight data from the operator."

On Jun 2nd 2015 the French BEA reported in their weekly bulletin: "During cruise at FL390, ENG 1 stalled and self-recovered, then ENG 2 stalled. ENG 1 was shut down. Aircraft descended to FL260. ENG 1 relight was successful. Aircraft proceeded to Shanghai without further incident." The occurrence was rated a serious incident and is being investigated by SAAIB.

An Airbus notification to A330 operators reported the event occurred when the aircraft was maneouvering to avoid adverse weather and suggested based on preliminary flight data analysis, that over a period of 13 seconds engine #1 stalled twice and recovered itsself, engine #2 stalled thrice recovering itsself each time, engine #1 was commanded to shut down shortly after. A #1 engine restart at FL370 did not succeed, the aircraft descended to FL260 where the #1 engine restart was successful. Engine continuous ignition was on, wing anti ice on, nacelle anti ice off. Both engines were examined including borescopic examination without any findings, both engines were declared servicable, engine #1 however was removed from the aircraft for a precautionary de-pairing of the engines.

On May 16th 2016 Singapore's AAIB reported in an interim statement, that the aircraft was enroute over the South China Sea southeast of Hong Kong when the aircraft encountered an area of adverse weather. The crew received indication that the right hand engine had stalled, but before the flight crew could take action the indication cleared as the engine self recovered from the stall, however, now the left hand engine was indicated stalled. The crew worked through the checklist, descended the aircraft to FL260, restarted the left hand engine and continued the flight with both engines operating normally. The occurrence was rated an incident, the investigation focusses on enroute weather and weather avoidance procedures, flight crew actions, ECAM system and engine stall.

On Aug 2nd 2018 Singapore's AAIB released their final report releasing following conclusions:

- The two engines encountered engine surges, one after another, while the aircraft was passing through an area of adverse weather.

- The engines, as per design, recognised the engine surges and self-recovered immediately.

- Engine No.1 was disassembled and the examination of the engine suggest that the engine surge in Engine No. 1 was most likely a result of the release of IPC rotor path abradable lining material. Although Engine No. 2 was not disassembled and examined, it is most likely that Engine No. 2, being as new as Engine No. 1, experienced the engine surge for a similar reason.

- The release of the abradable lining material into the combustion section of the engine resulted in a disruption of airflow through the compressor section.

- The “Eng Stall” ECAM message was timed to display for 60 seconds whether or not the engine had self-recovered.

- The pilot while carrying out the ECAM checklist noticed the “Eng Stall” message still displayed and concluded that Engine No. 1 was still in a stalled condition and decided to shut down the engine.

- Engine No. 1 was restarted without issue and the aircraft continued the flight to the planned destination.

The AAIB reported that the aircraft was enroute at FL390 about 130nm southeast of Hong Kong when the crew observed weather cells along their planned flight route and planned a deviation to the right. After turning right the crew decided for a flight path that would bring the aircraft through the weather system but avoid areas of higher cell intensity. They instructed the cabin to prepare for turbulence, activated the seat belt signs, made an according announcement through the PA system and reduced the airspeed to the recommended turbulence penetration speed. Wing Anti Ice and continuous ignition for both engines were activated. The aircraft subsequently entered a weather cell, the right hand engine surged, the crew observed an engine surge indication for the #2 engine and began working the related checklist, however, before completing the checklist the crew received indication that the #1 engine had also surged (the indication was programmed to remain active for 60 seconds irrespetive of whether the engine had recovered).

The ECAM displayed the actions for the #1 engine above the actions needed for the #2 engine, the crew thus worked the #1 items first and reached the point to decide whether to shut the engine down. As the surge indication was still active the crew believed the engine was still in a surge condition and decided to shut the engine down. The FDR however indicated that the engine had already self recovered, which was unknown to the crew.

The crew declared Mayday and considered the options to divert to Hong Kong or Guangzhou which both would require them to fly to the weather zone again or to carry on to Shanghai with the options of Xiamen and Hangzhou as alternates should a diversion become necessary, the route appeared clear of weather. The crew thus decided to carry on to Shanghai.

While enroute at FL260 after drift down the crew attempted to restart engine #1, which succeeded. All engine parameters appeared normal, the crew thus cancelled the Mayday.

Before passing Xiamen the crew consulted with dispatch and maintenance and maintained the decision to continue to Shanghai.

The FDR showed that both engines surged and recovered in quick succession, all within a span of 12 seconds. Engine #2 surged three times, engine #1 two times, their first surges were almost simultaneously.

Engine #1 was removed from the aircraft also under consideration to depair new engines. Engine #1 was examined in detail. The AAIB wrote:

There was a significant amount of grey coloured dust deposit found on the aerofoil surfaces of some Intermediate Pressure Compressor (IPC) rotor blades and stator vanes, on the Intermediate Pressure Turbine (IPT) nozzle guide vanes and on the High Pressure Turbine (HPT) stubshaft. The dust was identified to be composed mainly of aluminium and silicon, which were the main constituents of the IPC rotor path abradable material.

There were signs of heavy rubbing of the rotor path abradable lining at Stages 3 and 6 of IPC. Light rubbing in the rotor paths were seen at Stages 4, 5, 7 and 8, and these rubs were considered typical of operational wear.

There was heavy rubbing at Stages 3, 4, 5, 6 and 7 stator vane shrouds. The rubbing could be attributed to contacts with the air seals of the IPC rotor drum.

There were signs of rubbing on the HPC stator vane shrouds. According to the engine manufacturer, the condition of the HPC was typical of normal operational wear.

Apart from the heavier than normal abradable lining rubs, the wear and tear of the engine was considered typical of an engine of a similar age.

The investigation analysed the engine surges:

The investigation team believed that the engine surge in Engine No. 1 was most likely a result of the release of IPC rotor path abradable lining material. Although Engine No. 2 was not disassembled and examined, the investigation team believed that Engine No. 2, being as new as Engine No. 1, experienced the engine surge for a similar reason.

The following is the likely sequence of events leading to the surge of Engine No. 1:

a. The tips of Stage 3 and 6 IPC blades contacted and rubbed the abradable lining material.

b. The rubbing caused the abradable lining material to be eroded into small particles which were carried downstream in the airflow.

c. These abradable lining material particles entered the combustion section and ignited, causing a temporary disruption of airflow through the engine.

The release of rotor path abradable material has been known to cause engine surges. If the aluminium and silicon-based rotor path abradable lining is rubbed heavily, the eroded small particles are carried downstream into the core airflow. When the abradable material and air mixture exceeds a certain threshold, the mixture can spontaneously ignite downstream in the hotter stages of the HPC or in the combustor, resulting in an engine surge.

The engine surge would result in a reduction in the engine pressure ratio (EPR). The EEC was designed to recover the engine by reinstating the EPR to the commanded EPR. This reinstating of the EPR accelerated the rotors, which caused a further release of abradable lining material particles. This would, in turn, result in ignition of the abradable lining material particles and a further disruption to the airflow through the engine, thus causing multiple surges. In order to arrest this situation, the EPR had to be reduced. Otherwise, the lining material would continue to be abraded until there was no more contact with the rotor blades, and thus no further release of lining material particles into the airflow.

The AAIB analysed the flight crew actions:

As mentioned in paragraph 1.1.4, when responding to the ECAM message pertaining to Engine No. 1’s surge, the flight crew had to decide whether to shut down Engine No. 1. As the ECAM message pertaining to Engine No. 1’s surge was still displayed, the flight crew believed that Engine No. 1 was still in a surge condition and therefore decided that they would need to shut down the engine.

However, FDR data showed that Engine No. 1 had actually been self-recovering in the meantime but this was not known to the flight crew. The reason the Engine No. 1 surge message was still being displayed is that the Electronic Engine Control (EEC) system was designed to retain the ENG STALL message for 60 seconds even if the engine had been recovering in the meantime.

Had the EEC system updated the engine surge status sooner, the flight crew would have been given a truer picture of the engine’s condition, and would most likely not have decided to shut down Engine No. 1.