Description

On 17 October 2011, a Boeing 777-300 being operated by Singapore Airlines on a passenger flight from Shanghai to Singapore with line training in progress and a Safety Pilot present on the flight deck encountered a very brief period of unexpectedly severe turbulence whilst in the cruise at FL350 in night Visual Meteorological Conditions (VMC) and a small loss of altitude resulted. Sixteen unsecured passengers and five members of the cabin crew sustained mainly minor injuries but after assessment, it was determined that the flight to destination should be completed as planned and this was accomplished without further event.

Investigation

The event occurred in Vietnamese airspace but the Investigation was carried out by the AAIB of the Singapore Ministry of Transport as State of the Operator. DFDR and QAR data for the event were downloaded by the Operator and copies provided to the Investigation.

It was found that the left pilot seat had been occupied by a Senior First Officer undergoing command line training with a Training Captain occupying the right pilot seat as aircraft commander with a Safety Pilot in the principal supernumerary crew seat. The pilot under training had been designated as PF. It was noted that the incident aircraft was fitted with a version of the Honeywell RDR4B weather radar system with both automatic and manual tilt angle control, although it was noted that other aircraft in the same Operator’s B777 fleet were instead fitted with a version of the same radar which provided for only manual tilt angle control.

Prior to flight, the aircraft commander had briefed the cabin crew that turbulence could be expected to be encountered one hour into the flight and would last for two hours. About three hours into the flight, in the cruise with the AP engaged at FL350 in smooth flying conditions with the seat belt signs off and the left and right seat pilots’ weather radar displays on their NDs set respectively to 80 nm and 160 nm ranges and in automatic tilt mode. The 80nm display had shown a small green return almost on track ahead - a weather cell of about 3 nm diameter. The PF selected the tilt to manual mode and at a range of 60nm, requested and obtained clearance to deviate 10nm left of track upwind of the cell with this deviation commenced at a range of 40 nm from the cell. After 2.5 minutes, the PF returned the aircraft to a track parallel to the original one having judged that sufficient deviation had been made for what appeared to be a relatively small and isolated cell with the aircraft clear of cloud and the silhouette of the cloud at the cell position visible. Given this visual evidence and since the flying conditions remained smooth, it was not considered necessary to switch on the seat belt signs or reduce to turbulence penetration speed.

As the aircraft passed abeam the cell position some 4.5nm from the original track and the (green) returns from the edge of the cell, some new short and very fine green lines appeared on the display directly ahead and when the landing lights were turned on, “very light traces of cloud” were visible so the PF set the HDG 20° to the left away from the cell. Whilst the aircraft was in a shallow left bank to accomplish this turn, turbulence was encountered and the seat belt signs were selected on and speed reduction initiated but the aircraft (then) experienced severe turbulence that lasted for about eight seconds and occupant injuries were sustained, 5 to cabin crew and 21 to passengers, one of which was relatively more serious. FDR data showed that during the encounter, vertical accelerations between 1.66 g and -0.27 g were experienced and recorded vertical speeds had varied between 800fpm up and 1340 fpm down with approximately 100 feet of altitude loss.

After a doctor travelling on the flight had rendered assistance, he advised that four of the injured would need further medical attention on arrival. The aircraft commander determined that the appropriate course of action was to continue to complete the flight to destination and this was achieved without further event and a medical emergency was not declared.

It was found that two other Singapore Airlines Boeing 777 flights had followed the same route as the one being investigated, one two minutes behind at FL 310 and the other six minutes behind at FL 360. Both of these aircraft were fitted with more advanced weather radar equipment which had entirely automatic tilt angle control, a memory for recent radar scans over the whole range from ground level to 60000 feet and the facility for crews to select horizontal ‘slices’ through these scans in 1000 foot increments to enable detailed weather cell analysis to be undertaken if required. Both aircraft had deviated from the same intended flight path as the flight under investigation around the same cell by 11nm and 16nm respectively. Both crews reported that they had observed a cluster of cells rather than a single cell on their weather radar, with red cell cores visible.

It was established that the Operator routinely provided both crew training and supporting guidance on the use of weather radar to which all three crew members had been exposed. It was found that the “procedure suggested by the operator” to its 777 crews was “a track deviation of at least 20 nm in order to avoid a thunderstorm cell” but that there was no similar suggestion in the case of encountering weather cells less severe than a thunderstorm cell.

It was noted that, at the range and manually set tilt angle used by the investigated aircraft crew at the altitude at which they were flying, their weather radar would not have detected that part of the cell which was below 20000 feet. Published guidance to crews quoted by the Investigation did not give any specific advice on use of the manual radar tilt function but did say “use (the) radar antenna tilt function to scan the reflectivity of the storms ahead. Assess the height of the storms. Recognise that heavy rain below, typically, indicates high concentration of ice crystals above.”

The Investigation noted that the investigated aircraft crew had not made a PIREP after their severe turbulence encounter as suggested in Operator-issued guidance but that they subsequently accepted that such action should have been taken. A requirement in ICAO Doc 4444 PANS-ATM for AIREP reports to be transmitted by aircraft which have encountered phenomena such as moderate or severe turbulence was noted by the Investigation without comment.

A series of Safety Actions taken by the Operator, all aimed at improving the understanding of pilots of both weather avoidance and the correct use of both types of weather radar fitted to the 777 fleet were noted. One further “Safety Suggestion” was made as a result of the Investigation that Operator should consider replacing older ‘RDR 4B’ weather radar systems with the fully automated ‘RDR 4000’ model. However, the response of the Operator was that the older systems were no less effective in differentiating weather severity provided that they were used as intended and that pilot recurrent training to this end had been strengthened since the event investigated.

The Final Report was published on 14 May 2013.

Further Reading

• Loss of Control
• Turbulence
• Weather Radar
• Weather Radar: Storm Avoidance
• ATC Operations in Weather Avoidance Scenarios
• Airbus Adverse Weather Briefing Note - Optimum Use of Weather Radar