Description

On 14 September 2010, an Airbus A319 being operated by Sichuan Airlines on a scheduled domestic passenger flight from Chongqing to Wuxi encountered previously forecast convective weather during the daylight approach in Instrument Meteorological Conditions (IMC) during which SPEED, Stall Warning and an Terrain Avoidance and Warning System (TAWS) hard warning and a temporary loss of control occurred. The crew eventually recovered control of the aircraft and made an uneventful diversion to Ningbo.

Investigation

The event was identified as a Serious Incident and an Investigation was carried out by the CAAC assisted by Quick Access Recorder data and flight crew interviews. An examination of the aircraft by engineers in Ningbo on arrival found that it remained airworthy. The Investigation also found, on the basis of a flight crew telephone conversation with the Sichuan Airlines’ Head of Flight Operations following their arrival in Ningbo, the flight crew had been permitted to continue their planned duty.

It was found that the aircraft had continued towards its intended destination despite clear information that the intensity of convective weather in the vicinity of the aerodrome was increasing. ATC also advised that, although an improvement was expected within half an hour, other aircraft had diverted. As the aircraft was establishing on the ILS LOC, ATC advised that the previously-advised ‘light thunderstorm’ had now worsened such that “the terminal area is now covered by thunderstorm” and “strong lightning is observed to the west of the terminal area”. Nevertheless, the approach was continued.

After establishing on the ILS LOC for runway 03 with the AP and A/T engaged, the ILS GS was captured but QAR data showed that the thrust set had been insufficient to maintain airspeed during descent. As a result, the aircraft pitch attitude had begun to increase as the AP attempted to maintain the selected profile. The pilots appeared not to have noticed this unusual pitch attitude or the decreasing airspeed. The recorded data showed that, when the airspeed loss had reached 12 knots below the applicable Vapp of 126 knots the aural ‘SPEED’ Warning (which indicates a low energy condition) had been annunciated. The crew response this was contrary to the prescribed Quick Reference Handbook (QRH) response and ineffective. With the AP and A/T still engaged and the A/T response not sufficient to arrest the 7knots/second rate of airspeed reduction, 12 seconds later, the Alpha Protection function was triggered with the aircraft pitch attitude at 34° up. The Alpha Floor function and its automatic application of TO/GA thrust and disconnection of the AP followed almost immediately, but a concurrent and significant change in wind velocity meant that the Stall Warning was activated only one second later with actual airspeed at that time estimated as having been between 93 KCAS and 81 KCAS compared to the calculated actually-applicable stall speed as flown of 79 KCAS. During the 6 second stall warning activation, to which the initial crew response was not in accordance with the prescribed QRH drill, the aircraft entered a fully stalled condition with a right wing drop to in excess of 40° and the rate of descent reached 3924 fpm. QAR data showed that the minimum airspeed reached was 74 knots and that the minimum height above the ground, reached some 13 seconds after the Stall Warning had begun had been 884 feet. From this point, the crew had effected a recovery of control and begun a climb and there had been no further aircraft control issues. However, it was noted that the effect of the rapid descent whilst stalled had been to trigger an EGPWS ‘SINK RATE, PULL UP’ Warning.

Having confirmed that the aircraft had remained in Normal Law throughout the incident sequence, the Investigation noted that both Flight Augmentation Computers (FACs) had ceased to function in quick succession during the stall, although both had been successfully reset almost immediately. It was established that both the FACs and all other technical functioning of the aircraft systems had been in accordance with design. The triggering of a Stall Warning so rapidly after activation of the Alpha Protection and Alpha Floor function was attributed to the inappropriate crew response to the SPEED Warning, which itself had followed a failure to monitor the flight instruments. The period during which the aircraft had been fully stalled was attributed to the inappropriate crew response to the Stall Warning, but it was noted that crew had eventually been able to recover the situation.

Overall, it was also concluded that the presence of windshear in the form of a sudden increase in tail wind component had been a factor which had had a material bearing on the sequence of events but that “during this flight, the crew didn’t act properly in making decision about weather, operating equipment and handling flaps under the weather condition of the day”. It was further concluded that “although the Alpha Protection function was activated before stall, it was still quite hard to effectively stop the aircraft from stalling, because the energy produced by the weather at that time had exceeded that of (the correctly functioning) auto protection system”. It was also noted that despite the evidence that windshear had been present during the incident, the severity of it had been insufficient to trigger the Windshear Alerting System with which the aircraft had been fitted.

The formal Conclusions of the Investigation were as follows:

• The crew did not pay enough attention to the complex weather condition and their decision-making ability was weak. The crew had made wrong decisions in the severe weather condition. Although ATC informed the pilots of the weather conditions and other planes’ diversion, the flight crew continued to launch the approach.

• The alternative plan prepared for complex weather condition was insufficient. When wind shear became obvious, pilots took no resolute action to stop the approach.

• The crew failed to respond as per the Operations Manual. When the aircraft attitude was gradually deviating from norm, air speed decreased and the low energy warning appeared, the pilots intervened in the AP control by choosing the speed manually instead of pushing the throttle forward as required by QRH 2.03. This led to a complex situation, including increasing attitude, low airspeed, Alpha Protection triggered and stall.

• The Crew Resource Management was a chaos. When aircraft was in complex conditions, both pilots reacted instinctively, introducing to dual side stick inputs for as long as 12 seconds.

The Cause of this “serious air transport incident” was the “flight crews’ inappropriate decisions and handling under adverse weather conditions”.

Four Safety Recommendations were issued as a result of the Investigation:

• that Sichuan Airlines should improve their training about safety awareness and skills, preventing flight crews from reckless flight in severe weather conditions.

• that when aircraft manufacturers update or revise critical manuals about flight manoeuvre procedure, airlines should organise training of relevant time and quality.

• that when in complex weather conditions, flight crew should keep alert to all relevant flight data, improve their situation awareness, make early preparation for decisive action, instead of making reckless decisions and actions.

• that airlines should improve their training on safety information reporting; strengthen staff awareness about reporting safety information and protecting evidence. In particular, a report about a serious safety event should be as complete and detailed as required by regulations and evidence shall be protected.

The Final Report of the CAAC Investigation was dated 4 May 2013 and a copy in English translation was released by the French Bureau d'Enquêtes et d'Analyses (BEA) on 29 November 2013. Note that this translation does not include the Appendices referred to in the main text.

Further Reading

• Loss of Control
• Low Level Wind Shear
• Energy Management during Approach
• Flight Control Laws
• Aerodynamic Stall Awareness and Avoidance
• Pilot Handling Skills
• Decision Making
• Cross-checking Process
• Situational Awareness
• Crew Resource Management
• Airbus FOBN - Adverse Weather Operations - Windshear Awareness
• The “Barn Door” Effect by C. West, Ph.D., NOAA - a paper about pilots’ propensity to continue approaches to land when closer to convective weather than they would wish to get while en route.