Jetstar A320 enroute on Mar 12th 2014, alpha floor activation

Last Update: June 17, 2014 / 14:50:04 GMT/Zulu time

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Incident Facts

Date of incident
Mar 12, 2014

Classification
Incident

Airline
Jetstar Airways

Departure
Melbourne, Australia

Destination
Darwin, Australia

Aircraft Registration
VH-VQY

Aircraft Type
Airbus A320

ICAO Type Designator
A320

A Jetstar Airways Airbus A320-200, registration VH-VQY performing positioning flight JQ-7991 from Melbourne,VI to Darwin,NT (Australia) with 2 crew, had just cleared from FL360 to FL380 and had just reached FL380 abeam Mildura,VI (Australia) when the crew noticed the airspeed was increasing. The crew reduced thrust to idle, extended the speedbrakes and disengaged the autopilot in order to reduce the airspeed, which resulted in speed warnings and a brief activation of flight envelope protection function. The aircraft continued to Darwin for a safe landing without further incident.

Australia's TSB reported an investigation has been opened into the flight envelope protection event (later adding more detail about the sequence of events).

On Mar 18th 2014 the French BEA reported in their weekly bulletin that the aircraft was climbing from FL360 to FL380, after the climb was completed the airspeed increased and the crew disconnected the autopilot in order to control the speed. The speed then decayed resulting in Alpha Floor Activation. Australia's TSB is investigating the occurrence rated an incident.

On Jun 17th 2014 the ATSB released their final bulletin releasing the safety message:

This incident provides a reminder to pilots of all aircraft types regarding the potential for an aerodynamic stall. The stall occurs at a critical angle of attack. The airspeed associated with the stall angle of attack varies depending on the aircraft weight and load factor (such as angle of bank), and the configuration of flaps, slats and spoilers.

The Golden Rules for Pilots article in Safety First - The Airbus Safety Magazine, Issue 15, January 2013, states that on highly automated and integrated aircraft, several levels of automation are available to perform a given task; and the ‘appropriate’ level of automation depends on the situation and task. It advises flight crew to understand the implication of the intended level of automation. Being able to anticipate the reaction of the automated response is important in deciding whether to proceed to rule 4 and change the level of automation.

In this incident, understanding the automated response to a potential overspeed situation may have given the first officer more time to analyse and resolve the situation. Disconnecting the autopilot and autothrust led to a rapid increase in workload and the aircraft changing from a potential overspeed to a slow speed state.

The ATSB reported that the captain briefed the first officer of the next suitable aerodrome in case of an emergency, then left the cockpit temporarily. The aircraft was enroute at FL360 on autopilot in managed speed mode at 0.78 mach. Abeam of Mildura the first officer received clearance to climb to FL380, the first officer selected the new altitude into the autopilot maintaining managed speed mode. The aircraft climbed through FL373 when the first officer noticed the speed had increased to 0.81 mach and had engaged in a 3000 fpm climb, the speed trend indicator suggesting the aircraft would accelerate beyond the maximum mach number operating (MMO) of 0.82 mach. The first officer attempted to arrest the speed by selecting the speed back to 0.76, however the speed continued to increase and the speed trend continued to indicate acceleration. Taken the Airbus golden rule "take action if things do not go as expected" the first officer reduced the thrust levers to idle, which effectively disconnected autothrust, in order to reduce speed, extended the speed brakes and disengaged the autopilot in order to level off and maintain the assigned flight level 380. The aircraft however climbed above FL380. At FL383 ATC queried to confirm altitude, the first officer radioed they were descending back to FL380. The first officer subsequently engaged autothrust and returned the levers to the climb detent and momentarily engaged the autopilot but disconnected again and pushed the nose down in order to re-acquire the assigned flight level.

The aircraft began to descend and the airspeed dropped to below the lowest speed that autothrust would permit to select. The first officer applied nose up commands in order to level at FL380 and moved the thrust levers back close to but not to idle position, which reduced the maximum thrust available from the engines. The nose up inputs increased the angle of attack beyond the alpha floor, the alpha floor protection activated, the speed brakes were automatically retracted and the TOGA lock was activated.

At that time the captain returned to the cockpit, scanned the primary instruments, noticed the aircraft pitch at 0 degrees, the speed in the yellow band about half way between stall and lowest selectable speed, the speed trend accelerating and the aircraft at FL365. There were no indications of any other aircraft in the vicinity that could have been affected by the altitude busts, the captain spotted the Thrust Lock indication. The captain took control of the aircraft, double clicked the autothrust disconnect button to disengage the thrust lock and moved the thrust levers to the climb detent, noticed the speed brake lever was extended and moved it to the retracted position, set a pitch attitude of about +5 to +7 degrees corresponding to a climb of 700-1000 fpm.

ATC noticed the aircraft was now below FL380 and queried to confirm the altitude again, the first officer radioed they were now climbing to FL380, operations were normal.

The captain, cognisant of a gentle recovery to avoid a secondary flight envelope event, levelled the aircraft at FL380 and re-engaged automation.

The ATSB reported that the flight data recorder revealed that during the climb a tail wind component of about 20 knots turned into a head wind component of about 15 knots beginning at FL365, an effective windshear of 35 knots which resulted in 3005 fpm climb rate and a CAS increase from 256 to 262 knots. The Alpha Floor activated when the aircraft descended through FL370 at a CAS of 226 knots with a rate of descend of 4500 fpm.

The ATSB rehearsed the flight crew operating manual procedures, renewed by Airbus mid 2013, stating that the autopilot should be kept on in case of a climbing/descending overspeed event.

The operator revised their standard operating procedures. Their SOPs did not require both crew members to be on the flight deck during cruise. The operator introduced that during altitude changes both flight crew should be present on the flight deck.