Virgin Australia B738 near Adelaide on Sep 13th 2017, control inputs injure two cabin crew

Last Update: September 30, 2020 / 13:07:06 GMT/Zulu time

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Incident Facts

Date of incident
Sep 13, 2017

Classification
Accident

Flight number
VA-233

Aircraft Registration
VH-VUE

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

Airport ICAO Code
YPAD

A Virgin Australia Boeing 737-800, registration VH-VUE performing flight VA-233 from Melbourne,VI to Adelaide,SA (Australia), was descending towards Adelaide when the aircraft encountered turbulence causing injuries to two flight attendants, one of them sustained a broken leg. The aircraft continued for a safe landing on runway 23. The two flight attendants were taken to a hospital.

The occurrence aircraft remained on the ground for 22.5 hours before returning to service.

On Sep 15th 2017 the ATSB reported the occurrence described as "operational event" was rated an accident and an investigation has been opened. During descent on autopilot the airspeed vectors showed a speed increase, the crew disconnected the autopilot however could not prevent an airframe minor overspeed. As result one flight attendant received a serious and another one a minor injury.

On Sep 30th 2020 the ATSB released their final report concluding the probable causes of the accident were:

Contributing factors

- During a high-speed descent, a sudden decrease in tailwind associated with windshear caused airspeed to approach and exceed the aircraft maximum operating speed (VMO). The flight crew did not apply speed brakes to arrest the speed increase.

- In response to the airspeed rapidly increasing towards VMO, the captain (pilot monitoring) perceived a need to immediately intervene, and made pitch control inputs without following the normal take-over procedure and alerting the first officer (pilot flying).

- The magnitude of the captain's control input was probably greater than intended. This was influenced by a perception that the autopilot was not controlling the aircraft so an urgent intervention was required. The magnitude of the control input caused sudden pitch changes, resulting in the injuries to the cabin crew.

- Although the flight crew identified the risk of overspeed during the high-speed descent into Adelaide, they did not consider steps for mitigating that risk, or how they would manage an overspeed during the descent. This reduced the likelihood of the crew effectively responding to the unexpected increase in airspeed.

Other findings

- Although the weather forecast included moderate clear air turbulence and the aircraft was making a high-speed descent, the pilots perceived that flying conditions were smooth and elected not to activate the fasten seat belt sign. The cabin crew briefings did not mention the forecast clear air turbulence. The cabin crew were not secured prior to the sudden pitch changes, which increased the likelihood of injuries.

- The ground handling supervisor assessed there was a significant fall from height risk associated with the unsecured use of the catering truck. For that reason, the ground handling supervisor did not agree to the emergency services request to use that equipment to remove the injured cabin crew.

The ATSB analysed:

During a high-speed descent, the airspeed of VH-VUE increased unexpectedly and briefly exceeded the VMO limit of 340 kt. This was contrary to the operating procedures provided by the manufacturer and the operator, and contrary to the intentions of the flight crew. The captain responded to the sudden increase in airspeed by pulling back on the control column causing the autopilot to disconnect. This caused pitch changes that resulted in large changes to vertical acceleration and injuries to cabin crew at the rear of the aircraft.

The recorded data showed the development of the pitch changes was sudden, and that the onset coincided with the autopilot disconnect. The pitch changes dissipated after the large control inputs had ceased. From this, the ATSB determined that it was highly unlikely that atmospheric turbulence caused the pitch changes.

Although VMO was exceeded, it was not by an amount that required any structural inspections to ensure the ongoing airworthiness of the aircraft, according to 737 maintenance manual. The captain’s intervention probably reduced the magnitude of the eventual overspeed, and it is unknown to what extent the aircraft would have exceeded VMO had the captain not intervened.

However, the recorded data indicated that the autopilot was responding to the speed increase by raising the pitch of the aircraft. Furthermore, the pilots could have used the autopilot disengage controls rather than applying breakout force, and could have reduced the aircraft speed using the speed brakes.

Therefore, the safety hazards involved in this accident were primarily associated with how the captain acted to prevent overspeed, and the consequent effects of vertical acceleration on the aircraft and its occupants.

Development of the overspeed

The flight crew accepted an instruction from air traffic control to perform a high-speed descent.

Accepting a high-speed descent instruction was not unusual, and the ATSB did not find that the issuance or acceptance of a high-speed descent were factors that increased risk. However, targeting a higher descent speed reduces the margin between the target airspeed and airspeed limitations, and the risk of injury due to sudden control inputs will increase due to higher speed and increased kinetic energy.

The flight crew managed the descent using level change autopilot mode, with intermittent use of the vertical speed command mode. This was generally consistent with procedures and airspeed was generally stable at 320 kt during the early descent.

However, about 30 seconds after the turn towards COMLY, when VUE was descending through around 17,000 ft, a sudden decrease in tailwind associated with a windshear encounter led to a rapid increase in airspeed.

After observing the speed increase and trend towards the maximum operating speed (VMO), the first officer (FO) changed the autopilot mode from level change to vertical speed. This is commonly used by flight crew to reduce the airspeed because, in a descent, reducing the vertical speed raises the nose of the aircraft, which consequently also reduces the aircraft’s airspeed. This was also consistent with the procedures in the Flight Crew Training Manual, and bulletins produced by the operator.

However, vertical speed mode does not directly control the aircraft’s speed, and if a windshear results in a large and rapid airspeed change, it may not provide sufficient speed control. Use of control modes that directly control the airspeed, such as the level change mode, allow the aircraft to respond directly to the airspeed changes. This is why the automatic speed protection will change from vertical speed to level change mode. Noting, however, that for very rapid wind changes, as was seen on this occasion, even the automatic flight director system (AFDS) may not be able to prevent an overspeed.

A few seconds later, the captain, who was pilot monitoring (PM) responded to the increasing airspeed by pulling back on the control column, which resulted in the autopilot disconnecting. Two seconds after the autopilot disconnect, the VMO was exceeded by one knot.

Neither crew member applied the speed brakes to prevent the speed increase, nor during recovery from the overspeed. Although they need to be applied with care at high speed, the use of speed brakes would have reduced the likelihood of an overspeed without generating large flight loads and pitching motions.

Intervention to correct speed increase

The captain responded to the unexpected speed increase by pulling back firmly and abruptly on the control column to raise the pitch attitude of the aircraft’s nose. The captain took manual control of the aircraft without notifying the FO (who was pilot flying). This was not consistent with the normal process for handover and takeover. In situations where the pilot monitoring (not flying) perceives immediate action is required to avoid a hazardous situation, there will be a tendency for the transfer of control to happen more rapidly. However, it is still important for flight crew to formally identify who has control, to maintain clarity of the pilots’ roles.

Perceived urgency and lack of autopilot control

The large pull-back control input caused an autopilot disconnect and sudden changes in pitch attitude, resulting in injuries to the cabin crew.

The initial forceful pull back on the control column was in response to what the captain perceived to be a situation involving a nose-down attitude and an unexpectedly high airspeed indication. The sudden pull-back movement was a reflexive application of well-rehearsed basic flying principles, being consistent with an attempt to raise the nose of the aircraft, rather than an explicit attempt to disconnect the autopilot.

The captain indicated at interview that their pull on the control column was more forceful than planned. Consistent with this, the 49 lb backwards force applied by the captain is a large amount of force for a pitch up manoeuvre during high-speed flight, and was not consistent with procedures that cautioned against making large control inputs during high-speed, high altitude flight.

The ATSB considered the reasons for the captain’s large control input and the absent transfer of control. When the captain saw the airspeed increasing unexpectedly and approaching VMO, the captain perceived that the autopilot was not controlling the aircraft, and that an urgent intervention was necessary. The captain related this response to a reflex, impulsive response, with the implication being the action was rapid and without conscious deliberation.

It is likely that the captain’s perception of urgency affected how the captain responded to the sudden speed increase, and contributed to the captain responding rapidly, with a low level of conscious analysis. Research shows that when individuals perceive they need to respond rapidly to a situation, they tend to consider fewer options and less information (Dismukes, Goldsmith, & Kochan, 2015) and typically use rapid, associative and unconscious information processing, which is primarily influenced by pre-existing knowledge and beliefs (Klein, 2008). Research summarised by Means, Salas, Crandall and Jacobs (1993) suggests that in real-world settings, there is a speed/accuracy trade-off between rapid intuitive decisions and more time-consuming analytical decision making.

Other research describes how individuals interact with control systems changes depending on the perceived time available. According to Hollnagel (1998), where the individual perceives there is a large amount of time, he or she is able to look ahead and think about higher level goals. The individual can sample a large amount of information, and feed-forward to test the effect of actions.

However, when the individual perceives he or she has little or no time, the most obvious feature of the environment and the immediate needs of the situation will dominate the choice of action. There is no planning or analysis; the individual is essentially seeing and responding.

It was evident that the captain of VUE perceived there was no time to evaluate the sudden airspeed increase, and needed to respond urgently because of a perception that the autopilot was not controlling the aircraft. While an overspeed event may not be desirable, this perception to respond urgently was inconsistent with the documentation provided by the manufacturer and the operator about the aircraft safety around VMO. Overall, the documentation implied that minor exceedances of VMO were not hazardous to the safety of the aircraft.

In further extensive analysis the ATSB analysed that the captain obviously was concerned due to the operator's management activities that overspeeding the aircraft meant the aircraft was out of control and other crews exceeding Vmo had been subject to management review as result. However, the operator had changed the policies prior to the occurrence. The ATSB annotated: "In this event, it is possible that the captain’s concerns about overspeed were a carryover from the operator’s previous management of overspeed events."

Metars:
YPAD 130900Z 23017G29KT 9999 VCSH BKN030 11/03 Q1019 INTER 0900/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130900Z 23017G29KT 9999 VCSH BKN030 11/03 Q1019 INTER 0900/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130855Z 23019G29KT 9999 VCSH BKN030 11/03 Q1019 INTER 0855/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130830Z 22017KT 9999 FEW034 11/04 Q1019 INTER 0830/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130806Z 20016G26KT 9999 VCSH FEW034 10/06 Q1018 INTER 0806/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130800Z 20018KT 9999 -SHRA FEW020 SCT034 10/06 Q1018 INTER 0800/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130754Z 20018KT 9999 -SHRA FEW020 SCT034 10/06 Q1018 INTER 0754/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130730Z 24022KT 9999 VCSH FEW020 12/02 Q1017 INTER 0730/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130700Z 23021KT 9999 SCT034 12/03 Q1017 INTER 0700/1000 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130630Z 24026KT 9999 FEW020 12/03 Q1017 INTER 0630/0930 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130623Z 24022G33KT 9999 FEW020 12/02 Q1016 INTER 0623/0923 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130600Z 24026KT 9999 FEW020 SCT032 BKN090 12/03 Q1016 INTER 0600/0900 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130530Z 22022G33KT 9999 VCSH FEW020 SCT032 12/05 Q1016 INTER 0530/0830 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130530Z 22022G33KT 9999 VCSH FEW020 SCT032 12/05 Q1016 INTER 0530/0830 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130500Z 21018KT 9999 VCSH FEW020 BKN032 12/04 Q1015 INTER 0500/0754 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130454Z 20017KT 9999 VCSH FEW020 BKN032 12/04 Q1015 FM0510 23020G30KT 9999 -SHRA SCT030 BKN040 INTER 0454/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0754 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130446Z 25023G36KT 9999 -SHRA FEW020 BKN032 12/05 Q1015 INTER 0446/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0746 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130430Z 24020KT 9999 FEW020 SCT028 13/04 Q1015 INTER 0430/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0730 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130400Z 24019KT 9999 FEW028 BKN076 13/02 Q1015 INTER 0400/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0700 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130330Z 23021G31KT 9999 FEW028 BKN080 13/04 Q1015 INTER 0330/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0630 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130330Z 23021G31KT 9999 FEW028 BKN080 13/04 Q1015 INTER 0330/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0630 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130329Z 23021G31KT 9999 FEW028 BKN080 13/04 Q1015 INTER 0329/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0629 26020G30KT 3000 SHRAGS FEW015 BKN025=
YPAD 130300Z 24021KT 9999 FEW020 SCT028 BKN060 12/05 Q1015 INTER 0300/0500 23020G30KT 5000 SHRAGS SCT025 BKN030 INTER 0500/0600 26020G30KT 3000 SHRAGS FEW015 BKN025=
Incident Facts

Date of incident
Sep 13, 2017

Classification
Accident

Flight number
VA-233

Aircraft Registration
VH-VUE

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

Airport ICAO Code
YPAD

This article is published under license from Avherald.com. © of text by Avherald.com.
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