Virgin Australia B738 near Adelaide on May 9th 2015, control inputs cause injuries to flight attendant

Last Update: October 8, 2016 / 18:29:50 GMT/Zulu time

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Incident Facts

Date of incident
May 9, 2015

Classification
Report

Flight number
VA-436

Aircraft Registration
VH-YID

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Virgin Australia Boeing 737-800, registration VH-YID performing flight VA-436 from Sydney,NS to Adelaide,SA (Australia) with 83 passengers and 6 crew, was in a high speed descent (320 KIAS) through about 8400 feet MSL towards Adelaide, when the first officer, pilot flying, attempted to prevent overspeed by extending the speed brakes, the aircraft however continued to accelerate. Cabin crew was in the final stages of preparing the cabin for arrival. When the aircraft reached maximum speed the first officer pulled on the yoke overriding the autopilot, causing the autopilot to change to Command Wheel Steering, a procedure that was routinely used to manage overspeeds on descents, this time the first officer however felt a "pinch" on the yoke associated with a sudden pitch change and a high G-load. In reaction the first officer released the backpressure from the yoke rapidly unloading the G-forces. The cabin crew, about to take their seats, thought the aircraft had encountered turbulence, two cabin crew in the back of the cabin lost their footing, one cabin crew fell onto the floor and suffered a knee injury. The first officer stowed the speedbrakes, selected level change into the autopilot and reduced the speed to 250 KIAS. The aircraft continued for a safe landing in Adelaide.

Australia's TSB released their final report concluding the probable causes of the aviation occurrence were:

Although routinely used for high-speed descents, the selected speed for the descent of 320 kt was close the aircraft’s maximum speed of 340 kt and, in combination with the increased headwind as the aircraft descended through about 8,000 ft, increased the risk of an overspeed.

- After identifying the unexpected rapid increase in airspeed, the first officer applied sufficient back pressure on the control column to override the autopilot with the intention of avoiding the impending overspeed. This increased the g loading on the aircraft to an extent where safely walking in the cabin would be difficult.

- In response to feeling the increased g loading, the first officer abruptly released the back pressure on the control column, resulting in a sudden pitch change and reduced g loading that led to the cabin crew member losing their footing and sustaining a knee injury.

Other factors that increased risk

- The Virgin Australia Airlines Pty. Ltd. procedures did not require flight crew to update the QNH, temperature deviation or the descent winds in the flight management computer after departure. In some circumstances, this would reduce the accuracy of the calculated flight path and increase crew workload in managing the aircraft’s energy state.

- At the time of the occurrence, the Virgin Australia Airlines Pty. Ltd. training included a focus on the management of overspeeds. However, the associated simulator training session had not been completed by the crew, increasing the risk that they would not react in accordance with the published operational guidance material.

Other findings

- The earlier-than-usual preparation of the cabin for landing meant that, at the time of the occurrence, the cabin crew were in the final stages of that activity. This reduced the risk of more serious and numerous injuries to the cabin crew and passengers.

The ATSB reported that the VNAV PATH flight management/autopilot mode is primarily designed to maintain a selected vertical flight path, thus permitting minor airspeed deviations while prioritizing to remain on the vertical path. 10 knots above target speed the FMS would issue a message "drag required", which would usually prompt the crew to extend the speed brakes.

The ATSB wrote in describing the CWS-P mode: "Pushing a CWS engage switch on the mode control panel engages the autopilot (A/P) pitch and roll axes in the CWS mode. At the time of the occurrence, CWS-P could also be engaged by applying 40 lb back pressure to the control column to manually override the A/P. By doing so, a desired pitch attitude could be set. The ability to manually engage CWS-P and override the A/P was later removed from the flight control computer’s software to avoid inadvertent and undetected reversion from a descent mode to CWS-P. With CWS-P engaged, the A/P manoeuvres the aircraft in response to control pressures applied by either pilot. The control pressure is similar to that required for manual flight. When control pressure is released, the A/P holds the existing attitude. While the FO’s routine use of CWS-P as a means to manage overspeeds was not part of Virgin training, it was reported used by the FO and other pilots. This was because it was an effective way to handle an overspeed on descent without disconnecting the A/P, particularly when changing weather conditions meant that an overspeed could occur with little time for the crew to react."

The ATSB reported that between 10,500 and 8,136 feet MSL the head wind increased from 30 to 39 knots, the airspeed increased from 320 to 333 KIAS. During the next 5 seconds the aircraft descended to 7,790 feet, the head wind increased to 48 knots and the airspeed increased to 339 KIAS (Vmo 340 KIAS). The first officer applied a back pressure of 41.5lbs which resulted in a vertical acceleration of +2.14G. One second later the first officer released the back pressure to 2lbs, which resulted in a minimum vertical acceleration of +0.066G.

The ATSB analysed that high speed descents at 320 KIAS was routine for the flight crew, an overspeed event was not considered likely. The crew knew there was no turbulence forecast, but had also not been informed about the changing wind situation at Adelaide.

The ATSB analysed:

the FO identified an immediate need to prevent an overspeed, and did so by pulling back on the control column and activating control wheel steering-pitch (CWS-P). This had previously been successful for the FO in addressing an impending overspeed, but the difference in this case was that the force required was larger than experienced by the FO in past situations. The flight data recorded a 41 lb back pressure on the FO’s control column with a resulting 2.14 g loading on reversion to CWS-P. Given the altitude at the time, reversion to CWS-P was considered contrary to Virgin Australia Pty Ltd’s (Virgin) guidance and training in overspeed management. However, the use of the CWS-P mode was reported common among some pilots and, in this case, had possibly become a stored condition-action pattern.

Surprise is a cognitive-emotional response to something unexpected. It results from a mismatch between the individual’s mental expectations and what actually happens around them. Experiencing surprise is a combination of physiological, cognitive and behavioural responses (Rivera and others 2014). If a pilot is not expecting things to go wrong, then the level of surprise can result in taking no action, or the wrong action (Martin 2012).

With respect to cabin preparation for landing the ATSB analysed:

The procedures for preparing the cabin for landing required t eir commencement no later than FL 200. In this case, the flight crew identified that turbulence on descent may increase the risk of injury to crew and passengers and therefore decided to initiate the cabin preparation at about FL 270. Flight data for the descent showed that this provided an additional 2 minutes before the occurrence for the crew to secure galley equipment and prepare the cabin, including seating all passengers. This reduced the risk of more serious and numerous injuries to the cabin crew and passengers.
Aircraft Registration Data
Registration mark
VH-YID
Country of Registration
Australia
Date of Registration
Airworthyness Category
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TCDS Ident. No.
Manufacturer
THE BOEING COMPANY
Aircraft Model / Type
737-8FE
ICAO Aircraft Type
B738
Year of Manufacture
Serial Number
Maximum Take off Mass (MTOM) [kg]
Engine Count
Engine
PikhldkfpibhdffgdbchdhgpkkllkqkmjpldglmAnll c Subscribe to unlock
Main Owner
Imfgnhnhlkf hfpb npAd ghbfgnpgnjkihqhjd icdcdf hnjmfgfcklmifibnkcgingc ccmpcpkkhpnicjpfpiqg pl feb Subscribe to unlock
Main Operator
Cmp neflcchgcmjddcgmjgbgncigempkpkbmmfhehig kq d c lkdddgAmlgi dAjdjl ggffcmjdmdlfegpkkffhjqbcn Subscribe to unlock
Incident Facts

Date of incident
May 9, 2015

Classification
Report

Flight number
VA-436

Aircraft Registration
VH-YID

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

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