Aviation safety investigations & reports

Operational event involving a Boeing 737, VH-VUZ, near Launceston, Tasmania on 4 January 2013

Investigation number:
AO-2013-041
Status: Completed
Investigation completed

Summary

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On 4 January 2013, a Boeing 737-800 registered VH-VUZ, departed Launceston, Tasmania on a scheduled passenger service to Melbourne, Victoria. During departure, the crew selected level change (LVL CHG) as the vertical auto-flight system mode, and a climb speed of 250 kts. The crew intended to switch from LVL CHG mode to vertical navigation (VNAV) mode later during the climb, but inadvertently overlooked that selection.

Had the crew switched to VNAV mode as intended, the aircraft would have accelerated during the climb in accordance with a programmed speed schedule. In LVL CHG mode however, the aircraft climbed at a constant speed of 250 kts until passing about flight level (FL) 260, when the auto-flight system sequenced automatically to continue the climb at a constant Mach 0.62 (which was the Mach Number corresponding to 250 kts when the changeover occurred).

As climb then continued above FL 260 at a constant Mach 0.62, airspeed gradually reduced. The unintended vertical auto-flight mode and the gradual airspeed reduction went unnoticed by the crew until the aircraft was approaching FL 350, when a ‘buffet alert’ caution appeared on the Control Display Unit and the auto-flight system made a small reduction in aircraft pitch attitude. At that moment, the crew noted that the airspeed had reduced to near the top of the amber bar on the airspeed indicator, representing the aircraft minimum manoeuvre airspeed.

In responding to recognition of the minimum manoeuvre airspeed condition, the crew reduced the aircraft pitch attitude to the point that the aircraft entered a shallow descent. Soon after, the crew was able to establish an accelerated climb to the intended cruising level of FL 360. Recorded data indicates that that aircraft reached a minimum speed of 201 kts, about 6 kts below the minimum manoeuvre airspeed at that moment.

The occurrence highlights the importance of consistent attention to auto-flight system modes and aircraft energy state. Since this occurrence, the operator has introduced a procedure requiring the announcement of flight mode annunciation changes. The operator also included mode awareness briefings during a 2013 recurrent training program.

 

 Aviation Short Investigation Bulletin - Issue 30

Download Final Report
[Download  PDF: 208KB]
 
 
Alternate: [Download  DOCX: 259KB]
 
General details
Date: 04 January 2013   Investigation status: Completed  
Time: 0620 EDT   Investigation level: Short - click for an explanation of investigation levels  
Location   (show map): Launceston Airport    
State: Tasmania   Occurrence type: Control - Other  
Release date: 26 May 2014   Occurrence class: Operational  
Report status: Final   Occurrence category: Incident  
  Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-VUZ  
Serial number 39921  
Operator Virgin Australia  
Type of operation Air Transport High Capacity  
Sector Jet  
Damage to aircraft Nil  
Departure point Launceston, Tas.  
Destination Melbourne, Vic.  
Last update 14 November 2018