Qantas B738 enroute on Feb 25th 2013, autoapproach activated at FL390
Last Update: April 27, 2015 / 11:32:57 GMT/Zulu time
Incident Facts
Date of incident
Feb 25, 2013
Classification
Incident
Airline
Qantas
Flight number
QF-962
Departure
Canberra, Australia
Destination
Brisbane, Australia
Aircraft Registration
VH-VYE
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
The occurrence became known on Aug 6th 2013 by notification of opening an incident investigation by Australia's ATSB. The ATSB stated that the focus of the investigation is human factors and autopilot mode awareness, the ATSB is also looking into the criteria for autoflight systems intercepting glideslope and localizer signals. The investigation is estimated to conclude by January 2014.
The incident aircraft remained on the ground in Brisbane for about 10.5 hours following the occurrence, then departed for its next revenue flight.
On Apr 27th 2015 the ATSB released their final report concluding the probable causes of the incident were:
Contributing factors
- Depending on the auto-flight and instrument landing system frequency selections by the flight crew, the configuration of the auto-flight system logic on the operator's Boeing 737 fleet allowed the aircraft to capture and follow a glideslope signal despite not being established on the localiser.
- The flight crew inadvertently selected the approach push-button after reaching cruising altitude, which was not detected for an extended period, allowing the aircraft's auto-flight system to capture the glideslope signal at cruise altitude while still about 213 km from the destination.
- Following departure from Canberra the instrument landing system frequency for that airport, which was the same as for the system at Brisbane Airport, remained active on the aircraft’s navigation control panel, permitting the auto-flight system to capture and follow the glideslope signal as the aircraft approached Brisbane.
- Contrary to their intent, the flight crew did not re-engage the autopilot after the climb associated with the glideslope capture approaching Brisbane, resulting in the aircraft laterally deviating from the flight planned track.
Other factors that increased risk
- The captain conducted significant non-aviation work when free from flight duty, which had the potential to lead to ineffective rest and cumulative fatigue.
The ATSB analysed:
An incorrect or inadvertent mode selection is not ideal and in itself may not contribute to an undesired autopilot control input. However, this occurrence shows that a combination of conditions/selections can result in the autopilot climbing the aircraft to capture the glideslope.
The conditions for the autopilot-initiated capture of the glideslope were that the departure and destination ILS frequencies were the same, this frequency remained active in the aircraft’s navigation system and there was a subsequent selection of the approach (APP) push-button on the aircraft’s auto-flight system. This selection armed the glideslope and localiser, meaning that when the aircraft was within range of the Brisbane ILS signal, the glideslope would become the active vertical flight mode. The combination of glideslope as the active vertical mode and an auto-flight system that allowed glideslope capture despite not being on the localiser allowed for the autopilot to command the aircraft to capture the signal.
The Qantas Airways Limited (Qantas) procedures would normally prevent the capture of a glideslope as, in accordance with these procedures, the APP push-button would not normally be selected before the aircraft was on an inbound intercept to capture the localiser. In this occurrence, this defence was negated as the crew did not detect that they had inadvertently selected the APP push-button during the cruise phase of flight.
...
The APP push-button was manually selected by the crew soon after reaching the cruise altitude of flight level (FL) 410. As the crew were aware that the approach mode should not be armed that early in the flight, the selection was considered to be inadvertent.
With respect to the captain's fatigue levels the ATSB analysed: "The captain’s roster met the relevant requirements and did not appear to be unduly problematic, although multiple four-sector days with duty periods over 11 hours has the potential to increase the likelihood of fatigue. Of more concern is that the captain reported conducting an average of 10 hours work unrelated to their flying employment on days where they were not conducting flight duties. This level of work has the potential to lead to cumulative fatigue and interfere with a person’s ability to ensure that they are getting adequate rest periods between rostered flight duty periods. It is very difficult for an operator to control such activities and there is a significant onus on the crew member to ensure that they are appropriately rested prior to reporting for duty."
Aircraft Registration Data
Incident Facts
Date of incident
Feb 25, 2013
Classification
Incident
Airline
Qantas
Flight number
QF-962
Departure
Canberra, Australia
Destination
Brisbane, Australia
Aircraft Registration
VH-VYE
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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