Challenge Assumptions: ATSB on A330 with a u/s GPS
The Australian Transport Safety Bureau (ATSB) have issued their report on an occurrence were after take off, a ‘gut feeling’ resulted in challenging assumptions that had been made during flight planning, resulting in a change to the mid-Pacific alternate.
The Occurrence Flight
The ATSB say that:
On the morning of 4 August 2016, Qantas Flight QF61, an Airbus A330 aircraft, registered VH‑QPC, was prepared for departure from Brisbane Airport, Queensland, for a flight to Narita Airport, Tokyo, Japan. The flight was dispatched with one of the aircraft’s two global positioning systems (GPS) units recorded as unserviceable using Saipan Airport as their enroute alternate.
There had been a pre-flight discussion about this:
The captain initially thought that two GPS units were required at dispatch because the Saipan Airport runway 25 approach procedure required GPS. However, after further discussion they decided that they were mistaken as maintenance and flight dispatch were aware one GPS was unserviceable and the captain had very few past experiences of an incorrect serviceability requirement at dispatch. One flight crew member then mentioned that the flight crew operating manual indicated one GPS unit was required. The captain referred to the MEL [which] indicated only one GPS unit was required at dispatch. However, the associated operational procedures indicated that ‘primary means GNSS approval’ was required at dispatch if the alternate airport arrival procedure requires GPS navigation. ‘Primary means GNSS approval’ indicated the requirement for two operational GPS units.
…the captain…considered the reference to ‘alternate’ to be a reference to destination alternate airport and not to an [Extended Diversion Time Operation] EDTO alternate airport. They planned to use Saipan as an EDTO alternate airport …[which]…fitted with their expectation that the unserviceable GPS 2 MEL item was acceptable for their flight.
Satisfied, the aircraft departed from Brisbane with 12 crew and 231 passengers on board.
As QF61 travelled north along the east coast of Australia, the captain became uncomfortable with their decision to accept the aircraft with an unserviceable GPS. The captain reviewed the flight plan and the publications, and concluded they needed two serviceable GPS units for their planned flight to use Saipan Airport as an alternate. The captain identified Guam Airport as a suitable alternate airport in lieu of Saipan Airport and the flight continued to Narita Airport and landed without further incident.
The ATSB notes that in large organisations there may be multiple departments with responsibilities for the dispatch of an aircraft. Whereas procedures are normally executed within a department, processes often involve multiple departments. Cross-checks occurred within the [Integrated Operations Centre] IOC and separately among the flight crew during this incident flight, but the cross-checks were not conducted between the departments, where personnel had a different mental model of the situation. The flight dispatcher believed the GPS 2 MEL item would be cleared before flight and the captain believed the flight was planned to be released with the GPS 2 as an unserviceable MEL item.
ATSB Safety Message
This incident highlights the importance of personnel challenging their own assumptions when something does not appear right in the environment. After the dispatch of QF61 from Brisbane Airport, the captain experienced a ‘gut feeling’ that something was not right. Rather than ignore their sense of unease, the captain reviewed the flight plan and company documents, identified the problem and resolved the issue so that the flight could continue without compromising safety. Throughout the process, they kept the other flight crewmembers informed of the problem they had identified and their decision-making, which enabled the crew to provide feedback to the captain.
The learning from such a relatively minor occurrence would not normally be shared so widely. ATSB are to be commended for doing so. The concept of intuition / gut-feel among professionals is examined in depth in Gary Klein‘s excellent book: Sources of Power: How People Make Decisions
Other Safety Resources
UPDATE 5 August 2018: Improvised Troubleshooting After Cascading A330 Avionics Problems