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Altimeter oversight caused Nauru 737 to go below safe altitude

Nauru Airlines has enhanced pilot training in a range of navigation and approach areas after a serious incident involving a Boeing 737-300 that flew below the minimum descent altitude on a night approach to Kosrae on 12 June 2015.

Due to several delays, the aircraft (VH-NLK), operating the carrier’s inaugural scheduled service from the Marshall Islands to Kosrae, reached the island at night in deteriorating weather conditions, says the Australian Transport Safety Bureau (ATSB) in its final report into the incident.

“The only instrument approach available for use was an offset procedure based on a non-precision navigation aid,” says the ATSB. “The risk associated with this type of approach was amplified due to the need to use a 'dive and drive' style technique instead of a stable approach path, and that it required low level circling maneuvering from the instrument approach to align the aircraft with the runway.”

In addition, there was high ground in the area and the airport’s control tower was unmanned owing to the flight’s late arrival.

On the VH-NLK’s first approach, the crew did not follow checklist procedure, which called for changing the atmospheric pressure.

“Leaving the altimeters’ subscale set to the standard atmospheric pressure setting of 1013hPa, and not setting the subscale to the local barometric pressure of 1007hPa, resulted in the indicated altitude over-reading, such that when the altimeter indicated 500ft, the aircraft’s actual altitude was about 320ft above the mean sea level,” says ATSB.

During the approach, there were three enhanced ground proximity warning systems (EPGWS) ‘too low terrain’ warnings in the cockpit. The crew, thinking the issue was due to ‘map shift’ caused by an inaccurate flight management system information, disengaged all three alerts.

The flight crew then lost visual contact with the runway and initiated a missed approach.

“The captain was experiencing fatigue and the flight crew had an increased workload and stress due to the inaugural regular public transport flight into Kosrae at night in rapidly deteriorating weather. As a result, the crew’s decision making and task execution on the missed approach were affected, and the aircraft state, airspeed, and attitude were not effectively monitored by either crew member.”

Following the missed approach the crew climbed to 4,000ft. The crew identified that the altimeters were set to the wrong barometric pressure, and corrected this. Following this, the aircraft landed at Kosrae.

The pilot had 16,000hrs of experience, and the first office 3,300hrs. Overall, there were seven crew and 69 passengers on the aircraft. Passengers included Nauru’s president, its aviation minister, and the chairman of Nauru Airlines. The pilot had gotten little sleep before the flight owing to his mother’s serious illness, but felt obligated to fly owing to the presence of dignitaries aboard the jet.

“This occurrence highlights the importance of flight crews declaring any instances of acute fatigue and stress-inducing circumstances that may have an impact on their flying performance,” says the ATSB.

“Operators also need to remind flight crew of the importance of their decisions with regards to their fitness to fly. For flight crews, the importance of completing approach checklists and monitoring the approach at safety critical times is emphasized. For operators, the occurrence highlights the importance of incorporating dual-engine go-arounds into simulator training sessions.”

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