On the morning of 18 July 2014, a Boeing 777, registered A6-ECO, was on descent into Melbourne, Victoria via an ARBEY 4U Standard Arrival Route (STAR) for the Area Navigation U (RNAV-U) Required Navigation Performance (RNP) runway 16 approach. There was some cloud and showers in the area at the time, with the wind from the south-west.

The ARBEY 4U STAR required that the aircraft track from ARBEY to BUNKY, then to the Bolinda (BOL) non-directional (radio) beacon. The STAR chart depicted a minimum en route altitude of 3,400 ft and a minimum terrain clearance altitude of 3,700 ft, between BUNKY and BOL. The RNAV-U (RNP) runway 16 approach chart included an ‘at or above’ 4,000 ft altitude restriction at BOL when joining the approach from the ARBEY STAR.

As the aircraft approached BUNKY, air traffic control (ATC) cleared the crew to descend to 4,000 ft, and cleared the crew for the RNAV-U (RNP) runway 16 approach. ATC radar data shows that the aircraft overflew BUNKY at 5,000 ft, then continued descent, passing through 4,000 ft about 5 NM prior to BOL. Descent then continued, leading to an ATC Minimum Safe Altitude Warning alert as the aircraft descended through about 3,400 ft, about 4 NM prior to BOL. ATC questioned the crew about their altitude, and advised them that the relevant radar lowest safe altitude was 3,200 ft. Moments later the aircraft passed over BOL at about 3,000 ft and maintained that altitude until intercepting the vertical profile of the RNAV-U (RNP) runway 16 approach. The approach continued for an uneventful landing.

The operator’s investigation found that descent below the 4,000 ft altitude restriction at BOL occurred because the crew programmed the Flight Management Computer (FMC) to overfly BOL at a ‘hard altitude’ of 3,000 ft. The operator’s investigation found that the potential for deviation below the 4,000 ft minimum altitude restriction at BOL was increased by factors related to aeronautical charts and the FMC navigation database.

For operators, this incident highlights the need for careful attention to FMC navigation data management, particularly any procedures that relate to crew modification of navigation data. Operators should remain mindful that any manipulation of FMC navigation data by flight crew has the potential to introduce errors. Additionally, operators are encouraged to work closely with aeronautical information service providers to ensure that aeronautical charts (and any other operational information) are presented in a manner that minimises ambiguity and reduces the potential for misinterpretation. For flight crew, this incident highlights the need for careful attention to approach procedure documentation and FMC navigation data management.


Aviation Short Investigations Bulletin - Issue 43

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