Thai B773 at Melbourne on Jul 24th 2011, descended below minimum safe altitude
Last Update: February 19, 2013 / 15:18:57 GMT/Zulu time
Incident Facts
Date of incident
Jul 24, 2011
Classification
Incident
Aircraft Type
Boeing 777-300
ICAO Type Designator
B773
Contributing safety factors
- The pilot in command probably experienced an ‘automation surprise’ when the aircraft’s vertical navigation (VNAV) flight management system automatically changed from VNAV speed mode to VNAV path mode.
- The flight crew did not manage the aircraftÂ’s descent in accordance with the prescribed instrument approach procedure, resulting in descent below the applicable approach segment minimum safe altitude.
Other safety factors
- The flight crewÂ’s selection of flight level change mode during the approach increased the risk of inadvertent descent below the required vertical flight path.
Other key findings
- Following the tower controllerÂ’s initial instruction to go around, there was a delay of about 50 seconds before the flight crew selected take-off/go-around thrust and commenced a climb to the required altitude.
The ATSB reported the first officer was pilot flying on a VOR approach to Melbourne's runway 34, the aircraft had been cleared to descend to 3000 feet, visual meteorologic conditions existed with 8km visibility in rain. The autoflight systems were engaged in lateral navigation (LNAV) and vertical navigation (VNAV), a speed of 230 knots as well as 3000 feet were selected into the master control panel (MCP). At about 3300 feet the FMS switched from VNAV SPD to VNAV PATH to ensure compliance with the initial VOR approach altitude constraint of 3000 feet. As result the aircraft pitched up to maintain level flight. The crew however wanted the descent to continue, selected 210 knots and 2000 feet into the MCP and changed to flight level change (FLCH) mode. Flaps 1 were selected, 190 knots and 3000 feet selected into the MCP and the autopilot captured the altitude of 3000 feet, the FLCH mode automatically changed to altitude to maintain the selected altitude.
A minute later the crew selected 970 feet (effective minimum descent altitude (MDA) at the time) into the MCP and activated FLCH mode again, autothrottle retarded the thrust levers to idle and the aircraft descended while the autopilot intercepted the final approach course.
The crew reported the runway in sight and was cleared for a visual approach to runway 34 provided the aircraft was " ... established on PAPI and inside the circling area".
About 7nm before the runway threshold and an altitude of 1284 feet the crew disconnected the autopilot. The tower controller observed the aircraft both on radar and visually being low on the approach and queried "check altitude", 4 seconds later he instructed the crew to go around and carry out the missed approach procedure. The crew acknowledged "climbing", the aircraft continued to descend reaching the lowest altitude of 984 feet before slightly climbing again. 50 seconds after the instruction to go around the aircraft was at 1167 feet and the tower queried whether the aircraft was going around.
The crew responded they were climbing, then maintaining 1200 feet and stated they were on a visual approach, the controller replied "negative, missed approach runway 34, climb to 4000 ft", after which the crew climbed the aircraft to 4000 feet.
The captain then assumed control, positioned the aircraft for another VOR approach to runway 34 and landed the aircraft utilizing LNAV and VNAV automation modes.
The ATSB reported the captain provided testimony that he was surprised about the pitch up at about 3300 feet and had not seen this before. He attributed the pitch to a VNAV malfunction and was unsure whether the VNAV function would work after being re-engaged. Based on that assessment he instructed the first officer to use the FLCH function. While turning final they were flying through a rain shower obscuring his view to the runway and he was busy on radio, when he got sight of the runway the PAPIs were showing 4 reds and they were really low, hence he instructed the first officer to stop the descent and climb a little in order to regain the vertical profile, when the controller queried to check altitude they were already climbing.
The ATSB analysed that the pitch up at 3300 feet was in accordance with the system design in order to ensure compliance with the initial VOR approach altitude constraint of 3000 feet. At that time the FMS computed the required altitude for the vertical profile at about 3400 feet, the aircraft at 3300 feet was thus already below the computed vertical profile prompting the FMS to command the aircraft maintain level flight until establishing on the vertical profile. The captain had flown several non-precision approaches to Melbourne's runway 34 before and had not seen this behaviour, it can thus be concluded that the aircraft had always been at or above the vertical profile during these approaches.
The ATSB stated: "the unanticipated pitch-up caught the PIC (pilot in command, captain) by surprise to the extent that the PIC believed that the VNAV function was malfunctioning. In response, the PIC changed the MCP altitude to 2,000 ft and selected FLCH mode. While it was likely that those actions were intended to arrest the pitch change and continue the descent, they were symptomatic of ‘automation surprise’ on the part of the PIC, probably due to a lack of AFDS (autopilot flight director system) mode appreciation."
By selecting the MDA into the MCP and changing to FLCH the crew had the aircraft descent to the selected altitude at a high rate of descent resulting in the aircraft descending below the sector's minimum safe altitude of 1950 feet about 3nm before reaching 6.5 DME, after which the minimum safe altitude reduced to MDA. The aircraft reached its lowest altitude of 984 feet at DME 7.9 with the minimum safe altitude still at 1950 feet. The ATSB analysed: "Regardless of the level of automation being used, it was the responsibility of the flight crew to monitor the approach to ensure that the aircraft tracked and descended in accordance with the published procedure. In this case, the selection of FLCH mode, coupled with an inappropriate MCP altitude setting, resulted in the flight crew not managing the aircraftÂ’s descent in accordance with the prescribed instrument approach procedure."
With respect to the delayed go-around the ATSB analysed: "While the flight crew did arrest the aircraftÂ’s descent, the time delay between the tower controllerÂ’s initial go-around instruction and selection of go-around thrust was about 50 seconds. On this occasion, the tower controller issued the go-around instruction because the aircraftÂ’s was unusually low on the approach. Equally, however, that instruction may have been issued to ensure separation from other traffic or terrain. In that case, a delay of 50 seconds could have resulted in a more hazardous situation."
The airline took a number of safety actions as result of the occurrence.
Incident Facts
Date of incident
Jul 24, 2011
Classification
Incident
Aircraft Type
Boeing 777-300
ICAO Type Designator
B773
This article is published under license from Avherald.com. © of text by Avherald.com.
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