Nauru B733 at Kosrae on Jun 12th 2015, GPWS warning on final approach
Last Update: March 16, 2018 / 15:59:49 GMT/Zulu time
Incident Facts
Date of incident
Jun 12, 2015
Classification
Incident
Airline
Nauru Airlines
Flight number
ON-33
Departure
Majuro, Marshall Islands
Destination
Kosrae, Micronesia
Aircraft Registration
VH-NLK
Aircraft Type
Boeing 737-300
ICAO Type Designator
B733
The ATSB opened an investigation (aircraft on the Australian aircraft register) and subsequently annotated, that the investigation so far made clear the occurrence is more complex than initially thought, the investigation has been re-scoped with additional resources allocated to the investigation.
On Mar 23rd 2016 the NTSB reported that they have appointed an accredited representative representing the country of manufacture and design of the aircraft and joined the investigation by the ATSB.
On Mar 16th 2018 the ATSB released their final report concluding the probable causes were:
Contributing factors
- The flight crew did not complete the approach checklist before commencing the non-precision NDB approach into Kosrae. As a result, the altimeters' barometric pressure settings remained at the standard setting of 1013 hPa instead of being set to the reported local barometric pressure of 1007 hPa. The flight crew descended the aircraft to the minimum descent altitude of 500 ft as indicated by the altimeters, however, due to the barometric pressure setting not being reset, the aircraft descended to a height significantly below 500 ft.
- The crew descended the aircraft in IMC and at night below the approach profile for the Kosrae runway, resulting in EGPWS alerts. Terrain clearance assurance was eroded further by the flight crew not correcting the flight profile until the flight crew lost visual contact with the runway.
- The flight crew's belief that the EGPWS warnings were due to a decreased navigational performance and not terrain proximity led to their decision to inhibit the first EGPWS warning and not correct the flight path.
- Due to the captain’s fatigue and the increased workload and stress associated with the inaugural regular public transport flight into Kosrae at night in rapidly deteriorating weather, the crew’s decision making and task execution on the missed approach were affected.
Other factors that increased risk
- The crew’s recurrent training had not included B737-300 full thrust go-around simulations.
- The operator commenced regular public transport operations into Kosrae with the only instrument approach available for use being an offset procedure based on a non-precision navigation aid. 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 manoeuvring from the instrument approach to align the aircraft with the runway. Furthermore, there was very high terrain in close proximity to the runway and the airport did not have a manned air traffic control tower. For this occurrence, the risk was further elevated as a result of the approach being conducted at night-time in poor weather conditions. [Safety issue] (Emphasis added by ATSB)
The ATSB described the sequence of events:
The flight crew stated that, prior to commencing the descent for Kosrae, they obtained the weather and the local QNH.5 The weather had deteriorated from that forecast (see section titled Meteorological information). The flight crew also stated that, during the descent and approach, the local flight information service radio operator6 provided a considerable number of weather updates on the local airport conditions at Kosrae. Visibility was around 3 NM, rain showers were in the area with low cloud and wind ‘pretty much straight down the strip for (runway) 05’. The captain, as pilot flying, conducted the briefing for the non-directional beacon (NDB) /distance measuring equipment (DME)8 approach to runway 05 (Figure 1). The captain stated that, at this time, they had made special mention of the unusually low transition level9 of FL 55.
The captain stated that at most airports they operated into, the transition level was between FL 110 and FL 130. The crew then completed the descent checklist. They had decided that, based on the expected weather conditions, they would make two approach attempts, and if they ould not land, would divert to Nauru Airport, the nominated alternate airport. Prior to descending below the transition level, the crew did not complete the approach checklist, which consisted of one item: set the altimeters to the local QNH and crosscheck them. Leaving the altimeters’ subscale set to the standard atmospheric pressure setting of 1013 hPa, and not setting the subscale to the local barometric pressure of 1007 hPa, resulted in the indicated altitude over-reading, such that when the altimeter indicated 500 ft, the aircraft’s actual altitude was about 320 ft above the mean sea level.
At about 0856, the aircraft passed overhead the NDB at 5,000 ft, and continued the descent, tracking outbound on a heading of 300°, to about 10 NM from the NDB (10 DME). The flight crew were controlling the aircraft through the auto-flight systems, with an autopilot and the autothrottle engaged. At this point, the crew turned the aircraft left, and at 0901, the aircraft intercepted the inbound track to the NDB at about 1,800 ft. The crew selected the landing gear down at 1,500 ft, and flap 15 at 1,250 ft.
The crew stated that they established visual contact with the runway as the aircraft passed through 900 ft indicated altitude, about 5 NM from the DME. At about 740 ft indicated altitude, the crew selected flap 25. The crew elected to delay selection of the nominated landing flap of 40 degrees until they made positive visual contact with the runway. They did not subsequently select flap 40 on that approach.
As the aircraft descended to the minimum descent altitude for the approach of 500 ft, the captain selected the altitude hold (ALT HOLD) mode to level the aircraft at 500 ft indicated altitude. At 0903:13, an Enhanced Ground ProximityWarning System (EGPWS) Terrain Clearance Floor (TCF) alert (see section titled EGPWS alerts) sounded, and lasted for 5 seconds (Figure 2). The aircraft was over water, at 368 ft radio altitude.10 The crew reported that they were in visual meteorological conditions (VMC) at night, with the runway lights in sight. The crew stated that, at the time, they believed the EGPWS alert was due to a ‘map shift’ in the aircraft’s navigation position (see section titled The navigation function of the flight management system). The flight crew selected ‘terrain inhibit’, which cancelled the current EGPWS TCF alert. The crew were not aware that the EGPWS had its own internal GPS.
At 0903:19, the aircraft was at 4.31 DME, 480 ft indicated altitude and 340 ft radio altitude, and descending at about 313 fpm, when the EGPWS TCF alert again sounded, and lasted for 12 seconds. The aircraft maintained 480 ft indicated altitude for about 12 seconds, before descending again.
The crew reported losing visual reference with the runway when the aircraft was about 3 NM from the DME. In response to losing visual reference, the captain disconnected the autopilot and autothrottle and pressed the take-off/go-around (TOGA) switches on the thrust levers. At this time the recorded aircraft pitch angle was 9.5°. The flight data recorder data showed that TOGA was selected at 0903:47, at 448 ft indicated altitude, or 304 ft radio altitude (see Figure 3), and the aircraft was about 3.5 NM from the DME. At this time, the aircraft’s computed airspeed reduced to 129 kt.
The captain stated that he pressed the TOGA switches on the thrust levers once. In the Flight Director engaged go-around mode, one TOGA switch press results in a reduced thrust autothrottle setting, and two presses of the TOGA switch advances the autothrottle to full go-around thrust (see section titled Autothrottle go-around modes). The crew stated that the aircraft pitch angle was initially raised to 15°, however, the captain observed the airspeed decay and pitched the aircraft down to increase the airspeed. The first officer stated he called ‘sink rate’ twice. The captain then realised and rectified the situation, depressing the TOGA switch a second time commanding full go-around thrust.
At 0903:53, the aircraft was at 3.3 DME, and the third EGPWS TCF alert sounded, which lasted for 10 seconds. The aircraft was then at 384 ft indicated altitude, or 244 ft radio altitude, and descended 5 seconds later to its lowest radio altitude of 200 ft before climbing.
At 0904:04, the flaps were retracted to 15° and the aircraft reached its maximum pitch up angle of 16°. Two seconds later, the flaps were retracted to 10°. From the time the captain set the thrust to TOGA until the aircraft was stabilised on the missed approach path (at about 0905), the recorded aircraft pitch angle varied from -0.35° to +16°.
When the aircraft was established on the missed approach heading, the captain continued a climb to 4,000 ft. After stabilising the aircraft in the missed approach, the crew identified that the altimeters were still set to 1013 hPa and corrected them to the local area QNH. After repositioning overhead, the NDB at 4,000 ft, the crew then conducted a second approach and the aircraft landed at Kosrae without further incident.
The ATSB analysed:
The crew had briefed the new transition level at top of descent and had briefly discussed the need to conduct the approach checklist on passing through the transition level when they were descending through FL 130 (the usual transition level). The crew had put a plan in place to complete the checklist. However, as they were flying over the non-directional beacon at FL 050 and were looking at the runway in preparation for the commencement of the approach, workload began to rise and they forgot to return to the approach checklist and complete it, as per a prospective memory error. Thus, as the aircraft descended through the transition level, the altimeters were not set to the local barometric pressure from the standard pressure setting of 1013 hPa and, as a result were over-reading the aircraft’s altitude by 180 ft. The aircraft’s actual height was, on average, 120 ft lower than the aircraft’s indicated altitude.
The ATSB continued analysis:
The flight crew stated that they heard the EGPWS warning prior to their reaching the missed approach point (MAP) and that they had the runway lights in sight although the visibility was fluctuating, and decided to keep going because they had a visual reference for the runway. Furthermore, the flight crew thought the warnings were due to map shift. Therefore, the flight crew believed the warnings were false and inhibited the first warning due to this belief.
...
The investigation found that the approach was being conducted at night in intermittent visual conditions prior to the aircraft going into IMC conditions just prior to the MAP. In these conditions, the EGPWS warnings should have been treated as genuine and action taken immediately rather than the crew inhibiting the alert.
The ATSB continued:
The captain’s mother had become ill and her health had severely degraded by the time the captain arrived in Nauru from Brisbane. The captain stayed with her at the hospital for as long as possible before returning to his hotel where he had a disrupted night’s sleep of less than 6 hours. At the time of the EGPWS warnings, the captain had been awake for about thirteen hours and reported feeling fatigued.
...
On the decision to go-around, the captain pressed the takeoff/go-around button only once instead of twice, which resulted in a reduced thrust rather than a full power go-around. The aircraft was pitched up initially to 15° but was then pitched back down to increase the decaying airspeed. Soon after, the aircraft was at its lowest height of 200 ft by the radio altimeter. Given the increased workload of the crew, and the effects of stress and fatigue on the captain, the aircraft state, airspeed and attitude was not effectively monitored by either crew member following the goaround decision. Therefore, the execution of the go-around task and its attendant decision making was not performed effectively.
Incident Facts
Date of incident
Jun 12, 2015
Classification
Incident
Airline
Nauru Airlines
Flight number
ON-33
Departure
Majuro, Marshall Islands
Destination
Kosrae, Micronesia
Aircraft Registration
VH-NLK
Aircraft Type
Boeing 737-300
ICAO Type Designator
B733
This article is published under license from Avherald.com. © of text by Avherald.com.
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