Description

On 9 May 2008, the crew of a Boeing 737-800 (PK-GEF) being operated on a scheduled passenger flight from Denpasar, Indonesia to Perth made a low go around after observing vehicles on the closed section of the runway without apparently recognising where the expected temporarily displaced landing threshold was located. A second similar approach led the TWR controller to instruct the aircraft to go around but instead, the aircraft was observed to fly level at a low height before eventually touching down just beyond the correctly notified and marked displaced threshold.

Investigation

The occurrence was classified as an “Operational Non Compliance” by the Australian Transport Safety Bureau (ATSB) but a full Investigation was carried out into the circumstances. It was established that both flight crew were familiar with Perth but the occurrence arrival was their first since the runway works requiring a landing threshold temporarily displaced by 888 metres had commenced. The crew were aware of the Notice To Airmen advising of this work prior to making their approaches and it was noted that both the landing clearances they had received included reference to the displaced threshold. The prevailing weather conditions were good.

On the first approach, using a LOC-only procedure because the Instrument Landing System (ILS) GS was de-activated due to the displaced threshold, the crew had queried the presence of vehicles on the runway with ATC when about 15 seconds from landing and elected to conduct a go around. On the second approach, the TWR controller “recalled observing the aircraft on what appeared to be an approach to land on the closed section of the runway and instructed the flight crew to go around”. This instruction also “included information to assist the flight crew in identifying where the aircraft should be landed”. The flight crew of an aircraft waiting to depart reported that the 737 had flown level over the runway works area at between 30 feet and 50 feet agl before landing soon after the displaced threshold. The 737 flight crew subsequently stated that during the second approach, “they only heard a clearance to land and did not recall hearing the instruction to go around or any other information”. It was considered by the Investigation that the additional information on the location of the displaced threshold in the context of their “high workload” at that time “may have momentarily confused them” with the result that neither of them assimilated or acted on the instruction to go around.

The Runway activity which had required the (correctly marked) displaced threshold was found to involve lighting installation and to only involve partial runway closure between 0730 and1700 local time each day. This meant that there was no requirement to obscure the permanent threshold and touchdown markings and it was only necessary to identify the works/closed area using four 6 metre crosses on the runway surface. The flight crew involved subsequently stated that they had not seen these crosses on their first approach and had seen them “too late” on their second approach. They also stated that upon disconnecting the AP and checking the temporary Visual Approach Slope Indicator Systems positioned to guide aircraft to the correspondingly displaced touchdown zone, they had noted that “the aircraft was undershooting the approach path to the displaced threshold”.

The Investigation reviewed the process by which the intended work was initially promulgated - a “Method of Working Plan (MOWP)” advising of work which would involve the partial closure of runway 21 occurring when notified during the period 23 March 2008 to 9 May 2008. This document included draft NOTAMs which it was intended would be used at various stages of the work but there was no indication in the MWOP or the draft NOTAMs that the effect of those works on the final vertical profile from the from the MDA of the runway 21 LOC-only approach had been considered. It was found that the MWOP had been sent to the Operator’s Perth airport office and had not then been copied from there to the Operator’s Flight Operations Department in Indonesia. The latter advised that had they been aware of the plan, they would have issued a Flight Crew Notice to support pilot awareness.

The Investigation also reviewed differences between relevant Australian regulatory requirements and the corresponding ICAO SARPs. ICAO Annex 14 specified at the time of the occurrence that “when a runway threshold is temporarily displaced from the normal position it shall be marked as shown …and all markings prior to the displaced threshold shall be obscured except the runway centre line marking, which shall be converted to arrows” (the illustrations referred to in this text are reproduced in the Investigation Report). The form of displaced threshold markings detailed in Annex 14 for temporary threshold displacement required a line of arrow heads pointing to a transverse line across the runway marking the position of the temporary threshold, with ‘continue’ arrows replacing the normal centreline markings on the closed section of runway. Only permanently closed runways or runway sections were required to be marked with crosses and in this case, 36 metre crosses were recommended. Australian requirements - which had been met by the Perth airport operator - required only that the position of thresholds temporarily displaced for “less than 5 days” should be indicated by arrow heads on either side of the runway and there was no requirement to obscure the permanent markings on the closed runway section. For the situation at Perth, crosses were required on the closed runway section but only ones of 6 metres rather than 36 metres. The difference between the visibility from the approach of the small and large crosses was compared by the Investigation - see the illustration below:

The Investigation noted that although the Australian Regulator CASA had filed differences with ICAO in respect of the use of displaced threshold markings, they had not done so in relation to closed runway markings.

In seeking to understand the factors that may have led to the inadequate recognition by the crew of the runway works and their resulting actions, the Investigation concluded that:

• it was unclear why the visual slope guidance provided by the temporary Visual Approach Slope Indicator Systems had not been followed.

• although it might have been expected that on the second approach, the crew would have managed their final descent more diligently, the fact that they did not indicated that in this instance, the temporary markings used to alert pilots to the closed section of the runway were ineffective.

• the relative complexity of the instructions given to the crew when it became clear that they may be about land within the closed section of runway may have had unintended and confusing consequences.

• the use of 6 metre crosses rather than the ICAO-recommended 36 metre ones to show an area of closed runway had increased the chances that a visual approach to the normal runway TDZ would be flown.

• the ICAO-recommended obscuration of the permanent markings on the closed section of runway would have increased the likelihood that the crew would identify the displaced threshold.

• the issue of a Flight Crew Notice by the Operator could have increased awareness of the NOTAM’d work.

It was concluded that “the permanent runway 21 threshold and touchdown markings were not required to be obscured and were clearly visible to the flight crew. Those markings continued to provide approach and landing cues to the normal touchdown zone, which was located within the runway works area. The use of 6m closed runway markings, in lieu of 36m markings as recommended by the International Civil Aviation Organisation (ICAO), increased the risk of a flight crew conducting a visual approach to the still-visible permanent threshold/touchdown area.”

The formally stated Findings of the Investigation were as follows:

There were two Contributing Safety Factors:

• The existing runway 21 threshold and touchdown markings were not required to be obscured and were clearly visible to the flight crew. Those markings continued to provide approach and landing cues to the normal touchdown zone, which was located within the runway works area. [Safety Issue]

• The use of the 6 m closed runway markings in lieu of the recommended 36 m markings increased the risk of a flight crew conducting a visual approach to the permanent threshold/touchdown area. [Safety Issue]

There were four ‘Other Safety Factors:

• The combination of the instruction to go around with landing information, and high workload at that stage of the approach, may have momentarily confused the flight crew, with the effect that they did not assimilate and act on the instruction to go around.

• The distribution system that was used by the airport operator to disseminate the Method of Working Plan (MOWP) did not ensure that all users of the airport were appropriately notified of the planned runway works. [Safety Issue]

• There was no follow-up action taken by the airport operator to address the lack of responses from aircraft operators to the MOWP, as required by the receipt and acknowledgement system.

• There was no evidence of any consideration in the MOWP of the effect of the runway 21 works on the final approach profile necessary from the MDA of the runway 21 localiser (LLZ) approach. [Safety Issue]

There was one Other Key Finding:

• The runway markings associated with the MOWP were in accordance with the Regulator’s Manual of Standards (MOS).

Safety Action in response to the Safety Issues raised above was noted to have included the following:

• The Airport Operator undertook a review of its distribution list and procedures, to ensure that future correspondence is dispatched via hard copy and e-mail to all appropriate stakeholders. Although the airport operator’s receipt and acknowledgement system was not identified by the investigation as a safety issue, at the time of drafting this report, the airport operator was investigating methods for implementing a more robust verification system. The operator’s aim was to encourage a better response from recipients.

• The Airport Operator is in the process of implementing a revised process to manage critical airside works. That revised process will include the establishment of a safety group for each project, who will review the relevant project’s draft MOWP and assess all risks associated with the works and the effectiveness of any safety procedures. In addition, it is intended that during the initial planning stages of any works, the project manager and manager airside safety will determine the necessary restrictions to the works and to aircraft operations, consulting where necessary with the Civil Aviation Safety Authority (CASA) and with (ANSP) Airservices Australia. The consultation process will include an assessment of the likely impact of the planned works on the airport’s navigational aids and associated published instrument approach procedures.

• The Civil Aviation Safety Authority is, in response to the possible safety benefits of the use of the ICAO-recommended 36 metre closed runway markings in Australia when the affected permanent threshold and touchdown markings were not required to be obscured, intending to consult with the industry on a proposal to change to the ICAO system for closed runway markings.

The Final Report of the Investigation was published on 30 June 2009. No Safety Recommendations were made as a result of it.

Further Reading

• Runway Excursion
• Runway Visual Perspective
• Runway Maintenance
• Visual Approach Slope Indicator Systems (VASIS)
• Cross-checking Process
• Situational Awareness
• ATC Expectation Bias