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18 Sep 2018 - Report issued into VH-VOP 737-800 aquaplaning on landing

What Happened

On 11 May 2015, a Boeing 737 aircraft, registered VH-VOP and operated by Virgin Australia International, conducted a scheduled passenger service from Sydney, New South Wales to Christchurch, New Zealand. Shortly after midnight, the aircraft landed on runway 29 at Christchurch. Runway 29 was shorter than the main runway at Christchurch. The aircraft landed within the required touchdown zone, using full reverse thrust, speedbrakes, and the autobrake system engaged the wheel brakes. Recorded flight data showed that the aircraft initially achieved, and at times exceeded the selected AUTOBRAKE 3 target deceleration rate. However, after crossing the runway intersection, the aircraft did not continue to decelerate as expected and the crew believed the aircraft appeared to slide or skid. In response, the crew overrode the autobrakes and applied hard manual braking while retaining full reverse thrust for longer than used in normal operations. The crew also corrected a minor directional deviation. The aircraft came to a stop about 5 m from the runway end. There were nil recorded injuries or aircraft damage.

Extracts of the report are given below but the full 71 page AustralianTSB report is available here

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What the ATSB found

The ATSB found that, due to increased workload, the crew misperceived the runway surface conditions and believed it was damp when in fact it was wet. As there was no regulatory direction on how a damp runway was to be considered for aircraft landing performance purposes, the operator’s policy was to treat a damp runway the same as a dry runway. As a result, the crew established the aircraft’s landing performance based on a dry rather than a wet runway and the expected runway 29 landing performance was not achieved.

The ATSB also found that, several months prior, the operator had changed its policy whereby damp runways had previously been treated as wet runways.

Based on the crew’s observations and a review of the available recorded data, it was very likely that the surface conditions on the later part of the runway had degraded to the extent that they adversely affected the aircraft’s braking capability. It was also possible that the aircraft experienced viscous aquaplaning. However, the initial exceedance of the target deceleration rate, combined with the crew’s actions, likely prevented a runway overrun.

Further, a post-incident analysis of the flight data recorder by the aircraft manufacturer found that a 5 kt tailwind existed on final approach and landing. This also significantly affected the aircraft’s landing performance and further reduced safety margins.

Additionally, and along with the United States Federal Aviation Administration, the ATSB found that the 15 per cent in-flight safety margin applied to actual landing distances during landing performance calculations may be inadequate under certain runway conditions. In these conditions, additional conservatism is encouraged.

Safety message

This incident highlights the adverse consequences of crew experiencing a high workload during critical phases of flight, including missing important information needed to determine an accurate landing performance.

In addition, runway surface condition and braking action reports (intended for the benefit of other pilots landing aircraft after them) can be subjective, and the terminology used to describe these can be inconsistent. Considerable efforts have been made by organisations such as the United States Federal Aviation Administration to address this issue with the introduction of the runway condition assessment matrix.

Recorded information

Flight data recorder (FDR) analysis

The aircraft was fitted with a FDR and following the incident, the data was downloaded by the operator and provided to the ATSB. The aircraft’s recorded data was also analysed by Boeing. The data showed the following (Figure 9):

  • A stable approach with flaps 40 was performed.
  • There was a 5 kt tailwind and essentially zero crosswind just prior to touchdown.
  • The flare was initiated at a radio altitude of about 30 ft.
  • The aircraft settled onto the right main landing gear immediately followed by the left main landing gear.
  • Initial touchdown occurred at a computed airspeed of 138 kt, which was 3 kt above the flap 40 landing reference speed of 135 kt (VREF40).
  • The aircraft’s actual landing weight was 60,164 kg.
  • The aircraft touched down within the touchdown zone. Based on the crew’s report of the aircraft coming to rest about 5 m from the runway end, Boeing established that the aircraft touchdown point was about 920 ft (about 280 m) beyond the runway 29 threshold.
  • Immediately after touchdown, the autobrakes engaged, and the speed brakes and thrust reversers deployed.
  • With the autobrakes engaged, the AUTOBRAKE 3 target deceleration rate of 0.224 g was initially achieved and then exceeded to a maximum of 0.29 g.
  • At a computed speed of about 79 kt, shortly after crossing the runway intersection, the autobrakes were disengaged and manual braking commenced. During the application of manual braking, the aircraft’s deceleration reduced to below the AUTOBRAKE 3 target rate. The rate decreased despite the crew applying greater than AUTOBRAKE 3 pressure to override the autobrakes and the FO reported applying ‘hard’ braking with the captain assisting. Overall, the average deceleration from touchdown to the aircraft stopping was about 0.179 g. Refer to Appendix B for a graphical representation of the aircraft’s deceleration during the landing.
  • The commanded brake pressures during landing could not be verified as the alternate brake pressures were recorded instead of the main brake pressures (refer to section titled Main landing gear brake pressure recording switch).
  • A minor directional deviation was observed during the later stages of the landing to maintain runway heading, consistent with that reported by the FO (Figure 9).
  • Reverse thrust was reduced to idle when at a ground speed of about 20 kt. Reverse thrust was normally reduced when the aircraft’s airspeed was about 60 kt.

Figure 9: Aircraft’s ground path and recorded data after crossing the runway 11/29 and 02/20 intersection.

Image shows the aircraft’s ground path (in yellow) and recorded data after crossing the runway 11/29 and 02/20 intersection (the zigzag yellow line on the last section of the runway was the result of flight data recording limitations). Source: Google earth, annotated by the ATSB

Boeing simulations

Boeing conducted simulations using the recorded data available in an attempt to characterise the amount of brake pressure applied and the runway conditions. While assumptions were made regarding the runway condition and commanded brake pressure, the results indicated that ‘good’ braking capability was initially achieved until the aircraft crossed the runway intersection. Between the intersection and the runway end, the aircraft’s braking capability significantly reduced. Deceleration throughout the landing was initially consistent with the characteristics of a wet runway and then consistent with a flooded runway after crossing the runway intersection. Boeing further stated that, since runway 29 was not grooved, if standing water was present between the intersection and runway end, the aircraft’s braking capability would have been reduced.

Findings

From the evidence available, the following findings are made with respect to the reduced braking effectiveness during landing involving a Virgin Australia International Airlines Boeing 737-800, registered VH-VOP that occurred at Christchurch Airport, New Zealand on 11 May 2015. These findings should not be read as apportioning blame or liability to any particular organisation or individual.

Safety issues, or system problems, are highlighted in bold to emphasise their importance. A safety issue is an event or condition that increases safety risk and (a) can reasonably be regarded as having the potential to adversely affect the safety of future operations, and (b) is a characteristic of an organisation or a system, rather than a characteristic of a specific individual, or characteristic of an operating environment at a specific point in time.

Definitions of the following headings are provided in the section titled Terminology used in this report.

Contributing factors

  • Due to the crew experiencing increased workload, the crew misperceived critical landing information, which resulted in the aircraft’s landing performance being determined based on a damp (dry) runway rather than a wet runway.
  • The runway surface conditions combined with the autobrake selection resulted in the aircraft exceeding the landing performance limitations for runway 29 for both the reported and actual wind conditions.
  • Using the operator's Airport Analysis Manual to establish that the aircraft could land safely on runway 29 relied on the crew's judgement to determine the braking level required. While that judgement was consistent with the flight crew training manual, the Quick Reference Handbook indicated that a higher braking level than chosen was needed to meet the landing distance safety margin requirements.
  • It was very likely that the amount of water on the later part of runway 29 resulted in less than 'good' braking action. Along with the possibility of viscous aquaplaning and despite the crew applying hard braking, this water resulted in a reduced braking capability.
  • Several months prior to the incident, Virgin Australia Airlines/Virgin Australia International changed their policy on calculating landing performance for damp runways from referencing a wet runway to a dry runway. [Safety issue]
  • There was no regulatory direction from the Civil Aviation Safety Authority on how a damp runway was to be considered for aircraft landing performance. [Safety issue]
  • Virgin Australia Airlines/Virgin Australia International did not have a policy requiring crews to independently cross-check environmental information and landing performance calculations in-flight, removing an opportunity to detect crew errors. [Safety issue]

Other factors that increased risk

  • The operator provided guidance on landing performance through an uncontrolled mechanism, which did not provide assurance that crews had read, understood and applied the critical information contained within that guidance.
  • The flight following department did not highlight to the crew the changing weather conditions, which had the effect of minimising their landing options for Christchurch.
  • The subjective nature of assessing runway surface conditions and braking action, increases the risk of incorrect landing performance determination.
  • Civil Aviation Order 20.7.1B stipulated that a 1.15 (15 per cent) safety margin was to be applied to the actual landing distance for jet-engine aircraft with a maximum take-off weight greater than 5,700 kg. This safety margin may be inadequate under certain runway conditions, which increases the risk of a runway excursion. The corresponding guidance in Civil Aviation Advisory Publication 235-5(0) had not been updated to account for this. [Safety issue]
  • The inherent complexity of re-calculating landing performance data in-flight using the operator’s Quick Reference Handbook increases the risk of errors, especially during times of elevated crew workload.

Other findings

  • The operator's guidance was inconsistent with its policy for determining in-flight landing performance.
  • Initial aircraft autobraking above the required deceleration rate, combined with the crew’s application of hard manual braking and delayed stowage of reverse thrust, likely reduced the risk of a runway excursion.
  • The main landing gear brake pressure recording switch was faulty, therefore, the amount of brake pressure applied during landing was not recorded by the flight data recorder.

The full AustralianTSB report is available here

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