July 31, 2023 | Susan Napier-Sewell

Landing Incident Highlights Importance of Operator Risk Controls for Unforecast Weather

flight crew

A Fokker 100 flight crew was committed to conducting an approach in low cloud that extended below the minimum decision altitude to land at Paraburdoo, WA, Western Australia, as the aircraft’s fuel state was nearing the minimum fixed reserve following three missed approaches.

The Network Aviation operated F100 was conducting a scheduled passenger flight from Perth on 22 November 2021 and as the aircraft approached Paraburdoo the flight crew encountered unforecast weather, an ATSB investigation details.

“Having completed 2 missed approaches at Paraburdoo, the flight crew had lost confidence in their flight plan weather forecasts and were reluctant to attempt a diversion to an alternate airport without current weather information for the alternate,” said Transport Safety Director Dr. Stuart Godley.

“After the third missed approach, the aircraft did not have sufficient fuel to reach a suitable alternate and the flight crew were committed to landing at Paraburdoo.”

The flight crew conducted a RNAV GNSS approach to Paraburdoo’s runway 24, which required the crew to visually acquire the runway – the minimum descent altitude or MDA – at a height above the aerodrome of no less than 584 ft. The investigation report details that 25 seconds after the aircraft descended through the minima, the autopilot was disconnected and the pilot monitoring announced that they had sighted the runway and that they were on profile. At this stage, the aircraft was 293 ft. above ground level and 291 feet below the minima/MDA.

Flight data recorder information indicated a steady descent profile on the approach and a maximum of 5° heading change between the autopilot disconnect and landing.

“The actual weather conditions the flight crew encountered at Paraburdoo were below their landing minima and were continuing to deteriorate. The cloud base at Paraburdoo was difficult for the Bureau of Meteorology to forecast as detection of low cloud by satellite imagery was obscured by higher level cloud,” said Dr. Godley.

After their second missed approach, the crew attempted to obtain from air traffic control an updated forecast for Newman Airport for a possible diversion there.

“However, the crew did not express any urgency when making this request, which, in combination with air traffic control workload at the time, resulted in a delay of 15 minutes before an update was offered. By that time, it was no longer required as the aircraft no longer had sufficient fuel remaining to divert to Newman.”

The investigation notes that the crew had no other means of obtaining updated weather forecasts for potential alternates beyond contacting air traffic control, as the aircraft was not fitted with an operational ACARS digital datalink messaging system, and the aircraft was beyond the range of the nearest AERIS automatic en route information service (which broadcasts a range of weather information from a network of VHF transmitters). 

Meanwhile, there is an automated weather station at Paraburdoo, but it did not have a means of detecting the moisture content in the atmosphere above the surface.

“This increased the risk that low cloud below the instrument approach landing minima might not be forecast.”

Dr. Godley said the incident highlights the importance for all operators to consider how unforecast weather will be managed and ensure it is reflected in their risk management.

“This is so that safety assurance activities can review how effectively it is managed and provide feedback for management review.”

Other than a procedure that limited the number of missed approaches to 2, Network Aviation did not provide flight crew with diversion decision-making procedural guidance when encountering unforecast weather at a destination, the investigation found.

In addition, the operator did not include the threat of unforecast weather below landing minima in their controlled flight into terrain risk assessments. This increased the risk that controls required to manage this threat would not be developed, monitored, and reviewed at a management level.

“The ATSB acknowledges and welcomes that, since the incident, Network Aviation has implemented several proactive safety actions in response to safety issues identified in the investigation,” Dr. Godley said.

These include introducing several diversion decision-making tools for F100 flight crew, such as an amendment to their flight plans to include an ‘alternate summaries’ section for all flights, the top of descent arrival brief procedure to include ‘minimum divert fuel,’ and the introduction of an F100 Company Procedures Manual with pre-populated standard divert calculations for F100 destinations.

In addition, the operator has updated their controlled flight into terrain risk assessments to capture the threat of adverse weather.

Key points
– Fokker 100 landed in low cloud with the flight crew not visually acquiring the runway until below the minimum descent altitude to continue the approach;
– Aircraft fuel state was nearing the minimum fixed reserve after three prior missed approaches;
– Cloud base was forecast to be above the minimum descent altitude but lowered to below the minimum descent altitude;
– Crew did not have immediate access to weather conditions at alternate airports, which might have led to a diversion.

Read the report AO-2022-048 Unforecast weather and flight below minimum altitude involving Fokker Aircraft F100, VH-NHV Paraburdoo Airport, Western Australia, on 22 November 2021.

Content and photo source/credit: ATSB, “Descent below minima landing incident highlights importance of operator risk controls for unforecast weather,” publication date: 24/03/2023.

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Accident, Investigations
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