The Australian Transport Safety Bureau has published a final report from its investigation into a Saab 340's descent below the glideslope during approach to land at Sydney Airport.
On the evening of 24 October 2022, the Link Airways operated aircraft was approaching Sydney, in cloud and darkness, on a scheduled passenger flight from Canberra.
Air traffic control cleared the aircraft for the instrument landing system (ILS) approach to runway 34 left, and the aircraft, with autopilot engaged, intercepted the ILS localiser at an altitude and distance from the runway that positioned it close to being on the glideslope for the approach.
"However, unknown to the crew, an instrumentation fault resulted in the pilots being presented with erroneous on-slope indications, without any failure indication," ATSB Director Transport Safety Stuart Macleod said.
As the approach continued, the crew observed the aircraft did not begin to descend as expected, with the cockpit instruments indicating the aircraft remained on the glideslope.
"In response, the captain increased the descent rate before re-engaging the autopilot. The autopilot maintained the increased descent rate, and the approach continued until the aircraft descended significantly below glideslope and the ground proximity warning system generated a 'glideslope' alert," Mr Macleod said.
At about the same time, the crew assessed the approach as unstable and commenced a missed approach.
The crew subsequently identified the erroneous glideslope indications, and completed a safe landing using a different approach type.
"This crew faced a complex scenario," Mr Macleod said.
"The absence of any failure indications reduced the ability of the crew to identify the fault, which incorrectly showed the aircraft on the correct and expected approach path.
"This incident highlights the importance of using all available indications, and reacting early to a significant exceedance from expected performance or instrument indications.
"It also underscores the value of adherence to operational procedures to ensure safe aircraft operation," Mr Macleod added.
The precise source of the error could not be determined during the investigation, which included a teardown of the captain's display processor unit by the manufacturer.
While maintenance action could not be linked to the incident, the operator developed and implemented several maintenance-related safety actions following the occurrence.
"These included a standardised component reinstallation (re-rack) procedure based upon aircraft manufacturer guidance," Mr Macleod noted.
"This procedure is aimed to reduce faults possibly created during defect troubleshooting."
The operator also issued an internal notice to maintenance personnel that provided guidance on the required items to be included in maintenance explanatory text.
You can find here the final report: Flight instrumentation failure and descent below glideslope involving Saab 340, VH-VEQ, 9 km south of Sydney Airport, New South Wales, on 24 October 2022
