Created Tuesday, Aug 26th 2025 15:14Z, last updated Tuesday, Aug 26th 2025 15:14Z
An Alliance Airlines Embraer ERJ-190 on behalf of Qantas, registration VH-UYO performing flight
QF-1888 from Cairns,QL to Darwin,NT (Australia) with 49 passengers and 4 crew, was on approach to Darwin's runway 29 in visual meteorologic conditions passing the initial approach fix for the ILS approach, when the approach mode of the autopilot unexpectedly disarmed and reverted to basic flight director mode. The aircraft veered to the right, then left of the ILS approach course before intercepting the localizer again at about the final approach fix. Descending through 1000 feet AGL the aircraft was high and fast and at a high rate of descent. Descending through 500 feet the crew considered the approach stabilized though still too fast. The pilot monitoring discovered the flaps had not yet been set to landing configuration, the flaps were extended to landing configuration, the crew continued the approach for a landing without further incident.
ON Aug 26th 2025 the ATSB released their final report concluding the probable causes of the incident were:
Contributing factors
- On crossing the initial approach fix for the instrument landing system (ILS) approach, due either to a system synchronisation issue or the pilot flying inadvertently disarming the approach mode, the aircraft’s auto-flight system reverted to roll and flight path angle modes.
- Following the unexpected mode change, the pilot flying did not re-engage approach mode or disconnect the autopilot. This likely contributed to the aircraft deviating outside the required lateral tolerance of the approach below the minimum safe altitude while in instrument meteorological conditions.
- The flight crew did not discontinue the approach when the aircraft was unstable at the 1,000 ft stabilisation height as they incorrectly assessed that they could continue to 500 ft in visual meteorological conditions, with multiple stabilised approach criteria unmet.
- In the limited time available to stabilise the aircraft by 500 ft, the flight crew incorrectly assessed that the aircraft was stable and continued the approach, unaware that the pilot monitoring had inadvertently selected an incorrect flap configuration.
- Alliance Airlines' standard operating procedures were unclear about the criteria for continuing an unstable instrument approach to 500 ft when aircraft entered visual conditions.
The ATSB analysed:
Mode reversion
Nearing the initial approach fix (LAPAR) for the Darwin Airport runway 29 instrument landing system (ILS)-Z approach, the aircraft’s approach, localiser and glideslope modes were armed, with the correct ILS frequency set as the armed navigation source. As the aircraft flew by LAPAR, the localiser mode engaged for one second before the auto-flight system reverted to lateral roll and vertical flight path angle modes.
It is possible that this occurred due to the captain inadvertently pressing the approach pushbutton at the same time the localiser captured. The captain recalled that this occurred because they received a second ATC clearance for the ILS, however, at that time, they had only been cleared for the ILS once. They received a second clearance about 50 seconds later and a third clearance 40 seconds after that. The pushbutton was not recorded in the data but pressing it when approach mode was armed would disarm approach mode consistent with the mode reversion.
Embraer's analysis of the recorded data found that the ILS navigation display source had not engaged at the time localiser mode became active. Therefore, it was most likely that this non-synchronisation led the flight control and guidance system to invalidate the capture and revert to basic modes. The avionics manufacturer, Honeywell, advised that as the recorded data did not update as frequently as the system status, it was not possible to determine whether a synchronisation issue existed.
The mode reversion resulted in the aircraft maintaining the 20° right bank and approximately level flight path angle that were present at the time of the mode reversion.
Automation and approach continuation
Alliance’s procedures stated that if the auto-flight system was not doing what the pilot expected, they were to disconnect the autopilot and manually fly the aircraft. However, following the unexpected mode change, the captain, who was the pilot flying (PF), did not re-engage approach mode or disconnect the autopilot until about 200 ft above the aerodrome elevation, in accordance with normal landing procedures. This resulted in the aircraft deviating beyond the permitted tolerance from the localiser course and above the glideslope.
It took less than 3 minutes from the mode reversion at 3,000 ft, to the aircraft being established on the ILS and in the landing configuration, about 400 ft above aerodrome elevation. During that time, there were several changes to the lateral and vertical modes, selected heading and altitude, and significant variations in vertical speed, as the PF manipulated the auto-flight system to recapture the localiser and glideslope.
There were also several triggers for the flight crew to discontinue the approach. The flight crew reported that their en route weather deviations and thunderstorms in the vicinity of the airport may have led to perceived pressure to continue the approach, but that there was adequate fuel on board to discontinue and conduct a second approach.
Once outside the lateral tolerance of the approach, the aircraft was operating contrary to the air traffic control (ATC) clearance and descended below the minimum safe altitude, likely while in instrument meteorological conditions (IMC). The pilot monitoring (PM) reported that they did not call out the course deviations, as the PF was actively working to correct them. Instead, after descending about 500 ft, the PM advised ATC they were re-intercepting the localiser and was assigned the minimum vector altitude to ensure terrain separation. As a result, the aircraft then maintained altitude, while the glideslope diverged beneath the flight path.
The PM did not read back the ATC clearance to descend once they had intercepted the glideslope, consistent with being focused on monitoring tasks and their self-assessed 10/10 workload during the approach. The PM selected flap 2 as the aircraft captured the localiser, about 800 ft below the normal (but not mandatory) 3,000 ft flap 2 selection height. Alliance reported that this late selection had a similar effect to other recent approach incidents, where the associated delayed speed reduction reduced the time available to stabilise the approach.
As the aircraft passed the final approach fix (FAF) and then descended through 1,500 ft, it was established on the localiser but 3 dots above the ILS glideslope. Alliance’s procedures required the ILS to be flown within 1 dot of the glideslope, and stated that aircraft ‘should be’ on the 3° glideslope no later than 1,500 ft.
The PF reported stating to the PM that they were visual at about 2,500 ft, but the PM reported this occurred just as the aircraft approached 1,000 ft. Without a cockpit voice recording, it could not be determined when visual flight conditions were established.
At 1,000 ft, which was the stabilisation height for an instrument approach, the PF incorrectly assessed that although several stabilised criteria were not met, as they were then in visual conditions, they could continue (unstable) to 500 ft. Although this had been the procedure at the PF’s former company, Alliance required all stabilised approach criteria, other than airspeed, to be met at 1,000 ft. The PM was sufficiently unsure of Alliance’s policy to defer to the captain’s experience, as the senior base training captain who had conducted some of the PM’s training.
At 500 ft, the PM assessed that the stabilised approach criteria were met as, although the airspeed was still slightly fast, it was trending down, and they continued the approach. Although the flight crew reported having completed the before landing checks prior to 500 ft, shortly afterwards, the PM identified that they had inadvertently selected a take-off flap setting (passing about 1,100 ft) instead of a landing setting, which they had not detected earlier. The PM rectified the flap setting and an uneventful landing followed.
Although the landing was uneventful, had the incorrect flap setting not been rectified, thrust retard mode would not have engaged during the landing flare, and EGPWS warnings would have been triggered by the use of different tolerances than with the aircraft configured for landing. The use of selected heading and selected altitude to drive the aircraft’s trajectory meant that these may not have been set correctly had a go-around been initiated either by the flight crew or ATC.
A coupled ILS approach should require minimal flight crew intervention other than monitoring. Continuing an approach using inappropriate modes increased the likelihood of an unstable approach. International research showed that unstable approaches and failure to initiate a go-around could result in landing too fast or too far down the runway, a hard landing, runway excursion, loss of control, or collision with terrain. Although visual conditions reduce the risk of collision with terrain, they do not mitigate against landing incidents resulting from poor energy management.
Alliance stabilisation heights
Alliance provided stabilised approach criteria consistent with international and Civil Aviation Safety Authority guidance, including airspeed, flight path and energy management parameters, to reduce the risk of landing accidents. However, Alliance’s minimum stabilisation heights at which these criteria were required to be met, varied between approach types and meteorological conditions, and were not clearly documented.
Alliance specified a stabilisation height of 1,000 ft for instrument approaches, but under the heading Visual approach, permitted a 500 ft stabilisation height for an instrument approach in VMC, provided only the airspeed exceeded the stable speed criterion, and all other stabilised approach criteria were met. Additionally, elsewhere in the procedures for intercepting a glideslope, it stated that an approach must be stable before reaching 1,000 ft in IMC, 500 ft in VMC ‘or other altitude in accordance with company policies’. Alliance did not have a policy for transitioning from an instrument approach to a visual approach and advised that its expectation was for flight crew to continue an instrument approach even when entering VMC.
In this incident, the lack of clarity regarding which of the stabilised approach criteria were not required before continuing to 500 ft, resulted in the flight crew incorrectly assessing that they could continue to 500 ft in VMC while the criteria were not met for:
- speed
- rate of descent
- glideslope
- landing configuration
- before landing checks complete.
International Air Transport Association guidance stated that using a single stabilisation height for one type of approach, regardless of weather conditions, can reduce confusion and make it easier for operators to track stabilised approach compliance using flight data monitoring. Clear stabilisation heights also support flight crew decision-making to initiate a go-around.
Related Flight:
QF1888,
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