Vueling A320 at Birmingham on Aug 26th 2020, Alpha Floor on second missed approach
Last Update: August 20, 2020 / 19:27:07 GMT/Zulu time
On Aug 20th 2020 the AAIB released their final report concluding the probable causes of the serious incident (as well as a second serious incident handled by the same report in December 2019) were:
The aircraft did not maintain the correct vertical profile because the pilots were not sure when to commence the final descent. The depiction of the descent profile on charts provided by the operator may have contributed to this uncertainty.
In the first event it is likely that the increased workload of an unplanned missed approach contributed to the pilots not configuring the aircraft correctly for the go-around, resulting in the aircraft entering the alpha floor protection mode. In the second event, having also commenced the final descent late, the pilots did not maintain the correct profile thereafter because the type of approach required them to manage the vertical flight path manually, and they were not familiar with the flight mode they were using.
The AAIB summarized the sequence of events:
After an uneventful flight from Barcelona, the aircraft positioned for an RNAV2 approach to Runway 33 at Birmingham Airport. Both pilots were experienced on the aircraft and the co-pilot was acting as the handling pilot. The weather at the time was good with light winds reported and no cloud below 5,000 ft agl.
The aircraft was at 4,000 ft approximately 11 nm south of the airport when ATC cleared it to descend to 2,000 ft and carry out the RNAV approach (Figure 1). The pilots read back the clearance correctly but, thirty seconds later, the aircraft had not changed altitude and they contacted ATC to request descent. ATC again cleared the aircraft to descend to 2,000 ft and to carry out the approach. The aircraft was 10.5 nm from the runway when it started descending. At 9.4 nm it was at 3,800 ft, 1,000 ft above the correct profile.
When the aircraft was 3 nm from the runway, ATC cleared it to land, at which point the aircraft was at 2,000 ft, 660 ft above the correct profile. The pilots continued the approach, but at about 0.3 nm from the threshold and at 470 ft, they announced they were going around. ATC cleared the aircraft to climb to 4,000 ft and gave radar vectors for a further approach.
Shortly after the aircraft began climbing, the commander took over as handling pilot and informed ATC that the crew had experienced a navigation problem on their initial approach, requesting a localiser/DME approach for the second approach. ATC accepted the request and provided radar vectors to position the aircraft to commence the approach. When the aircraft was on base leg, ATC cleared it to descend to 2,000 ft, but the crew mistakenly read back the clearance to descend only to 3,000 ft. This mistake was missed by ATC and was not corrected.
The aircraft descended to 3,000 ft whilst positioning to establish on the localiser, during which it was given further clearance to descend with the approach. When the aircraft began its final descent from 3,000 ft it was about 7 nm from the runway and crossed the final descent point, 5.1 nm from the runway and 200 ft above the correct profile altitude.
Initially the crew continued the approach, but then informed ATC they were too high and requested a left turn. In response, ATC instructed the crew to turn left onto a heading of 240° and to climb to 4,000 ft. The crew commenced the turn 2.5 nm from the runway, descending through 1,900 ft. At the same time, they selected a climb to 4,000 ft using the open climb mode, leaving the landing gear down and full flaps set. They did not select the toga thrust mode appropriate for a standard go-around manoeuvre. This caused the aircraft to pitch up to about 10° nose-up.
The aircraft began to decelerate, and the crew changed to the vertical speed mode, reducing pitch to about 1° nose-up. However, the aircraft entered the alpha floor protection mode, automatically setting toga thrust and causing the speed to increase.
The commander then set the thrust levers to prevent the aircraft exceeding the full flap limiting speed. With pitch reducing, the aircraft continued to descend and ATC again instructed the crew to climb. The crew selected a climb of about 900 ft/min still using the vertical speed mode and the aircraft, having descended to 1,300 ft (about 940 ft agl), then started to climb.
The aircraft climbed to 4,000 ft and ATC gave further vectors for another localiser/DME approach. The aircraft then landed without incident.
The AAIB analysed:
It was possible to complete the approaches successfully at the point the aircraft were originally cleared to do so. In the August incident, the aircraft’s speed was not managed early in the initial approach and the crew were not certain of the correct descent point, leading to an increasingly difficult situation for them to manage. In the December incident, not maintaining the correct profile early in the initial approach again led to difficulties maintaining the correct flight path.
The approaches were continued whilst not meeting the stable approach criteria, and go-arounds were carried out late in the approach, both of which reduced safety margins as highlighted in previous safety studies.
Having gone around, the subsequent approaches should also have been safely achievable. In the August incident the commander chose to change the type of approach, which placed additional pressure on the pilots in setting up the aircraft and re-briefing. Positioning the aircraft further from the airport before commencing the subsequent approach would have allowed the crew more time to prepare.
The commander in this case stated that he did not wish to alarm the passengers by conducting a further go-around but did not explain his plan thereafter. The aircraft was in VMC and, if his intention was to reposition visually for another approach, this might explain why the aircraft was not reconfigured for a go-around (nor toga selected) when ATC instructed the aircraft to climb. The result was both a further descent and increase in angle of attack which triggering of the aircraft’s alpha floor protection system. Even when the climb was initiated, the crew continued without changing the aircraft’s configuration, indicating the startle and high workload likely to arise from this unintended situation.
The pilots of the aircraft involved in the December occurrence chose to conduct a localiser/DME approach on both occasions. The aircraft did not maintain the correct profile on either approach. When ATC vectored the aircraft for an RNAV approach this caused the pilots to doubt that they were conducting the correct type of approach.
The December incident involved a high rate of descent being selected to regain the appropriate approach path. The operator’s own investigation suggested the crew may have overlooked the fact that there was no glideslope for the aircraft to capture, resulting in it continuing its descent below the correct approach profile. Unlike the first incident, this occurred whilst the aircraft was in IMC, which removed any visual cues for the crew and resulted in a significant departure below the correct profile, taking the aircraft below the minimum safety altitude for that part of the approach.
The challenge faced by both crews in managing their descent has been the subject of discussions between the operator and air traffic service provider. ATC commented that had the incorrect readback of the cleared altitude been perceived and corrected, this might have prompted the crew on that occasion to continue their descent.
Different chart providers have different ways of depicting approach profiles. However, the AIP remains the source document and ATC will naturally rely on this, rather than individual operator’s charts, when managing air traffic. Where differences exist, it is desirable for operators and ATC to ensure their effect is understood.
This article is published under license from Avherald.com. © of text by Avherald.com.
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