Investigators have found that an Air France Airbus A318’s stall-protection system activated when the aircraft captured a false glideslope, after its crew lost situational awareness while dealing with a rushed approach to Toulon-Hyeres airport.
The aircraft (F-GUGD) had been arriving from Paris Orly on 20 December 2019.
It had been cleared to descend to from 11,000ft to 1,900ft and the crew selected this assigned altitude on the flight control panel.
Although Toulon’s runway 23 was in use, the controller had remarked about meteorological conditions and the crew – believing the minima would exclude a runway 23 approach – opted instead for runway 05.
The aircraft, heading south-east, turned left to head north-east for the initial approach fix at the PALME waypoint.
But during the turn the A318 became subject to a strong tailwind component, initially rising from zero to 50kt and remaining above 40kt during the descent to around 3,000ft. This tailwind reduced the time window available before the aircraft reached the runway.
Although the crew was offered an orbit to provide additional distance in which to reduce altitude, it was turned down in favour of a straight-in approach.
French investigation authority BEA says the first officer, who was flying, knew the approach could be aborted if it became unstable and the captain backed this reasoning.
But the crew “underestimated” the effect of the tailwind on the approach path, it says, and “did not sufficiently assess” the feasibility of capturing the runway 05 glideslope from above. It adds that the pilots paid attention to the indicated airspeed but “did not take into account” the high groundspeed and descent rate.
While 12nm from the runway the aircraft was still at 5,170ft – about 1,350ft above the level expected for a standard 3° glideslope – and travelling with an airspeed of 237kt but a groundspeed of 307kt.
The crew, cleared for the approach and established on the localiser, started configuring the A318 for landing.
But the pilots did not realise that the aircraft was still set up to level off at 1,900ft – the altitude to which it had earlier been cleared – and did not detect the autopilot’s change of mode when it switched to capture this selected height.
The aircraft passed the final approach fix, 5.3nm from the runway, at 2,200ft. This was still 500ft above the published altitude of 1,700ft.
But it levelled at 1,900ft, with the result that it stopped closing in on the glideslope from above and instead deviated further from it.
BEA says the crew, finalising the landing configuration, “did not realise the aircraft was level” and did not have the runway in sight, because it was obscured by cloud.
Having failed to reach the proper 3° glideslope the aircraft instead intercepted the false 9° glideslope signal, a phenomenon created by the nature of the ILS’s electromagnetic lobes. This phenomenon reverses the signals to the aircraft’s guidance systems, thereby causing the autopilot to generate pitch axis commands in the wrong direction.
Upon encountering the false glideslope, the A318’s autopilot commanded nose-up attitude and its pitch rose from 1° to 30° in the space of 20s. The pitch-up caused the airspeed to fall and the autothrust commanded higher engine power, until the increasing angle-of-attack and low-energy state triggered an audible speed warning to the crew and the stall-protection system engaged.
BEA remarks that, as the aircraft had pitched up, an air traffic controller had ordered the crew to abort the approach but this was not read back by the pilots.
The crew disengaged the autopilots and set the engine thrust to go-around power, while the first officer made nose-down inputs. BEA says the airspeed declined to a minimum of 96kt.
While the aircraft climbed at about 1,000ft/min along the runway centreline, the first officer initially maintained a 15° nose-up attitude. But this allowed the aircraft again to exceed the angle-of-attack threshold, triggering the stall-protection system again for a few seconds.
The crew started retracting the A318’s flaps and the aircraft’s airspeed rose above the ‘VLS’ level – the lowest speed the autothrust can be ordered to follow – where it had languished for 46s.
After stabilising the aircraft at 4,000ft the crew was offered radar vectors for a second ILS approach to runway 05, and the jet subsequently landed without further incident. None of the 114 passengers and five crew members was injured.
BEA says the crew identified the presence of the tailwind but focused on its effect on landing performance rather than the impact on the A318’s trajectory. The pilots’ attempt to catch up with the glideslope from above, combined with the tailwind’s reducing the time to reach the runway, resulted in the crew’s losing situational awareness and failing to realise the aircraft was still set up to level before reaching the glideslope.