French investigators have determined that a Swiftair Boeing 737 freighter landed long at Montpellier in windshear conditions before overrunning and coming to rest with its nose partly submerged in water.

The aircraft, having carried out a VOR-DME approach to runway 12L at night in heavy rain, touched down 1,500m beyond the threshold – more than halfway down the 2,600m runway.

French investigation authority BEA states that the aircraft encountered windshear as it descended from 200ft to 100ft. Over the space of 4s the wind shifted from 350° to 70° and fell from 30kt to 8kt, replacing a substantial tailwind with a light headwind component.

The 737 had already been high on the approach, and the wind change increased airspeed and lift, but the consequences on flight parameters were “not detected” by the crew, says the inquiry.

It overflew the threshold at 100ft with a speed 34kt above the approach reference of 136kt, and began to flare.

But the crew did not retard the thrust levers to idle at this point. The power reduction to idle was only carried out 9s later once the jet had descended to 20ft.

The aircraft touched down with only 1,100m of runway remaining.

BEA says the crew “did not detect the long landing and did not carry out a balked landing”, pointing out that the runway was not equipped with lights in the touchdown zone or on its axis.

“The length available after the touchdown point was very probably not sufficient for the aircraft to be stopped safely within the runway limits,” it adds.

Analysis indicates the high speed on landing could have contributed to the jet’s hydroplaning. It passed the far end of runway 12L at around 70kt, travelling over the safety area and stopping with its nose in the Etang de l’Or, situated less than 200m beyond the runway. None of the three occupants was injured.

While it attributes the overrun to the undetected windshear and long landing, the inquiry states that the pilots did not sufficiently prepare for the approach.

BEA says the first officer, who was initially flying, had previously flown ATR turboprops for Swiftair and had obtained his 737 type rating only six months earlier. He had relatively limited experience on the aircraft, around 186h including 93h under supervision.

The crew had initially briefed for an ILS approach to runway 30R following the service from Paris Charles de Gaulle on 24 September 2022.

But the runway in use subsequently changed to 12L. Although the crew prepared for a non-precision VOR-DME approach, there was no briefing for the change – there was no clear decision on the choice of descent mode, or a discussion on the specifics of the approach including its 3.66° slope or its offset from the axis.

BEA adds that the crew had “low” awareness of the weather situation, particularly the threat from cumulonimbus cloud in the vicinity and rapidly-evolving conditions, focusing “principally” on the adapting to the runway change. The inquiry also says “it is probable” that weather radar settings did not enable the crew to notice the presence of a storm cell close to the airport during the approach.

“Despite the information available, the crew seemed to have unconsciously taken insufficient account of this information which could have prepared them for the weather conditions they were going to encounter during the approach,” it states.

Absence of a briefing increased the crew’s workload as the aircraft descended towards the runway, with late discussions taking place about descent rates, at a time when the non-precision approach also presented the pressure of altitude-distance checks.

Several standard callouts were omitted on final approach, among them a 4nm altitude-distance check and a stabilisation callout at 1,000ft, when the aircraft was still nearly 200ft above the correct slope.

“These omissions may be a further illustration of the deterioration in the crew’s performance due to the high workload,” says the inquiry. “The absence of these callouts also contributed to a reduction in situational awareness with regard to the stabilisation criteria.”

BEA adds that the crew did not make several callouts regarding minimums, nor a stabilisation callout at 500ft – a height at which the pilots opted to change roles, with the captain taking over as the flying pilot.

This swap also complicated the situation, says the inquiry, because the first officer did not switch to the monitoring task but remained focused – as he had been while flying – on external references.

Although the aircraft was fitted with an enhanced ground-proximity warning system, capable of detecting windshear, the function to generate a caution to the pilots had not been certified for the 737. The system internally detected the windshear at 65ft, but the absent functionality meant no caution was issued in the cockpit.

Swiftair informed investigators that it took several safety measures in the aftermath of the accident, says BEA, implementing wind limitations “more restrictive” than Boeing’s, introducing relevant simulator training, clarifying operational policy regarding runway conditions, and improving familiarisation with the weather radar control panel.