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ANALYSIS: An-148 pilots' opposing inputs during fatal dive

Analysis of the crew actions during a fatal Saratov Airlines Antonov An-148 flight last year shows the two pilots applied opposite inputs to the control column after a ground-proximity warning sounded.

The twinjet's captain had pushed the aircraft into a 30° dive in response to an apparent dramatic loss of airspeed – a false indication owing to the icing of the aircraft's pitot-static sensors.

Its ground-proximity warning system issued a "pull up" alert at about 1,500m (5,000ft) altitude, as the aircraft descended at more than 9,800ft/min.

The first officer intervened as the jet passed 1,200m, pulling on the control column in a bid to bring the nose up.

This meant the pilots' actions were "multidirectional", says the Russian Interstate Aviation Committee, with the captain pushing nose-down with a force of 412N while the first officer countered with a 382N nose-up command.

It states that the opposite inputs effectively cancelled one another out, and the elevators "practically did not deviate" from their position. The An-148 remained in a dive.

But at a height of 300-400m the flight-data recorder shows the two pilots both suddenly started pulling on the control columns.

"Most probably, the aircraft emerged from the clouds at this time and the pilots realised the ground was rapidly approaching," says the inquiry.

The sudden nose-up inputs generated a 4.2g load on the An-148 but was insufficient to arrest the descent in time, and the aircraft – still in a 30° dive and entering a 25° right bank – struck the ground at around 430kt, completely disintegrating with the loss of all 71 occupants.

In-depth analysis of the flight-recorder data shows the extent to which the crew was misled by unreliable airspeed data, after they overlooked activation of the pitot-static heating system just before take-off from Moscow Domodedovo on 11 February last year.

From the moment of take-off for Orsk, just after 14:21, a warning stating "no heating" of the pitot-static sensors was displayed on the instrument panel, and remained for the entire flight.

Although icing alarms were armed, the outbound crew received no indications of icing conditions – unlike the prior inbound service to Moscow, when the An-148 had descended through icing conditions from 2,500m to 1,400m.

Gear and flaps were retracted by the time the jet had climbed to 640m. The crew did not mention a notification displayed during slat retraction, says the Interstate Aviation Committee, which it highlights as evidence of a slack attitude to the aircraft's central information system.

The aircraft followed the OKREM 14G departure, and was instructed to climb to 7,000ft, before being transferred to Moscow departure control.

Flight-data recorder information shows that, at about 1,100m (3,600ft) and 250kt, a discrepancy between the aircraft's actual and recorded airspeed began to emerge.

Controllers instructed the aircraft to climb to 11,000ft. As the pilots were working through the checklist for passing the transition altitude, 3min 40s into the flight, they received a speed-comparison alert.

This showed that one of the three speed sensor channels had already been rejected, and that the readings of the other two were starting to differ by more than 5.4kt. The pilots' airspeed indications were showing around 245kt, and the rejected channel was significantly lower at 230kt, while the actual estimated figure was about 265kt, says the inquiry.

As the aircraft climbed through 2,000m the captain's airspeed indication started to fall back rapidly, while that of the third speed channel began to rise, and the comparison alarm was again triggered.

The crew disengaged the autopilot and investigators believe the captain pushed his control column forward, taking the aircraft from 5° nose-up to 5° nose-down, putting the jet into a descent at up to 3,900ft/min.

When the action was queried by the first officer, the captain's "hold" reply suggested that he was attempting to prevent the aircraft's declining speed from falling below 215kt.

As the speed sensor readings varied, the captain's speed channel was suddenly rejected in favour of the third channel which was registering 259kt – meaning that his displayed airspeed abruptly leapt by more than 50kt.

At the suggestion of the first officer, who had noticed rising airspeed indications on his side, the crew disengaged the autothrottle and retarded the An-148's thrust levers to idle before advancing them, and then retarding them again to near-idle.

The aircraft continued to descend, from 2,050m to 1,760m, and its airspeed increased; that on the captain's side showed more than 300kt, and an overspeed warning sounded. Investigators estimate the actual speed was even higher, around 313kt.

Subsequently the aircraft was put into a climb, at a rate of up to 2,100ft/min, reaching a maximum height of 1,900m. The captain's airspeed reading began to fall rapidly – the result of further deterioration from sensor icing – and the aircraft was put into another dive, with a 16° nose-down attitude, while the engine thrust levers were advanced initially before being retarded again.

The crew did not discuss any of the notifications being displayed on the central information system, says the inquiry.

Just before 14:27 the captain, in response to a further drop in his speed indication, pushed the aircraft into its final dive, 30° nose-down, and advanced the thrust levers to the 'maximum continuous' position before once again retarding them.

The enhanced ground-proximity warning system sounded, in accordance with its design, stating: "Terrain ahead, pull up."

Two seconds later, in response to the warning, the first officer acted to try pulling the aircraft out of the dive but – with the captain still pushing the jet nose-down – the attempt was unsuccessful.

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