Spanish investigators have determined that a Swiftair ATR 72-500 stalled while climbing through icing conditions as a result of the pilots’ failure to manage the flight correctly and their “inappropriate” use of automation.
The turboprop (EC-KKQ) lost airspeed and height and rolled to excessive bank angles during the Alicante-Madrid service on 9 September 2017, and the captain’s response suggests he initially did not recognise that the aircraft was stalling.
Eight minutes before the stall the aircraft had entered icing conditions which then worsened into severe icing.
Investigation authority CIAIAC says the pilots received a degraded-performance warning but did not carry out the associated checklist.
They “did not consider” the ATR’s behaviour as they fixated on its climb, says the inquiry, forcing the aircraft to its limits and then attempting to climb “beyond” these limits.
Their focus on climbing resulted in their selecting autopilot modes which were prohibited in icing conditions, including vertical-speed mode, leading to a severe degradation in performance and the stall just above 17,000ft.
The ATR lost more than 1,660ft in altitude in 33s and experienced a series of uncommanded pitch attitudes – from 6° nose-up to 11° nose-down – and bank angles, rolling from 58° left to 11° right and then back to 41° left. It also reached a maximum angle of attack of 19.6°.
CIAIAC says weather information had been provided to the crew anticipating that much of the flight would take place in icing conditions, with moderate icing at 14,000-15,000ft over the waypoint NARGO.
“The reality of what happened in the flight showed that, in effect, icing occurred near that point, only it was worse than forecast,” the inquiry states.
But the crew did not adhere to procedures during the climb, selecting a climb rate above the published aircraft performance and failing to maintain speed above the minimum for severe icing.
Three minutes before the event, the aircraft was unable to keep climbing and, 35s before, the speed began to drop.
“These are clear indications, and are described as such in the procedure, that the aircraft’s performance was degrading,” the inquiry says, pointing out that the crew had already been issued cautions of ice build-up and increasing drag.
“The crew only considered the visual indications [of icing] and did not take into account the rest of the information.”
Investigators catalogued a series of deviations, including the failure to use the target speed of 170kt for icing conditions, climbing in ‘pitch’ mode despite none of the conditions for this mode being present, and failing to follow procedures – including disengaging the autopilot – when the aircraft’s performance degraded.
Owing to the autopilot’s remaining active, says the inquiry, “the abnormal feel of the flight controls could not be checked” despite the performance indications.
Sixteen seconds after the degraded performance caution turned on, the aircraft reached 16,200ft and was unable to climb further, holding this altitude for 90s.
Despite the evidence for a loss of performance, the crew commanded a series of actions to force the aircraft to climb beyond its capabilities – changing the power-management selector to maximum continuous thrust, and switching the autopilot to the vertical-speed mode with a climb rate of 500ft/min, even though this mode was banned during icing.
When the aircraft did not react as the pilots expected, they increased the climb rate to 1,100ft/min in a bid to break through cloud.
“According to the crew’s statement, they were about to clear the cloud layer and thought that climbing – and not descending – was the best option to escape the icing,” says the inquiry, adding that the crew was “completely focused on this objective” and “ignored the cautions” from the aircraft.
CIAIAC also points out that, although the first officer was the flying pilot, the captain violated task assignments by carrying out certain activities including changing the power-management setting. The captain had much more experience than the first officer – some 18,000h against 1,400h – and this probably influenced the first officer’s ability to oppose any decisions.
The actions during the climb were “inconsistent” with procedures and recommendations from the operator and ATR, says the inquiry. “Though some were initially inconsequential…others did have consequences,” it says.
Cockpit-voice communications were not available to the investigation. But flight-data recorder information shows that, as the aircraft stalled, the captain applied four nose-up inputs over the course of 21s – suggesting he “did not seem” to recognise the stall, and appeared fixated on climbing.
The first officer, however, did appear to identify the stall, because he made two nose-down inputs to the controls at the same time as the captain was making inputs in the opposite direction.
Throttle levers were moved to the maximum-power position, despite this not being part of the stall-recovery procedure, and the required extension of flaps to 15° was not carried out.
Despite the events only one person, one of the 22 passengers, sustained minor injuries during the incident. The crew recovered the aircraft and proceed to Madrid, although the inquiry has also highlighted that the ATR conducted a high-speed and unstable approach during its arrival.