US investigators believe the pilots of a Hawker 900XP were insufficiently prepared for a post-maintenance stall test, during which the aircraft encountered icing and entered a fatal spin.
The executive jet (N900VA) had undergone routine maintenance at Grand Junction airport, Colorado, including removal of the wing leading-edges and de-icing panels for inspection.
Its crew, comprising two pilots, planned to fly to Gig Harbor in Washington on 7 February last year and conduct the required stall test en route.
Two aviation weather advisories – for moderate icing throughout the aircraft’s climb and cruise altitudes for the test – were issued about 2h 30min before the flight, says the US National Transportation Safety Board.
“The presence of these conditions should have given the flight crew pause before they executed the stall test, which required visual meteorological conditions and no icing,” the inquiry states.
Flight-data and cockpit-voice recorder information indicate that the crew “intentionally departed” without engine ice-protection, it adds, and “likely” without airframe ice-protection.
Weather analysis showed the aircraft flew in instrument conditions while climbing from 5,000ft to 16,700ft and could have accumulated up to 1mm of ice on its wings.
Required conditions for the stall test included flying at least 10,000ft above ground level – but lower than 18,000ft above sea level – and 10,000ft above cloud.
During the climb, however, the crew asked Denver air traffic centre for an altitude block covering 18,000-20,000ft.
The aircraft levelled at 20,000ft, some 5,000ft above cloud and 2,000ft above the maximum prescribed altitude for the test.
Investigators found “no evidence” the crew attempted to verify that the external surfaces were free of ice, despite the icing conditions and despite the jet’s being equipped with an ice-detection spotlight system that could have been used to illuminate the wing fairings.
The aircraft was also fitted with an ice detector that needed manual activation by the crew, because it was not directly connected to the airframe ice-protection system.
After the crew discussed the stall test – which involved checking the stick-shaker and stick-pusher activation – the aircraft decelerated, its pitch increased, and its flaps retracted, consistent with the test commencing.
But although the stick-shaker activated at just under 118kt, the aircraft entered a stall at the same time, giving the crew “no warning”, says the inquiry. This was probably the result of ice contamination on the wing.
It states that the jet abruptly banked to the right, reaching more than 80°, and the crew responded with full left aileron and full aft control column input, as well as increased thrust, which “aggravated” the stall.
“The flight crew’s attempted remedial action suggested that they were insufficiently trained for the flight,” it adds.
Although the captain had participated in a stall test four years before, his involvement was probably limited, because he was a first officer at the time. The first officer on the Grand Junction flight had not previously taken part in a stall test.
Both had attended separate simulator sessions, covering stall warning and identification, but these were not designed to teach full stalls or prepare them for a possible uncommanded roll.
As the aircraft rolled into the right bank, it started pitching nose-down. The bank reached 84.7° before the jet rapidly rolled left – through full inversion and back upright – while its pitch continued deteriorating, to more than 75° nose-down.
Analysis shows the aircraft rapidly descended, “consistent with a flat spin”, says the inquiry, before striking the ground about 25nm northwest of the airport. Neither pilot survived the accident.
