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Cuban probe details balance errors behind fatal 737 crash

Cuban investigators have detailed the gross errors in weight and balance which preceded the departure of a Boeing 737-200 which subsequently stalled as it attempted to climb away from Havana.

As the Global Air aircraft, operating a domestic service to Holguin, lifted off from runway 06 its crew struggled to stabilise instability in the roll axis. The aircraft pitched nose-up and its airspeed bled away, before it crashed just 40s after becoming airborne.

Four of the 113 occupants – comprising 107 passengers and six crew – were initially found alive in the wreckage by firefighters but three of them subsequently succumbed to injuries in hospital, leaving just a single survivor.

While Cuban regulator IACC revealed earlier this year that balance miscalculations had led to the 18 May 2018 accident, investigation authority CEIAA has detailed the extent of the errors in an analysis released in August.

Global Air had subcontracted to a company called Eagle Aviation the preparation of a preliminary load and balance sheet, which would then undergo final amendments by the captain before approval.

But CEIAA says that "large errors" in this case were "not corrected by the crew", nor were last-minute changes – relating to luggage and passengers – taken into consideration.

It points out that the number of passengers in the forward cabin was given as 62 when it had capacity for 54, and the weight in the cargo compartments was "incorrect". The fuel weight at take-off exceeded that calculated by some 5,000lb.

The load sheet presented to the crew put the take-off weight at about 99,900lb (45.3t) but recalculation by investigators produced a figure of just over 104,000lb, while the zero-fuel weight was wrong because the weight of luggage was lower than planned.

Crucially the centre-of-gravity was listed as 17.4% of mean aerodynamic chord but the recalculation put it much higher at 28.5%. The inquiry says this put it "very close" to the permissible operating limit of 29%, while the stabiliser trim setting was off by more than two units.

Although the weight and centre-of-gravity were technically within the aircraft's capabilities, the inquiry points out that the balance miscalculation and the position of the trim "undoubtedly caused unstable behaviour" from the aircraft and made piloting the jet "difficult".

Immediately after rotation and lift-off the aircraft started banking excessively to the right, reaching 33.5°, and there was a marked increase in pitch even while the control column was being pushed forward.

Bank angle warnings sounded and the stick-shaker activated and, 11s after becoming airborne, the captain took control of the jet from the first officer.

"It is the captain's prerogative to assume the controls at any time in order to preserve safety," says the inquiry. "But this unplanned transfer always has some negative influence on the piloting of the aircraft."

As the jet reached a height of 85ft, travelling at 139kt, its pitch had increased to 27.2°. The pilots tried to counter the right bank by rotating the control wheel to the left, applying 117lb of force, and pushing the left rudder pedal to level the wings, and they were briefly successful in neutralising the roll.

But flight-data recorder information shows the right-bank tendency persisted, and it rolled again to 23.6°, before the pilots' efforts pulled it into an opposite left bank of 13.6°.

This correction and the moderation of pitch to 16.5° allowed the aircraft to approach an "acceptable" condition to continue flight, says the inquiry, but it then banked a third time to the right, reaching 28.6°, and the airspeed declined from 130kt to 122kt owing to the high nose-up attitude and "probable unco-ordinated actions" on the flight controls.

While the captain managed to reduce the right bank to 9.3°, this was only because of the significant left-rudder input and the loss of aileron authority owing to the reduced airspeed.

The aircraft's pitch rose to 20.5° and the 737 entered a fourth right bank, of nearly 30°, before pitching up to more than 25° and stalling.

Oscillation of the aircraft became more rapid, with the captain's labours to counteract the bank resulting in a 17°/s roll manoeuvre which swung the jet into a 39° left bank, before it quickly rolled back – this time at nearly 28°/s – into a right bank of more than 72°.

Some 13s before impact the crew issued a distress call, declaring an emergency. The aircraft's airspeed had fallen 116kt and the inquiry says the low height offered little margin to lower the nose sufficiently to recover from the stall.

It descended in a sharp right bank and struck the ground close to the entrance road to one of Havana airport's terminals.

CEIAA says that a "chain of errors" led to the loss of control, with a preponderance of those involving human factors.

"When analysing the flight profile, it can be considered that the aircraft initially responded to the pilot's commands," it states. "But subsequently the flight parameters degraded until they declined to an abnormal state and control was lost."

Global Air is a Mexican-based charter carrier also known as Damohj Airlines.

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