Sharp manual overspeed
correction injured 737's cabin crew
10
mAY, 2018 - SOURCE: FLIGHT
DASHBOARD - BY: DAVID KAMINSKI-MORROW - LONDON
UK investigators
have determined that a Boeing 737-800 captain's heavy-handed manual response to
a developing overspeed resulted in a serious injury to one of the flight
attendants.
The inquiry into the upset on
the Ryanair jet (EI-EBW), which had been descending towards
Manchester on 14 January last year, found that the captain had been startled by
a sudden increase in airspeed towards the maximum operating Mach limit.
Air Accidents Investigation
Branch analysis revealed that the 737 had started a descent from 40,000ft
having been cleared to 20,000ft. Air traffic control had requested that, upon
speed conversion, that the crew fly at 270kt rather than the carrier's standard
245kt.
The 737 began to descend while
travelling at M0.77-0.78 but, owing to a strong Jetstream, the wind speed
increased rapidly and – during descent through 36,700ft – the aircraft's
airspeed rose to a maximum of M0.818.
While the autopilot was engaged,
the captain felt that it was not correcting the overspeed.
"Thinking that he had little
time to react, he simultaneously pressed the autopilot disengage button on his
control wheel and pulled back on the control column," says the inquiry.
"His intention was to avoid the overspeed as smoothly as possible using
manual control inputs."
But flight-data recordings
showed that the captain quickly exerted a 42.7lb (190N) force on the control
column. Although the inquiry could not determine whether the autopilot
disengaged as a result of the button-press or the column input, it says that
the force applied was twice that required for autopilot disconnection.
"At the time [the captain]
believed he was managing the manoeuvre gently," it states. "But with
hindsight he suspected that startle effect caused him to exert more force on
the control column than intended."
The abrupt manoeuvring caused
two cabin crew members to lose their balance and one suffered a broken ankle.
Investigators point out that the
lower air density at higher altitudes reduces the operating envelope, and that
manual flying in such conditions demands particularly careful handling and the
avoidance of large control movements.
Boeing's training manual states
that changes in wind speed could lead to overspeed. While the autothrottle is
able to provide "aggressive" speed control near operating limits, the
manual says short-term overspeed can nevertheless occur under certain
conditions.
It advises crews, in such cases,
to deploy partial speedbrakes slowly until a noticeable reduction in airspeed
in achieved, and retract them equally slowly once the airspeed falls below the
operating limits.
Both pilots subsequently
completed a recurrent simulator session which focused on overspeed recovery.
Investigators have highlighted a
similar event, two months after the Ryanairincident, which involved
a Qantas 737-800 that experienced increasing headwinds during descent
towards Canberra on 13 March last year. The crew used manual input to avoid
overspeed, and two cabin crew members were injured.
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