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. Ryanair says it "accepts the findings" of the inquiry, and points out that there were "no safety actions or instructions issued to Ryanair as a result of the report”.
Investigators have highlighted a similar event, two months after the Ryanair incident, 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.