History of the Flight
On 14 January 2017 at 1645 UTC, a Boeing 737-8AS (registration EI-EBW) was descending toward Manchester Airport on a commercial passenger flight. The aircraft, with 6 crew and 89 passengers on board, was at Flight Level 400 in the London terminal control area. The crew requested descent clearance from ATC to coincide with the top of descent point calculated by the flight management computer (FMC). The aircraft was heading north-west, and the pilots were aware of a forecast northerly jetstream during the descent.
ATC cleared the aircraft to FL200 and requested a speed of 270 kt on conversion, higher than the operator’s default 245 kt. The descent began at 0.77-0.78 Mach with 109 nm to touchdown. The autopilot and autothrottle were engaged, with the autopilot coupled to the pilot flying’s (PF) flight guidance in accordance with standard operating procedures. The flight path was controlled by LNAV and VNAV PATH modes, with the FMC ECON speed schedule. No turbulence was present and the passenger seatbelt signs were off.
Overspeed and Pilot Response
Recorded data showed that from FL392, the windspeed displayed on the primary flight display (PFD) began to rise gradually. Passing FL367, the windspeed increased more rapidly, rising 22 kt over 28 seconds. The aircraft’s speed increased from 0.78 Mach to a maximum of 0.818 Mach, triggering an overspeed warning. The commander recalled that the speed trend vector extended rapidly into the overspeed warning zone, indicating an additional 15-20 kt.
Believing the autopilot was not correcting the condition and having little time to react, the commander simultaneously pressed the autopilot disengage button and pulled back on the control column. In one second, the force on the control column changed from -0.51 lbs to +42.76 lbs. Immediately after disengagement, the overspeed protection logic caused the vertical flight mode to revert from VNAV PATH to LEVEL CHANGE, which commanded a pitch-up to reduce speed.
The commander stated he had not noticed the increasing windspeed indication on the PFD. Both pilots noted and discussed the airspeed increase. The commander perceived a startle effect due to the rapid speed increase toward MMO and the magnitude of the impending overspeed, and suspected this caused him to exert more force than intended.
Injuries
Two cabin crew members standing in the rear galley fell to the floor. One sustained a fractured ankle. All passengers remained seated throughout the event. No damage to the aircraft was reported.
Operating Procedures
The operator’s Operations Manual Part A covers pre-descent considerations, including determining the top of descent adjusted for wind, and conducting the descent with VNAV and ECON speed. The manufacturer’s Flight Crew Operating Manual (FCOM) and Flight Crew Training Manual (FCTM) form part of standard procedures. The FCOM emphasizes threat and error management during descent briefings, including anticipation of overspeed. The FCTM provides guidance on overspeed recovery: leave autopilot engaged unless it is apparent it is not correcting the overspeed; if manual inputs are required, disengage the autopilot but be aware this may cause an abrupt pitch change. The commander stated he was aware of these procedures and automatic protections.
Aircraft Information
The aircraft’s Flight Control Computer had software version P8.0. This update removed the automatic transition to control wheel steering (CWS) mode when force override was applied through the control column with the autopilot engaged. The manufacturer’s analysis indicated that the overspeed protection logic caused the mode reversion after autopilot disengagement.
The operator has issued additional guidance to pilots regarding overspeed recognition and recovery.