Thomsonfly 737 stalled on approach says UK accident report

[TallDutch]

The airspeed of a Thomsonfly Boeing 737-300 on approach to Bournemouth airport, UK, dropped to 82kt (151km/h), the aircraft stalled, and the maximum pitch-up during the crew's go-around manoeuvre was 44°, according to an Air Accidents Investigation Branch report. The crew recovered control of the aircraft successfully and landed safely from a second approach.

More info can be found here:
http://www.flightglobal.com/articles/20 ... ident.html (http://www.flightglobal.com/articles/2009/05/21/326834/thomsonfly-737-stalled-on-approach-says-uk-accident.html)

[Matt Molnar]

Yikes.

[LGA777]

Yikes is right, I did not realize a 737 could still fly at only 82 knots, glad this one had a successful outcome.

Talldutch thanks for posting.

LGA777

[DHG750R]

More info here..http://avherald.com/h?article=419f2f9e&opt=0

all this took place at or below 2500ft, at night and in IMC - that airplane was in immediate danger.

From AvHerald:

A Thomsonfly Boeing 737-300, registration G-THOF performing flight BY-3894 from Faro (Portugal) to Bournemouth,EN (UK) with 132 passengers and 5 crew, performed an ILS approach to Bournemouth's runway 26 in night and IMC conditions, however the autothrottle system disconnected by itsself unrecognized by the crew, so that the thrust levers remained in their idle position. When the speed fell below the planned approach speed, the commander took control and initated a go-around, during which the crew lost elevator authority and could not prevent the nose to pitch up to 44 degrees and airspeed decay to 82 KIAS. Eventually the crew was able to recover the airplane and perform a safe landing.

The Air Accident Investigation Board (AAIB) rated the event as an accident and released their final report identifying following causal factors:

1. The aircraft decelerated during an instrument approach, to an airspeed significantly below the commanded speed, with the engines at idle thrust. Despite the application of full thrust, the aircraft stalled, after which the appropriate recovery actions were not followed.
2. The trimmed position of the stabiliser, combined with the selection of maximum thrust, overwhelmed the available elevator authority.

Contributing were:

1. The autothrottle warning system on the Boeing 737-300, although working as designed, did not alert the crew to the disengagement of the autothrottle system.
2. The flight crew did not recognise the disengagement of the autothrottle system and allowed the airspeed to decrease 20 kt below Vref before recovery was initiated.

The air safety report filed by the captain did not reflect the severity of the incident and just indicated, that there had been an incident so that the data recorders should be read out, so that the AAIB was notified of the incident only 12 days later, after the airline had read out the quick access recorders (QAR) 11 days after the incident.

The first officer was pilot flying for the leg. The crew had briefed an ILS approach to runway 26 with a Vref of 129 KIAS for flaps 40 and planned to use an approach speed of 135 KIAS (Vref+6).

The flight had been routine until after intercepting the glideslope. The autopilot captured the glideslope, while the crew lowered the gear and selected a lower speed into the master control panel (MCP). The autothrottles responded as expected by pulling the thrust levers to idle.

About 20 seconds later the autothrottle system (ATS) disconnected and the ATS disconnect warning (a red flashing "A/T P/RST" indication) was triggered. No manual disconnect was recorded. The disconnect was not recognized by the crew however, the thrust levers remained in idle position throughout the remainder of the approach.

The flaps were extended up to flaps 40 according to speed schedule. When the speed fell to 125 KIAS, the captain called out "SPEED" and took control of the airplane commanding a flaps 15 go-around. He manually disconnected the ATS (the press of the disconnect buttons recorded by the QAR), which also stopped the red flashing "A/T P/RST" announciator indicating the autothrottle had disconnected about one minute after the announciator had been activated, and pushed the thrust levers slightly forward. Within 1.5 seconds at a speed of 110 KIAS the stick shaker activated. In the following 2 seconds the throttle levers were moved into their full forward position. The autopilot changed from localizer and glideslope to command wheel steering (CWS) pitch and roll. The attitude of the airplane had been steadily increasing using elevator trim and had reached 12 degrees nose up, the automatic pitch trim had stopped at 4.9 degrees (7.9 units) of stabilizer trim.

The commander pushed the yoke forward in anticipation of the nose up moment caused by the engine acceleration. This arrested the pitch and brought the nose down to 5 degrees nose up. The stick shaker stopped operating. The minimum recorded airspeed was 101 KIAS.

Four seconds after the thrust levers had reached full forward position - the engines were accelerating through 81% N1 and the airspeed was increasing - the TOGA mode (takeoff/go-around) activated causing the autopilot to disengage. The pitch angle increased again, the stick shaker activated again. The pitch angle increased to 22 degrees nose up, where it seemed to stabilise at a speed of 118 KIAS. Both engines produced 96-98% N1 in excess of the Go-Arond thrust of 94% N1. The angle of attack began to decrease, the stick shaker operation ceased.

When the flaps retracted past the 25 degrees position, the nose began to pitch up again at an increasing rate. When the flaps reached the 15 degrees position, the pitch angle had increased to 27 degrees, the stick shaker reactivated, full nose down elevator was applied and the airspeed began to decay.

When the pitch angle reached 36 degrees, the TOGA mode disengaged and the speed had dropped to 107 KIAS. The pitch reached 44 degrees nose up, the airplane stalled, however, without a change in elevator position the pitch rate reversed bringing the nose down. The angle of attack still increased and the airspeed fell to 82 KIAS with a pitch angle of 33 degrees.

Within the next 10 seconds the pitch angle reduced to 20 degrees, the airspeed began to rise rapidly. 5 seconds after the minimum speed was recorded the thrust was reduced to 86% N1. The nose down pitch rate increased reducing the pitch angle by 15 degrees in the next two seconds. The aircraft stabilized in a 5 degrees nose up attitude, the speed increased and the commander regained control. TOGA was re-engaged at a calibrated airspeed of 147 knots.

The commander levelled off initially at 3000 feet before climbing to 4000 feet and positioning for a second approach. During the second approach both autopilot and autothrottle system worked as expected. The airplane landed safely 16 minutes after the upset began.

The AAIB reported, that G-THOF was involved in another uncommanded autothrottle disconnect on October 22nd 2007 with airspeed decreasing to 128 KIAS, however no resulting upset.

The AAIB issued a safety recommendation to the FAA to have the effectiveness of the autothrottle disconnect announciation verified and improved, another safety recommendation to the EASA to review airplane certification to ensure, that disengagement of autoflight controls including autothrottle is suitably alerted to flight crews. Another safety recommendation requires an update to the flight crew operation manual's stall recovery procedures to include, that forward trimming may be needed to enhance pitch control authority.