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Idle Approach

Idle Approach

Causalfactors

BY MARK LACAGNINA

The 737 stalled after the disengaged without notice. Idle Approach

either pilot was aware that As airspeed had decreased, the autopi- None of the 132 passengers or five the system had lot had increasingly trimmed the 737 crewmembers was injured, and there disengaged with the nose-up to maintain the glideslope. The was no damage. The AAIB’s investiga- levers at idle during an instru- pitched up further as thrust tion of the Sept. 23, 2007, incident led to Nment system (ILS) approach to from the underwing-mounted recommendations to ensure that flight Bournemouth (Hampshire, England) increased as the commander advanced crews are effectively alerted to the disen- © Philippe Ballerstedt/.net © Philippe . The -300 initially the thrust levers. gagement of an autoflight system and to decelerated according to the flight The combination of the nose-up clarify procedures for recovering from crew’s expectations. However, after final trim and the application of maximum an impending . extension, the commander noticed thrust “overwhelmed” the eleva- that indicated airspeed had dropped tor, the report said, but neither pilot Night Instrument Conditions 10 kt lower than the target speed. He considered retrimming the . The aircraft was en route on a sched- was moving the thrust levers forward Both pilots were pushing their control uled flight from Faro, Portugal. The to initiate a go-around when the stall- columns against the stops when the commander, 56, had 11,280 flight warning system activated. aircraft finally stalled and descended hours, including 420 hours in type. He The flight crew’s subsequent ac- in a steep nose-up attitude. The com- had served as a 757/767 for tions to avoid the impending stall were mander was able to recover from the the operator before upgrading as a 737 inadequate, said the U.K. Air Accidents upset only after reducing thrust to commander in 2006.1 The first officer, Investigation Branch (AAIB) in its the go-around setting, which restored 30, had 3,170 flight hours, includ- final report on the serious incident. authority. ing 845 hours in type. He had flown

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twin- regional aircraft before being the pitch of the aircraft to minimize glideslope employed by the operator in 2006. deviation and adjusting the stabilizer angle to “Before departing Faro, the crew discussed keep the aircraft in trim.” the weather at Bournemouth, uplifted additional The report said that the approach was stable fuel to permit two approaches and decided on a and that there was no sign the crew was “rushing full-flap (flap 40) landing,” the report said. the approach.” However, the pilots momentarily Night instrument meteorological condi- became distracted when the first officer increased tions prevailed at Bournemouth, which is on the the illumination of his map light to read a placard southern coast of England. Surface winds were showing the flap limit speeds before asking the from 220 degrees at 14 kt, visibility was 4,000 commander to select flap 40. About this time, m (2 1/2 mi) in light rain, and the ceiling was airspeed began to decrease rapidly. overcast at 400 ft. Cleared to conduct the ILS approach to 26, the crew calculated a ‘I Have Control’ landing reference speed (Vref) of 129 kt and After selecting flap 40, the commander also se- decided to add six knots for the final approach. lected 135 kt — the planned Vref plus 6 kt final As the captured the glideslope at approach speed — on the MCP and completed 2,500 ft, the first officer, the pilot flying, asked the landing checklist. “The commander stowed the commander to extend the , the checklist on top of the instrument panel, and Idle Approach select flap 15 and begin the landing checklist. He when he looked down he saw an IAS [indicated also selected a lower speed on the mode control airspeed] of 125 kt,” the report said. “He called panel (MCP). The autothrottle system moved ‘speed.’ The [first officer] made a small forward The autothrottle (A/T) the thrust levers to idle to reduce airspeed to the movement with the thrust levers, and the com- annunciator flashes selected value. About 20 seconds later, the auto- mander called, ‘I have control.’” so often during disengaged. “The disengagement was The aircraft was descending through 1,540 approach that it may neither commanded nor recognized by the crew, ft with a 12-degree nose-up pitch attitude and be perceived as a and the thrust levers remained at idle through- airspeed slowing below 110 kt when the com- nuisance message. out the approach,” the report said. mander moved the thrust levers full forward. Indicated air- As he did so, the activated to warn speed initially of an impending stall (Figure 1, page 30). The decreased normally commander engaged the autopilot’s control at about one knot wheel steering mode and moved his control per second. “As the column forward, reducing the pitch attitude to speed decreased 5 degrees nose-up. “The stick shaker operation A/P A/T FMC P/RST P/RST P/RST below 150 kt, flap stopped, and the minimum airspeed was 101 kt,” 25 was selected,” the the report said. “A small, apparently unintended TEST report said. “The application of right induced a right roll.”

autopilot tracked the As low-pressure rotor speed (N1) glideslope accurately, increased though 81 percent, the /go-

Susan Reed gradually increasing around (TOGA) mode activated. “The autopilot

www.flightsafety.org | AeroSafetyWorld | August 2009 | 29 Causalfactors

full forward. “Both Selected Plots From QAR Data pilots reported [dur- ing post-­incident 200 Angle-of-attack interviews] that they Pitch attitude 180 had no pitch control Calibrated airspeed

Stick shaker Airspeed (kt) authority,” the report 160 said. 60 140 Calibrated air- speed (CAS) de- 40 120 Autothrottles disengaged creased below 107 kt 20 100 as the pitch attitude Degrees reached 36 degrees 0 80 and the left bank increased beyond 13 2247:00 2247:20 2247:40 2248:00 2248:20 2248:40 2249:00 2249:20 2249:40 degrees. The TOGA Coordinated universal time mode disengaged. A right control QAR = quick access recorder input brought the Source: U.K. Air Accidents Investigation Branch level before Figure 1 the 737 stalled with a nose-up pitch attitude of 44 degrees. disengaged, the pitch attitude started to increase “With no change in elevator position, the again, and the stick shaker reactivated,” the pitch rate reversed from positive to negative al- report said. “A corrective roll input was made to though angle-of-attack continued to increase as bring the aircraft wings-level, and although the the aircraft started to descend,” the report said. control column was positioned fully forward, “Despite reducing pitch, the airspeed continued the nose-up pitch increased to 22 degrees.” to decrease for a further five seconds to a mini-

N1 increased to nearly 98 percent, which mum recorded CAS of 82 kt when the pitch was is above the rated go-around thrust setting of 33 degrees nose-up.” 94 percent. The pitch attitude stabilized briefly The commander regained control af-

at 22 degrees, and the stick shaker ceased as ter reducing N1 to 86 percent. Pitch attitude airspeed increased to 118 kt. However, the pitch decreased rapidly to 5 degrees nose-up, and attitude again began to increase when the crew airspeed increased to 147 kt. “The commander selected flap 15, the go-around setting. initially leveled the aircraft at 3,000 ft before “A small continuous left rudder input started climbing to 4,000 ft and self-positioning for a a left roll,” the report said. “As the flaps reached second approach,” the report said. The com- flap 15, the pitch angle was increasing through mander remained as pilot flying during the 27 degrees and the left roll was increasing second approach, which was conducted without through 7 degrees. The stick shaker reactivated, further incident with the autopilot and auto- full nose-down elevator was still being applied, throttles engaged. The 737 was landed at 2301 and the airspeed began to decay.” local time. After to a stand and shutting down ‘Full Forward Stick’ the engines, the commander told the operator’s The first officer called “high pitch,” and the base engineer that there had been an incident commander replied, “I have full forward stick.” and that, although he believed the aircraft was The first officer also held his control column serviceable, the operator likely would want to

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review the recorded flight data. “No The investigation did not resolve report said, noting that the 737 did not defects were entered in the technical why the autothrottle system disengaged have, and was not required to have, log,” the report said. “The engineer as- during the approach. Manual disen- an aural indication of autothrottle sured the commander that the opera- gagement is achieved by selecting the disengagement. tional flight data monitoring (OFDM) autothrottle switch on the glareshield As a result, AAIB recommended information was sent from the aircraft panel to “OFF” or by pressing a push- that Boeing and the U.S. Federal by an automatic mobile telephone- button on either . The QAR Aviation Administration review the based data link.” data indicated that neither of these effectiveness of the autothrottle system actions had been taken. disengagement warnings in 300-, 400- Questions Unanswered The uncommanded disengagement and 500-series 737s and improve them The next morning, the commander of the autothrottle system could have if necessary. AAIB also called on the advised the operator’s safety depart- resulted from detection of an internal European Agency to ment of the incident and completed fault by built-in test equipment. “Due to review Certification Standard 25 for an air safety report (ASR). The AAIB the delay in notification of the incident, transport category to “ensure report said that the ASR “contained the aircraft had completed more than that the disengagement of autoflight limited information” and “did not 10 flights, and therefore the fault his- controls, including autothrottle, is suit- depict the event accurately.” Not real- tory information from the incident had ably alerted to flight crews.” izing the seriousness of the incident, been overwritten,” the report said. Post- The incident investigation revealed the operator did not file a mandatory incident tests of the autothrottle system that the flight crew did not apply nose- occurrence report with the U.K. Civil revealed no faults that could cause an down trim to regain elevator author- Aviation Authority. uncommanded disengagement. ity. The flight crew training manual The OFDM analyst who read Why the pilots did not see the (FCTM) and the quick reference hand- the ASR was not a pilot and flagged flashing red light on the instrument book (QRH) for the 737-300 both say the event for further review by a panel that warns of autothrottle that the first action in response to a stall pilot representative. An OFDM pilot disengagement also was unanswered. warning or a stall is to apply full thrust. representative was on duty in the The annunciator is a small rectangular However, only the FCTM advises that safety department that day but was pushbutton lens in the upper center of the aircraft’s nose will pitch up as the too busy with other tasks to review the instrument panel. Labeled “A/T P/ engines accelerate and that the stabi- the incident aircraft’s flight data. The RST” —“autothrottle, push to reset” — lizer must be trimmed nose-down to report said that the seriousness of the annunciator also generates a flash- assist in pitch control. “The [QRH] drill the incident was not identified and ing amber caution light when airspeed does not mention the use of pitch trim,” appropriate action was not taken until is 10 kt above or 5 kt below the selected the report said. the next pilot representative came speed or decreases to “alpha floor,” or Based on this finding, AAIB called on duty again at the OFDM office 11 1.3 times the stalling speed. on Boeing to “clarify the wording of days later. “The autothrottle warning … flashes the approach-to-stall recovery [in the “[The aircraft] was not subjected amber routinely for extended periods QRH] to ensure that pilots are aware to an engineering examination to during the approach phase of flight,” the that trimming forward may be required ensure its continued airworthiness and report said. “It is likely that flight crews to enhance pitch control authority.”  remained in service throughout this are subconsciously filtering out what is This article is based on AAIB Aircraft Accident period,” the report said. Data recorded perceived as a nuisance message.” Report 3/2009, “Report on the Serious Incident by the voice recorder and flight Investigators identified “a num- to Boeing 737-3Q8, Registration G-THOF, on data recorder during the incident were ber of other events” that involved Approach to Runway 26, Bournemouth Airport, overwritten, and the AAIB’s incident uncommanded and unrecognized Hampshire, on 23 September 2007.” investigation was limited to interviews autothrottle system disengagements and analysis of the flight data captured in 737s. “Consequently, the efficacy Note by the quick access recorder (QAR) for of the autothrottle warning became of 1. The operator was Thomsonfly, which the OFDM program. interest during the investigation,” the became Thomson Airways in 2008. www.flightsafety.org | AeroSafetyWorld | August 2009 | 31

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