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BA038/17JAN PEKLHR - Initial Report Update
NTSB: National Transportation Safety Board (米国運輸安全委員会)経由で BAW038/17JAN PEKLHR に関して、AAIB: Air Accident Investigations Branch (英国の航空事故調査委員会)が先日発表した Initail Report の update が入電しました。
エンジンが推力を失った過程がより詳細になってきました。
先の報告の通り、当該機は EGLL Rwy 27L へ ILS Approach を行っていました。問題が発生するまでは全くもって順調で、所謂 Stabilized Approach でありました。
オートパイロットで ILS Approach を実施していましたので、エンジンはカップリングされた Auto Thrust で制御されています。
オートパイロットによる進入過程で、Auto Thrust から両エンジンに推力増加の指示が出ました。
(※素人の下種な推測:既に Final Flap はセットされていたでしょうから、おそらくは風の影響を補正し、Vref + α の速度を維持し、VNAV Path は on Glide Slope を維持すべく推力補正をしたのだと思います)
その指示( Auto Thrust から両エンジンへの推力増加指示)から約3秒後、右翼の No.2 Engine の推力が低下しました。そのおよそ8秒後、続いて左翼の No.1 Engine の推力が低下しました。
「推力が低下した」の意味するところは、エンジンが停止した訳ではなく、また、完全に IDLE Retard でもなく、Flight IDLE よりは若干(推力は)出ていました。
しかし、Auto Thrust が指示した推力には達していませんでした。
DFDR, ADAS から得られた情報では、燃料は充分あり(これで燃料切れの可能性はまずなくなった)、Autothrottle 及びエンジン制御システムは、〔推力に異変が起こる前も、その後も〕適切な指示を出していました(これで FADEC: Full Authority Digital Engine Control システムが全滅したとの可能性も薄くなった)。
AAIB では、
・なぜ Autothrottle からの指示に反して推力が減ってしまったのか?
・なぜ、その後の PF による Thrust Lever の操作にエンジンが反応しなかったのか?
について、考えられ得るシナリオを、Boeing 社,Rolls Royce 社,および英国航空との緊密な協力のもと、解明することになります。
先ずは、燃料タンクからエンジンのインジェクタまでに至る燃料経路(その制御系も含めて)を徹底調査するようです。
未だ全容解明には程遠い状態ですが、たとえ些細なことであっても確認できた情報を迅速に公表する姿勢に、AAIB が Boeing, Rolles Royce, British Airways に止まらず、世界中の 777 のオペレータや、その Flight Crew に対して気を配っていることが伺われます。
AAIB からの当該 Update 原文も入手しましたので以下に引用します。
Accident to a Boeing 777-236, G-YMMM, on 17 January 2008 - Initial Report Update
Accident to a Boeing 777-236, G-YMMM, on 17 January 2008 at 1243 hrs
Initial Report Update 23 January 2008
Since the issue of the Air Accidents Investigation Branch (AAIB) 1st Preliminary Report on Friday 18 January 2008 at 1700 hrs, work has continued on all fronts to identify why neither engine responded to throttle lever inputs during the final approach. The 150 tonne aircraft was moved from the threshold of Runway 27L to an airport apron on Sunday evening, allowing the airport to return to normal operations.
The AAIB, sensitive to the needs of the industry including Boeing, Rolls Royce, British Airways and other Boeing 777 operators and crews, is issuing this update to provide such further factual information as is now available.
As previously reported, whilst the aircraft was stabilised on an ILS approach with the autopilot engaged, the autothrust system commanded an increase in thrust from both engines. The engines both initially responded but after about 3 seconds the thrust of the right engine reduced. Some eight seconds later the thrust reduced on the left engine to a similar level. The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust.
Recorded data indicates that an adequate fuel quantity was on board the aircraft and that the autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.
All possible scenarios that could explain the thrust reduction and continued lack of response of the engines to throttle lever inputs are being examined, in close cooperation with Boeing, Rolls Royce and British Airways. This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles.
Further factual information will be released as and when available.
また、BBC UK 版にも記事が出されていましたので、参考までに以下に引用します。
BBC NEWS | England | London | 'Engines running' in crash plane
'Engines running' in crash plane
Both engines of the British Airways jet that crash-landed at Heathrow Airport were still running when it came down, investigators have said.
But the engines did not respond sufficiently to a thrust request as it came into land, the Air Accident Investigation Branch (AAIB) said.
The AAIB said it was now focusing on the Boeing 777's fuel supply system.
US investigators have noted six previous engine failures in the same type of aircraft, it also emerged.
All 136 passengers and 16 crew on the flight from Beijing survived the crash-landing on 17 January.
The AAIB's investigation update comes as the US National Transportation Safety Board's website listed the previous engine failures.
The most recent was in September 2006, when a Malaysia Airlines' Boeing 777's right engine shut down near Brisbane, Australia.
A UK aviation industry source told the Press Association seven engine failures was "not a large figure" given the aircraft's long flight history.
About 1,000 Boeing 777 jetplanes have been sold since the model's first commercial flight in 1995.
In its update, the AAIB said the Boeing's twin Rolls-Royce engines initially responded to the request for thrust, but after three seconds the thrust of the right engine reduced and after eight seconds there was a thrust reduction in the left one.
Recorded data shows the aircraft had enough fuel and its automatic throttle and engine control systems had worked as expected, the AAIB said.
The AAIB said it was now carrying out a "detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles".
Last Updated: Thursday, 24 January 2008, 16:08 GMT