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AAIB BA38 B777 Initial Report Update 23 January 2008

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AAIB BA38 B777 Initial Report Update 23 January 2008

Old 24th Jan 2008, 10:03
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AAIB BA38 B777 Initial Report Update 23 January 2008

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.
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Old 24th Jan 2008, 11:46
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fuel pumps

NCM - no offence taken!
My colleague Tom Symonds points out the possible significance of the AAIB highlighting the fact that they are examining the fuel supply from the tanks to the engine fuel nozzles.
Previous posters have mentioned something called fuel-waxing. Would we be barking up the wrong tree to look into this possibility more?
What other explanations could there be for enough fuel not getting through from the tanks to the engines?
That's a completely open question.
Shoey
(PS as for pprune getting the story ahead of BBC News -- in case any of my bosses are reading this, we were examining the reports and first broke the story within minutes. the difference between pprune and the Beeb is that fact-checks don't have to be made on pprune before publishing! In the past we've received tipoffs about incidents which upon examination have turned out to be false-alarms or very minor.)
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Old 24th Jan 2008, 11:59
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Theres probably quite a list for fuel not being able to get to the engines, including Fuel Management Unit failure, Fuel flow transmitter, Fuel Pump Failures or Control Valve Failure. Having reduced fuel flow for whatever reason would cause the engines to reduce power.

You still have the EEC and FADEC which tie in with all this, so you could endlessly speculate on what went wrong.

To add, the report said that the aircraft had enough fuel left to power the engines, so you can rule out that the tanks didn't have enough fuel to power the engines.

Last edited by pavvyben; 24th Jan 2008 at 12:22.
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Old 24th Jan 2008, 12:02
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From AAIB initial report (my colour/bold) :

" ......... Initial indications from the interviews and Flight Recorder analyses show the flight and approach to have progressed normally until the aircraft was established on late finals for Runway 27L. At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond. The aircraft speed reduced and the aircraft descended onto the grass short of the paved runway surface. ......."

From updated report (my colour/bold) :

" ...... 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. ....."

No wonder there will be lots of speculation - it would be good if the AAIB read their own reports. A "poor" reading of their second report and one can "hide" behind poor grammar, otherwise one might think they were covering up a mistake in their initial report.
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Old 24th Jan 2008, 12:05
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All of which sounds like nasty [or restricted (icily waxed)] fuel clotting up either the high pressure fuel delivery - or (more likely) the air or fuel reference pressures sensed by the FADECs (some of which air comes from air data and some from inlet sensors etc etc).
Dagger, are you saying that if FADEC senses an error in pressure the response will be to throttle back?
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Old 24th Jan 2008, 12:07
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moggiee,
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.
Not fuel starvation per se, but possibly reduced fuel flow.
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Old 24th Jan 2008, 12:32
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BA normally land with 6-8 tonnes worth of fuel in the 777. This includes contingency fuel, Diversion to an alternate airport (LGW probably), and a standard reserve fuel figure (30mins) and then any extra fuel that the captain would wish to take.

The fuel would be contained in the wings tanks at this point, very little if any, would be remaining in the centre tank.
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Old 24th Jan 2008, 12:39
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Saturn V and MM Flynn..points taken

MM Flynn: Point 1 taken. Thank you.

Saturn V: Can't quite agree. During my engineering and investigation days (before LHS 733 days), fuel-starvation was defined as: "Fuel-flow, possibly zero, but in any case below that which was/is required or demanded under the prevailing conditions."

Sorry to be pedantic.

Cheers bm
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Old 24th Jan 2008, 12:43
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A question please, about the engine controls.

Is the mechanism for matching thrust demand to engine fuel flow open loop or closed loop. What I mean is, in the event that engine EPR/N1 is not what is being commanded by the thrust levers, does the fuel control unit keep trying to increase fuel flow until it is, or does the fuel metering unit simply set a fixed fuel flow and expect the engine to accelerate to an EPR setting concomitant with it?
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Old 24th Jan 2008, 12:58
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Thrust Limit Anomaly

Howdy All

This is slightly off topic and while it's a problem that affects thrust - I can't see how it bears on BA38, since they had commenced final approach and I'm sure that crew advanced thrust manually. However it's something worth highlighting for those operating the 777, particularly instructors who should be pointing this out to students in the sim.

I believe there is a software anomaly in the 777 thrust limit setting system which in specific situations can limit auto-throttle thrust below that required to maintain level flight. Details can be found in a document I wrote a while ago on my web site at http://www.infinidim.org - direct link is http://www.infinidim.org/Docs/Thrust...ly%20Brief.pdf

Essentially it revolves around the FMC setting CRZ thrust on approach when VNAV is engaged after flaps have been extended, when according to the Book GA thrust should be annunciated. In specific situations (Engine Out, Gear Down, Flap 20, Hi Weight, Hi OAT) this results in insufficient thrust to maintain height. I have advised my company's management, and Boeing about it a few years ago, but the issue is still there - whether it's a fault or not seems to be in dispute.

I know, I know - this doesn't relate to BA38. It sounds like they pushed the thrust levers up above what the autothrottle was commanding, and did not get a reponse.

However two points to be made are that as good as the 7 is, it's not perfect - maybe that's why we're still there - and when you reach down and push the thrust levers, you should get the thrust, whatever automatics are engaged. A Pilot's aeroplane.

Regards.
 
Old 24th Jan 2008, 13:12
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I'm not a pilot, just a controller. I've read every post on this. Do you think it might be expedient in the short term to do away with the "160 to 4" with a view to stabilised approach further out and at greater height ? I refer to comments about power reduction at "4" and then power demand after speed shedding.
I repeat that I'm not a pilot, but maybe ATC can help if it is considered to be worthwhile as an interim safeguard during the investigation cycle.
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Old 24th Jan 2008, 14:01
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FADEC Freezing

No Country Members said (post 23)
Dagger, are you saying that if FADEC senses an error in pressure the response will be to throttle back?
The FADEC is electro-mechanical because it bootstraps its determinations from porting into itself a number of reference air and fuel pressures (which are derived from a variety of locations). As the AD on the GE engines said: (see link to the AD at http://tinyurl.com/2nx3ym)

"...action requires visually inspecting Ps3 and P3B sense lines and full authority digital engine control (FADEC) Ps3 and P3B sensing ports and fittings, cleaning Ps3 and P3B fittings and sensing ports, purging the Ps3 and P3B systems of moisture, and, if necessary, blending of high metal, nicks, burrs, or scratches on Ps3 and P3B fitting threads.

This amendment is prompted by seven reports of loss of thrust control due to corruption of the signals to the FADEC caused by water freezing in the Ps3 sensing system. The actions specified in this AD are intended to prevent loss of thrust control due to corruption of the Ps3 and P3B signals to the FADEC which if it occurs in a critical phase of flight, could result in loss of aircraft control."

I'm sure that you can see that "purging the Ps3 and P3B systems of moisture" is no permanent fix. You could probably also appreciate that, as in icing-caused accidents, the evidence is lost when the ice melts. It's Similar to the situation where waxy deposits in super-cold fuel isn't there when the FAA tech gets to tear down the item. Some untoward developments are a function of the environment - and therefore transient. A 12 hour transpolar flight at very abnormally cold temperatures (way below ISA) followed by a prolonged low power descent (not much nacelle heating and much hot bleed air being drawn off for anti-ice/de-ice).

It's worth repeating that the AD went on to say: "FAA has received seven reports of loss of thrust control (LOTC) on General Electric Company (GE) Model GE90 turbofan engines installed on Boeing 777 series aircraft. Five LOTC events occurred in-flight and two occurred on the ground. The five in-flight LOTC events were temporary in that the engine recovered and continued to operate normally for the remainder of the flight.

Investigation

The investigation revealed that water can accumulate in the Ps3 and P3B pressure sensing system, which can freeze in the full authority digital engine control (FADEC) sensing ports or pressure line. Frozen water can result in a
restriction or a blocked signal to the FADEC. This blocked signal can cause a corruption of the FADEC signal and result in abnormal engine start characteristics on the ground or lack of engine response to commanded thrust levels in flight. Although there have been no LOTC events attributed to icing of the P3B sensing system in the field, inspections have identified moisture in this system, which could freeze and corrupt the P3B signal to the FADEC as
well. This condition, if not corrected, could result in LOTC due to blockage of the FADEC sense lines, which if it occurs in a critical phase of flight, could result in loss of aircraft control.
.
Simultaneous LOTC Events
.
The FAA is especially concerned about the possibility of simultaneous LOTC events on both engines installed on the Boeing 777 series aircraft due to common mode threats, such as certain atmospheric conditions that may result in ice in the Ps3 or P3B pressure sensing system and causing corrupted signals to the FADEC in both engines.
.
Interim Action
.
Both Ps3 and P3B pressure systems incorporate weep holes that allows drainage of water in the lines that may accumulate from condensation or ingested water; however, the field events and the investigation have determined that these design features may not always be effective in eliminating water from these systems. GE is assessing design changes that will prevent water from freezing in these systems and causing corruption of the signals to the FADEC.

The requirements of this AD may change based on the ongoing investigation of the root cause and field inspection results, and future rulemaking may be necessary."

So did they examine the RR Trent for a similar failing? I'd guess not. Never kick over rocks if you really don't want to know what might crawl out.....

So to answer your question: "...the response will be to throttle back?" Not really. As the FAA blurb says: "...which could freeze and corrupt the P3B signal to the FADEC" - which is my guess as to what happened on BA038. The FADEC lockups on engines 1 # 2 probably occurred sometime earlier and at different times. It's only when symmetrical power was demanded by the a/throttles that the "freeze" became apparent. I'd be surprised if that isn't to be the final revelation.
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Old 24th Jan 2008, 14:05
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I'm not a pilot, just a controller. I've read every post on this. Do you think it might be expedient in the short term to do away with the "160 to 4" with a view to stabilized approach further out and at greater height ? I refer to comments about power reduction at "4" and then power demand after speed shedding.

I repeat that I'm not a pilot, but maybe ATC can help if it is considered to be worthwhile as an interim safeguard during the investigation cycle.
No, leave it as it is. They were in a stable approach at 600'/2nm. In my opinion, it would have made no difference it they were at 1200'/4 or 2400'/8.

Maybe the fact that they were not stabilized until short final is what saved them from landing 2 miles short instead of 2000' short.

Think about it.....
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Old 24th Jan 2008, 14:26
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That second report confirms my initial suspicion, that the engines were not in fact at idle. Something occurred to me while I read it though. What would have been worse for them, being stuck just above flight idle, or crossing the numbers, to find the engines stuck at approach power

What do you guys think?
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Old 24th Jan 2008, 14:48
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At last, some useful info from the AAIB.

'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.'

Gotta be a clue in those time differences.

How many things can be ruled out by the fact that both engines didn't reduce to the lower thrust simultaneously?
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Old 24th Jan 2008, 15:03
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Perhaps cancelling A/T and advancing No1 throttle only; might have produced more thrust on the left hand engine to enable them to reach the runway.
The crew did manually advance the thrust levers. They may have also cancelled (turned off) the A/T but this would have no effect, because manually moving the thrust levers overrides the A/T.
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Old 24th Jan 2008, 15:29
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It's starting to remind me of the typical carb ice accident where the evidence melts before the investigators get to the carburetor.

Similarly any wax in the fuel and sensor lines likely had time to go back into solution and any ice time to melt

If the sensor lines retained the fuel, the investigators could check it for freezing point and moisture content.

The investigators could also put a temperature sensor/recorder on the lines and see if the sensor lines are exposed to a lower temperature than the tanks. However the proximity to the engine suggests it would be warmer there.
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Old 24th Jan 2008, 16:52
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Just came across this...

Airworthiness Directives; Boeing Model 777-200 and -300 Series Airplanes Equipped With Rolls-Royce RB211-TRENT 800 Series Engines
PDF Copy (If Available):

Preamble Information
AGENCY: Federal Aviation Administration (FAA), Department of
Transportation (DOT).

ACTION: Final rule.

SUMMARY: The FAA is adopting a new airworthiness directive (AD) for certain Boeing Model 777-200 and -300 series airplanes. This AD requires revising the airplane flight manual to provide the flightcrew with new ground procedures for shedding core ice during long taxi periods in freezing fog with visibility of 300 meters or less. For airplanes unable to perform the shedding procedure after prolonged taxiing in freezing fog with visibility of 300 meters or less, this AD requires certain investigative and corrective actions. This AD results from reports of engine surges and internal engine damage due to ice accumulation during extended idle thrust operation in ground fog icing conditions. We are issuing this AD to prevent internal engine damage due to ice accumulation and shedding, which could cause a shutdown of both engines, and result in a forced landing of the airplane.

DATES: This AD becomes effective February 27, 2008.

FULL TEXT CAN BE FOUND AT:

http://www.avionews.com/index.php?co...ante=index.php
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Old 24th Jan 2008, 17:00
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Hello Phil (Today, 1202z),

Are you being a little unkind to the AAIB? The statement in the initial report that the engines "did not respond" to an autothrottle demand at approximately 600 ft turns out, a week later, not to have been the whole story. But does it make it wrong?

We are now told that the engines did respond to the first demand initially, but soon reduced thrust one after the other. We can infer that the autothrottle subsequently sent a series of demands, to which the engines did not respond, as stated in the initial report.

There is currently no evidence to the contrary, and the handling pilot would have noticed if there was an autothrottle problem. This is because, on the B777, the throttle levers move to reflect the demands of the autothrottle. When the handling pilot intervened and pushed the levers (even further) forward, the engines still failed to respond properly, if at all.

As for suggestions of "pedantry", from southern Africa, I doubt that any accident investigators, employed at Farnborough these days (if ever), fall into that category. The expressons "rigour", and "not jumping to early conclusions" would be more appropriate in my experience.
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Old 24th Jan 2008, 17:26
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Airworthiness directive.

Dagger, thank you. Very interesting and informative indeed.
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