PPRuNe Forums - View Single Post - AAIB BA38 B777 Initial Report Update 23 January 2008
Old 24th Jan 2008, 13:01
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Dagger Dirk
 
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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.
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Simultaneous LOTC Events
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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.
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Interim Action
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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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