In response to the post by OVERTALK at
#268 of this thread,
VNV2178B said:
The problem with P30 icing on R-R engines has been known and compensated for at least since the RB211 FAFC equipped units. I can see no reason why Trents are not similarly fitted.
Even if P30 is lost the FADEC will fail to reversionary mode using speed and temperature rather then EPR as the main control parameters.
I am not allowed to speculate further.
However to quote further (below) from the AD itself, it would appear that the FAA had genuine concerns about the possibility of LOTC (Loss of Thrust Control) on the Triple 7 - which is exactly what happened to Flight BA038 after an extended period of cruise in very cold temperatures at height - during a 12 hour trans-polar operation. In addition, any prolonged CDA descent at near idle power has the potential to:
a. Allow greater than normal cooling within the nacelles
b. Disguise the fact that the engines are "locked at/near idle" due to the FADEC's reference pressure sensing lines becoming obstructed by waxy ice crystals.
I'm not aware whether or not the FADECS are just similar (or the same) on GE, Trents and P&W, however the possibility remains that the problem is replicable across the GE's and Trents. I doubt that it is just a mere coincidence.
QUOTE FROM the Airworthiness Directive: (
link at http://tinyurl.com/2nx3ym)
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.
SUMMARY: This amendment adopts a new airworthiness directive (AD) that is applicable to certain General Electric Company GE90 series turbofan engines. This 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.
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 rule-making may be necessary.