The BA038 Likely Cause   - Paraffin Wax?
 
More on the sources used by me in that 09 Oct 2006 "the UNTHRUSTWORTHY 777" ASW article [to be found at this link)]
 

 Granted that the following AD (link) applies to GE90's and not the Rolls Royce TRENT engine, but it does appear to confirm that there are paths where FADEC signals can be corrupted.


SUPPLEMENTARY INFORMATION: The Federal Aviation Administration (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's (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.

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 rulemaking may be necessary.

 
from: =>

General Electric Company GE90 Series Turbofan Engines

... (FAA) has received seven reports of loss of thrust control (LOTC) on General Electric Company (GE) Model GE90 turbofan engines installed on Boeing 777 ...
rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/b51bf59705afd3e4862568820061cbea - 28k - Cached - Similar pages

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Click here for a photo of:

BA038 on late finals (on its final flight)

 Icing may not have been an immediate issue here but a contributory one to the clogging of sensor points/connections (moisture).... perhaps because of fuel "waxing". Waxing occurs when aviation fuel is frozen and rewarmed (think in terms of how a blood clot can cause a stroke). FADECS use reference air and fuel pressures. Loss of those due to obstruction or waxing clogs can lead to the FADEC's logic becoming confused or unresponsive.

The fact that it was picked up by the FAA on the G.E. powerplant rather than a Rolls-Royce one (on the same aircraft design) should make little difference to the underlying cause. Just because the FAA didn't put out an APB (i.e. an Airworthiness Directive) on the Trent engine, doesn't mean they're infallible to the same control/FADEC issues!... it just meant they hadn't investigated or dealt with them at that time.

Operators with RR's would have looked away (understandably), but the FAA's remit was only to explore what was going on in ITS (much bigger) aviation industry, where G.E. engines dominate, and had led to previous (unrelated) issues (IIRC)...

And if this BA workhorse 'was' doing the same route frequently, then it looks likely that moisture could have been a <contributory> cause in losing control ...at that time of descent, whilst the fuel pressure/electricals were working overtime!

What would have triggered it on this occasion? Well as many pilots have remarked, the temperatures at height were the coldest they'd ever experienced - i.e. down to as low as -70 degrees Celsius on that day.

And to those who pooh-pooh the simultaneity of the event, remember that it could have happened to the individual engines at any time during the CDA approach from cruise altitude. CDA? Continuous Descent Approaches are being used world-wide now by ATC to save fuel. Courtesy of the FMS (Flight management System's Computer), pilots can set idle leaving cruise altitude and the autothrottle, via the FADEC (Full Authority Digital Engine Control) only needs to respond to a call for more power (to offset a drag increment) once the gear and flap are selected down upon intersecting either/both centerline or ILS glidepath. Only at that point would it become obvious that the engines were "frozen" at idle.


Fuel Waxing

"...in severe winter temperatures aviation fuel may cloud and eventually become 'waxy' (where the blend in the fuel turns to a jelly like substance), making it

 impossible to pump..."

Paraffin is a common name for a group of alkane hydrocarbons with the general formula CnH2n+2, where n is the number of carbon atoms. The simplest paraffin molecule is that of methane, CH4, a gas at room temperature. Heavier members of the series, such as that of octane C8H18, appear as liquids at room temperature. The solid forms of paraffin, called paraffin wax, are from the heaviest molecules from C20 to C40. Paraffin wax was identified by Carl Reichenbach in 1830.

Paraffin, or paraffin hydrocarbon, is also the technical name for an alkane in general, but in most cases it refers specifically to a linear, or normal alkane —whereas branched, or isoalkanes are also called isoparaffins. It is distinct from the fuel known in Britain and South Africa as paraffin oil or just paraffin, which is called kerosene in much of the U.S., Australia and New Zealand.

The name is derived from the Latin parum (= barely) + affinis with the meaning here of "lacking affinity", or "lacking reactivity". This is because alkanes, being almost non-polar, are very unreactive. Paraffin wax is mostly found as a white, odorless, tasteless, waxy solid, with a typical melting point between about 47 °C to 64 °C, and having a density of around 0.93 g/cm3. It is insoluble in water, but soluble in ether, benzene, and certain esters. Paraffin is unaffected by most common chemical reagents, but burns readily.

During aircraft operation, fuel is withdrawn from fuel tanks on the aircraft and is passed to a filter at the inlet to the engine fuel system. To prevent blocking of the fuel filter due to the formation of wax or ice in the fuel when the fuel is very cold, a heat exchanger is provided on the engine or the aircraft, in the fuel line upstream of the filter to warm up the fuel before it enters the fuel filter. Conventional heat exchangers use hot air bled from the engine compressor which produce an adverse effect on the engine efficiency when they are operating. The times when the greatest heating is required, i.e. when icing of the fuel filter is most likely, are often those times when the engine is not operating at maximum power, for example, during a lengthy descent from high altitude, or at idling in cold climates.

Jet A (US) has a higher freeze point than Jet A-1 (i.e. it freezes more readily). The current spec for the UK is what used to be called DERD 2494 now renamed DefStan 91-91. The spec is - 47 degrees Celsius. The problem with the freezing /waxing theory is that Chinese jet A-1 equivalent (used to be RP-3, now called jet fuel number 3) typically has a lower freeze point (-52C) than Jet A-1.

That's not to say it could not have happened or that the particular delivery could not have been waxy. But there will have been retention samples taken and freeze point is a test that will have been carried out by an approved lab. BA do take a keen interest in the fuel that goes into their aircraft and I remember being on the receiving end of an assessment visit in Nairobi with painful clarity. It resulted in us buying a whole bunch of new equipment.

The problem is that the normal freeze point for aviation Kerosene is minus twenty or so. Dual purpose kerosene, which can be used in both aircraft and in domestic appliances has both a low freeze point and a high smoke point.

Mistakes can happen. Adulteration can happen.

 

Click here for a photo of:

BA038 at its very hard (moment of) touchdown  (port oleo has just pierced upper wing surface)

 

 
FADEC Freezing

No Country Members said (post 23)
 
Quote:
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.

 

   
 
I stand Corrected

In an earlier post (#12) on this thread, I had said:
 
Quote:
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.....
It's been pointed out to me (via a PM) that this is incorrect (i.e. that the FAA had considered at the NPRM stage (of the AD cited in post 12) that the Trent 800 is equally vulnerable to the icing/waxing malady) - which was my surmise from the start.... and of course tends to confirm the validity of the FADEC freeze theory expounded at post #12.....
.
text received:...all below


In a recent post, you rhetorically asked if they considered the RR Trent in the AD, and I think you surmised the answer was negative.

I think this paragraph below is germane, and show that the intent was to include them with the GE, as there was no reason to expect them to respond differently (bold added):



---Quote---
Requests To Specify Lack of Events With Trent 800

Boeing and Rolls-Royce ask that we clarify the summary and paragraph (d) of the NPRM to specify that Trent 800 series engines (the subject of the proposed AD) have not experienced engine surges and internal engine damage due to ice accumulation during extended idle thrust operation in ground fog icing conditions.
We acknowledge that Trent 800 series engines have not experienced the stated events, as described in the "Discussion'' and "Similar Engine Models'' sections of the NPRM. As stated in the preamble of the NPRM, both the Trent 700 series engines and the Trent 800 series engines *have a similar compressor design, and therefore may be subject to the same unsafe condition. *We find that this information need not be added to the AD Summary section, or to any regulatory paragraphs of the AD.
---End Quote---

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http://www.avionews.com/index.php?co...ante=index.php

 

  Link - The Cold Fuel Management Story

Link - The Unthrustworthy 777