phil gollin

From the NTSB press release posted above :-

"......... These recommendations are being issued in response to the findings in two investigations - an accident and an incident - involving engine thrust rollbacks on Boeing 777-200ER airplanes powered by Rolls-Royce RB211 Trent 800 Series engines. In both cases a build-up of ice (from water normally present in all jet fuel) on the fuel/oil heat
exchanger (FOHE) restricted the flow of fuel to the engine, resulting in an uncommanded engine rollback. ............" (my bold)


Fascinating.

According to this, the two investigations have been completed (at least they have specific "findings" and are factually known) as opposed to theories or speculation !

Likewise, they have supposedly explained where the additional water came to cause the problem.

Unforunately they haven't bothered to highlight other engine installations in other planes which need similar flight restrictions.

.

EMIT

To Phil Gollin

Quote
Unforunately they haven't bothered to highlight other engine installations in other planes which need similar flight restrictions.
Unquote

This problem occurs specifically on the mentioned RR engine, due to its specific Fuel Oil Heat Exchanger design. Other jet engines on other aircraft have other designs of FOHE which have not exhibited this problem, so do not loosely assume that they also need restrictions. READ information please, when it is provided is such ample detail.

infrequentflyer789

I think it is credible that the test rig investigations have moved forward substantially from the last official reports, particularly with another incident providing another dataset. This is supported by the image provided.

Because I don't think this is an issue any more. It looks from the details and the image that we are not looking at a big slug of ice collecting elsewhere and getting pulled down the pipes by the increased fuel demand, but rather it is small amounts of ice acreting at the entrance to the small diameter FOHE tubes. Increased (cold) fuel flow (as thrust was demanded) would then both cool the area and probably pull the ice into the tubes, blocking them.

EDIT: The above was based on the NTSB statement and the FOHE image, now looking at the AAIB report it is clear that the ice acretes on the pipework and is swept onto the FOHE inlet.

Leaving aside possibilities of ice in tanks not being detected or removed at sumping, there would still have been 5L or so (according to the AAIB) or water in the fuel. Looking at the FOHE image, I don't think you need anything near 5L of water to create a serious problem. Just the mechanism for small amounts to crystalise in a bad place - which is what appears to have now been established.

If you look back on the thread there are plenty of posts identifying the Trent plumbing as different, particularly in the location of the FOHE. The Trent 800 FOHE may well be different again, sufficiently for this not to happen on other Trents.
The publication of the image strongly suggests to me that they have got this ice acretion repeatable on the test rig, in which case testing other FOHEs could have already ruled out this particular problem.

repariit

This from a Seattle newspaper . . .
NTSB wants Boeing 777 engine redesigned

"Boeing engineers, working in the laboratory, determined that the heat generated by the Rolls-Royce fuel-oil heat exchanger is not adequate to prevent moisture in the fuel from freezing. When that happens, ice can form that blocks fuel to the exchanger, starving the engines."

airfoilmod

To avoid Ice on FOHE, "Max Thrust is applied....". Sorry, isn't that what got This a/c into trouble in the First Place.??

To Avoid Ice on FOHE, "Idle Thrust is required...." Sorry, DELTA??

These "Mitigating procedures" were addressed. NTSB claims Another Incident (Accident) is "likely". Applied Power solutions are NOT working.

Will it now be "co-ordinated blending of high EPR followed by Idle, followed by" ....? A Secret Handshake?? Shaman? Tarot?

The way RR proposes to redo the HE frankly isn't too confidence inspiring either.

READ ##2204, 23, 42, 49, 54, 58, 59, ........ and others, ad, well, nauseum......

ReRead Osmus' letter. This isn't just "very cold temps". Neither is it "Chinese Fuel". Phil, this is a TRENT thing. Fuel has water in it, no mystery.

AF

bsieker

To avoid the ice buildup, max climb thrust for step climbs had been mandated a couple of months ago in certain cold conditions.

This has proven to be insufficient in some cases. The Delta flight in question did two max. climb thrust step climbs.

To remove an ice blockage that has formed nonetheless, a few seconds of idle thrust will melt it away by changing the balance between hot oil (only a little less) and cold fuel (a lot less).


Bernd

airfoilmod

I cannot even infer the point you may be making. The AD is working?

It is not? It may be working? Give it time? What is your point?

AF

Delta's Idle melting excursion cost 8,000 feet. Could it have been more?
Have you seen the terrain they were flying over? At 31,000 feet they could Glide what, 90 miles? Not enough. At 31k feet, they were at most 19,000 feet AGL, Get my point?

Low Flier

AAIB .pdf Report

bsieker

airfoilmod,

sorry for being obscure.

You said in your post that both actions (max. thrust, and idle thrust) were to avoid ice build-up.

They aren't.

One was supposed to avoid it, the other to remove it.

And I also can't see how max thrust brought any aircraft into trouble. It was only that max. climb thrust (which is usually on the order of only 30% of maximum (takeoff/go-around-) thrust) didn't keep them out of trouble.

Or maybe I misunderstood the first part of your post.

I get the point, as does the NTSB, that the idle melting procedure is not optimal, to say the least. That is why the Trent 800 FOHE design is being changed. It may take a while. What to do in the meantime? Obviously I have no answer to that.


Bernd

airfoilmod

Thanks Bernd. On finals, it would seem, if not already established, that increasing thrust caused Rollbacks. If so, Demonstrably the fix doesn't work. Parsing "How Much Thrust" is a fool's game. I have alluded to vibration and Acoustics on enhanced EPRs many times; this may dislodge deposits upstream the FOHE, sending them down to the Face. It also may dislodge ice accreted at the Face, further blocking Flow. That Idle may melt accreted Ice and Open the Line is wonderful, but How much is anyone willing to gamble? If it doesn't work, what then Glide Idle?

AF

FE Hoppy

Very interesting report. Quite a few unknown unknowns are now known unknowns.

airfoilmod

Reasonable to conclude from the evidence is the fact that Ice can migrate downstream to block the Face of the FOHE. Add to that the fact that Ice may have built up on the Face independently of additional "dislodged Ice". Yet the FOHE wasn't designed to be an Ice Melter; It is to be relied upon now to not only melt its own independent accumulations, but that released by "Thrust, Pitch, Turbulence"....etc.?? This is a type of Ice I suggested earlier, "Amoebic, migratory, slurry, etc." It seems to be too much for any makeshift lashup, (with respect).

AF

Post #2176 3 Feb. 09

Smilin_Ed

Since there have been no reported incidents with the other brand(s) of engines, it would seem prudent to consider the FOHE design(s) of those other brand(s). Could other FOHE designs not be adapted to fit the RR engines? Someone with specific experience with both/all types of B777 engines comment please.

chris weston

Airfoilmod sir, with the greatest possible respect you are showing your age here .............

In most (state) schools today raw results, such as the attainment of say a Grade "C", do not repeat not, rate as "satisfactory" or anything else; hitting your Mininimum Attainment Grade ("MAG") just might.

The Ofsted concept these days is more subtle and target driven as a function of your achieving "Value Added" cf datum MAG - datum MAG usually being defined as a function of a statistically derived average points score at the previous Key Stage.

On a personal note, the perception today is that in school all must win prizes and this mechanism is pretty good at diluting simplistic achievment driven elitism.

Do not expect me further to defend the current system...........

CW

phil gollin

Water quantities.

The AAIB Interim report no. 2 (my bold) states (page 7) that the

"... accident flight might have contained up to 70 ppm of dissolved and entrained (suspended) water ...."

whilst their post accident samples showed

".... Fuel samples taken from G-YMMM after the accident indicated that the water concentration in the fuel taken from the left main tank sump, APU line and Variable Stator Vane actuator was approximately 40 ppm ......".

However, the tests took place using 90 ppm something like 28 to 125% more than was in the crashed aircraft - as

".......the industry standard for continuous system operation tests, aiming to condition the fuel with 90 ppm of water. ......"

So, not only is the incident report rather cagey about what actually happened, but the tests do not seem (unless I missed something) truely representative of what happened. The report still doesn't explain where the additional water might have come from.

===========

However, it does look like there is a whole new bunch of research on fuel/ice mixtures to come about.

.

airfoilmod

Young Chris, As I understand STAR, it is an "indexed form" plateau, sensitive to the augmented mean of specific prior record, adjusted for performance diminution caused by variable but "smoothe" forms. Graphically, a hyperbolic (sic) paraboloid, truncated at x and x-y, replacing that of which we cannot speak.

AF momentus continuum, continuum truncare

HarryMann

Phil, I beleive it does at least allude to where it came from...

The varied (and higher levels) of water can come from less than homogeneous distribution in the tank, but mainly from prior upstream ice melting, either from the boost pump screens or from pipewrok less far upstream.

As I took it to mean ppm at the point of interest e.g. in the pylon pipework or adjacent main tank pipework.

I believe they also inferred that concentrations could vary wildly from time to time in the fuel passing through the critical pipework sections, up to possibly 125ppm.

.. without of course re-reading or quoting directly.

deScally

Would anybody be willing to make a guess as to the likelihood of the FAA ordering a worldwide grounding of the 777-200ER with the Trent 800 engine type until such time as a redesigned FOHE can be approved and installed? Nonexistent, small, moderate, significant...? Perhaps a silly question in which case my apologies, but it looks to this non-expert that the airworthiness directive now in place is far from ideal.

airfoilmod

#2273 #2274..... Probability ??..... Moderate to Severe.

AF (my opinion) (Have you read the thread?)

deScally

airfoilmod; thanks for enlightening me. I had read the thread, just looking for some clearer opinion re: likelihood of grounding the fleet. I await the FAA's action with bated breath. Thanks again.