airfoilmod

If, unlike the GE and PW designs, RR had designed a system with HE bypass for Fuel, based on Pressure, this thread would not have appeared.
A packed up FOHE fuel side, causing cavitation and no flow, would have been bypassed by a simple gate valve in front and behind the Plug.
Electrically (solenoid) operated slaved to a pressure sensor.

AF

phil gollin

Airfoilmod,

Where do you get your scientific information ?

There seems not to have been enough ice to cause the icing - unless you know better (if so please state how you obtained it).

There is no report stating what is formally believed to have happened (again unless you know something that you are not saying).

And, unless this accident is properly explained a similar one affecting a totally different aircraft/engine configeration might occur.

You seem to be taking too much credence in partial information.

.

Swedish Steve

Sooty, I see you have worked for RR so I must bow to your knowledge. I only know what I was taught on the AMEL course, and read in the AMM (which is a bit basic on the B777)

The last iteration of RB211 the -524G/H on the B744 and B763 had a very complicated system on two FCOCs with bypass valves for low fuel temp and high fuel temp. But these only controlled the flow of oil, not the flow of fuel.

But engines transfer large quantities of heat to the oil, which is passed to the fuel.
The Trent 800 also has an AOHE, air cooled oil cooler. This is controlled by the FADEC to cool the oil when the FOHE can't cope. If the AOHE control system fails, the valve is driven full open. It is covered in the MEL and can be locked part open. There is a caution in the MM
"Make sure you lock the AOHE in the open position. If the AOHE is not locked correctly, oil and fuel temp can rise much higher than normal and cause damage to the engine"

Do you have any reference to how the FOHE Oil bypass valve works? The AMM just says low temp.
The operating oil temps are 50-185 degC, and norm about 100-120. What is low?

Machaca

Phil:

For each side, something on the order of 20,000 gallons of jet fuel with an enormous number of tiny suspended ice particles transited through the fuel system for upwards of ten hours. The accretion of ice is to be expected.


It is perhaps likely that thousands of flights in similar profiles over the years have been close to succumbing to ice accretion induced trouble, only to have landed in time.

Some by minutes, others by hours. That doesn't change the phenomenal statistical success achieved to date.

You may feel better if you join a rheology forum and a fluid dynamics forum and demand their immediate combined action on sub-zero kerosene ice and its accretion properties in tubular delivery systems.

precept

Following is a link to the latest AD note regarding the roll back incident.

Justia Regulation Tracker Airworthiness Directives: Boeing Model 777?200 and ?300 Series Airplanes Equipped with Rolls-Royce Model RB211?TRENT 800 Series Engines , - Federal Aviation Administration - 9565?9568 [E9?4650]

Below is an alternate link if Justia link is not available:

FR Doc E9-4650


Partial content of AD:


Since we issued AD 2008-19-04, we received a report of a single-
engine rollback as a result of ice blocking the fuel oil heat exchanger
(FOHE) on a Model 777 airplane equipped with Rolls-Royce Model RB211-
TRENT 800 series engines. The data confirm that ice accumulates in the
fuel feed system and releases after a high thrust command, creating
blockage at the FOHE and resulting in the inability of the engine to
achieve the commanded thrust. Examination of the data from the rollback
shows that the second of two maximum thrust step climbs was performed
approximately 40 minutes prior to the thrust rollback. Ice was released
within the fuel system during the step climbs and formed a restriction
at the FOHE of the affected engine, as evidenced by an increase in
engine oil temperature. Further analysis of the data shows that ice
accretes in the fuel system more rapidly and at warmer fuel
temperatures than previously indicated, and ice may build up gradually
on the FOHE before causing the engine to rollback. The data from this
event, in combination with Boeing fuel lab testing, demonstrates that
reducing the fuel flow to minimum idle levels will clear any ice
accumulation at the FOHE within a few seconds.
All of the testing and research has been conducted on Boeing Model
777-200 and -300 series airplanes, equipped with Rolls-Royce Model
RB211-TRENT 800 series engines. Initial review of other Model 777
airplane engine combinations has not revealed the same vulnerability to
the identified unsafe condition.

HarryMann

< 50C

e.g. 40C

vapilot2004

Trouble with this idea is the ice would muck up the works downstream. Pressure regulators and flow metering devices have much smaller passages than the FOHE's.

airfoilmod

Actually not. To gain surface area for rapid transfer, the HE has numerous "tubes" of small diameter to hasten thermal gain. There is a picture of one on thread, by Machaca. (my impression of said pic, af)

I say "Gate Valve" for a reason, they are as large or larger than the Fuel line itself, and for the reasons you state.

AF

vapilot2004

Interesting. So you're saying the combined cross-sectional area of the FOHE's is actually smaller than a metering valve or nozzle? Or are you referring to only one of many tubes within the exchangers?

I had always envisioned a fin-on-tube arrangement for these FOHE's. Perhaps this type is only for the air cooled variety.

sooty655

The bypass valve is opened by differential pressure, basically similar to a relief valve. You were right that there is no thermostatic bypass, only the diff pressure one.

I haven't been able to find out the oil temperature at which the differential pressure would open the valve, but I agree with HarryMan it is likely to be around the bottom of the normal operating range.

No finned tubes in the FOHE. It has a bundle of straight through small bore tubes for the fuel to flow through. The oil passes through a jacket around the tubes, with baffle plates to cause a zig-zag flow to maximise contact with the outside of the tubes.

As you said, finned tubes are normally used on air cooled arrangements, to maximise contact area for the air.

Sooty

airfoilmod

"Restriction" rapidly diminishes the "area available for transit". One large pipe is best if what you want is Flow, related to viscosity and demand, etc. It really isn't critical the cross-section, but the available gross area.
Several small pipes can perform as well as one big one. Introducing another variable, Ice, changes everything. The small openings pack first, but may remain open if demand is, say, Idle or flight idle. Increasing demand may be the same as further restricting the cross section with constant flow. I don't have any more information than anyone else, and quite possibly less than some, but the bypass, while preventing a case that Boeing proposes, eliminates heat exchange. The GE has a recirc feature that may make the ice more susceptible to transit, eg forestalling refreeze.

If the Fuel is VERY cold, it may cool the Oil too much, also a bad thing.
While it's interesting to discuss these problems, my basic concern is the same as Phil Gollins. I'm a convert to ETOPS and have high expectations, I'm no engineer, but I dislike the lack of information forthcoming from the authority. So far, I like Boeing's approach better than the other's.

M.Mouse

You think that the AAIB and Boeing are acting individually without sharing of information and in full knowledge of each other's actions?

The information which is known and is necessary to be promulgated to reduce the risk of the events recurring is being promulgated.

What exactly is your beef except you think every move by the AAIB should be publicised and dissected before a definitive, accurate and full report is ready? Perhaps it is all a big conspiracy.

airfoilmod

I suppose you hadn't heard of the second rollback. If everything you say is true, evidently enough information was not forthcoming; perhaps Delta wasn't informed. The thing you have with conspiracies is a little over the top. I take note that you are annoyed at my irritation.

Let me be blunt. If BA038 had started descent a bit sooner, and was the least little bit lower than they were, it would have been disastrous. If Delta couldn't relight, and lost a second engine, Have you ever flown over the Rocky Mountains?

AF

Bis47

My memory can fail ...

Did I really read that the Delta Airplane that recently experienced a roll back over the Rockies also was originated from China?

If true ... that is too much of a coïncidence.

jfill

It was Delta Flight 18, from Shanghai to Atlanta. Happened at 39,000ft. The crew descended to 31,000ft and restarted. Continued on to KATL

Flight_Idle

FOHE, I find it suprising that the fuel is directed to flow through the narrow bore pipes & oil throught the larger bore outer casing. Why not have it the other way round? the heat exchange would surely be the same?

There still seems to be some confusion over how the correct oil temperatue is maintained. Does the oil flow switch on & off to give the desired result? if so, this could effect ice melting, which has been said to take only a few seconds when engine power is demanded? (That was said on one report, but clearly did not happen when engine power was demanded on the incident in question)

Smilin_Ed

I think there wasn't enough time for the ice to melt in this situation. They had been at low power settings for a while and it simply was too late to melt any/much before they hit the ground. I think that's why the new procedure calls for periodic power increases during letdowns.

airfoilmod

And from the info available, it would appear that how the Power is increased would play a part. If the FOHE has ice in the Fuel passages, care must be taken to avoid a sudden increase that disturbs the ice upstream, allowing it to drop and block completely the exit of the Fuel passage from the FOHE. The increase commanded by the A/T's produced large EPR's with accompanied Vibration; the FOHE is mounted on the Fan Shroud, after all, and full chat creates monumental vibration and acoustic energy. This might have shaken loose enough ice to pack up the exit, causing Rollback to ~1.15~. Again, the similarity of system performance would "defeat" the "expected" isolation of each engine from the other; at some point, systems are identical at the end of the pathway.

AF

sooty655

The differential pressure across the oil side of the exchanger will increase with cooler oil and decrease with warmer oil. So as the oil cools, the differential pressure valve will allow more oil to bypass the exchanger. However, this would be a progressive, rather than an on/off effect.

Sooty

airfoilmod

As Designed. If I'm not mistaken, the FOHE's intended purpose is to cool Oil not Heat Fuel. Warm Fuel is not a designed consequence, though obviously it gets taken into account. Down the road, if I may anticipate remediation, It will be considered insufficient to mitigate Icing Fuel Lines with Applied Power Profiling. There will need to be a mechanical solution. There is too much room for PE in a short term ("Advance the Throttles, this way, at this time," etc.). So a (Automatic) bypass will be installed, or Fuel Filters to sort small particle ice, or Fuel Heaters to melt ice, etc.

Perhaps one solution might include Fuel Bypass coordinated with Oil Bypass. If the Oil is sufficiently cool to bypass the FOHE, why continue to supply the Engine with FOHE derived Fuel? Link the bypasses to activate simultaneously??

AF