shawk

Fluidic resonance in a tuned circuit tends to be low frequency and is far less likely to be attenuated by the aircraft structure.
Interestingly, this noise would be very similar to tire rumble at landing.

Cavitation noise tends to be high frequency and is likely to be attenuated by the aircraft structure.
Below is a spectrum for cavitation noise



Typical fluid resonance detectors operate between 2Hz and 300Hz.
Unfortunately, I can not find a spectrum for fluidic resonance that is not limited by copyright.
Those interested in further exploring this topic might Google: Bayesian Modelling of Fluid Flow in Pipelines.

tanimbar

Shawk,
You say that "cavitation noise tends to be high frequency and is likely to be attenuated by the aircraft structure".

The cavitating high pressure fuel pumps (HPFP), that I mentioned might be the source of the reported strange noise, are integrated with, and driven by, the engines.

Lots of local pipework and other metal objects (nacelle etc) in the vicinity available to resonate with the HPFP, plus, a large hole at one end to let sound out.

Would much attenuatation occur and how would the other resonating objects alter the frequency range?

Of course, if your interesting fluidic resonance idea is correct then the sound is important evidence of causation. If it is the result of the cavitation then it is not important.


Regards, Tanimbar

lomapaseo

One would have thought that any significant sounds reaching the ears of ground spotters etc. would have been recorded either as acoustic signals on waterfall plots on the CVR or as forced low-frequency responses on the DFDR from various inputs around the aircraft.

On the other hand less specific sounds (white noise) or those left in the memory of observers have a tendancy to only exist in the minds of theorists.

shawk

You may have experience with a water faucet that sometimes makes a loud, low rumble or rattle that is accompanied by a small volume of water that comes out of the tap in discrete spurts of water. That's fluidic resonance in a pipe. Generally, you have to stop the flow to stop the effect. With the resonance gone, water flows normally.

I suspect that high frequency cavitation noise similar to the frequency plot would be inaudible through a typical thickness of aircraft aluminum at 10 meters. A lot depends on the placement of the pump and the shape of the surrounding structure but not much high frequency sound is likely to escape an airframe. From what I've seen, aircraft piping is very well attached and damped to prevent vibration that will cause rapid metal fatigue, so it might not act as a good acoustical conduit for high frequency sound.

If the fluidic resonance theory has much basis in reality, the likely noise is somewhere between 16Hz and 32Hz. This sound frequency range has a very good chance of escaping an airframe and be audible on the ground.

But I'm not certain that a CVR will record audio at 16Hz to 32Hz. They seem to be optimized to record human voice and use brick wall digital filters to not record below 100Hz.
The DFDR may not be sensitive enough to record the airframe effects of a small mass of fuel oscillating at 16Hz to 32Hz.
I hope that experts will add their knowledge to what parameters could be recorded.

ARINC

Thanks NoD

I was chatting to a FADEC "Expert" last week when the subject came up and he was of the opinion that as there are quite a few parameters in the FADEC system that are not logged on the DFDR and that if affected could cause a roll back to flight idle, then EMI still remains a possibility.

barit1

Does anybody know for certain the clock speed of the Trent FADEC?

I once knew of a FADEC clocking at 120Mhz, with predictable coupling to VHF comm frequencies.

pilotbear

The unusual sound heard by pax could have been the flaps retracting, which without engines spooled up would probably be the most prominent and "not normally heard sound' at that point on the approach.

And considering the obvious technical prowess of most people here I am surprised that you have difficulty with the OFF mode for your PC/MAC when you don't like what you are reading instead of this persistent fascist approach of telling people what they can and can't write, think or do. Get a life eh?

Speculation or conjecture no matter how ridiculous is where discoveries and ideas come from, talk out the most ridiculous first and fine tune the way to the truth.

HarryMann

To whom?

It's fairly common knowledge that GOOD human hearing is in the range 50 Hz to 20,000 the upper frequency decreasing rapidly with age (and usually lower for the male gender)..

It rolls off rapidly below 50Hz to prevent damage from the energy content in low frequency sound.

Bottom string (E) on a bass guitar is around 42 Hz, and it's 2nd harmonic (84Hz) is usually more understood by the ear than the fundamental, which itself is more felt rather than heard.

32 Hz or less? Something by-standers commented upon, amongst the melee of other sounds from an aircraft so unusally close - nope, don't think so.

Neither do I find it credible that cavitation noise within a pump within an engine nacelle would be heard, or commented upon, whatever its frequency... over the mass of aerodynamic and other turbomachinery noise - IMHO of course!

HarryMann

From both tanks, a nebulous volume of cold fuel - within 8 seconds of each other?

airfoilmod

It sustained damage from Cavitation, but even with that was "able to function normally". The squishiest suspect is of course, Fuel.

1. Ten degrees "off spec.". Not "better than"; OFF SPEC.

2. Fuel contained some amount of water, deemed "less" than "excessive."

3. Water and Jet don't "mix" (non-soluble)

4. Tank Temp., at -34C would freeze water immediately

5. Boeing Captain Carbaugh posits "Ice Crystals", a "Temperature issue"

6. The Heat exchanger has yet to be exonerated from involvement.

7. Fuel and Water could have transited the LP pumps

Possible: Given a seriously cold-soaked engine Pylon and barely warm heat exchanger, ice crystals could accumulate in the exchange matrix of the exchanger, blocking flow to HP pumps.

For this to have happened, the timing sequence of engine thrust loss needs to be seriously considered to satisfy many possibilities.

1. The Fuel/Ice "mix" would have to be inordinately homogeneous between the two main tanks.

2. If the ice crystals had already been formed, further "freeze" may not have been possible anyway.

3. The Fuel/Ice emulsion, at flight idle, transits the Fuel works fine, no loss of supply or even thrust.

4. At Throttle up, the Fuel is moving more rapidly, ice is "collecting"
in restricted spaces, but the engines respond fully before the accumulation can get ahead of the supply.

5. At full flow, the ice crystals gather rapidly in the exchanger and elsewhere, constricting the Flow of Fuel. #2 drops power.

6. Drawn from Port Tank, the Fuel ice "emulsion" is perhaps 1.5C warmer than the Starboard Tank, and #1 drops 8 seconds later. (Port Tank is in sunlight throughout the flight, the inner portion of the Starboard Tank is in the shadow of the Fuselage.)

Why Crystals and not surface accretion? Agitation, and lack of nucleation. Any "Slurry" will shed its solids at turns and pressure foci differentials. It has to be Fuel Fault.

Airfoil

tristar500

Lords, Ladies and Gentlemen...

In a nutshell...

How many times has this type of incident happened before?

How many times have aircraft flown through extreme weather conditions (inc freezing temps etc etc) with NO problem at any time during the flight?

Whats so different from this particular flight, to any other commercial flight?

As far as I know, Boeing and BA have issued NO change to operating procedures on the B777 (I stand to be corrected if they have)... Would Boeing have grounded the B777-RR powered, if anyone had lost their lives?

Maybe this is a case for the guys at CSI to solve, since the CAA,AAIB, RR, BA and Boeing seem to be at odds...

Bis47

As far as I can remember early comments :

- Multiday rather cold winter operations
- Fuel tank icing messages
- Actual flight much higher than initial operational flight plan (to satisfy ATC request) => hence extra landing fuel.
- Very low OAT at a given time (- 76°c = ISA - 20). What was the TAT at that time ?Somewhere around minus 50 °c ? Quite a cold spot ... for how long?
- no descent to lower altitude over Germany while many other commercials were requesting FL 250 that day.
- no "low fuel temp" warnings, while other airliners got that same warning ...

So ... each fact not exceptionnal. But the whole set still quite infrequent ...

The culprit might be a deficient fuel temp warning system ... and crew putting too much confidence in that system, instead of "crosschecking" fuel temp information with indicated TAT and met temperatures forecast.

I guess (see previous post a few pages above) that crews didn't wait for Boeing or the CAA to take extra margins ... Any captain worth his pay would not want to replay that scenario !

Actually, the operational flight plan was calling for lower altitudes, avoiding very low temperatures ... Is the program already including some margins against limitating low temps? Perhaps ... BA knows the answer.

PeePeerune

For Info.

I believe that the B777 fuel system has a suction feed system from tanks so that in the event of a.c fuel pump failure (of anykind),the engines will still receive a flow of fuel via suction ,albeit at a slightly reduced rate.

Suction Feed
-Suction feed line bypasses pumps.
-During climb air from fuel tank may collect in suction
feed line, inhibiting suction feed. Air will deplete
eventually after reaching cruise.
-Once air dissolved suction feed can occur with no
limitation.
-Time to deplete depends on altitude, fuel temp and
type of fuel.

L337

One wonders if the fuel temp sensor was reading correctly?

But, seems too easy a solution.

arcniz

Even if the tank fuel temp sensors used for flight-deck reporting WERE reading correctly, one may wonder if their reported readings were representative of the fuel temps actually in the dir0 ect path of flow to the engines?

As one very experienced in precision temperature measurement, I know uneven distribution of temps in a large volume (of anything) is a near certainty.

If pockets of colder fuel were present in the wing tanks, perhaps already gelled and stratified into layers or globs of denser fuel materially colder than the tank fuel temp sensors could see, then is it not possible that descent-related changes in aircraft attitude and acceleration could possibly shift the coldest and waxiest fuel into the feed path at the critical late stages of descent, with exactly the consequences observed?

A relatively simple enhancement to flight-deck data might be to add reporting of temps IN the actual flow path near the engines. Likely some of this data is already sensed and available within the FADEC -- and possibly available in a usable form.

Green-dot

To CONF iture, post #1138:

The 777 has no transit lights. Spar valve positions are displayed on the fuel synoptic and fuel management maintenance page. These have to be selected on the lower MFD (or left/right inboard MFD) to enable monitoring of the spar valves.

15 seconds.

28V DC

Spar valve control system (fuel control switches and control relays): yes

Spar valve actuators: no. Limit switches remove power from the spar valve actuators after commanded position is reached.

Continuous 28V DC power is provided to the control relay.

Fuel control switch moved to RUN energizes the open coil of the control relay which then sends a power signal to the spar valve actuator. When the spar valve reaches the open position, power through the limit switches is removed from the spar valve actuator.

Fuel control switch moved to CUTOFF energizes the close coil of the control relay which then sends a power signal to the spar valve actuator. When the spar valve reaches the closed position, power through the limit switches is removed from the spar valve actuator.

Yes , if a spar valve actuator jams while in transit. This wil generate a fault indication on EICAS due to a disagree between control relay position and spar valve actuator position.

Or (highly improbable but not impossible):

A temporary reverse logic to the control relay. Example:
With fuel control switch in RUN and temporary reverse logic to the control relay, this moves the spar valve actuator from open to closed position. No alert is generated because there is no disagree between control relay and spar valve actuator. Actuator moves to commanded position.

If the temporary reverse logic is corrected in less than 15 seconds, the spar valve moves [open->close->open] and will be in transit for almost but less than 30 seconds without ever reaching the closed position. Result, obvious limitations to the fuel flow. No alerts are presented and no visual indications if fuel synoptic and/or fuel management maintenance page are not displayed at moment of occurrance.



see my post #590 on page 30 of this thread:
Code One . . . .


regards,
Green-dot

airfoilmod

From an ETOPS perspective, I think some folks have a hard time entertaining how both engines could respond similarly to a single or series of faults. The timing isn't so exclusive of what appears to be reality. There is a "range" of possibilities that could easily have trapped both Powerplants in a "simultaneous" cycle of failure. Subtle changes, or commands, or even trimming, result in subtle responses from the A/C. Large inputs can create their own problems.

Possible: BA038, established in flight, is exposed to fierce low temps.
There is some amount of Water in the Fuel, in some "state".
On descent, Equilibrium is established, the "system" is constant
Engines have established stasis, low operating temp. sipping fuel

1. At low altitude (~800feet) the Autothrottles command significantly increased thrust.

2. The engines respond, accelerate, and make commanded power.

3. Everything impacting the new thrust level is instantly at high level

4. Fuel flows rapidly increase, as does temperature in the engines

5. Stasis collapses, Fault (s) appear and overwhelm the system.

Possible: The isolation of the powerplants from each other works to prevent simultaneous failure, but given a demand in the nature of a sudden command for full power, the scale of the demand can "override" functional isolation. What I'm trying to say, is that though the Fuel was contained in separate tanks, it was of the same make-up, exposed to very similar conditions, and could have easily created unrecoverable circumstances in two engines within 8 seconds.

Back to stasis. The profile of the descent and the extended period of low power and uncommonly cold and perhaps contaminated Fuel created a homogeneous environment for the final hole in the cheese. I think the restriction was closer to the HP pumps than spar valves, LP stand pipes, or system piping upstream from the pylons and heat exchangers. The instantly present Vibration and fuel demand, including sonic energy and increased temp. may have served to melt/dislodge pack up the heat exchangers with the ice crystals that may have collected prior to throttle up. Within 8 seconds of either engine? Possible.

Airfoil

Machaca

The Trent 800 FCOC (black) and fuel & oil tubes:










Cutaway of similar FCOC (fuel flows through matrix of small tubes):

arcniz

I fully agree with your assertion that a core vulnerability in ETOPS redundancy is that the top-level systems (i.e. the aircraft, external nav-comm environment, crew) are unitary and therefore not able to be much more redundant.

As you observe, a variant of the same theme is that the recent history of the aircraft is also common-mode for even the redundant systems, esp the power train. When the two halves of the aircraft are somehow identical, certain unanticipated failure modes related to patterns of usage may affect them equally at roughly identical points in time.

Perhaps the logical extension of this observation - especially if the principle eventually is shown to be causative in the case of BA038 - would be a future requirement to ensure that the two halves of an ETOPS aircraft may use many common components but must be deliberately asymmetrical in regard to the form, fit, and function of sustaining flight-critical systems. A logistics nightmare, of course, but probably a reasonable extension of the no-fail-all concept.

Nature handles this problem quite neatly in many living things, including ourselves, by making the redundant critical appendages, sensors, and control systems from the same general parts and pieces, but different in right-left (etc.) symmetry. A substantially trickier design asymmetry might be needed to cover the many plausible common-mode failure symmetries possible with aircraft.

Oldlae

Arcniz,
Several hundred posts ago and I have spent some time trying to find them but it was too time consuming, it was explained that the jet pumps in the tanks are designed to keep the fuel in the tanks in motion to prevent cold spots or stratification from forming. As I understand it, the jet pumps were found to be working correctly.