India Four Two

Oldlae,

All liquids are compressible to some extent, even water.

There are some values for Jet A1 quoted here:

http://www.eng-tips.com/viewthread.c...=155581&page=1

I have no idea whether this is material to the BA038 accident, although with the low pressures involved, I suspect not.

barit1

Both the plumbing and the fluid therein (fuel, possibly with air or vapor) have elastic properties: they store energy when under stress, and give that energy back when the stress is removed. That's all it takes to have the potential for resonance, which was discussed a few pages back. I trust this is being studied until some better candidate emerges.

lomapaseo

Agree

But what is the forcing function and the response

barit1

THAT, of course, is the $64 question (am I showing my age?)

One excitation possibility is the gear tooth passing frequency (assuming a gear-type HP pump).

Flight Safety

Since both engines initially responded to the autothrottle with increased thrust, that meant the restriction or obstruction was not in effect yet, at least not initally. Then within 3 secs or so, the right engine rolled back to EPR 1.03, with the restriction or obstruct in effect, with low fuel pressure present and HP pump cavitation damage taking place. The same sequence occurred for the left engine, although the rollback in power to EPR 1.02 occurred 7 seconds later. In other words the restriction or obstruction developed slightly later for this engine.

This suggests to me that the sudden increase in fuel flow required for the power increase, is what actually triggered the formation of the restriction or obstruction (whatever it was), which apparently was not present in the first few seconds of increased fuel flow.

I'm not sure what that means, but this appears to be the sequence of events. High fuel flow was available initially, then the restriction or obstruction developed (perhaps precipitated by the sudden increase in fuel flow), then low fuel pressure and HP pump cavitation followed and remained in effect until impact with the ground.

Of course the million dollar question is, what was the nature of the restriction or obstruction, and how could a sudden increase in fuel flow rates possibly cause it?

ChristiaanJ

Flight Safety,
Without any more AAIB input we're back to guessing games....
But your post makes for an interesting question.

What was the fuel flow in m/sec or ft/sec ?
In other words, if it was indeed some kind of obstruction, how far did the 'slush puppie / chinese noodles / wad of chewing gum' travel after being dislodged, to arrive after 3 secs on one side and 7 secs on the other side?
Maybe there's a clue there?

CJ

airfoilmod

Daniel Bernoulli, whose work with Hydraulics may have a hint to help the discussion. His most widely known discovery was that a fluid (inviscid Fluid, not "compressible") that was accelerated in a tube, created a concomitant reduction in pressure within. The Fuel line in 038 was subject to sudden demand. Cavitation at the HP (both) has been described and bears further thought. As an "incompressible" mass, the Fuel in the Line would transmit the focal cavitative results on the High side of the Pumps back through the system. In essence, the massive pressure at the Pump Lobes would communicate back through the Fuel and have an instantaneous effect on other mobile mecnanical devices in Line. Air "cavities" would have developed and perhaps failed backflow valves, Spar valves, and LP (boost) pumps back through the system. The Fuel in the Line is a "Hammer" that takes the shape of the System, but retains its abilities to hamper normal supply throughout the entire Fuel System.

Airfoil

(Suggesting there may have been no "obstruction", other than the Fuel itself, having created an insufficient flow due to inertial reluctance which may have failed other intersections)

ChristiaanJ

airfoilmod,
Phenomenon also known as "water hammer" to your local plumber.
Known to every fuel and hydraulic system engineer.
As an explanation for what happened on BA038 it seems somewhat far-fetched.
Not to mention, that any "fuel hammer" effect is nearly instantaneous. It does not take 3/7 seconds to manifest itself.

airfoilmod

And the theory has more to do with resonance and harmonics having an effect on mechanicals than a "Hammer". I used the term to convey the communicative effect of pressure differentials in an "instantaneous" way.

"Hammer" is an effective way to understand cavitation, notwithstanding your derisive retort to its use. I didn't think it would bother anyone, I'll be more careful with my vocabulary.

RGDS Airfoil

(BTW, if you've noticed Water Ram, you will also have noticed its brother, Recoil. That vibration, and "return,rebound" of the Ram, stops flow.)

cwatters

Anyone got a phase diagram for jet fuel?

airfoilmod

Rather coy way of introducing doubt and suspicion of the Flight Crew, No?
Coupled with your statement on another thread, why don't you just man up and state your point.

cwatters: Awesome, now you're talking.

Airfoil

arcniz

That may be hard to come by. Problem is that typical fuels are not an "it", but a complex and widely varying collection of complex components blended to meet a set of specs. A batch could consist primarily of ground-up gerbils and would likely not be rejected if it met all the specs.

Similar might be said for various of the aircraft systems involved -- they are designed to meet reasonable specs derived from science, engineering, and experience. Sometimes circumstances arise that have not been fully contemplated in the specs, and unanticipated results ensue. BA038 may well be one of those situations where tried-and-true standards and specifications did not prevent a unique special case from having adverse effect.

PickyPerkins

So did the UA 767 referred to in my link:
http://www.pprune.org/forums/showpost.php?p=4000971&postcount=1101

The engines on the 767 performed just fine in suction all the way from sea level to 41,000 ft.

What they could NOT handle was a cessation of boost pressure during the climb.
The engines spooled down 14 seconds after the boost pumps were turned off.

Exposed leaky pipes and seals were not involved because the tanks were full.

This seems a clear indication to me that the boost pumps were doing something which the HP pump could deal with so long as boost pressure continued.
But when boost pressure was discontinued, the engines spooled down after a delay of 14 seconds.

That something which boost pumps were doing could have been bringing air out of solution.
Which was OK with boost pressure on, but not OK when the boost ceased.

In the case of BA038, does it really matter whether air bubbles did or did not cause cavitation damage?

The AAIB says although there was cavitation damage, it was not sufficient to prevent the fuel being pumped. The damage was not significant wrt pumping capacity.

What matters is - why did the fuel stop advancing through the system?

More than 30 years ago a Royal Aircraft Establishment repot by WGS Lester, “Temperature and Fluid Effects on Cavitation in Aircraft Fuel Pumps” showed that air coming out of solution could cause the volume output of centrifugal type boost pumps to fall markedly.

He was not concerned about cavitation damage, only about whether the pump continued to deliver fuel.

With air dissolved in the fuel, his test pump didn’t stop pumping, but the output volume fell.
Sound familiar?

And the fall off in performance was greater when the fluid was cold.
Sound familiar?

In fact the pump output was greater when the test liquid was boiling (no dissolved air) than when it was cold (with dissolved air).

Some of his test results are shown in:
http://www.pprune.org/forums/showpos...postcount=1102

Fuel saturated at room temperature becomes highly supersaturated when cooled.
It typically remains supersaturated for many hours in aircraft fuel tanks at altitude.
This was mentioned by Lester, and confirmed decades later in Boeing test flights.

Bottom line
The difficulty in finding the restriction to fuel flow maybe because there was no restriction to flow, just a fall-off in boost pumping capacity due to air coming out of solution from cold fuel, and a mixture of air bubbles and fuel delivered to the HP pump.
Just my 2cents. Maybe 1 cent.

airfoilmod

You may be intermingling two separate causes of Fuel restriction or Flow reduction. Your reference to dissolved air or air (Gas) in the Lines reducing pump efficiency sounds a whole lot like "vapor Lock" (or Block). It is a serious problem in pressurised Fuel systems, but one wonders of its relevance here. Also, a "foaming" supply of Fuel would mimic starvation, even if Flow continued. Cavitation, having been demonstrated, has stolen all the attention, and is an unrelated (presumably) failure, unconnected to dissolved gases in Fuel (to my understanding).

Airfoil

Joetom

Just a point if interest.

Fuel flow could have been restricted for a long period of time.

Think of Spar or LP valve being in a position between open and closed, Engine will run Ok upto a point, but when FMV/FMU needs more flow than avail, problems will start, problems may be delayed a little with the head of fuel down stream of restriction, but problems on the way.

barit1

What might the 777 cruise fuel flow have been just before TOD? (call this point A)

And what fuel flow might have been demanded on a normal close final? (call this point B)

Between these two points, fuel flow was at a very low level. Is it possible the restriction existed (without symptoms) at A, and didn't become evident until thrust advance at B?

airfoilmod

Or Point B plus 3 and then plus seven seconds, the time needed to make the restriction critical ? Think you're on to it.

snanceki

It never ceases to amaze me how "creative" individuals can be.
After all, this is the purpose behind brainstorming!

Only problem is that each "theory" has to be "understood" and either moved forward to the "possible" file or discarded due to some misunderstanding/lack of knowledge by the proposer.

I was particularly "impressed" by your "definitive" conclusion that there was adequate fuel on board after resolving the frost pattern on the underwing from a photo a few seconds before the crash landing.

Now the various "theories" put forward in the last few pages of posts, many seem to focussed around the cause of the "restriction" without any data in support of the nature of any such restriction.

In order to try to get you guys to be creative in a slightly different direction I wonder what are your thoughts are on the "size" of the restriction?

By this I invite your energy into determining the following:
1. What fuel consumption (flight idle?) would exist on approach prior to the incident. Ltrs/sec or whatever.
I don't have any idea what these big fan jets consume even at idle but its certainly "significant"!
2. What capacity (volume) exists in the fuel lines between pickup and engine spray nozzles?
Is this the same on both sides of the aircraft or slightly different and if so which side has the greater volume and why?
3. Is it possible to resolve this volume more accurately. i.e. what %age of the previous volume exists between pickup and lets say the spar valve, heat exchanger or HP pump.
4. How long would it take to use this volume with 100% "block" at whatever thrust setting was likely to have been called for during final to offset flap etc (which wasn't delivered when requested).
I appreciate that full thrust was at some point selected but it was a much lower power request that highlighted the problem..
DO THESE FIGURES MAKE ANY SENSE?
So we could deduce 75%, 50% etc restriction vs duration.
Unless these figures make sense, discussing the possibility of Bernoulli type restriction etc seems a little premature (although possible) especially since such an occurrence has not been experienced during multi million previous landings. So what was different this time around?

As a professional engineer, but unfamiliar with the "detail" of aircraft installations, the 7/8 secs is in my view highly relevant.
The difference between 7 and 8 secs on the other side may NOT be relevant but explained by deficiencies in the measurement (DFDR sampling rate) process or subtle differences in the level of thrust commanded. However it MAY be significant if indeed the capacity of the system is indeed different LH to RH.
However a "significant" amount of fuel would be used during a 7 secs period. So how about a few FACTS even if these are derived from from first principles.
The AAIB have access to the real data so I guess they will have already been through an exercise something like this.

Maybe you will just overlook my request / maybe not!

Finally, am I correct in my understanding that BOTH LH and RH boost pumps WERE WORKING and that this was actually confirmed by the AAIB...I'm too lazy to go and reconfirm! If so why the discussion about what would happen if the pumps weren't running and the possibility of air being drawn in with low tanks etc.

CONF iture

At first sight the 7 or 8 seconds difference could have been explained by a single fuel source going through a fuel crossfeed valve ...
I believe that kind of engine must be around 1000kg/H at ground idle, so flight idle would not be far from 1 Liter every 2 seconds

ChristiaanJ

Thanks!
Half a litre per second is as good a ballpark figure as any.
Now what's the diameter of the fuel feed pipe?