Posts about: "Hydraulic Pumps" [Posts: 87 Pages: 5]

From the detail in this video:

" Just a short video of the Boeing 787 RAT being driven by an attached hydraulic motor. This test is performed to check the RAT's hydraulic pump and electrical generator are functioning correctly. The motor that is bolted onto the back of the RAT is driven using an external hydraulic rig that is feeding the motor with hydraulic fluid at 4200psi at 40GPM ."

FP.

I'd like to mention something that, while unrelated, does shed a bit of light on how computerised systems can misinterpret input data and take the wrong action. I spent 40 odd years as an electronics engineer involving complex systems, it can be surprising just how careful one must be in systems that sample data and then process it for decision making in software.

On August 9th 2019, there was a significant grid failure in the UK leading to load shedding (removing supply to many consumers, including Newcastle Airport) which started when a series of several lightning strikes in Hertfordshire caused a trip out of generators at Little Barford combined-cycle gas turbine generation plant. This was followed by the shut down of the power concentrator and grid connector from the Hornsea1 off-shore wind farm, significant changes in the grid frequency away from the acceptable limits which is what triggered further load shedding.

The reason I mention it is that Hornsea1 going off line was due to the software that operated the concentrator/connector sensing large voltage transients due to the lightning strikes 120 miles away, however these transients were only of the order of 10us length spikes with nominal 20ms cycles at 50Hz on the grid. In old reliable grid equipment that had been in use for decades such spikes would have been ignored because the large rotating machine inertia would make them irrelevant. Other systems went into various states of shut down for protection reasons, some of the Siemens Class 700 trains had to be reset by the train crew, others required a Siemens engineer to drive to each train and reload its firmware. The train protection mode occurred because the grid frequency on the 25kV AC supply went below 49.8Hz, this was a programmed default and it turned out to have been a very conservative one, the trains could have continued to operate normally at even lower frequencies but someone decided to write the programs without actually testing what a sensible limit was. The whole very widespread problems this caused could have been avoided by not acting instantly on microsecond transients in a 50Hz system.

Is it possible that the monitoring systems on a Boeing 787 also sample the electrical system voltages and currents at a relatively high frequency, and thus in the event of a transient of some type perhaps over-react to this event by taking precipitate action instead of waiting a short time before re-sampling again. I have seen a suggestion that an alternative explanation for the "bang" heard by the survivor in seat 11A might have been the sound of a Bus Tie Contactor closing, which in itself suggests something quite important relating to the electrical system. The 787 is unusual in that it uses variable frequency AC generators whose outputs are rectified and then inverted to other AC voltages and also quite high DC voltages, some in the 250-300V region.

I hope that some hard information is going to come out from the investigators soon, but given that the flap mis-selection idea is effectively debunked and we know that the main undercarriage either started its retraction cycle with bogies tilting forwards or that falling hydraulic pressure caused the same thing to happen, then the only thing that fits the observed flight path is loss of thrust on both engines which could have either preceded or followed an electrical failure. We also know that the RAT deployed and in the relatively undamaged tail cone the APU inlet was open or opening indicating a likely auto-start of the APU due to the parameters to trigger that occurring.

I would like to know how many tests of the electrical/computer interactions in 787 development involved arcing/shorting voltage/current transient testing. Is this the sort of thing that is done at all? The EECs (FADECs) in the engines are self-powered via magnetos and self-controlling in many circumstances, but perhaps something caused them to think that the thrust levers had been retarded, and such a thing might have been down to the effect of electrical transients on the various signals received by the EECs.

I have read the original 65+ pages of the thread, but I have not seen any discussion of this particular idea. Maybe that is because the 787 is quite a significant departure from Boeing's previous design practices with totally different electrical systems, higher pressure hydraulics and no doubt other aspects as well.

What do you all think?

I originally was onboard with the truck tilt seen in the one video being proof that the gear handle was selected rather than the flap handle. But after watching several videos and an "endless" web search for info, I also have started to think that there was a problem before the gear handle was moved.

In a normal retraction, the main gear doors begin to open before the truck tilt is complete (roughly when the gear trucks are "level", seen around 8-10 second marks of this video ). The nose gear doors open at the same time the main gear tilt starts (seen in Jetstar video linked below). It's too blurry in the video/stills of the accident aircraft to definitively say the nose doors are not open, but I'd say no. And it's pretty much a certainty that the main gear doors are not open. And finally I think it is also pretty clear that the main gear trucks are tilted down to the retract position.

I have no confirmation of this, but I read (or heard someone say in a video) that the truck tilt actuator was a "single acting" actuator, meaning hydraulic pressure held it in the toe up position and it was spring loaded to the retract position. Which means if hydraulic pressure was lost due to loss of power to the electric driven center hydraulic system, then the main gear trucks would tilt forward on their own without moving the gear handle. The Jetstar burst tire video somewhat backs this up, in that the blown tire caused a leak to the center hydraulic system and an alternate extension was required (thus the reason the main doors are down during the landing). No hydraulic pressure means the main gear trucks remained pointing nose down during the landing. Video here

One comment here, and maybe I am mis-understanding your comment, but the landing gear only operates via the center hydraulics. It does not matter whether the Left/Right engine driven hydraulic systems are operative or not. The RAT will only pressurize the primary flight control portion of the center hydraulics.
No idea. I only got that info from the Master MEL on the FAA website. According to the MMEL the aircraft can be dispatched as long as there is one of each type sensor working on each main gear. (AIs MEL could be more restrictive)

Are you saying that the RAT cannot power retraction of the gear, neither via its own hydraulic pump nor via its generator providing power to the centre system pumps?

Or could it be that it's capable of delivering enough power (via either route) to move the tilt actuator but not a humungous retraction jack?

The RAT provides hydraulic power only to the flight control portion of the C hydraulics. Wing and tail flight controls only. Non return valves prevent power to other C hydraulic powered systems like the gear.

The RAT provides electrical power only to critical flight instrumentation (mostly Captains), navigation and communication. The same critical equipment that the Battery will provide. If all electrics was lost, the main Battery would provides standby power until RAT is fully deployed. The RAT electrical power would not be able to power C hydraulic electric pumps.

Should we talk about the RAT being an electrical generator? No? Okay then, carry on.

Did you not read my full post?

The 787 RAT has both a generator and a hydraulic pump.

There’s been some debate over what electrical and hydraulic systems are powered. According to one unauthorized and Not For Flight -labeled FCOM copy that seems quite authentic:
The list of inoperative systems include most/all secondary fligh controls powered by the center hydraulic system. There we can conclude that centre hydraulic system electric pumps are not available when operating on APU RAT (or battery) only electrical mode.

The landing gear is run off of the center hydraulic system which is pressurized by electric pumps. The left and right hydraulic systems run their respective thrust reversers and some flight controls, and that is it.

Thankyou I didn't know that and I'm not questioning you. I guess it doesn't matter electrically driven or engine driven, they cannot share the same power supply the gear must be retracted within 12 seconds. this in my opinion improves the argument for total electrical failure. Unless of course gear up was never selected..There must also have been a substantial loss of thrust on at least one engine.

I still consider the forward truck tilt is a massive clue to a C system Hydraulic failure prior to wheels-up, which must have been caused by loss of electrics (since C is solely electrically powered), so I am still wondering if hydraulic failure happened before or after the loss of thrust.

While thinking about the consequences of a total hydraulics failure around time of rotation (caused by a suspected dual engine failure), here is a new observation. I searched the previous threads to see if anyone lese had noticed it.





In the rooftop video, as it's just approaching the treeline, there are spoiler deflections visible just behind the engines on each wing.

With a total hydraulics failure, the pilots control column using direct wiring, will only control this spoiler pair and the stabiliser. The RAT does not control this spoiler pair (hydraulically), only the most inboard spoilers pair. Its an interesting observation because it means this spoiler pair were being deflected electrically, either by the battery or RAT. But if the RAT provides emergency C hydraulic power why didn't the RAT powered spoilers deflect instead? Does this mean the RAT was unable to provide emergency hydraulic at such low airspeed?

I think this also re-affirms the critical loss of power (dual engine failure), and rules out many other theories. It tragically lost all power, both engines, all hydraulics and electrics (apart from battery and RAT).

I am wondering if anyone else has noticed other flight control deflections?

(Tried posting link to youtube but unhelpfully frames entire video)

Well said thanks &
“A ram air turbine (RAT) pump converts mechanical input power into hydraulic power for the center system flight controls. The RAT is in the right, aft wing-to-body fairing.”

To make things clear, just check this B787-related alert service bulletin dated 25 Nov 2014 (my bold):



787 RAT hydraulic pump location

RAT types vary significantly by aircraft family. The 777 and 787 types (along with most BBD aircraft) are indeed dual hydraulic-electric (lifted from the 2010 FCOM available online):


Other types are different. The A350/A380 do have an electric-only RAT with adequate electric flight controls.

The A320/A330 have a hydraulic-only RAT with a separate hydraulic-driven electrical generator. The 757/767 are similar except the generator is optional.

Some used an electric RAT to drive an electric hydraulic pump.

Be careful when attempting to transfer knowledge from one type to another.

DISCLAIMER: Poster (a) is one of the (apparently quite numerous) lawyers following this thread; (b) a long-time forum lurker and aviation enthusiast who loves studying FCOMs for fun (to each his own, I guess); (c) has followed and read this thread from the start.

What I cannot do is add new theories or uncover any new facts the actual experts have not already thought of. However, since summarizing and structuring information is one thing lawyers tend to regularly do (and sometimes even do well), here is my attempt at a useful contribution to this thread: an attempt to summarize the main theories discussed here since day one (which I think hasn't been done for quite some time) in the hope that a birds-eye view will be helpful to those who have not read everything since the beginning or might even trigger some new flash of inspiration for someone more knowledgable than me. I have focused on the cons since there does not seem to be enough evidence to come to any positive conclusion.

I shall try to be concise and to refrain from personal evaluations of my own. Of course, no disrespect whatsoever is intended towards all those who have contributed to this thread and to the individual theories, one or combinations of which may turn out to have led to this tragic outcome. That arguments can be made against every single theory that has been propagated seems to be the result of the highly improbable and unusual nature of this deplorable event and certainly not due to any lack of knowledge or reasoning skills in this forum.

DEAR MODS: If I have distorted anything or if, meaning well, should have achieved the opposite \x96 I guess you know where the delete button is\x85

Anyway, here goes:

A. Misconfiguration or wrong takeoff data
Widely refuted, since

• rotation, takeoff and initial climb seem normal;
• likely extreme errors would have been required to have such tragic effect (the fuel tanks should have been only about half full, so not close to MTOW);
• there is strong evidence that at least some flaps were extended for takeoff (post-crash photo, perhaps also visible in video from behind)

B. Flaps retracted post-takeoff instead of gear
Still brought up from time to time. However, widely disregarded due to

• the fact that with two working engines an inadvertent flap retraction should easily be recoverable, even with gear down;
• strong indications that hydraulic and electric power were lost (audible/visible indications of RAT extension, survivor statement, lack of engine noise, position of MLG bogies).

For a while, the forward tilt of the bogies as first part of the retraction cycle was seen as additional evidence that the gear had been selected up. However, it has been pointed out that the forward tilt and the opening of the gear doors occur almost simultaneously so that it seems unlikely that hydraulic power was lost in the split second between bogie tilt and gear door actuation. It is now assumed the forward tilt of the bogies was merely a consequence of the hydraulic power loss.
It should be pointed out that the question of "RAT in or out" was for a while the most contentious in this thread.

C. Low-altitude capture
Still argued, even if refuted by many since

• inconsistent with apparent loss of hydraulic/electric power;
• PF would have been flying manually (however, A/T reaction would have been unexpected for the PF);
• should have been recoverable (unless one assumes that the crew (a) remained unaware of the changed FMA annunciations although alerted by the unexpected FD commands; and (b) was so startled that an A/T thrust reduction was not noticed and corrected, even though the PF was apparently sufficiently alert not to follow the FD commands).

D. Loss of both engines at or shortly after rotation
Various possible reasons for this have been discussed:

I. Bird strike/FOD

• Would have to have occurred simultaneously due to lack of rudder/aileron input indicating symmetric thrust.
• No remains/traces on runway, no visual indications (flocks of birds, flames, structural engine damage).

II. Fuel-related III. Improper maintenance
Unclear which maintenance measures could possibly have been performed that would have resulted in simultaneous loss of both engines. No apparent relationships between malfunctions reported by previous passengers and essential systems.

IV. Large-scale electrical fault (e.g. due to water in E&E bay)
The engines will continue to run if electrical power is lost. FADECs are powered independently.

V. Shutdown of engines by TCMA
A parallel is drawn to the ANA incident. However, this would require not only a fault in the air/ground logic but also a sensed discrepancy between T/L position (not necessarily idle) and thrust output on both engines simultaneously.

VI. (Inadvertent) shutdown by flight crew VII. Malfunction/mishandling of the fuel cutoff switches (most recent)

It is very, very, very close:

Both engines failed: yup, both engines have failed.
Triple hydraulic pressure low: either you've been hit by a SAM/uncontained engine failure causing massive fluid leaks, or both engine driven pumps have failed (likely because the engines have failed) and all four electric pumps have failed (because the engines have failed).
Loss of all electric power to flight instruments both sides: total AC electric loss, and I think battery/static inverter too? Given four generators and four buses, either massive electrical failure (swimming pool in E&E bay) or engines have failed. Note failure of an individual contactor that can tie two buses together should not cause a quad-bus outage.
Loss of all four electric motor pumps: total AC failure, see above.

thanks for posting the conditions, while likely it deployed as a result of a simultaneous loss of both engines, not the only circumstances then.

Isn't "close to rotation" a little broad? "Close to" can be before or after. If before, and with about 4,000 feet of runway remaining, why did they take off at all? How did they take off, for that matter?

Can anyone do the Momentum / Energy calculations to work out how high the plane would have travelled at the normal climb gradient purely on the momentum it had at rotation? I'll try, see how far I get. A stone, fired into the air at an upward angle, begins to slow down and curve towards the earth the moment it leaves the catapult. It appears to me that the plane climbed at approximately a steady speed until about the 200 ft mark, so I submit that it had adequate climb thrust up to "about" that point.

Which, as confirmed in the earlier thread, is about where GEARUP is typically called. I say those two events are linked, led by GEARUP, but it could be coincidence. Though I don't think so. Coincidence usually refers to unrelated events and that would be very hard to say, here.

On that point, the gear, as far as I can establish (not openly published according to Google), weighs around 8-odd to 10 tonnes. Typically, retracts in about 10 seconds. I estimate it's no more than a 2 metre lift. As far as I can work out (using 3m to make the value higher), that requires about 30kW (rough estimate, budgetary figure, not accounting for it being a curved path, so it's probably higher closer to fully up), but whether wind pressure affects it, I have no idea. Anyway, 30kW isn't a huge (additional) load on a 225kVA alternator. Less than I'd imagined.

Now I'm wondering how big (power ratings) the hydraulic pump and motor are? No doubt, they're driven by a VSD. Can anyone comment, please?