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

Under most circumstances you would apply the benefit of the doubt but as the RAT deployed (something there is a lot of evidence for) it strongly suggests this. Yes, the RAT can deploy for other reasons but that would imply an even greater level of coincidence than two engines failing in a short period (3 hydraulic systems, 4 generators, etc.). The distance they went until ground contact also ties in with a loss of pretty much all thrust, as does the audio recording of idling/windmilling engines. There is also the fact, which may turn out to be an assumption, that any failure of other aircraft systems should not affect engine operation as a) the engines are effectively self-powered in flight and b) the engine controls on the flight deck are part of an isolated system powered by the FADECs.

I would be very surprised if the thrust levers were not firewalled early on, in fact with such determination that they went through the instrument panel!

On a wider observation, professional commercial pilots like the Air India ones in this accident go through regular simulator training according their own SOPs, which when dealing with things like thrust loss during or after the takeoff roll are likely pretty similar or even identical to the manufacturer\x92s guidelines; if they did differ it would be because they were more conservative in application. Boeing standard is to do nothing until 200\x92AGL other than control the aircraft in yaw, pitch and roll. Above 400\x92AGL you can start doing some drills, if applicable. This assumes, of course, that you can get to these heights in the first place.

I would put forward that in this accident, the crew immediately found themselves in what Boeing call \x93Special situations\x94 or \x93Situations beyond the scope of normal procedures\x94. We don\x92t know yet whether there was a thrust loss or total failure at the outset; we don\x92t know if the RAT deployed due to sensed failures or control operation. As a trainer, the captain would have known the implications of actioning the dual engine failure memory items, especially near the ground, but if you\x92ve tried everything else and are still going down then what is there to lose? This is not to suggest this is what happened, just to fill in the blanks in terms of possibilities. Whatever did occur likely put them outside the realm of SOPs in short order, which is a difficult situation at the best of times, especially as for your whole flying career you have been trained and assessed at your ability to conform to those SOPs as accurately as possible in the takeoff phase.

Listed here as 37GPM at 4750PSI each for the EMPs, and 27GPM for the EDPs - most unusual to have beefier EMPs than EDPs. Source looks like it could be a wealth of other information.

37GPM at 4750PSI is ~76kW before considering pump, motor, and converter losses. Ouch.

Very surprised they kept the demand pumps for left/right systems the same size given they only do flight control and perhaps reverser loads - and reverser operation off an EMP is presumably rare as it implies the EDP failed (or was MELed) without the corresponding engine.

777 centre EMPs were apparently only 6GPM 3000PSI with gear/flaps using the air-driven demand pumps.

(we may be re-approaching 'hamster wheel' territory)

The fixed doors on the landing gear are set at an angle that will ensure they assist gravity deployment in an emergency. During retraction that means the effort to stow the gear is increased due to these air loads. This is is standard on all the large aircraft I have worked on.
As for hydraulic pumps, they are limited to how much flow they can produce. The pressure drops significantly during large control movements and the landing gear actuators in particular need a large flow to keep them moving.
When all pumps are operating, engine driven or otherwise that pressure drop is limited, when down to just one small RAT driven pump there's only so much it can do and the design ensures that control of the aircraft can be maintained on just RAT power.
There won't be enough power from the RAT generator to power emergency aircraft systems and large hydraulic pumps. This is why it has its own small hydraulic pump.

Let me try and summarise one possible scenario and then link in some of the better posts provide evidence relating to it:

• In error, PF reduces power to idle and/or reverse at a speed after V1 (either deciding to reject, or for some unexplained reason e.g. the recent BA incident at LGW)
• Decision is changed to continue take-off, thrust levers moved to TOGA
• Let's say the thrust inputs are similar to NM985 and TCMA is triggered; and engines shut down around the time of rotation
• A/C rotates achieving a maximum speed in the region of 184kts

Relevant "ruling out" questions, with links to posts that add new information:

Q: Would the a/c have enough kinetic energy a 184kts to climb to 100-150ft agl and then reach its final position if the engines had failed at, or just, before rotation?
A: Theoretically possible - see calculation here . NB, the a/c actually flew 1.5km from the end of the runway and 2.3km from the likely point of rotation.

Q: Doesn't the forward position of the gear mean that power failed after the pilots had selected gear up?
A: Inconclusive - had hydraulic power had been lost prior to rotation, the gear could also be in this position - explanation here

Q: If the throttle levers were brought to idle during take-off, would the A/C have applied autobrake, reversers and speedbrake?
A: Yes, although there is a built in delay before reverser and speedbrake actually deploy - see here .

Q: Is the ADS-B data consistent with this scenario?
A: Yes, e.g. the Flightradar data shows the aircraft decelerating rapidly (12 knots in 4.2 seconds) from close to rotation. However, it's not clear how accurate this data is. For one, the altitude data is +/- 25 feet, second, while I was under the impression FR would have received airspeed data from the a/c sensors, this post suggests maybe not.

Q: Does TCMA activation require the thrust levers to be at idle or does it function when the thrust levels are above idle, but where the actual thrust is above that commanded?
A: No, the latter is true (i.e. idle is not required) - confirmed here - there are of course many protections against false activation

Q. Did AI171 have the same software version / logic paths as NH-985
A. Unknown. That a/c had Trent 1000s so to some extent the software is different, but we understand the TCMA logic is broadly the same regardless of engine. I have not seen a post clarifying whether the TCMA software has been updated /changed via SB since 2019 to account for this incident.

Be grateful if posters could refrain from speculative responses "e.g. I think this is unlikely because I feel x". I am not opining on how likely this sequence of events is, simply trying to summarise whether or not this theory has been ruled in or out.

I also recommend this post for a summary to read before posting. .

Boeing specifications say that the RAT will provide hydraulic power within 6 seconds and electrical power within 10 seconds. That would be the worst-case scenario, so it should usually be a bit less than that. Almost everything that gets electrical power from the RAT can also be powered by the main battery. So you don't have to wait for the RAT to spin up before you have instruments.

The engine-driven hydraulic pumps should still work for at least a few seconds after flameout. There's also a small amount of stored energy in the hydraulic systems even after the pumps stop. So even with that 6-second delay for the RAT, there shouldn't be any significant interruption in hydraulic power for the primary flight controls.

May I ask where this information of load shedding comes from please
In my experience the APU supplies enough power to run all systems. Hydraulic pumps, fuel pumps etc

On the 767, 757 and A330 anytime you are in single generator operations the aircraft is load shedding. The 787 with a totally different electrical system might function differently.