Posts about: "APU" [Posts: 151 Pages: 8]

There was banging prior the failed generator in the 787 Emergency landing....plus loss of altitude, direction, and EE Does all elec load go to the opposite engine? Because if it does and that engine either fails or gets shut down mistakenly ....OOPS no Thrust, no Electrical power, only instruments from APU. If the APU started on climb, we know exactly what occurred....

Have been thinking about this a bit in this thread. First I thought it might be the APU spooling up, but apparently the APU isn't very audible in seat 11A - good 787 sound insulation. Cabin Air Compressors are not started either if power loss induces APU start, so that might not be it either. Since his sequence doesn't give a good idea of when, is the RAT a potential source of this noise? would it be audible in the cabin? Apparently the RAT is variable pitch constant speed, so as the aircraft speed decayed from 170kt to 120kt, would it become louder as the RAT prop pitch adjusted to the lower airspeed?

It's a great question!

Does the RAT make a banging noise on deployment?

Several hundred posts ago, a link to a PPrune thread re 787 RAT deployment was posted. I am sure everyone posting has at least seen if they have read the thread....

I apologise, if my thoughts have already been posted - please delete if this is the case, I cannot find them in the main thread though

None of us know if there was no engine failure, single engine failure or double engine failure.

If RAT was deployed, we do not yet know whether it was automatic or manually deployed by a very experienced captain because "We have no power. What harm can it do now?" (Electrical power, not thrust) Would the Captain also elect to start the APU in the few seconds he had?

In the thread re 787 RAT deployment, some one states that a single engine failure, due to the small rudder size on 787-8, automatically throttles back the remaining engine as the rudder will not be able to correct the course. I am not clear about the guards around this - be they height restrictions, speed restrictions or % of power delivery. If there are any guards in the software. It may have been stated and I missed it or didn't understand.

However, as someone involved with critical software design & development, if the generators were "playing up", which is highly possible given passenger observations on previous flights, could there be a window, if the aircraft experienced a problem with say no 1 engine ( suggested in video "analysis" despite the aircraft tracking right ) whereby the loss of electrical power triggered the software to "throttle back" No 2, and that again limiting any recovery of No 1, if the generators on 2 didn't perform/react as planned. Software always has holes.

The primary flight deck screens have battery backup - but do they lose power when the main buses go offline - and/or again when the RAT delivers? Or is it seemless? Previous posts mention both scenarios but with no answer. Are there flickers, resets, reboots? All distracting at best and time limiting at worst.

I think a pertinent point posted earlier, was that the problems seems to have begun with "gear up", a lot of load on the electrically driven, hydraulic pumps.
Seemingly started, but obviously not completed.

I believe the 2 guys sitting in row 0 dealing with this, were just passengers from the moment it left the gate - for whatever reason. The mayday call, by whichever pilot - although no transcript officially published - was probably a last ditch attempt to alert ATC asap to a situation with a clear outcome. Very sad. It is bad form to point the finger before any useful facts are confirmed.

So, I suspect generator problems & a hole in the software and/or logic due to timing issues caused by generators appearing to be on/offline -maybe rapidly - restricting thrust by design..

if the engines were spooling down, packs down, the ambient noises would be low enough that I would think anything is a possibility\x85

Hello,

I joined today to comment on this thread. I have read about 90% of the posts. Not 100%--apologies if I repeat somthing already discussed.

My daily driver is a 777.

--All of discussion regarding the previous leg and IFE / pack issues is highly unlikely to be related to the accident. IFE is installed in such a way it is isolated from AC systems (unless AI had it installed in the same manner as SwissAir 111) The AC may have been hot and possibly 1 pack was inop. (Could have been other reasons for warm interior temps) bMEL requires 1 of 2 operational. The previous sector people were breathing so 1 pack was working. Again, a pack issue is unlikely to be related to the accident.

--Fuel cut offs / possibly intentional?

--Please refrain from "any pilot would have firewalled the throttles, no pilot would ever...." There are so many accidents where pilots did things which are unfathomable. AirFlorida 90--if the thrust had been simply increased the plane would have flown normall / the Aeroflot crashes when the pilot allowed his teen son to play pilot, another where one pilot dared the other he could land with his eyes closed (or blind folded) / the airliner.that made a complete take off with the configuration horn blaring from the start of the TO run / Colgan 3407 stickshaker and stall but the Capt commands full up elevator / i could go on and on

--APU on for every TO. NO!!! Waste of fuel, higher maintenance costs due to more run time ect. Partnair 394 crash due to APU running in flight (lots of swiss cheese--too much to go over here)

That APU door is clearly open (inflight position) indicating an autostart sequence had commenced which would only occur for a loss of all AC Busses


APU door open in the inflight position

Could it be opened by the crash impact as well??

This is a constant-pressure hydraulic system, not a little hydraulic ram on a logsplitter. While I assume there are some overpressure relief valves, they're not relevant here.

It uses a variable displacement pump to maintain 5000PSI constant pressure. The swashplate angle is varied to adjust pump output flow: more devices consuming fluid, more flow to keep the pressure up. If the pumps cannot deliver enough fluid, the swashplate reaches the full flow position and the output pressure decreases until flow consumed equals flow produced. Very much like a constant-current constant-voltage power supply.

Running in that area of maximum flow is 100% expected under some conditions, especially if an engine or EDP fails and the electric demand pump is supplying a whole hydraulic system sized for the larger EDP (although I think this would be less of an issue on the 787 as the L/R systems don't do much, but the same variable-displacement pump design has been around for a LONG time including on the 737).

And again, there's a VFD between the aircraft electrical bus and the pump motor, because the pump is 400Hz and the aircraft is wild-frequency. VFDs are very very good at isolating faults unless you are actually looking at a sustained overload on one of four generators .


Virtually every bus will have a feed and one or more cross-ties or back-feeds. A failed contactor is 100% designed for and with possibly the sole exception of RAT-only flight, entirely designed around. Plus, of course, flight on batteries only or PMGs.

No bus is essential on a modern aircraft.

Boeing treats everything electric as a black box but the A380 has this beautifully overkill drawing - given both have 4x generators, 2x APU generators, and a RAT, it should not be entirely dissimilar levels of redundancy:

Note that the reason for some links having two contactors in series (e.g. BTC5/6 or BTC7) is because this is spread across two separate units, so that a fire and total loss of one leaves ~half the aircraft powered and totally flyable.


As per TDR, built into the FADEC logic.

Power-open power-close is very common in commercial/situations where you don't want to be wasting energy 24/7 and don't have a defined position for the valve/damper in case of power loss. Done a bunch of them in ductwork and electrically operated windows - your car likely has them, for example.

No evidence of engine failure - Not true. No engine noise on video where you would expect TO or TOGA power to be in use. Mayday call stating No Power

No evidence of RAT deployment from a poor image . - You can argue Not from the Image but...: 2 independent audio analysis of the video audio shows the sound comes from a deployed RAT plus JB's video. Plus the guys who live in SEA having heard 100's of RATs deployed during test flight have stated that the sound is a RAT.

No evidence of electrical failure . - Not true. Reported cabin emergency lights going off, FR24 feed stopped just as in the 737 South Korea incident in December. APU intake door partially open at crash scene, suggesting an APU autostart.

Now you can call into question the above evidence but to state there is none, is simply not true.

In reply to you, I am solely stating that it's a thrust-side problem. I think I somewhat misinterpreted your post as it looks like you might have been saying that anyway.

In general, I think it's looking like dual engine failure/shutdown cutting electrics. I agree that why it occurred is very unclear. Outside chance of total electrical failure causing dual engine failure rather than the other way around, but that would perhaps be even more concerning a design failure.

Similar to Jeju, we also have what is looking increasingly like a loss of ADS-B data at the moment things went wrong, not just a loss of coverage.

That gives:

• Sound of RAT
• Visible RAT
• (edit: APU door open implies APU autostart)
• Loss of ADS-B out
• Near-total loss of thrust.

The alternate theories seem to be a) flaps (basically discounted), b) suction feed failed after total electrical failure, or c):

• A/T rolled engines back
• Crew interpreted this as dual engine failure
• Crew didn't push throttles forward
• Crew did switch each engine off & on again and maybe deployed RAT manually as well.

I think it has been suggested that the upload only happens every 30 minutes or so.

It is most likely the RAT deployed in the picture as it is also most likely the APU initiated an AUTO START, by evidence that the intake flap is partially open. Given it takes 20-30 seconds to fully open, the resting position looks about right given the time line that all 4 VFSGs went off-line.

So one thing to keep in mind, the RAT can be deployed manually, but also comes automatically when certain conditions arise, everybody here is assuming it\x92s only on dual engine failure but there are 4 more conditions that trigger the RAT,

- all three hydraulic system pressures are low

- loss of all electrical power to the captain and first officers flight instruments

- loss of all four EMP\x92s (electro motor driven pump) and faults in the flight controls system occur during arrival

- loss of all four EMP\x92s and an engine fails during take off.

This all comes directly from B787 FCOM,



If we assume that what our survivor saw is correct, maybe it was an electrical failure, the aircraft had electrical issues in Delhi during departure and I checked the crash video again, I don\x92t see the strobe lights (neither wing or tail) and also no Anti collision light either. this might also explain the self starting APU on loss of the electrics (engine driven generators). That could also result in a loss of situational awareness with the speed, because of no indication, even the HUD would not work. The mayday call would still be doable because the radios work from the battery.

Any thoughts?
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Given the OAT ond TOW they most likely used the APU for airconditioning\x85

If you mean for takeoff, that\x92s not a thing on the 787

I wonder if the APU and its 2 x 225 Kva generators was serviceable…….

After Kegworth some UK operators mandated APU on at TO, switching off passing 10000 feet and on again with the lights below 10000. These operators were flying B734 and B757. Presumably cost changed that SOP.

Misty.

Have you considered that what you have described won’t deploy the rat, initiate the gear retraction sequence or cause the auto start of the APU, all of which there is a fair bit of compelling evidence for?

This is a regular test during maintenance at my airline (British operator of 787s)
Carried out as part of a 12k check.

Fuel level in the wing tanks made to be between 3100 kg & 3400kg. Engines are started, APU shut down and boost pumps are selected off.
As long as the engines keep running, it\x92s test passed. (Just have to remember to fire the APU back up before shutting the engines down!)