Posts about: "APU" [Posts: 100 Pages: 5]

787 gear and flaps/slats are both on the centre system, powered by 2x big electric pumps and no EDPs, so retraction should be minimally impacted by engine failure assuming electric power was still available and reconfiguration worked. Note the 787 has two generators per engine so generator failure is also unlikely to contribute, unless both engines failed taking out all four generators (and presumably no APU running).

From that thread, I believe it was discussed that on most/all other large transports, deploying the RAT re-powers the CVR/FDR. The 737 didn't have that happen because no RAT. You may still get a few second gap while the RAT deploys.

The 787 has 2x Enhanced Airborne Flight Recorders (EAFR), which each record both cockpit voice and flight data. I expect they are also fitted with the dedicated batteries that the Jeju was a year or two too early to require. Per the NTSB , the forward recorder has a 10-minute backup battery.

Hopefully flight data is not going to be an issue for this investigation.

The suggestion is that the buzzsaw/propeller sound is the RAT; it does sound a bit like an interference pattern, but you don't get the engine roar with it.

It's also maybe visible in a few stills (e.g. post 64).

Two 225kVA generators per engine (plus two on the APU). Directly driven with no constant speed drives so they are variable frequency. That is converted to high voltage DC and distributed around the plane. So, four or six independant sources.

Several load distribution centers, where the DC is supplied to various loads and in some cases converted to 28Vdc, 400Hz AC, or other levels as required by the loads. The switching between alternate sources would depend on each load buses criticality. There are also those lithium ion standby/starting batteries available. So, theoretically very redundant. But highly complex systems also suffer from the possibility of hidden faults which may escape certification testing and analysis. We'll see.

You have heard AC with the RAT deployed HUNDREDS of times? The RAT is a last resort, when all other power sources have failed. All engines, the APU and (possibly) batteries on AC as the 787. Yet you have heard hundreds of RAT deployments passing overhead? Yeah, sure.

Just a stupid question: Why do they not run the APU during the take-off phase of flight when failures could be catastrophic?

Yep, total rubbish - 45 years airline flying - deployed it only once and never saw it anywhere else.

when some(most?) airlines are imploring their pilots to use lower flap settings to save 2-3kg in fuel per approach running an APU that can burn 20-40kg/min on take off\x85..

​​​​​​​The APU might seem to use that much - but the performance of the engines are surely increased as they do not generate aux power any more. One should compare apples with apples.

The AC wasn't working while at the gate on the previous leg, so it may be that they APU was broken. Been on several filghts where this has occurred, so the APU presumably isn't on the MEL.

If it isn't listed in the MEL then it can't operate the flight.

Without getting into how an MEL actually works, here's an excerpt from an actual 787 MEL.



AGCU = APU Generator Control Unit, VFSG = Variable Frequency Starter Generator. IDK what exactly the AGCU does nor why the left one is important.

There are people in the world who have a different, less limited lived experience from you.

As noted in my profile, I live in the Apple Maggot Quarantine area. Specifically, I live in Lynnwood, Washington directly under the approach path for Paine Field's runway 34 Left, and I've been there since 2007. I lived in Mukilteo from 2000-2007, which is more next to the runway than under it. I also own and operate a small aircraft based at Paine Field and am outside on the ramp at least every other day. Boeing manufactures the 777 and 787 at Paine field. Two thousand, nine hundred and fourty-six of them have been made since 2000, when I moved here. The RAT is deployed and tested during EVERY SINGLE first flight of every aircraft Boeing produces that has a RAT installed. And sometimes it requires a re-test on subsequent flights.

So yeah, I have heard a deployed RAT, from the ground, HUNDREDS of times. I've heard it while preflighting my airplane, I've heard it while mowing my lawn, I've heard while lying in bed. And this is exactly what they always sounds like. Another poster compared it to the sound of a T-6 Texan, which is really quite similar. You can find dozens and dozens of videos on YouTube of aircraft landing at Paine Field with the RAT out. It's not rare, it happens daily.

For those still under the impression that the takeoff or rotation was at the runway end... In the video of the CCTV camera you can see rotation happens basically behind the small structure in the foreground. Unless I am severely mistaken... Knowing the approximate rotation of the camera, one can simple draw a line until it intersects with both the structure and the runway:



Yes, it is just an approximation, but I'd say they still had a decent chunk of runway left, and not in any way close to taking out antennas on the opposite end.

So... likely not at MTOW, not using the full runway, normal rotation, followed by videos of it with RAT deployed which should only happen upon loss of all electrical/hydraulical sources, which should only really happen on loss of both engines without APU running, combined with barely (if any) sound produced by the engines in said videos... It leads me to form some sort of image of what happened. Could still be completely wrong.

I have done a RAT deployment test flight on a previous type, and even though I was prepared for it, it was still a bit of a shock to see the cockpit go mostly blank before reconfiguring itself. I don't know how quickly the 787 would reconfigure, if there is any downtime at all, but that would not have been a thing you'd expect at 100-200ft AGL. I do not know how many of us would quickly overcome the startle.

That is the opposite of how an MEL works. Is it school holidays?

Especially when the video is clearly taken on the ground, when you could easily expect source switching due to shutting down an engine for single-engine taxi, then switch to APU, then ground power.

IIRC the CVR battery (in this case EAFR battery) is required to power at least the cockpit area microphone, if not the pilots' mics.

Once the RAT deployed at least some data should have come back.

This has been discussed upthread and has happened before (on a 777 IIRC) but did not result in a crash.

It does not explain the RAT and generally you would expect crews to shove the thrust levers fully forward.

SLF here, retired physicist, but with much engineering (esp. systems engineering) background and considerable interest/experience in fault-finding in complex (not aircraft) systems.

I think it is helpful here to work through some possible failure scenarios in some detail. You could usefully partition these into two separate groups: “RAT was deployed” and “RAT was not deployed.” I’ll mostly follow the former here.

\xb7 By following this path, I think we can exclude incorrect flaps setting or premature flap retraction as the primary cause of the crash. It’s difficult to see how improper use of the flaps would be correlated with RAT deployment. Everything in this case points to a loss of engine thrust.

\xb7 The first question is, why did the RAT deploy? As I understand it, manual deployment by a pilot is possible, or automatic deployment caused by detection of major electrical or engine failures. I haven’t found an authoritative, detailed discussion of this, or about the time to deployment, which is relevant here as there is so little time involved.

\xb7 According to tdracer , if the primary issue was a major electrical failure, that should not have caused any engine rollback. Thus, absent pull back of the throttles (which surely would have been corrected by the pilots), there should not have been a loss of thrust.

\xb7 Thus we are left with engine rollback as the likely underlying problem. Absent other issues, a single engine rollback should not have been a major problem, so dual rollback, unlikely as it might be, seems a reasonable conclusion.

\xb7 This is consistent with the reported mayday call, although that report needs confirmation.

\xb7 It is difficult to understand a dual engine rollback. Various causes have been suggested but ruled “unlikely” here. However, it is not possible to rule out a unicorn event, like the dual engine rollback experienced by BA 38. Leaving aside the cause, it is useful to look at the consequences.

\xb7 There would have been a major loss of electrical power (apart from battery backup), assuming the APU was not running. I don’t know if is possible the APU might be used at takeoff (e,g., to unload the main engines), or if any evidence from the tail photo at the crash site provides a meaningful indication (e.g., intake door status).

\xb7 Are there other indications of loss of electrical power? The reported statements of the surviving passenger may have some relevance, but I would want to see the results of an interview by crash investigators.

\xb7 What about the loss of Flight Aware ADS-B data shortly after takeoff? There have been a few mentions of this, but not much discussion. Could this indicate loss of electrical power?



I hope this is of some use. I’m happy to defer to professionals or others here for better information/analysis.

The nose gear angle suggest that the "gear-up" was selected and a dual engine failure happened at the same time, with the hydraulics failing and possible RAT deployment.. The APU was not running and everything, except the flight recorders, might be powered down and restarted at a very low altitude.

Very scary.

Combining all the bits and pieces of info from this thread so far, IMO we can theoretically sequence it thus using the video from the left:

00:18 Rotation. Normal takeoff config.
00:24 Gear up starts. per Raffael with FF.
......... FR24 ADSB last transmission (71ft, 172kt) just before runway threshold. Matches with video aircraft altitude at 1/2 wingspan.
......... ? Full power flameout leaves N2 ~ 60%; Airspeed < 200k so N2 will decay to 15% in 8-10s?
......... ? Takeoff EGT of 900C needs 25-35s to fall below 250C ?
00:27 Gear up stops. per Raffael with FF. Bogies tilted.
......... ? APU starts. 20-55s to 95%N?
......... Per 787 dual-engine fail/stall memory items, PM initiates Fuel Cutoff and Run.
00:28 Visible loss of thrust. Alt ~ 200ft using aircraft wingspan as measure.
......... Matches with eyewitness "within 5-10s ... it was stuck in the air".
......... Per 787 dual-engine fail/stall memory items, PM initiates RAT Switch for 1s. Whether auto or manual, the RAT initiates.
......... RAT "bang" heard by survivor
......... RAT coming online accounts for eyewitness "lights started flickering green and white".
......... Per 787 QRH below 1000ft, PF makes no change to Main Landing Gear and flaps, aircraft pointed straight for best glide.
00:31 Descending visibly, somewhere beyond the runway threshold. Alt ~ 200ft using aircraft wingspan as measure.
......... ? Because EGT > 250C FADEC blocks fuel (T-HOT hot restart inhibit?) so no relight though N2 > 15% ?
......... 787 glide ratio between 16:1 to 25:1 with MLG down, Flaps 5. About 15-20s and 3-5000ft of glide from 200ft?
......... Some flap accounts for the ground pictures.
00:34 ? N2 has presumably decayed to 15%, FADEC flips to X-START: airspeed outside envelope? No hope of relight now.
......... PM/PF transmits Mayday?
......... Video showing RAT deployed.
00:46 APU reaches some fraction of 95%N (APU sound accounting for survivor's perception of thrust?).
00:48 Impact. 4200ft from descent start, 3990ft from airport boundary road. 17s from visible descent start.

if this is a valid sequence, the only remaining question is why the dual-engine failure at ~200ft agl?

with condolences to the families and people affected.

It might appear from this photo that the APU intake flap is in the \x91not closed\x92 position and generally not damaged.
The APU will automatically start in flight , irrespective of SW position, if 3 or more engine VFSGs are lost.

The B787 is a way way different and much more complex and sophisticated plane than your Gulfie. The B787's two outermost (left & right) hydraulic systems are primarily driven by the engines, mechanically driving the hydraulic pumps. The center hydraulics are primarily electrically driven, and power the main flight controls, amongst other things like the gear. The left and right ones power the main flight controls as well, some of the less important flight control surfaces like spoilers and thrust reversers -- pardon me for not having the exact list of things. They also have a backup/supplementary electric pump each. Each of the two main engines has redundancy for the power plant a.k.a. VFSG (and motor-driven pump?) in its own right as well. All three hydraulic systems work together in a redundant fashion when it comes to the primary flight controls. The RAT can provide both electrical and mechanical sources of hydraulic support, if I'm not mistaken. The flight instrument and information systems can also be powered from two backup batteries, the APU power plant itself, and/or the RAT.

Good spot, but it is possible the actuator that operates the door is damaged.
Unlikely, but possible.
This does add more credance to the complete power loss scenario.