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Someone Somewhere
2025-06-19T12:25:00 permalink Post: 11905981 |
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. Last edited by Saab Dastard; 19th Jun 2025 at 15:45 . Reason: Reference to deleted post removed 10 users liked this post. |
Capn Bloggs
2025-06-19T12:25:00 permalink Post: 11905982 |
Here we go again.
Originally Posted by
LGB
I am also thinking that Air India would follow Boeing procedures in that the left seat pilot will move their right hand away from the thrust levers at V1, and thus, at 400', the thrust levers are not guarded or monitored?
Originally Posted by
LGB
Even if thrust levers were pushed forward, is there some kind of logic related to FMC and-or FADEC or other involved systems, which regardless of thrust lever position commands IDLE thrust to the engine?
.
Originally Posted by
LGB
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Remember that Airbus accident where the aircraft thought it was landing, while the pilots wanted full thrust, and they crashed into a small forest because some kind of idle is all they were afforded by the system?
Originally Posted by
LGB
​​​​​​​
If the engines of this 787 thought it was in the rollout or final part of the flare, it might also command thrust levers to idle?
Last edited by T28B; 19th Jun 2025 at 14:33 . Reason: formatting assistance 6 users liked this post. |
Capn Bloggs
2025-06-19T12:55:00 permalink Post: 11906000 |
Originally Posted by
syseng68K
I guess it depends on the model
Not so the Boeing fuel switches: they can be relatively easily "sat" in the middle, on the centre raised bit and could be bumped either way. Hence our (non-787) FCOM saying make sure you jiggle them when you put it in the On position to confirm it's locked there. 5 users liked this post. |
Bap7788
2025-06-19T20:21:00 permalink Post: 11906338 |
Lower than calculated lift at Vr
Hi all,
Sorry it’s going to be a long one but seeing the level of competency here, I though it would be the perfect place to get my answers. From the precious messages read and answers received, I have a scenario to run. I am more than happy to be told wrong from point 1). I don’t have the knowledge some of you do. Please let me first start by saying that I am not trying to incriminate anyone. Hundreds of CRMS debriefs and accidents reports show us that unfortunately sometimes, the holes in that swiss cheese just do line up. It is far too easy for any of us, seating here, to judge any of the sequences happening in a Flight Deck. Mistakes happen, regardless of training and experience. We all do mistakes, every day, in every line of work. DISCLAIMER: I know that the consensus is a dual engine failure due to either TCMA bug or any sort of mecanical/software/wear and tear. I do hear a RAT (I don’t see it though) and I do find the audio analysis quite compelling. It is at the top of my list as probable cause. I am just exploring another scenario, based on the AC’s profile and state from grainy video and poor audio. 1- Let’s assume that we do all our perf calculation correctly. Is it possible that the OPT would spit up a F15/20 take off with the conditions on the day on a 787? 2- If so, let’s say we have performances for a F15/20 TO in the FMC. Now let’s assume we select F5 for TO (not in the FMC, physically). Would there be an FMC message, or would that trigger the T/O warning on the 787? If it doesn’t, we now potentially have an aircraft on the heavy side, with already a lift penalty on a high density altitude day. 3- Please bear with me, I know so far I have made an awful lot of suppositions and assumptions. Murphy’s law dictates that what can happen will happen albeit not on the same day. As it was answered to me by someone who was obviously seeing where I was going in a previous post, it’s a lot of swiss cheese to line up. 4- Take off roll goes on, Vr F15/20 comes and we rotate at a speed lower than we should for our actual F5 setting. My buddy calls for GEAR UP, I retract flaps to F1. Another lift penalty. Is there enough thrust now, or are we then already to deep on the back end of the drag curve ? I do understand that this is not testable in a simulator. I am asking if someone with a 787 OPT and/or FCOM and knowledge views this scenario as possible or not (especially regarding the FMC message and the T/O warning). That is all. thanks for the help ! Last edited by T28B; 19th Jun 2025 at 22:23 . Reason: formatting assistance |
Capn Bloggs
2025-06-20T15:49:00 permalink Post: 11907075 |
Disclaimer: the numbers I mention are from publicly available sources, namely Wiki (for the ZFW weight calculation) and a Boeing FCOM dated 2010, and my own estimations.
Strange, as I would have estimated this quite differently based on layman's intuition. If one assumes average values, then the approximate flight profile of AI171 according to layman's guidance certainly fits a situation in which the engines failed at or even very shortly before rotation.
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Is VR about 20 to 30 knots above the landing speed?
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Would these 20 to 30 knots of additional energy be sufficient to lift the aircraft to a good 200 ft during and after rotation?
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If the angle of attack is then successively reduced, wouldn't the airplane still have enough lift to glide for a few seconds before losing all or nearly all lift?
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Wouldn't it be the case that if the thrust had only ceased five seconds after rotation, the aircraft would then have reached a good 250 ft with the engines still running and then another good 200 ft in normal conditions before the speed was used up to about 150 kn?
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AI171 probably didn't reach an altitude of 400 to 500 ft above ground (in relation to the airport), did it?
@Brace , I think you're exaggerating the residual thrust effect at lower RPMs. Of course 70% would get you round the pattern but you're at a much lower drag config and you're going much faster, again less drag. And are improved-climb takeoffs in the 787-8 even a thing? I can't see a two-stage rotation. I've made up a YT combo video: 10 users liked this post. |
nachtmusak
2025-06-20T21:17:00 permalink Post: 11907329 |
As has been gone over before, the auththrottles normally stay in HOLD until 400 AGL at which point they engage in THR REF xxx VNAV SPD to whatever value was programmed in the RTOW.
BUT Say the MCP is mis-set to 200\x92 or so. VNAV never engages and the expected mode (from others; I`ve never tried it) would be SPD xxx ALT. The modes still engage regardless of whether or not the airplane is on autopilot; the autopilot itself just follows the flight director commands (and the PF certainly woudn`t follow the FD in this case). Autothrottles are independent of autopilot. I am actually looking for an answer from a pilot (or at least someone with an FCOM who can share any relevant passages) because nothing I have been able to look up on my own suggests that this is the case. All secondary sources I can find just say that the autothrottle is inhibited under 400 feet on takeoff, with my impression being that the crew is expected to manage thrust manually during that phase of flight. The incidents I was able to turn up involving an aircraft attempting to capture a target altitude at takeoff specifically don't help either: - G-ECOE: Flight was completely normal until the crew engaged the autopilot at 1350 feet, at which point the aircraft started diving to the target of 0 feet. However it involved a Dash 8 and not only is that not a Boeing aircraft, it doesn't have an autothrottle to begin with. Nothing to conclude from this. - F-WWKH: Again automated deviation was triggered by the crew engaging the autopilot a few seconds after takeoff as part of a test flight. However again also not a Boeing aircraft (an A330) and the selected target altitude was 2000 feet so the autopilot tried to pitch up to capture it. Not sure if anything can be concluded from this. - A6-EQI: The most relevant, being a Boeing 777, with wide speculation being that the selected altitude was left at 0 feet. However the preliminary report is very thin so there's little to go on in the way of factual information, but the problem seems to entirely have been that the pilot flying was following the flight directors without question. There's zero indication of any loss of thrust, in fact they seem to have nearly entered an overspeed condition partly due to the shallow climb angle implying that the engines were doing just fine. So it doesn't seem like the selected altitude caused the autothrottle to do anything. I am sorry if it seems like I'm banging on about this autothrottle point a bit much but as an engineer it just seems completely backwards to me. What exactly is the point of the HOLD mode or of setting those specific gates (80 knots, 400 feet) if the autothrottle can so easily come out of it? The design might as well not have it at all and just leave the autothrottle in THR REF for takeoff then - what would be the difference? |
Shep69
2025-06-20T21:52:00 permalink Post: 11907348 |
To be clear, are you stating categorically or guessing (or neither, and I'm misreading you and you mean something else) that the 787's autothrottle will come
out of HOLD
and into SPD mode
below 400 feet
by design in response to the altitude that's set in the mode control panel, entirely by itself, without the autopilot engaged? Not that this is the way it would behave in normal flight - that this is the way it is
designed
to behave while it's in HOLD mode for takeoff?
I am actually looking for an answer from a pilot (or at least someone with an FCOM who can share any relevant passages) because nothing I have been able to look up on my own suggests that this is the case. All secondary sources I can find just say that the autothrottle is inhibited under 400 feet on takeoff, with my impression being that the crew is expected to manage thrust manually during that phase of flight. The incidents I was able to turn up involving an aircraft attempting to capture a target altitude at takeoff specifically don't help either: - G-ECOE: Flight was completely normal until the crew engaged the autopilot at 1350 feet, at which point the aircraft started diving to the target of 0 feet. However it involved a Dash 8 and not only is that not a Boeing aircraft, it doesn't have an autothrottle to begin with. Nothing to conclude from this. - F-WWKH: Again automated deviation was triggered by the crew engaging the autopilot a few seconds after takeoff as part of a test flight. However again also not a Boeing aircraft (an A330) and the selected target altitude was 2000 feet so the autopilot tried to pitch up to capture it. Not sure if anything can be concluded from this. - A6-EQI: The most relevant, being a Boeing 777, with wide speculation being that the selected altitude was left at 0 feet. However the preliminary report is very thin so there's little to go on in the way of factual information, but the problem seems to entirely have been that the pilot flying was following the flight directors without question. There's zero indication of any loss of thrust, in fact they seem to have nearly entered an overspeed condition partly due to the shallow climb angle implying that the engines were doing just fine. So it doesn't seem like the selected altitude caused the autothrottle to do anything. I am sorry if it seems like I'm banging on about this autothrottle point a bit much but as an engineer it just seems completely backwards to me. What exactly is the point of the HOLD mode or of setting those specific gates (80 knots, 400 feet) if the autothrottle can so easily come out of it? The design might as well not have it at all and just leave the autothrottle in THR REF for takeoff then - what would be the difference? I am guessing because although I flew the 777 I never tried a low altitude capture before VNAV engaged — and it`s been a few years). But think it probably would. As one goes through 50’ LNAV engages; VNAV is normally armed prior to the EFIS check if it`s to be used (which it usually is). So in this scenario LNAV would have been engaged but since VNAV is armed but never engages my guess is that the automatics would engage in SPD/LVAV/ALT. I could be wrong. The PF would have been hand flying (and obviously not following the flight director with autothrottles engaged). HOLD is present in many other regimes of flight; all it means is that the auththrottle (right now) is not controlling the throttles and they stay where they are—and the PF can move them if desired at will. Fr` instance, when descending in FLCH or even VNAV SPD the throttles will usually be in HOLD. (To me this usually meant `hold` the throttles—and tweek them in descent as required). Thrust can be modulated to adjust rate of descent (the throttles become vertical speed levers). On altitude capture in the case of FLCH or path capture in the case of VNAV SPD (in descent) the auththrottles kick in and it becomes SPD/xxx/ALT (or VPTH or VALT as the case might be). Most everyone knew the autothrottles would not engage below 400` and that FLCH in descent at very low altitudes was not an appropriate mode — and they did not activate providing low speed protection in the case of Asiana. IIRC our throttles went into HOLD at 60 knots and stayed there until VNAV activated (THR REF—takeoff thrust). It was also possible that the autothrottles under some environmentals wouldn`t fully achieve takeoff thrust setting (EPR or N1 depending on which engines) and they could be manually moved in HOLD to achieve it. Although I don`t remember that as ever happening. But at this point it`s a guess because I never did it (MCP set at low altitude on takeoff with VNAV never engaging). Perhaps someone else has. Last edited by Shep69; 20th Jun 2025 at 22:05 . |
Shep69
2025-06-20T22:29:00 permalink Post: 11907366 |
I suppose an informed guess is the best answer I can get here, short of someone having access to an actual sim or a Boeing engineer chipping in. Though I am curious that you are guessing that it
would
engage, when in the example you give to point out the autothrottle being inhibited at low altitudes (the Asiana accident at SFO) the autothrottle
didn't
engage - not even to provide low speed protection, a potentially life-and-death matter. If that accident was my only datapoint my guess would be that if it's strict enough to not engage for stall protection, it's strict enough to not engage for altitude capture.
Also to be clear I do know that the autopilot and autothrottle are independent - I have been talking about the autopilot because as I listed, in the incidents I could find where the aircraft automatically tried to capture a target altitude on takeoff, the autopilot was first engaged. So my impression was that until then the aircraft might provide guidance (like in the Emirates case) but will not actually do anything to change the thrust, pitch, etc parameters that have been set. |
Someone Somewhere
2025-06-22T11:01:00 permalink Post: 11908441 |
Always possible, however since a pilot made a radio call there was some
emergency leve
l power available, which suggests the EAFR would be powered.
The Jeju recorders were okay if I recall correctly, they just had no input, was that the case? Somoeone made a good point above about the German Wings FDR/CVR being available the next day after the aircraft was aimed at the ground like a missile. These things are built tough, as you know, this may be type specific but…. ![]() (from the online 2010 FCOM) ![]() (from the maintenance training ) The 787 battery fire report says the two recorders are on the left and right 28VDC buses. I don't think those get powered on RAT by the looks of it. I would wager you get whatever is on the 235VAC 'backup bus', plus the captain's and F/O's instrument buses via C1/C2 TRUs. You won't get all of that (like the F/O's screens) because the 787 energises/de-energises specific bits of equipment, not just whole buses. Losing recorder power looks entirely expected.
SLF Engineer (electrical - not aerospace) so no special knowledge
Perceived wisdom may be applicable in normal circumstances but not when all the holes line up. For example I've seen it quoted many times that the engine FADECs are self powered by the engines, the TCMAs-whether part of the FADEC or a separate unit, similarly self contained within the engine. The perceived wisdom seems to be that there is no common single fault which can take out both engines. And yet we're also told that the TCMA function can only function in ground mode and receives ground-air signals from a combination of inputs from Rad Alts and WOW sensors. There is therefore a connection from the central EE bay to the engine. Yes I'm sure the Rad/Alt and WOW sensor processing will use different sensors for each side and powered from different low voltage buses. However as an analogy, in your house your toaster in the kitchen may be on a separate circuit from the water heater in the bathroom, each protected by a fuse at the main switchboard. In normal operation a fault in one cannot affect the other. However a lightning strike outside the house can send much higher voltages than normal operation throughout the entire system and trash every electrical appliance not physically disconnected at the time. Now I'm not suggesting the aircraft was hit by lightning but FDR has proposed a single event, buildup from a water leak entering one of the EE bays at rotate. It would be possible for one or more of the HV electrical buses to short so that all the low voltage buses go high voltage. I have no knowledge of how the FADEC / TCMA systems connect to or process the Ground-Air signals but there is a single fault mechanism whereby high voltage could be simultaneously and inappropriately applied to both engine control systems. It would be unfortunate if this failure mechanism did cause power to be applied to drive the fuel shut off valve closed. Since the likelihood is that we're looking at a low probability event then perceived wisdom about normal operations and fault modes might not be applicable. Weight on wheels appears to go into data concentrators that go into the common core system (i.e. data network). Presumably there is a set of comms buses between the FADECs and the CCS to allow all the pretty indicators and EICAS alerts in the cockpit to work. The WoW sensors might flow back via that, or via dedicated digital inputs from whatever the reverse of a data concentrator is called (surely they have need for field actuators other than big motors?). Either way, left and right engine data should come from completely different computers, that are in the fwd e/e bay (or concentrators/repeaters in the wings, maybe) rather than in with the big power stuff in the aft e/e bay. 8 users liked this post. |
NSEU
2025-06-30T13:35:00 permalink Post: 11913630 |
If rigorously applied, an "engine thrust balancer" would cause the good engine to fail if something happened to the other one. Surely there is some logic in there somewhere to give up and disconnect past a certain amount of adjustment??
* as for why not before, probably because it didn't happen that way or in Boeing's worst nightmare some weird corner case in the software that does this if certain parameters are in rare combination. 1 user liked this post. |
NSEU
2025-06-30T13:59:00 permalink Post: 11913644 |
There are several ways that the HPSOV can close: An EEC (engine ECU) can close the upstream Fuel Metering Valve (FMV) electronically, so the HPSOV will lose its opening pressure. The HPSOV can be acted on by a Shutoff Solenoid Valve (which directs fuel pressure in an opposite manner to the pressure coming from the Fuel Metering Valve). Unfortunately, the diagram I am using is truncated, and I can't see if the Shutoff Solenoid Valve is magnetically latched in its last commanded position like typical fuel shutoff valves. Nor can I see what controls it. I suspect things like the respective cockpit fire handle and fuel cutoff lever, but also EEC commands. There is probably a copyright on the diagram, so I won't post it here. Perhaps someone can fill in the gaps for me? |
island_airphoto
2025-06-30T14:55:00 permalink Post: 11913678 |
So it has an automated VMC departure preventer? That is pretty clever as long it never goes off when not needed.
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Gupeg
2025-07-01T03:07:00 permalink Post: 11913998 |
787 Fuel System
![]() On the A320 if the centre fuel pumps are selected on pre-start, they run for 2 mins after start and then turn themselves off until Flaps selected to 0 (i.e. well after takeoff), when they turn themselves on. As far as the crew are concerned they were selected on from pre-start onwards [long retired A320 so forgive me if in error]. If certification demands this 'complexity' it would seem surprising the 787 does not have a similar system? Are we sure the 787 centre tank 'higher pressure' pumps are:
3 users liked this post. |
Capn Bloggs
2025-07-01T04:06:00 permalink Post: 11914010 |
Originally Posted by
Megan
Switch over to the centre tank to feed all engines typically takes place at 10,000'
![]() 3 users liked this post. |
AerocatS2A
2025-07-01T09:26:00 permalink Post: 11914140 |
Hold your horses there
Bloggs
, I didn't say they did, I said centre tanks were typically turned on at that altitude (using a certain 737 operator as a guide). As the check list that you posted shows the centre pumps will automatically turn off because of load shedding once an engine is started.
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BuzzBox
2025-07-01T12:10:00 permalink Post: 11914226 |
It's exactly the same on the B777 - the centre fuel pump switches go on before start if the FUEL IN CENTER EICAS message is displayed. The switches go off again when the FUEL LOW CENTER message is displayed. On the ground, the B777 needs two power sources for both centre tank pumps to operate, so one pump is normally shed until after engine start. The centre tank pumps output about three times the pressure of the main tank pumps. Fuel is fed from the centre tank until the centre tank pumps are selected off.
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MaybeItIs
2025-07-01T12:20:00 permalink Post: 11914234 |
Originally Posted by
MaybeItIs
...\xa7 25.903(b) includes the words: "in at least one configuration," It doesn't, that I can see, state that that configuration must be used during takeoff, though common sense would say it should. I also don't see any evidence that engine driven fuel pumps alone must be able to handle this scenario: provide enough fuel flow for takeoff and climb, even while the pitch is rotating, even in a hot environment with significant weight, even while the gear is stuck down. A lot of other posters here have stated that according to FCOM instructions, the normal, accepted 787 Takeoff configuration is "Both sides draw from centre" if the Centre tanks have enough fuel in them. I think (maybe wrongly) that this (prior few posts) is the first time this exact point has been raised. I hope I'm correct there. If not, my humble apologies. The great thing about this forum and sadly, this tragic accident, is that it's drawing a few previously little-known worms out of the woodwork. 1 user liked this post. |
Sailvi767
2025-07-01T13:23:00 permalink Post: 11914271 |
Sorry, you missed the point I was trying to make. \xa7 25.903(b) does say that the fuel system
must
be able to operate
in an isolated, two-sided mode (for a twin engined jet), such that nothing on one side, such as bad fuel, will adversely affect the other engine. Of course, during Takeoff, both sides drawing fuel from a Centre Tank containing a lot of contaminants (e.g. Fuel Bug matter, water) is a scenario that could bring down the plane. We are all aware of that. But the point I was trying to make is that although \xa7 25.903(b) requires "at least one configuration" that separates both systems entirely (such as Left engine drawing from Left Main Tank, and Right from Right) which
can
be configured, the Rule doesn't appear to make that compulsory for Takeoff.
A lot of other posters here have stated that according to FCOM instructions, the normal, accepted 787 Takeoff configuration is "Both sides draw from centre" if the Centre tanks have enough fuel in them. I think (maybe wrongly) that this (prior few posts) is the first time this exact point has been raised. I hope I'm correct there. If not, my humble apologies. The great thing about this forum and sadly, this tragic accident, is that it's drawing a few previously little-known worms out of the woodwork. \x93Fuel from the center tank is transferred to the inner tanks to keep the inner tanks near full. The FCMC automatically controls the flow from the center to the inner tanks by operating the inner tank inlet valves. The center tank pumps run continuously when there is fuel in the center tank and provide fuel to the fueling manifold. When the inner tanks are full, the inner tank inlet valves are commanded closed until the inner tank fuel is burned down to approximately 4400 lbs below full. At that point, the inner tank inlet valves reopen to allow the inner tanks to fill, and repeat. \x93 |