Posts about: "MLG Tilt" [Posts: 76 Pages: 4]

In of a 787 take-off, first the inner main landing gear bay doors open, and then the bogies tilt downwards. In the first video of the crash, the bogies appear to be tilted downwards but said doors remain closed?

early doors then after a certain time they close. Or gear left down for brake cooling in the MEL

That is a -9. On -9 and -10s the inboard gear doors open at lift off automatically without gear up being selected. The accident aircraft was a -8 and does not have that feature. Main gear bogies tilt nose down, then gear doors open, then retraction as seen in this video

The video of the crash shows the gear tilted down, which should mean gear up was selected, however something kept the sequence from continuing (loss of hydraulic pressure for example).

Discussed here

Was wondering about this since bogie attitude was first mentioned a while back. Perhaps someone familiar with the electrical/hydraulic aspects of main retraction on a -8 could comment on what (or loss of what) would prevent the sequence from continuing.

You're right it is frightening. In this instance they didn't avoid a sink did they and the nose wasn't raised. Flaps add drag, OK not much at a takeoff setting but a little. If you retract them without changing power and without increasing climb, speed will increase. It's physics. This plane started descending. You think a plane retracted flaps, started descending and in the process lost around 60 knots (probably more actually, that's based on an average). Also the landing gear actually appear to just start the retraction process, the bogies tilt up which is the first phase, and then it suddenly stops. So that rules out 'pulling the wrong lever'.

So much confirmation bias here. A lot of people settled on flaps being the cause at the start and are now completely blind to all the overwhelming evidence saying it had nothing to do with flaps. I don't blame you, even professionals in critical jobs fall victim to it, see various miscarriages of justice after police got fixated on one suspect and refused to accept all following overwhelming evidence to the contrary. Hopefully air crash investigators are a little more open minded, and not quite as determined to blame the dead guys.

With respect I can\x92t see that the two videos we have available to us as primary evidence are sufficient to draw any conclusions, they certainly don\x92t provide overwhelming evidence of anything other than the aircraft crashed with the inevitable horrendous consequences. Although I stand by my unsubstantiated theory that this was a flap/config issue of some sort, I\x92m only basing this on the scant evidence available publicly and I certainly aren\x92t resting my hat on it. Very much expect that there may be a root cause that nobody expects or there may be many factors at play here that have allowed the holes to match up.

One thing is for certain though and that is that, due to the number of 787 aircraft in service and the ominous nature of the event, we may see preliminary findings released fairly swiftly.

In the take off video I think there is a subtle yaw to the right at lift off (the moment it begins being obscured by the building in the foreground). There is then a massive blast of dust on the airfield on the left side of the aircraft as it lifts off. Later video audio suggests the engines were not at full power ( or even operating - just wind noise) and that the RAT was whirring. It also shows the fuselage wheel bogies were tilted forward - so wheels up has commenced - but the gear doors are not open.

Possible sequence: right engine failure at rotation, firewalling of left engine, lift off and positive rate, gear up lever activated, wheels tilt forward, left engine failure, RAT deployed, insufficient hydraulics to Open gear doors to complete gear up sequence, ongoing dual engine failure?

Why the need to make the two theories fit in such a convoluted manner? Inadvertent flap retraction was theorised because people assumed that was most likely to have caused the loss of lift - hardly anyone believed dual engine failure was possible. Now we know that the loss of lift was indeed almost certainly caused by a loss of engine power, why all these awkward attempts to reverse engineer it to still fit the flap retraction theory? People need to realise they are only trying to make the flap retraction theory fit because of cognitive biases. If we had all been told immediately, there was dual power loss and the RAT deployed, no-one would even be considering flaps. Its confirmation bias of past assumptions, that's all.

As has been said many times as well, the landing gear retraction process appears to start as the bogies tilt, and then suddenly stops. Which rather suggests they did pull the gear lever. Based on the videos and the amount of speed the plane lost in the very brief sequence ovents, I'd say that the plane lost power a lot earlier than it would have in your theory.

and how do you explain the gear starting the retraction sequence (bogie tilt) and then stopping? A lot of mental gymnastics going on here to try to make a preconceived scenario fit the facts

On item 1, the TCMA issue should have been fixed, it does fit the sort of issue that occurred here. TDRACER can talk to that, and has done in 2019 and again in post 792. As to flap auto retraction, the B787 like all Boeings has a gated flap lever, and the flaps are only able to move independent of the lever by flap load relief. That would not have caused a loss of thrust, and in this case it is evident that the event is a thrust loss not a CL loss.

On item 2, the video shows no asymmetry at any time, so there is only a symmetric failure of the engines possible. Back on a B747 classic, you could chop all 4 engines at the same time with one hand, on a B737, also, not so much on a B777 or B787. I would doubt that anyone used two hands to cut the fuel at screen height. Note, there was a B744 that lost one engine in cruise when a clip board fell off the coaming. Didn't happen twice, and it only happened to one engine.


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Neila83 is correct, the gear tilt pre retraction is rear wheels low, and at the commencement of the selection of the retraction cycle (generally), the first thing that happens is the inboard MLG doors start to open below the wheel well and then the bogie is driven to front wheels low. (There is also an option that the inboard gear doors start to open early as a result of WOW sensing to improve the SSL climb limit). [my bad, for the B788 Capt Bloggs informs us the gear door sequence is after the tilt, not before, the B789 has the before tilt, the option for the door open at rotate is separate]

The inboard doors do not appear to have opened in this case, yet, the gear is forward wheels down. This appears to be out of sequence. TD may have better knowledge on the options that exist with the B788, but this is not looking good at this time.

There is enough in the way of anomalies here to end up with regulatory action, and airlines themselves should/will be starting to pore over their systems and decide if they are comfortable with the airworthiness of the aircraft at this moment. A latent single point of failure is not a comfortable place to be. Inhibiting TCMA might be a good interim option, that system could have been negated by having the ATR ARM switches....(Both)... ARM deferred to the before takeoff checks. The EAFR recovery should result in action within the next 24-48 hours. Boeing needs to be getting their tiger teams warmed up, they can ill afford to have a latent system fault discovered that is not immediately responded to, and the general corporate response of "blame the pilots" is not likely to win any future orders.

I think we are about to have some really busy days for the OEM.


Not sure that Neila83 is that far off the mark at all.

I have no knowledge about 787, but every landing gear system I\x92ve worked with did not sequence Left and Right MLG to retract and extend with each other. In fact it is typical for left and right side to move at slightly different speed due to different friction etc. Retraction assymmetry is probably not a clue to this mystery.

@FDR, you made a comment earlier re the gear sequencing. It appears that you were referring to the 787-9 (doors then tilt). The 787-8 is different, according to this post and this YT video:


In it, the -8 tilts the bogies then opens the doors.

Many thanks. It still is a concern that the tilt has occurred but no doors have opened.

Miscellaneous comments:
I have seen in manuals for other airliners that because the bogie tilt is by a hydraulic actuator, gravity deploying the gear means the gear doesn't tilt to landing position.

With the loss of centre-system pressure*, would you expect the bogies to tilt naturally? I.e. spring pressure holds the gear in the stowed tilt, a hydraulic cylinder pushes the gear to the landing tilt. No pressure means the gear returns to the 'stowed' tilt.

The tilt actuator is designed to be overridden when the bogie hits the ground, so perhaps it has some kind of intentional bypass and doesn't stay in place without continually applied hydraulic pressure.

If so, that would also point towards total loss of electrics and no attempt to raise the gear.

* 787 centre system is powered by two electric pumps, plus the RAT. The RAT hydraulic pump only powers flight controls, not the landing gear.


Electric loss:
Surely even total AC power loss shouldn't result in engine loss, even if the RAT doesn't come online. The FADECs have their own alternators, bare minimum flight control computers and actuators are available on battery (though probably result in some equivalent of Direct Law), and boost pumps are unnecessary at low altitude. Left/right EDPs will remain active if the engines are running at any serious speed; providing flight controls.

Poor crew reaction to ending up in direct law is possible but it's hard to see the electrical issues as a cause, not a symptom.

Anti-terrorism squad looks to be doing an excellent job (/s) and recovered a convenience-store-grade CCTV recorder, probably from somewhere on the campus it crashed into:
https://economictimes.indiatimes.com.../121823103.cms

Does the gear tip forward in this case? Because it's tipped forward in the crash aircraft.

Plane crash near Ahmedabad..

Really? What would you do when you realised you stuffed up the power settings for takeoff? Just sit there for many seconds and crash?? Of course not. You'd slam the throttles to the firewall, just like the EK 407 captain did. Even if you then forgot the gear, it'd still fly. And the Miami incident is a red herring because once the aircraft was airborne, it was always going to fly, because the takeoff thrust was correct.

Hopefully the investigation will tell us these things. Captains, PFs and PNFs are not necessarily infallible when it comes to thrust management - look at the Yeti 691 accident - they increased power settings but it didn't actually increase thrust at all, for the reasons explained in the investigation report.

I think they meant the retraction had "commenced". A previous post said the bogeys on the -8 tip forward, then the doors open. In the video, the bogeys are tipped forward but the doors remain closed.

Plane crash near Ahmedabad..
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A previous post suggested that the gear were held tipped back by hydraulic pressure, so perhaps the fact that the bogeys were tipped forward could indicate an earlier loss of hydraulic pressure, rather than retraction having commenced.

There have been a couple comments regarding the tilt of the bogies not corresponding to the landing configuration which have taken this as an indicator for an attempted (but failed) retraction.
I don't think anybody has so far confirmed which of the two positions the bogie would have without hydraulic pressure, but I would strongly think it is the one used in the retraction/extension cycle and not the landing configuration, for the simple reason that otherwise the gravity drop would potentially not work (I assume it is tilted for the stowing because it would otherwise not fit).
Maybe someone with concrete knowledge can confirm this?

This would then only confirm that the bogies were unpressurized (likely because of loss of hydraulics, but of course could also still be a partial retraction that stopped for some reason)

A slight decrease in two engines is still far better than a loss of one engine, and that has to be manageable.

787 has four main AC buses. Cannot find information on the standby/emergency buses but I would probably expect two.

There's a list of equipment operable on battery/RAT here, but I'm not sure which (if any) is the transponder (26:10):
I think the post you replied to was in the context of mandating APU on for takeoff. Could just say "on if available", though.

I can't see how gear pins would stop you doing anything more than raising the gear. They don't cause engine failure, RAT extension, or uncommanded flap retraction.

If you had gear pins and an engine loss, I could maybe see climb rate being zero or slightly negative. Not the brick impression we see here.

I did raise this earlier... FCOMs say that the bogies remain in the stowed tilt after a gravity drop, but I don't know if that's because the gear has springs to hold it that way without hydraulics, or just they close the valves on the hydraulics so it stays in the last commanded position without pressure.