Posts about: "Gear Retraction" [Posts: 243 Pages: 13]

On the 747, Weight on Wheels (WoW) depends on prox sensors on the landing gear (i.e. gear compression). I don't know how that's done on the 787.
The reason we used both Radio Alt and WoW is that both can give erroneous indications on certain conditions - RA can be 'fooled' by dense rain or even really dense fog (the signals bounce off the water and falsely indicate on-ground), the prox sensor system can subject to HIRF/Lightning interference.

TCMA acts quickly, but it does require some persistence, so an input glitch won't activate it (mainly N1, which is measured every 15 milliseconds).

What sort of 'confirmation' do you have in mind - the regulator mandate that resulted in TCMA basically says we can't take credit for the flight crew.

This has just appeared in the Times (of London) which is usually one of the most reliable sources of information in the UK.

"A loss of engine power is emerging as the most likely cause of the crash of the Air India Boeing that killed at least 279 people at Ahmedabad on Thursday. The Boeing 787-8 series appeared to have suffered from lower than normal thrust from its General Electric GEnx engines as it took off and failed to climb more than 450ft before crashing, video and reports from the Indian authorities have indicated.The new information has eclipsed an early focus on the unusual configuration of the aircraft\x92s wing flaps and landing gear."

https://www.thetimes.com/world/asia/...rash-vhqw6b7v3

(subscription required to read the article)

If you read the thread, you would know:

The RAT was almost certainly deployed. 4 different sources.
The Flaps were not retracted. Visible at the accident site plus many other sources agreeing they were indeed down.
APU will autostart when all engine power is lost. Potentially explaining why the inlet door was open or partially open at the accident site. Mentioned in several previous posts
On a 787-8, the main bogies tilt as the 1st action of the gear retract sequence. As stated in previous posts. I don't think this happens unless gear is selected up. So the conclusion was, gear was selected up. One caveat, IIRC, there was some discussion around a failure could have caused the bogies to tilt without Gear up being selected but I don't recall the outcome.
As for the Air France remark, un-necessary IMHO. Let's respect the crews please.

I previously speculated the forward truck tilt was proof the gear had been selected UP and the retraction sequence was interrupted.

I’m not so sure now and believe there is a different conclusion from this non-normal gear position.

In normal retraction sequence the gear doors open almost instantaneously after the forward truck tilt. It does seem coincidental the tilt was completed while no indication of the doors opening is visible on the rooftop video, which would suggest hydraulic failure at that exact moment; this precise timing of interruption in the retraction sequence feels unlikely. So is there a more likely answer for the forward truck tilt that does not involve movement of the gear lever?

I suspect it’s more likely that C hydraulics lost power prior to rotation, as a consequence the truck could not tilt rearward during rotation as it normally should. Therefore it’s probable it always stayed in a neutral or forward tilt position from the take off run until we see it in the rooftop video. If the gear was behaving normally, and the crew had omitted to retract, it should be hanging rearwards. Watch any 787-8 takeoff video and you can see at rotation all 4 main wheels stay on the runway as the aircraft rotates. Just after wheels up they tilt rearwards. It’s a very subtle position change.

If the gear was always in a neutral or forward truck tilt position then this undermines the theory that retraction sequence was interrupted. It insinuates the C hydraulic and electrical failure happened prior to main wheels lift off.

For this reason I believe we cannot assume that gear UP was selected nor that retraction was interrupted. I’m seeing lots of social media posts which suggest the forward tilt means gear was in retraction and I don’t believe it was now.

I think the truck tilt position is key to understanding the timeline of system failures and whether the automatic RAT deployment was triggered by power failures or engine(s) failure. The question remains, did loss of center hydraulics happen before or after loss of thrust?

Not a pilot, but an engineer with some experience in accident / incident investigation.
One thing that strikes me in this and the previous closed threads is the lengthy back-and-forth discussion about potential technical faults that could lead to this accident.

However, there has been relatively little discussion about the #1 cause of all airplane accident - pilots' mistakes.

Except for the initial conversation about mistakenly retracting flaps instead of landing gear, which eventually was dismissed in favor of engine failure as there has been an almost unanimous agreement that even if such mistake was made, it was recoverable.

But what if the pilots made more than one consecutive mistakes?

Mis-diagnosing the first abnormal behavior and making things worse by performing another action that instead of correcting the first mistake, turned the situation unrecoverable. The history of aircraft accidents is full of examples when a first, relatively benign issue was made much worse and eventually tragic by pilots' subsequent actions.

Frankly, I think it is much more likely that the pilots made several consecutive mistakes than eventually led to the crash than a 1-in-a-billion unrecoverable mechanic failure. I'm looking forward to the CVR and FDR transcripts and I suspect that we will learn that after whatever happened first (pilot's mistake or minor technical failure), it was subsequent pilot's action that turned a manageable situation into a tragic accident.

I do not wish to speculate on the cause of this horrible accident but this vey situation happened to a good friend of mine, former Tornado pilot with 25 years on airliners, as a captain and PF on a B777. In his words he was initially totally confused why the aircraft stopped climbing. He engaged the autopilot and quickly realised the problem but they came very close to stalling.

I would not like to place money on how quickly I would recognise the, highly unusual, situation of flaps being retracted with the gear remaining down and react appropriately.

The Air India flight was circa 30 seconds from lift-off to crashing.

I am sure an initial report will soon publish the basic facts.

I originally was onboard with the truck tilt seen in the one video being proof that the gear handle was selected rather than the flap handle. But after watching several videos and an "endless" web search for info, I also have started to think that there was a problem before the gear handle was moved.

In a normal retraction, the main gear doors begin to open before the truck tilt is complete (roughly when the gear trucks are "level", seen around 8-10 second marks of this video ). The nose gear doors open at the same time the main gear tilt starts (seen in Jetstar video linked below). It's too blurry in the video/stills of the accident aircraft to definitively say the nose doors are not open, but I'd say no. And it's pretty much a certainty that the main gear doors are not open. And finally I think it is also pretty clear that the main gear trucks are tilted down to the retract position.

I have no confirmation of this, but I read (or heard someone say in a video) that the truck tilt actuator was a "single acting" actuator, meaning hydraulic pressure held it in the toe up position and it was spring loaded to the retract position. Which means if hydraulic pressure was lost due to loss of power to the electric driven center hydraulic system, then the main gear trucks would tilt forward on their own without moving the gear handle. The Jetstar burst tire video somewhat backs this up, in that the blown tire caused a leak to the center hydraulic system and an alternate extension was required (thus the reason the main doors are down during the landing). No hydraulic pressure means the main gear trucks remained pointing nose down during the landing. Video here

The doors on a 788 don't open prior to tilt. The gear doors open for 30 seconds automatically after takeoff on the 789 and 78-10 in anticipation of selecting gear up, but not the 788.

WHEN something catastrophic happens, like dual engine failure, that then creates a query about any "duality" between two standalone systems that really should have nothing whatsoever in common... except the PF.
Nothing in common? Is that really the case for the 787-8 in the Air India 787 crash?
Look at these three TCMA-related links in the order presented and note the proforma prescriptive caveats in the first two:

https://downloads.regulations.gov/FA...tachment_1.pdf
https://downloads.regulations.gov/FA...tachment_1.pdf
https://patents.google.com/patent/US6704630B2/en

TCMA is designed to detect and accommodate single failures within the EEC/FADEC, preventing a failure from jeopardizing the safe operation of the aircraft.
Implementation:
It involves implementing specific software changes within the engine's control system (EEC).
Regulation:
After some incidents, the design change was mandated by regulators, with a deadline for production aircraft by December 31, 2018, and a retrofit plan for existing aircraft.
Boeing 787 Application:
The TCMA feature is specifically relevant to the 787-8 equipped with GEnx-1B engines, but it may also be applicable to other 787 variants using the same engine type.

The first two links are respectively the request for and FAA affirmation/approval for a GENx-1b software system called TCMA (Thrust Control Malfunction Accommodation). TCMA is the system that precludes High Uncommanded Thrust (HUT) after touchdown by fuel-chopping the engines. It is designed to avoid runway departures. One input is power-lever position. It's then fair to say that (additionally) Air/Ground sensing is quintessentially vital (as to when the system is "armed" and can do this fuel-chop). The third link is the complex description (with diagrams) of the patent application's design functionality of TCMA.
FROM THE 3rd link above:
"​​​​​​The method of the present invention compares the engine's actual power level with a threshold contour defined by the TCMA software package. When the TCMA software package determines that a thrust control malfunction has occurred, based on the engine's power level exceeding the threshold contour, the engine is shut down by the TCMA circuit." It is also notable that it says within the 3rd link that "Typically the aircraft is allowed to operate for a limited period of time with just a single operative processing subsystem."
That Air India 787 was not long out of maintenance.
We are then motivated to ask "what dictates the Air/Ground sensing". Is it just a Weight-on-Wheels microswitch or a RADAlt? (or both? or triplicated micro-switches?). We may then ask: "Did Air India implement the post-5G changes to their RADAlts that concentrated on maintaining their auto-land capability (in the face of 5G interference with RADAlts?) I seem to recall that the FAA's dictums on this pointed out that it was an individual nation's responsibility to both control their 5G frequency spectrums and implement changes to Radar altimeters that would work interference-free in critical phases of flight. What has the Indian regulator done in this regard as the responsible entity? The whole shemozzle, starting with the US Federal Communications Commission (FCC) spectrum allocations, was an ongoing fight between the telecom giants and their getting their new mobile tech to market.

So where are we going with this line of causal reasoning? The only commonality/duality between left and right engines is the software driving the TCMA as monitored by the TCMA software incorporated in each engine's EEC. Most pundits have identified the gear-tilt as evidence that only the centre electrically-driven pump can do the gear-tilt if the engines' other two hyd systems are suddenly both in QUIT mode (which accords also with the instant RAT deployment and loud noise heard by the sole survivor) - and an ensuing transition from climb-out to a deadly sinking and commensurate attitude change for speed maint.
My unavoidable conclusion is that the selection of gear UP and the breaking of the gear downlocks (and WOW sensing and energization of the RADALTs) called upon the TCMA to fuel-chop the engines (via the TCMA functionality in each engine's EEC).
We could start by looking at the No Break Power Transfer (NBPT) tech used in modern airliners. This has led to Gen Control Panel meltdowns in 777's due to GEN contact meltdown. I know of one instance when a 773 was reduced to a RAT only landing enroute and another where a disastrous MEC fire occurred after start on pushback at LHR. A description of the systems glitch often experienced is at the following link. It's quite apparently a "gear-up" hiccup with potential damning consequences for smooth TCMA operation. As to be seen in the quality videos, a fuel-chop provides no real clue (such as engine failure/smoke/fire classically does). An uncommanded "reset" of the two engine's TCMA's upon gear retraction (link below) is trackable to be the sought after "duality" leading to a "both simultaneously quit" engine failure. These momentary electrical glitches and instant "resets" are described in the two links below. Food for reasoned thought?

https://tinyurl.com/yn5ce4tz

https://tinyurl.com/3kkh6n3d

One comment here, and maybe I am mis-understanding your comment, but the landing gear only operates via the center hydraulics. It does not matter whether the Left/Right engine driven hydraulic systems are operative or not. The RAT will only pressurize the primary flight control portion of the center hydraulics.
No idea. I only got that info from the Master MEL on the FAA website. According to the MMEL the aircraft can be dispatched as long as there is one of each type sensor working on each main gear. (AIs MEL could be more restrictive)

If a gear retraction remains in "tilt", then what does that interregnum status say in response to various systems' interrogations of the WoW sensors? Is it stuck in the Netherworld of betwixt and between ?
Perhaps the safer solution should be an additional circuitry micro-switch feed of all three "up and locked"? (that agrees with the gear-handle).
Also waiting to hear what the 787-8 RADALT status is? Was it ever 5G emanations-proofed?
Is there any software provision for the TCMA to know for sure that the gear is neither up nor down? I would doubt it. Does that uncertainty affect the thrust-lever's positional input?

As a pilot with 40+ years of professional flying, currently on the B777, and with some background in modelling aircraft performance, I've got a few observations, having read all posts.


Flaps & Takeoff

The flaps were out for takeoff, which was normal: normal rotation rate, right place on the runway. Debris also shows the flaps were out at impact. The plane's flight path completely matched what you'd expect from a flight with flaps set correctly.


Flight Controls & Pilot Action

To my mind, the controls were clearly working right up to the very end of that tragic flight. This also strongly suggests the plane never had any asymmetric thrust, because there's no sign of yaw, rudder, or big aileron/spoiler movements. The pilots seemed to guide the plane accurately and consistently, just like an experienced crew would. It looks like they were highly skilled from the start: immediately after rotation, they lowered the nose a bit to keep the plane flying. They also bleed off speed to avoid ground contact. Eventually, with no speed left, the plane descends, but it still looked like they were in control. The instinctive pitch-up right before impact is what a lot of pilots would do.


ANC

Why transmit to ATC ? beautifully described by EGPI10BR


Aviate: The aeroplane has decided it doesn\x92t want to do that any more

Navigate: Not many options on where to go

Communicate: May as well let people know it\x92s going to be a bad day and to get there ASAP.


Booster Pumps

Engines don't actually need booster pumps at sea level to get takeoff thrust. (tdracer)


Restart

There's no noise of the engines spooling up at impact. The mobile phone footage just has the RAT noise, normal airframe sounds, and impact noises. I didn't hear any engine spooling up. That witness talking about a "revving, starting noise" sounds to me like he was trying to describe the RAT noise, which we're all pretty familiar with now.


Engine Spool Down

When I did this for real in an actual plane (we had an uncontrollable engine, but also three good ones), stuck at about 30% maximum thrust, at 220 knots, it was an instantaneous spool-down \x96 really dramatic. The yaw came on so suddenly it felt like a heavy catering truck had slammed into us at speed.


Engines

They appear to get the plane to the correct speed and position on the runway as commanded, then at a time very close to rotate they stop providing thrust. The rest of the flight is consistent with gliding flight.


Rotate vs. Air/Ground Switching Vs Pilot action

(This is my take, it might seem counterintuitive, if you know better help) On a normal takeoff at rotation, liquids don't slide backward any more than they do during the takeoff roll itself. They slide back during the takeoff roll, and at rotation, there's actually a slight reduction in those backward forces. The main force is still pushing them back, but because the wings are now generating lift, there's more drag than when the plane was just rolling on its wheels. So, if liquids have pooled at the back of something at rotation, during and just after that maneuver, they'll actually ease forward a bit. The main force is still pushing them back, but it's less so. My take is if something sudden happens around rotation, I think it's probably more connected to something like the air/ground sensing or pilot action, but it is still possible that pooled liquids sloshing forward a little could also cause it.
Pilots have to do a few things at rotate time, pull back in the controls, not the thrust levers in error and then move the landing gear lever. Over the years there's been a few action slips, so unintended control movements.

Are you saying that the RAT cannot power retraction of the gear, neither via its own hydraulic pump nor via its generator providing power to the centre system pumps?

Or could it be that it's capable of delivering enough power (via either route) to move the tilt actuator but not a humungous retraction jack?

If the 787 is not different to all other aircaft, the WoW senses and putouts only one parameter that is always 1 or 0. There is no netherlands in between. The physical motion the switch is sensing migh have not reach it other end at the time the switch changes its output, but it is irrevelant. If there is a requirement to know that the motion has reaches it\x92s other end, another switch (or other type of sensor) is needed.

Using up and locked as signal to prevent TCMA activation is obviously more dangerous as it allows TCMA to function while the aircraft is in the air.

Even in the same aircraft there might be different logics to determin if the aicraft is on the ground or in the air, depending what is the priority. Is the priority to know that the aircraft is in the air or on the ground or even NOT in the air. Some of those logics could use up-and-locked sensors and the gear handle position.

Pooled water around the moisture barrier is just looking for a suitable access point to drain into. The 3 events I am aware of in the B744 were all during manoeuvring, 2 at rotate and one during a descending turn.


Based on approximate distances and heights, the time line, the aircraft configuration, surface wind, the published L/D (gear up reduced to a guess for gear down and RAT out), and probable off-optimum speed for maximum L/D in that configuration, it's my opinion (aeronautical engineer, unused ATP rating, and glider pilot, national contests) that we can't rule out both engines being at idle or very low thrust at or shortly after rotation, rather than shut down.

If power was lost at the moment gear was selected ‘up’ it would have started the process only for it to stop, hence why the forward tilt was present - perhaps

Investigators will try to establish a single source or truth. For them, that\x92s an ideal outcome. But due to the nature of investigations, the aim gets derailed as established facts versus possible/potential scenarios based on missing links of the chain are pieced together. But for sure, commercial considerations and liability wont enter into a proper investigation. The task of the investigators is to determine the how and why.

Theres a saying, and it may even have been the title of a book\x85 Lift is a gift but Thrust is a must. In this case, certainly one or both were absent.

For 2 (maybe 3 if it was a training flight) professional people, the day started with waking up, getting ready, saying goodbye to loved ones, who they believed they would see again soon in 2-3 days. Sadly, that won\x92t happen, and it\x92s the job of the investigators to find out why and liability isn\x92t a hindrance to them.

They will look at every piece of evidence and recreate the events. But it will take time.

information worthy of note is:

A) The gear was still down.
B) It would appear (due to some work by amazing members) that the RAT was indeed deployed.

Other observances:

1. Bits flying off the aircraft. (From experience of operating in this area, they like to fly kites, and waste thermals and floats around, notably plastic bags) I have no idea if that la what we see or not.

2. Generally in India, they love using their horns. So whilst you can compare the frequencies of a potential RAT to a motorbike, appreciate there would be a million horns going off too.

3. Some posters have spoken of the \x93startle effect\x94 like they know what they are talking about. The most startling effect of a professional flight deck during an emergency is how calm it is. There aren\x92t hands flying around everywhere. In fact:

V1 > Rotate > Positive Rate > Gear Up > Confirm FD/AP Modes is adjusted to:
V1 > Rotate > Positive Rate > Gear Up > Silence the bell (Or your SOP Variation) FD/AP modes and the PF flies the aircraft.

the next step is to identify the problem, agree on it and then perform the actions. That won\x92t happen below 400\x92.

4. There\x92s no company in the world where Pilots are being fed and watered between V1 and 1000\x92 so spilling drinks on run switches isn\x92t a thing on this departure.

5. Temperature inversion. Yes it\x92s possible and it degrades performance. However, if it\x92s present, it\x92s usually announced on the ATIS. IF other aircraft have reported it.

6. Fuel contamination. Without knowing the systems, yes it\x92s potentially possible, but it would appear no other aircraft have reported being affected by it.

I think, this accident is especially interesting to Professional Aviators and Engineers because I think none of us would ever believe that it could happen. The aircraft is highly automated, the crew have been properly trained and the operation was a regular or possibly training flight.

A query I have is, do later Gen aircraft like the 777/787/747 A330/A350/A380 constantly send Airframe/Engine data home to ops/engineering/oem\x92s. Is it likely the data is out there?

Anyway, I just wanted to post this to reassure the travelling public that Pilots don\x92t try to shut down engines before they raise the gear.

That is one conclusion (which I now suggest is unlikely), because you are assuming:

1. The hydraulic failure happened exactly when the truck had tilted forward but the split-second before the gear doors could open. That exact timing seems too coincidental.
2. The crew selected Gear Up. We have no proof of this. I speculate the crew never got as far as "Positive Rate...Gear Up" because they were already engrossed in flying the aircraft and processing their thrust problem. If an electrical problem had developed (as evidenced by the RAT deployment) the flight instrumentation would have been flickering and a flurry of silent master caution alerts would be very distracting.

I suggest it's more likely that the truck remained in a forward tilt from the takeoff run because the hydraulic failure happened prior to rotation, therefore keeping it in this unusual position. This answer does not rely on the gear lever position either. It also insinuates that their hydraulic problems occurred probably between V1 and VR. See my earlier post .

yeah the low MCP alt setting/alt capture doesn\x92t make a whole lot of sense- the plane didn\x92t pitch forward it just failed to climb/lost lift

that\x92s not conducive with what happened nor does it explain why the gear is still down (although seemingly selected up given the boogie tilt) or the RAT deployed (if it really was)