Posts about: "Takeoff Roll" [Posts: 59 Pages: 3]

The Dash 8 doesn't have an autothrottle IIRC, so I wasn't sure what to think about the fact that in the Flybe incident the takeoff and climb were completely normal until the autopilot was engaged. However I thought that an autothrottle on its own (without an autopilot's input) will simply target its given speed or power setting? I am not sure how or why an autothrottle would change its target speed/power uncommanded.

Actually...doesn't the autothrottle normally enter HOLD mode once the aircraft enters the high-speed regime during its takeoff roll? And only re-engages its actual selected mode at 400 feet (or maybe 200 feet?), and in the time in between the crew is supposed to have manual control of thrust? It seems backwards to me that any automated systems would ever override the pilot's requested thrust on purpose at such a sensitive stage of flight and I thought that the autothrottle goes into hold for exactly that reason. Coupled with the fact that it would be extremely early in the flight to turn on the autopilot...I'm sure I have a very simplified understanding of how this all works. But I'm struggling to see how - unless the crew had also made some other egregious mistake in their preflight preparations in addition to entering the wrong target altitude - it could go so badly in this specific manner so quickly after lift-off.

SLF but: I still really don't understand how this is supposed to happen, not merely hypothetically, but congruently with how short and low the flight we can actually see is.

My understanding (and others have corroborated this) is that in a standard departure the autothrottle is armed and starts the takeoff roll in THR REF mode. It goes into HOLD mode when the IAS passes 80 knots (obviously still on the ground). While it is in HOLD mode, the autothrottle is physically inhibited from moving the thrust levers. It then automatically re-engages at 400ft AGL (though I am not sure how the altitude is measured), and begins to operate as requested by various human and computer systems.

If something about my explanation is wrong, please let me know - but if it is correct, then how would the autothrottle roll back thrust drastically in what looks like the first few seconds of the flight? I do understand that what you describe is how the autothrottle would behave when it is active, but it sounds to me like it is by design not supposed to actually be active during the critical time that we are looking at no matter what automations are armed to be activated once the aircraft is safely away from the ground. Unless the crew did something to cause it to engage - and I'm not sure what that would even be. What would they plausibly be doing before even retracting the landing gear?

My only guess was that depending on how the altitude is measured to determine whether the 400ft gate has been passed (radio altimeter? pressure altitude?), the autothrottle might have come out of HOLD mode (along with VNAV if armed) at a lower altitude than it was supposed to due to some mechanical fault or crew error. But that's already a bigger kettle of fish than just altitude capture...

At 100 knots during the takeoff roll the systems take a snapshot of the barometric altitude at that point. The 400' for VNAV engagement, if it is armed, is based on that datum.

Thank you - it doesn't sound like it should be possible for a non-faulty autothrottle to activate below 400 feet then. Even if e.g. the "wrong" starting altitude is measured on the runway (wrong QNH, etc), as long as it is measuring 400 feet from that altitude, then it shouldn't matter.

Small correction, the 625 ft pressure altitude data point (~100 ft AGL taking temperature, pressure and field elevation into account) is the last received ADS-B data point, which was right around the end of the runway. The aircraft crashed quite a bit further on from that, ~1.5km from the runway end. If you take perspective into account it also appears to continue to climb for a few seconds after passing over the runway end. I think some people in either this thread or the first thread have argued that the ADS-B data cutting out could be indicative of power loss (like with the Jeju Air 2216 accident) and not just spotty coverage, and when I looked at other flights I was inclined to agree - coverage is spotty on the apron and runway, but once planes start climbing the updates become consistent. If that really is when the engines died, ballistics suggests that the aircraft could well attain another 100 - 200 feet off its sheer momentum before the inevitable descent.

Re: mayday call transmission, isn't that easily battery powered? At least on the captain's side.

Sorry, maybe the context was lost - I was responding to a theory that did argue that the aircraft was automatically trying to capture a target altitude that was incorrectly set too low. That has happened before, but the incidents I could find looked very different from this one (one Dash 8 and one A330, both involved the crew activating the autopilot).

Yes. I simplified. The point stands that the throttle needs to be pulled back, as it was in the ANA event, because that was a landing and not a take-off.

First, you posted a good summary. I'd have added "unanticipated hardware fault" and "unanticipated software fault" as generic causes.

Note that the thrust lever actuators are wired to the FADECs, and that the TCMA gets the T/L position from that. For TCMA to trigger, it has to determine that its FADEC (on that engine) failed to achieve a commanded reduction in thrust. So we're either looking at a weird, unprecedented edge case, or a FADEC failure, or both.

It has been mentioned before that this capability existed as part of the N2 overspeed protection: the FADEC would shut down a runaway engine by cutting its fuel before it disintegrates.
The thrust lever sensors are wired directly to the FADEC (and hence the TCMA). No data bus is involved with this item.

With a MCAS crash, it required a hardware problem with an AOA sensor, used as input to a correctly working MCAS, to cause the aircraft to behave erratically. With a correctly working TCMA, I believe it'd require two hardware problems to get TCMA to shut down the engine, as there'd have to be an implausible thrust lever reading, and a FADEC/engine failure to process it within the TCMA allowed range ("contour"?). On both engines, separately and simultaneously.

That leaves a software problem; it's not hard to imagine. The issue is, at this point it's just that: imagination. I could detail a possible software failure chain, but without examining the actual code, it's impossible to verify. We simply don't have the evidence.
I could just as well imagine a microwave gun frying the electronics on both engines. An escaped hamster under the floor peeing on important contacts. A timed device installed by a psychopathic mechanic. There's no evidence for that, either.

This process is a way to psychologically cope with the unexplained accident, but because it lacks evidence, it's not likely to identify the actual cause. We've run the evidence down to "most likely both engines failed or shut off close to rotation, and the cause for that is inside the aircraft". Since the take-off looked normal until that failure, we have no clues as to the cause hidden inside the aircraft. We need to rely on the official investigation to discover and analyse sufficient evidence. The post-crash fire is going to make that difficult.

"Both engines failed or shut off close to rotation" explains all of the evidence : it explains an unremarkable take-off roll, loss of lift, absence of pronounced yaw, loss of electrical power, loss of the ADS-B transponder, RAT deployment, the noise of the RAT banging into place and revving up, emergency signs lighting up, a possible mayday call reporting loss of thrust/power/lift, and a physically plausible glide from a little over 200 ft AAL to the crash site 50 feet (?) below aerodrome elevation .
It explains what we saw on the videos, what the witness reported, where the aircraft ended up, and the ensuing sudden catastrophe.

I don't believe we have evidence for anything else right now—I'd be happily corrected on that.

-----
Edit: the evidence of the crash photo with the open APU inlet door, and the main gear bogeys tilted forward, are also explained by the dual engine failure/shut off.

You're absolutely right, Musician! Your text in bold print is what happened! And you and I and many other pilots know what the most probale cause for that is. What evidence do we need?
The EAFR will tell the story, but the reason for the crash will always remain a "mystery" because the B787 was not equipped with EPTPR's! ( E nhanced P ilot's T hought P rocess R ecorders)

I think AI171 will go down in history with MSR990 an MH370.

I would, of course, presume, that take-off roll performance was within expected limits, otherwise they would have aborted by V1. They reached VR before running totally out of runway, and achieved a short-lasting climb. What one single point of failure occurred very shortly after aircraft went nose-up and would it be possible that the fuel feed in some way affected by virtue of that angle in the context of some failure?

Is this a rhetorical question?

Runway performance seems to be still under some speculation but I thought it was fairly solidly shown that they rotated in about the normal position.

Two events that I see are rotate (g-forces/deck angle causing fuel sloshing) and weight-on-wheels going false due to lift-off.

The engines would be fed separately by the left and right pumps in the centre tank as there's >34t of fuel onboard; one pump per engine. I am not sure how physically separated they are in the tank.

Isn't "close to rotation" a little broad? "Close to" can be before or after. If before, and with about 4,000 feet of runway remaining, why did they take off at all? How did they take off, for that matter?

Can anyone do the Momentum / Energy calculations to work out how high the plane would have travelled at the normal climb gradient purely on the momentum it had at rotation? I'll try, see how far I get. A stone, fired into the air at an upward angle, begins to slow down and curve towards the earth the moment it leaves the catapult. It appears to me that the plane climbed at approximately a steady speed until about the 200 ft mark, so I submit that it had adequate climb thrust up to "about" that point.

Which, as confirmed in the earlier thread, is about where GEARUP is typically called. I say those two events are linked, led by GEARUP, but it could be coincidence. Though I don't think so. Coincidence usually refers to unrelated events and that would be very hard to say, here.

On that point, the gear, as far as I can establish (not openly published according to Google), weighs around 8-odd to 10 tonnes. Typically, retracts in about 10 seconds. I estimate it's no more than a 2 metre lift. As far as I can work out (using 3m to make the value higher), that requires about 30kW (rough estimate, budgetary figure, not accounting for it being a curved path, so it's probably higher closer to fully up), but whether wind pressure affects it, I have no idea. Anyway, 30kW isn't a huge (additional) load on a 225kVA alternator. Less than I'd imagined.

Now I'm wondering how big (power ratings) the hydraulic pump and motor are? No doubt, they're driven by a VSD. Can anyone comment, please?

Under most circumstances you would apply the benefit of the doubt but as the RAT deployed (something there is a lot of evidence for) it strongly suggests this. Yes, the RAT can deploy for other reasons but that would imply an even greater level of coincidence than two engines failing in a short period (3 hydraulic systems, 4 generators, etc.). The distance they went until ground contact also ties in with a loss of pretty much all thrust, as does the audio recording of idling/windmilling engines. There is also the fact, which may turn out to be an assumption, that any failure of other aircraft systems should not affect engine operation as a) the engines are effectively self-powered in flight and b) the engine controls on the flight deck are part of an isolated system powered by the FADECs.

I would be very surprised if the thrust levers were not firewalled early on, in fact with such determination that they went through the instrument panel!

On a wider observation, professional commercial pilots like the Air India ones in this accident go through regular simulator training according their own SOPs, which when dealing with things like thrust loss during or after the takeoff roll are likely pretty similar or even identical to the manufacturer\x92s guidelines; if they did differ it would be because they were more conservative in application. Boeing standard is to do nothing until 200\x92AGL other than control the aircraft in yaw, pitch and roll. Above 400\x92AGL you can start doing some drills, if applicable. This assumes, of course, that you can get to these heights in the first place.

I would put forward that in this accident, the crew immediately found themselves in what Boeing call \x93Special situations\x94 or \x93Situations beyond the scope of normal procedures\x94. We don\x92t know yet whether there was a thrust loss or total failure at the outset; we don\x92t know if the RAT deployed due to sensed failures or control operation. As a trainer, the captain would have known the implications of actioning the dual engine failure memory items, especially near the ground, but if you\x92ve tried everything else and are still going down then what is there to lose? This is not to suggest this is what happened, just to fill in the blanks in terms of possibilities. Whatever did occur likely put them outside the realm of SOPs in short order, which is a difficult situation at the best of times, especially as for your whole flying career you have been trained and assessed at your ability to conform to those SOPs as accurately as possible in the takeoff phase.

The excellent #724 post by user989 really should be seen as the defining statement on what is currently known.

I’d like to add a complimentary test to user989’s logic on TCMA faults.

Regardless of whether the ‘aircraft on ground’ signal was incorrect after rotation it would have been correct during the takeoff roll. IF there was an unrevealed fault in a thrust lever position signal THEN why didn’t TCMA activate during taxiing or the takeoff roll?

Such a fault occurring spontaneously in just the few seconds after rotation is way way down the probability table. Such a fault occurring spontaneously on both separate (think ETOPS) engine control systems is surely vanishingly unlikely.

They may be out there but you’d have to ask if TCMA is implicated where are the lower consequence precursor events in the 787 fleet? These might be spurious TCMA action on one engine or faults with ‘aircraft on ground’ found during maintenance or engines not responding to thrust lever position and so on.

Change Analysis would ask what happened differently in the few seconds after rotation on this flight that separates it from all other 787 takeoffs and why at that particular time ?The interim report will provide some answers until then please let’s confine this thread to fact based technical discussion and debate.

Re-reading this I did briefly consider suggesting engine overshoot of thrust lever positions and FADEC shut down on N1 overspeed but that leaves a lot of WHY and WHY both engines questions so I dismissed it.

You mention here the a/c roll and after rotation but there is also a short period during take-off where the wheels are in the process of leaving the runway. There is also the situation where the MLG was suspended at an angle. What was the WoW value at this stage?

Also during this period, the Rad Alt may not have been giving useful values given its proximity to the ground

These two factors alone could increase the possibility of an error quite considerably.....

The autopilot would NOT be engaged below 400’ (or 200’ in the 78–although I doubt anyone engages it that low. The autopilot and autothrottles are separate systems but do interact. The autothrottles typically WOULD be engaged from the start of the takeoff roll; using the TOGA levers to set takeoff thrust).

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.

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.

Both the B777 and B787 have a minimum autopilot engage height of 200'. I would suggest it is often the best course of action after an engine failure to engage the autopilot at 200' to dramatically reduce the workload!

When talking about low altitude capture below the 400' VNAV engagement height, although it has been talked about, would mean (mis) setting the MCP altitude to less than 400'. I really find that implausible. You are correct though that the modes would be SPD | LNAV | ALT should this occur.

All correct.

The lack of autothrottle automatic activation when the autothrottles are in HOLD mode and the speed has decayed below minimum manouevring speed has been addressed. The autothrottle will now automatically activate if, when in HOLD mode, the speed decays below minimum manouevring speed with FMC software after BP4 on the B787 and after AIMS V17B on the B777.

Since the Asiana accident many airlines prohibit use of FLCH below 1,000' and the lowest altitude setting when using FLCH is then, of course, 1.000'

Almost fully correct, the speed when autothrottles transition to HOLD on takeoff is 80kts. on both the B777 and B787.

Yours is very good logic, as far as it goes.

But I'd posit these points (without making any assertions about the probabilities of the scenarios).

There is a flaw in the logic arising from the categorical assertion that the 'aircraft on ground' signal 'would have been correct' during the take off roll: "Regardless of whether the ‘aircraft on ground’ signal was incorrect after rotation it would have been correct during the takeoff roll. IF there was an unrevealed fault in a thrust lever position signal THEN why didn’t TCMA activate during taxiing or the takeoff roll?"

What IF the 'aircraft on ground' signal into the TCMA systems was INcorrect during taxi and the take off roll, thus disabling the TCMA functionality during that phase of the flight, making no difference in any event because the engines were operating normally in accordance with the thrust lever settings? In that case, any error in, for example, the thrust lever signals and engine signals to the TCMAs would not have had any consequence. Maybe the signal reversed and stayed INcorrect after take-off. Of course, that's why we are all craving to find out from what source/s the TCMAs get the 'on ground/in air' input/s and what other systems use the same source/s.

And I reiterate the point that the TCMA is "just software". I haven't seen anyone dispute the suggestion that the thing 'common' to all 4 channels of the TCMA is 4 copies of software.

In the earlier thread there was a statement about the 'obsession' of some with TCMA. I'm not 'obsessed' with it, but confess a prejudice towards trying to find a cause that is not a result of crew error. I therefore also have an attraction towards fuel contamination, but have difficulty in believing that fuel contamination would result in such a 'clean', immediate and simultaneous reduction in thrust from both engines after they'd operated normally during the take off roll.

I found descriptions on the systems of the 787 were easily discovered online, and while I have no hands-on experience of aircraft related matters, I do have experience in wider electrical theory and maintenance.

THRUST Asymmetry PROTECTION.
"For an engine-out condition, Thrust Asymmetry Protection (TAP) reduces thrust on the operating engine to ensure there is sufficient rudder for directional control. TAP reduces thrust when the airspeed decreases below approximately V2 on a takeoff or below approximately VREF on a go-around. Once speed is increased above V2/VREF, TAP increases thrust."

From what we know so far, it does seem the engines were not producing sufficient thrust, during a period when it would also be crucial to maintain electrical output for the electro-hydraulic systems and critical electrical loads.

Reduced electrical output could explain the failure of the gear to complete retraction, maybe caused by a generator failing at the worst possible moment.

If there was an EFATO, the ability of the remaining generators to provide sufficient power might become questionable, as is highlighted with the load shedding system.

Other features which are unique to the 787 could be contributing factors in explaining the accident.

It is known the 787 will generally employ an extended take-off roll, and a relatively higher V1 and Vr, and also climb out less steeply than other aircraft. Using more of the runway would reduce the margin for aborted take offs.

With the evident lack of thrust early in the climb out, and failure to retract the gear, if V2 had not been maintained, the TAP system would have reduced thrust even further. Manually increasing thrust will be inhibited.

I’m a 767 Pilot, so I’m not sure if this applies exactly to the 787. Referring to Loose Rivets post mentioning a longer than usual takeoff run, perhaps a scenario I’ve seen in simulator sessions may have occurred. On the 767 we have a Thrust Mode Selection Panel (TMSP). To demonstrate a potential error that could occur, an Instructor would select “ Climb thrust” on the TMSP instead of a takeoff thrust setting prior to takeoff. This can inadvertantly happen. In any event, if the crew mistakenly took off in a Climb thrust instead of a takeoff thrust, the takeoff roll will be much longer. With the high temperature conditions that day, the climb out would have been anemic, possibly prompting the crew to believe there was an engine thrust issue. In the simulators, when this issue was demonstrated, most crews didn’t realize what was happening and continued the takeoff anyway resulting in tail strikes, and or very sluggish climb performance. There are protections to avoid this, but the insidious nature of this scenario has caught many a crew.

None of the above makes any sense. There are excellent videos of the takeoff. The aircraft was rock solid in yaw. The takeoff roll and rotation appear normal. Had the flaps been up they would have had a tail strike before getting airborne. The videos show a normal takeoff roll, normal rotation and normal initial climb. At some point below 200 feet AGL all thrust is lost. Here is a video of what a single engine failure looks like before reaching acceleration height. Note also how the wing looks when it reaches about the same distance the Air India video is shot from. At flaps five they appear up.