Posts about: "VNAV" [Posts: 45 Pages: 3]

Minimum autopilot engagement height on the B787 is 200'.

On the B787 if VNAV is engaged and the aircraft captures an altitude below the programmed acceleration height then the vertical mode transitions to VNAV ALT and commences acceleration to the current flap setting limit speed -5 knots.

Because they are in `hold` after the takeoff thrust is set by engaging the TOGA levers. VNAV will engage at 400` AGL and set them at THR REF — referencing takeoff or climb thrust as programmed.

From one of the previous scenarios if capture at a way low altitude occurred they`d (IIRC correctly) engage in SPD mode resulting in a significant loss of thrust as they attempted to maintain speed assuming a level off. Presumably after this when the PF continued a climb (regardless of flight director) they`d come forward after this loss of thrust to attempt to meet commanded speed—V2 or V2+15 or whatever—but there`d be a lag. But I`ve never misset the MCP to a way low altitude in flight or the sim (not a holier than thou thing) so I`m not entirely sure how how the autothrottles would behave.

More than one crew has failed to push them to the stops when things went south.

And the reverse is true; when the second MCAS accident happened IIRC the autothrottles stayed in THR REF due to task saturation by the crew. Exacerbating the condition and ultimately resulting in an accident.

My comment was in relation to the scenario being discussed: a low-level level-off because the altitude selector was set properly before takeoff. And in any case, that Air India jet didn't look like it had climb thrust on as it topped out and started descending; it was pretty much gliding.

Assuming then that VNAV in the 78 engages at 200` AGL vice the 400` of the 777 ?

Yes, we know it climbed. I suggested use of the granular data to show how high it was, in the context of the 400ft mode changeover point.

I don't have access to a current 787 manual, but have found a couple of unverified references to the VNAV engagement on the net. It appears the VNAV will engage at 400ft also. Happy to be corrected though.

Level-off point, approx 11sec after liftoff:
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On the B787 the VNAV, if armed, engages at 400'.

Hmmmmm\x85.then I wonder how the auto throttles will behave if a low stop height if the MCP is mis set \x97 potentially below 400\x92 AGL. Will they stay in hold, will they go to SPD, or will they engage in THR REF.

I never tried this in the sim on the 777 so I\x92m not sure. I was grateful that the 777 would change to SPD mode on initial altitude capture (as opposed to the -400 VMo here we go). It\x92s been a few years in any case though.

VNAV hasn\x92t engaged yet in that scenario. So do I get SPD LNAV ALT ?

Usually on takeoff LNAV and VNAV are armed. During the takeoff roll the autothrottle system goes in to HOLD mode at 80kts.

At that point the FMAs read:



At 50' LNAV engages and the FMAs change to:




At 400' VNAV engages and the FMAs change to:




The height is referenced to a barometric snapshot taken during the take off roll at 100kts.

If an altitude is captured before VNAV engagement (totally bizarre to capture an altitude of less than 400') then the FMAs would change to:

SPD | LNAV | ALT

Thanks much — pretty much same as 777. Was wondering if someone had set MCP to 200 in error what autothrottles would do. What I’m wondering is if the aircraft were being hand flown to a low capture like that if the autothrottles would initially pull power back from THR REF to anticipated level off setting for SPD which would be bug speed (V2 to V2+15). And might be a fairly dramatic reduction of power. Granted since its speed on autothrottles at some point power should come back up. But I wonder if the autothrottles would respond to what they thought for level flight initially.

Could explain the witnesses thought that power came back up before they hit but witness recollections post traumatic event as well as second hand reporting need some time to settle in before accurate facts come out.

The autopilot would not have been in and the pilot would have been hand flying maintaining the takeoff attitude. The flight director might have commanded a level off which obviously would not have been followed.

Minimum autopilot engagement altitude is 200\x92 for the 78 (from others on the forum). It\x92s 400\x92 AGL on the 777.

My interest is in how the auto throttles would have behaved in such a situation\x97where VNAV would not be engaged yet and the automatics gone into SPD/ALT. ; would they have pulled off thrust assuming a level off and then come back in to maintain bug speed (perhaps confusing the crew in the process ?).

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.

Dont know the 78 but presumably the AT commands HOLD at 60-80 kts and stays there until once above 400\x92 either a config change or flight guidance mode selection. If the FD commanded a level off below 400\x92 it\x92s been suggested that the AT would transition to SPD mode.

This is a daft question probably, but if the automation commanded a level off below 400\x92 and the AT transitioned to SPD\x85 would the logic have a brain fart and revert back to HOLD? I\x92m guessing not to be honest. I wonder if people have managed to try this in the SIM.

Not saying at all that is what happened here. Just interested in the logic.

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.

Reported 625' pressure altitude, elevation 189', QNH 1001.

So they only got to around 100' height, half the wing span of a Boeing 787? I think it looks higher than that.

189' elevation, QNH1001, the pressure altitude should be around 550'. First readout from ADS-B is 575', highest is 625'. I did not look up if runway departure end is much different there than aerodrome elevation.

Interesting that the readout of pressure altitude doesn't get lower again, after 625', which could support the total loss of all AC (and most DC power?). Transponder possibly stopped working, but one of the pilots was able to transmit a mayday call. So they must have had some kind of electricity available.

So what I mentioned about THR REF / VNAV SPD is probably not applicable. Yes, I did not account for the QNH initially, which makes closer to 100' than 400' AGL.

I will revert to await the official findings then, with a substantial loss of electricity seeming more and more plausible.

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).

I am specifically asking about the case without a malfunction because my understanding is that the autothrottle explicitly does not affect thrust between passing 80 knots on the runway and passing 400 ft AGL.

Neither does (or should?) any autopilot mode (except LNAV? at 50 feet?) be active under 400 feet. Armed yes but not actually engaged.

So how would either of these things have any effect on an aircraft that's just lifting off unless they're faulty? Everything I can turn up seems very clear that the autothrottle, VNAV, etc only engage at 400 feet AGL on departure. In fact, both altitude capture incidents I've been able to turn up were only triggered when the autopilot was engaged. Why would any pilot be engaging the autopilot bare seconds after lift-off?

There are some cases, type specific for example, where the conditions could preclude A/T actually being engaged for departure.

No idea of 787

There are other cases where maybe A/T is U/S

Not saying any of those things were relevant here. Just answering your question.

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’ 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.

And the autothrottles would initially be expected to reduce thrust to whatever value the squirrels think is appropriate to level flight. They WILL advance (how fast I don`t know) to maintain speed on auto throttle but this would be confusing. That`s the scenario I`m wondering about as well as a failure of the auththrottles themselves; it`s as plausible as anything else they engaged to reduce thrust for some wild reason.