Posts by user "nachtmusak" [Posts: 35 Total up-votes: 0 Pages: 2]

Hello,

Would anyone happen to know (or to know where I could find) the drag coefficients of a 787 in various configurations? I've seen some numbers and charts floating around the internet, but I don't know if I can trust them.

For context I am trying to plot various possible flight paths (well, essentially ballistic trajectories), in hopes of pinpointing if/when the aircraft lost thrust

Non-aviation engineer here: I have a question about the low level altitude capture theory that I've been a bit hesitant to ask, since no-one else seemed to be bringing it up.

My understanding of altitude capture is that the autopilot will automatically adjust both thrust and pitch to intercept the requested altitude. However to my eyes there is very little pitch adjustment in the CCTV video of the plane taken from behind, until the very end of the video when it pitches up somewhat (obscured by buildings, more visible in the smartphone video). Please correct me if I'm wrong but I'd have thought that if the autopilot was trying to capture a very low altitude it would start pitching down (quite noticeably!) to do so, not remain at what looks like 10+ degrees nose-up. I honestly struggle to reconcile what I'm looking at in the video with an attempt to level off at 0ft, 200ft, or any of the other mentioned low-level altitudes.

Also maybe I'm missing something 787-specific but generally doesn't the autopilot have to be activated for the aircraft to automatically attempt to capture the pre-selected altitude? That was the case in this incident involving a Dash 8 and a target altitude of 0 feet that I am reading about ( https://www.gov.uk/aaib-reports/aaib...-dash-8-g-ecoe ). I'd have thought the PF would still be hand-flying the departure at the point that things went wrong, considering that the gear hadn't even been retracted yet...

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...

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 can't speak to the glide ratio but the other numbers are rather off.

For example the distance between the last received ADS-B point (23.066541\xb0N, 72.623189\xb0E) and the impact site (23.055547\xb0N ,72.612406\xb0E) comes out to 1.6467km (~5403 ft) using the Haversine formula. That's significantly less than 2km (~6562 ft).

The aircraft also keeps climbing past the last received ADS-B data point, and visibly attains more than both 100 ft and 200 ft in height above the ground (easily judged by the wingspan of 197 ft), though not by much in the latter case.

How does a non-faulty autothrottle pull power to idle at the critical time we are witnessing (i.e. just before or just after takeoff)?

I don't mean to keep asking this, but I still haven't gotten an answer. Unless the allegation is that the crew did not arm the autothrottle properly, it just doesn't make sense to me.

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?

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 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.

Disclaimer: I'm not a pilot, only a non-aviation engineer, and so I'm a bit hesitant to criticise this. And yet:

The determination of the height AGL seems quite incorrect - it completely ignores the local pressure and temperature, which very much need to be corrected for. Applying those corrections even using rough rules of thumb (~30 ft per mbar, ~4 feet per \xb0C per 1000 ft) gives a figure that's around ~100 feet AGL. The follow-up sanity check also fails even without knowing the correct math because if you match the ADS-B data (timestamp + location) to when & where the aircraft lifts off and starts to climb in the CCTV video (versus just assuming that the peak height seen in the video matches the last ADS-B data point), the aircraft is very much not 300 feet above the ground when its transponder reports an altitude of 625 feet.

Also the estimation of the glide ratio with flaps 5, gear and the RAT deployed being 3.5 to 1 seemed incredibly low to me, but I'm the first to admit that I have nowhere near as much knowledge of gliding performance as I do of weather math. So I looked up the closest incident I could think of: Air Transat 236, an A330-200 (so of a pretty similar shape, wingspan and wing area as a 787). According to the final report (link: https://www.fss.aero/accident-report...1-08-24-PT.pdf ), the aircraft arrived at a fix approximately 8 nautical miles (48609 ft) from the runway at an altitude of approximately 13000 ft, at which point the crew decided to execute a 360 turn to lose altitude as well as to extend the slats and landing gear during the turn (the RAT of course had already long been deployed at this point), both to prepare for landing and to help further lose altitude. Sure there's some rounding here, and my understanding is that "flaps 1" on Airbus aircraft deploys only leading edge and not trailing edge devices, but this already suggests that their expected glide ratio was significantly higher than the raw 48609:13000 ratio (~3.74:1).

They re-established themselves on final in their landing configuration at an altitude of ~8000 feet and a distance of 9 nautical miles (54685 ft), so let's say that the true distance was somewhere between 8 NM and 9 NM to account for rounding. That gives a glide ratio of between ~6:1 and ~6.83:1. But on top of that, the crew still had to execute a series of S-turns to lose enough altitude to actually make the runway, so their "dirty" glide ratio must have been even higher than that. Unfortunately I don't think it's possible to determine conclusively what the ratio was since we don't know how many track miles were added on by the turns (the flight data recorder stopped when the engines flamed out and human testimony only goes so far), and again they did not have flaps extended, but I think it's fair to say that a glide ratio of 3.5:1 is a wildly low estimate for an airliner of the 787's calibre even with the gear down.

Sorry if this is off-topic or too much rambling, but considering how much speculation there tends to be both in this thread and elsewhere about real-world glide performance (especially in non-ideal configurations), hopefully these details are helpful.

There is easily-correctable available data with the aircraft's altitude at pretty much the end of the runway and it is not at 200 feet (it's around 100\xb112.5 feet).

As the aircraft visibly continues to climb past that height (and for a longer period than ADS-B data covers, if the camera's perspective casts doubt on that), it seems rather clear to me that it reached its peak height past the end of the runway.

In light of this I find the fact that people keep calculating a glide from the runway to the crash site to be a bit strange. Wouldn't the first step of any math be to try to determine where it started descending?

SLF: I have a small observation but I'm not sure what it means or if it has any relevance to the accident.

Previously I had assumed that the ADS-B data cut out at the same time as power was lost, so I imagined that whatever caused the fairly clear loss of thrust would have happened not too long before. But this report throws a bit of a wrench in my understanding of that.

According to the report, the fuel cutoff switches transition from RUN to CUTOFF at or very shortly after 08:08:42 UTC. Both engines' N2 values pass below minimum idle speed and the RAT begins supplying hydraulic power at about 08:08:47. Does this not imply that the generators have already been lost? With the APU also being off (the APU inlet door is noted to start opening at 08:08:54), I would have expected ADS-B data to cut out at or before 08:08:47. But curiously FlightRadar24 at least claims to have received data frames from the aircraft until 08:08:51.640970, almost five seconds later and almost ten seconds after the transition to CUTOFF (though the last frame containing coordinates comes at 08:08:50.871005).

Could anyone with relevant experience confirm how long it would take for AC power to be lost in this situation? Also, is it usual/unusual for a preliminary report like this to mention if/when the flight recorder switched to its independent power supply? I imagine it would definitely be in the final report, but I'd hoped it would be easily observable enough to be in this one.

Beyond idle curiosity I'm asking because the report also says the no. 1 engine's cutoff switch transitioned from CUTOFF to RUN at "about 08:08:52", which oddly coincides with the last ADS-B data frame at 08:08:51.640970, and that seems important somehow. Or more likely I'm just ignorant of some quirk of the 787's electrical system.

For reference FR24's CSV containing all ADS-B frames supposedly received from the aircraft can be found in their post here: https://www.flightradar24.com/blog/f...rom-ahmedabad/

I don't think this answers my question? Evidently there is an interruption in power, the (quite stable, at that point) ADS-B stream cuts out and doesn't return for the final twenty seconds of flight. And there are other indications that non-essential loads were shed, like the survivor remembering what must have been emergency exit lighting in the cabin. Though I would have liked explicit confirmation of AC power loss happening or not in the report.

The timing of data loss seemingly lining up with the first cutoff switch being flipped back to RUN seems a little too coincidental to me, but again hopefully someone with relevant knowledge has a mundane explanation for this.

Regardless of what actually happened in that cockpit, reasoning like this seems completely backwards to me. Since when do humans reliably, instantly recognise that they've just made a mistake? If they were that attuned to their actions they're almost certainly attuned enough to not make the mistake in the first place, especially if we're talking about an action slip. I've e.g. absentmindedly added salt to my tea instead of sugar and I certainly didn't immediately realise that I'd done that - in fact it took a good few seconds even after sipping and spitting it out for my brain to catch up to what must have happened. Even if an observer had pointed out to me before drinking it that my tea had salt in it, I would just have been confused because obviously I wouldn't do something that silly (spoiler alert: I did).

On top of that I feel like people are overestimating how long ten seconds actually is, especially considering some of those seconds are reported to have been taken up by confused dialogue (that isn't even reported in its entirety). I thought it was common wisdom that accidents are never down to one thing; it would be light-years from being the first time that suboptimal crew response turned a maybe-recoverable error into a definite disaster without an ounce of malice in the mix.

It may not be my place to say this, but it's been confusing and more than a little shocking to see professional pilots so quick to ascribe criminal intent to one of their colleagues.

Of course deliberate pilot sabotage has occurred in the past, nobody is disputing that. But personally I'm aware of far more cases of pilot mistakes without malice as the root cause of an accident than of all the confirmed and possible cases of sabotage put together, and I'm sure that there are even more cases of the former that I've never heard about (and I don't mean pilot error in general, I'm referring to things like e.g. taking off with an improper configuration).

I think people are not actually thinking through how the situation would play out IF it was an honest mistake. Ten seconds is no time at all for either pilot to:

- notice the degrading performance (and/or warnings)
- scan the instruments and controls for the problem
- see (on their display and confirm on the pedestal) that fuel has been cut off
- ask the other pilot why they did that (because neither pilot would believe they were the one who did so - that is how action slips work)
- get a response that they did not (again, see above)
- snap out of confusion and actually do something about the situation

Pilots have reacted with far less alacrity in plenty of accidents (even in cases where the day was ultimately saved) and it was not my impression that the aviation industry accused them of criminal intent for it. Surely there is a middle ground between robotic hyper-competence and literal murder? Don't get me wrong, there is a solid chance that it turns out to have been the deliberate murder of hundreds of people. But to me at least it seems extremely uncharitable to confidently declare that that's what happened off a very loose timeline, or to paint people who are considering the possibility of a mistake as just hiding from the truth.