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| nachtmusak
2025-06-20T21:17:00 permalink Post: 11907329 |
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\x92 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. 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? |
| Shep69
2025-06-20T21:52:00 permalink Post: 11907348 |
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 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. Last edited by Shep69; 20th Jun 2025 at 22:05 . |
| nachtmusak
2025-06-20T22:24:00 permalink Post: 11907362 |
The autopilot would NOT be engaged below 400\x92 (or 200\x92 in the 78\x96although 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 \x97 and it`s been a few years). But think it probably would. As one goes through 50\x92 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\x97and 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\x97and 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 \x97 and they did not activate providing low speed protection in the case of Asiana. 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. 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. 1 user liked this post. |
| M.Mouse
2025-06-20T22:29:00 permalink Post: 11907367 |
he 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.
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.
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'
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.
2 users liked this post. |
| Shep69
2025-06-20T22:59:00 permalink Post: 11907385 |
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. 1 user liked this post. |