At acceleration altitude (800') speed on MCP set to 211 knots. MCP speed, PFD speed target and magenta bug all at 211. Stil in TO/GA, so following the pitch bars gives 231 knots.

Ok. Now, do you agree during the first climb portion before acceleration, speed bug was at 166, and MCP speed at 146? Do you remember what happened at the first "click" or change that you did to the MCP window? Say you increased to 147 knots in the MCP speed window, did the speed bug (which was at 166) jump back to 147 by that time? Also, and maybe I have a slight misunderstanding of the system here, is it correct that FD would still command selected speed (not V₂ anymore!) plus 20 at any time TOGA is the active pitch mode? It's interesting that you say all three of PFD, speed tape bug and MCP read 211 (which is then different behavior than what I see), while the FD bars STILL command plus 20 of now selected speed (231 in your case).I would have assumed once you manually alter the speed, the FD bars would command selected speed under any circumstances and not continue to add 20 knots to the selected speed. Sorry I feel like I start to nitpick here, but now that I got into it I'd actually like to track it down.

Guys, We had this TO/GA + MCP speed behavior tested on the real airplane (yes, real in this case, not a sim) - this is what happens (it's wrong in the original NGX release): - TO/GA is a *flight director* mode. It is not an auto-throttle mode or anything like that. The vertical FD command bar pitches to maintain V2+20 when in TO/GA mode - period. (technically it's MCP+20 at the time TO/GA is pressed - this is why you put V2 into the speed window in preflight) MCP and speed bug show V2 during this if left untouched. - If you click the MCP speed window up one click, it causes the window and the bug to jump to V2+20 and then up normally from there. It is not possible to have the MCP and bug showing two different values - this was main error before. It does NOT cause the FD bar to command a higher airspeed you might dial in past V2+20 though as long as TO/GA is still the active mode. You MUST engage another mode first before it will do this. LVL CHG or VNAV are what are allowed in most airline SOPs - you'd never use V/S because it doesn't protect airspeed. (VNAV won't allow you to select speed unless you use SPD INTV obviously - it blanks the MCP speed window and climbs at the FMC calculated ECON speed) This is how it acts in the NGX in the current build I have, but I'm not sure if it's made it to the public build yet - there's a new hotfix coming soon that'll have the logic correct for sure.

This is how it acts in the NGX in the current build I have, but I'm not sure if it's made it to the public build yet - there's a new hotfix coming soon that'll have the logic correct for sure.

OK, no point then spending more time on the way the current build behaves. Looking forward to the update

- TO/GA is a *flight director* mode. It is not an auto-throttle mode or anything like that. The vertical FD command bar pitches to maintain V2+20 when in TO/GA mode - period. (technically it's MCP+20 at the time TO/GA is pressed - this is why you put V2 into the speed window in preflight) MCP and speed bug show V2 during this if left untouched.

Correct. It all stays at V2 until liftoff, where the bug will jump to +20 with everything else unchanged.

- If you click the MCP speed window up one click, it causes the window and the bug to jump to V2+20 and then up normally from there. It does NOT cause the FD bar to command a higher airspeed you might dial in past V2+20 though as long as TO/GA is still the active mode. You MUST engage another mode first before it will do this. LVL CHG or VNAV are what are allowed in most airline SOPs - you'd never use V/S because it doesn't protect airspeed. (VNAV won't allow you to select speed unless you use SPD INTV obviously - it blanks the MCP speed window and climbs at the FMC calculated ECON speed)

Ok, then this behavior is not reflected on my build here. The issue wasn't so much about what the FD commands, but rather about the ongoing discrepancy between the speed bug on the tape and the MCP and PFD readouts, even when you change the setting on the MCP. V/S was only an example here, just for the sake of leaving TOGA as the active pitch mode. And doing so would in fact resynchronize MCP speed, PFD readout and speed bug.

This is how it acts in the NGX in the current build I have, but I'm not sure if it's made it to the public build yet - there's a new hotfix coming soon that'll have the logic correct for sure.

Ok, this could of course be a reason. I will likely have a way older build than what you have. Luckily enough the ticket system is so slow right now that filing the ticket has probably not even succeeded, so one less thing for you to worry about... I will check it out in the SP. BTW Just realized how I named the thread... MCP Speed "Bug"... quite ambiguous in that case. I really meant there was a bug with the MCP speed, not the speed bug... LOL

Edited September 21, 201114 yr by badderjet

Well, another one regarding speeds, speed bug and FD commands. Should an engine fail during takeoff, the commanded speed after liftoff should be V₂ if speed was <V₂, current speed if V₂<current speed<V₂+20, or V₂+20 if speed was >V₂+20, as per FCOM Vol II 4.20.12. Right now, as I observe it, speed will first of all go to V₂+20 just as if the engine hadn't failed at all. Shortly after that (last time I checked around 100' RA) it will snap back to V₂, no matter what the actual speed was. Now to be honest the FCOM is again a little short on information, as it only describes what the FD bars will command, not if the speed bug should react accordingly as it would with both engines running. So I wouldn't know if the speed bug would reflect what the FD commands, but if you strictly follow the FD, you'll attain V₂ and not a single knot more no matter what your speed was before. I tried all three conditions and I get the same results. Speed snaps ultimately to V₂, and both FD and speed bug exhibit this behavior. Has this been reported or confirmed? As always, any thoughts are appreciated.

Etienne,I think what you're seeing there is the first part of the FCOM statement on how it works:"The pitch command continues at 15 degrees until sufficient climb rate is acquired. Pitch then commands MCP speed (normally V2) plus 20 knots." THEN "If an engine failure occurs during takeoff, the pitch command target speed is:•V2, if airspeed is below V2•existing speed, if airspeed is between V2 and V2 + 20•V2 + 20, if airspeed is above V2 + 20." The speed bug should never move off V2 without manually moving it or selecting an AP mode - that's fixed in SP1.

So as it turns out, I was correct !! Your MCP speed will not automatically change to V2 + 20 while you are still in TO/GA !!!

I never said it would. It actually never will. The original issue was a continuous discrepancy when resetting the MCP, but luckily this has been confirmed fixed already, read above.

I think what you're seeing there is the first part of the FCOM statement on how it works: "The pitch command continues at 15 degrees until sufficient climb rate is acquired. Pitch then commands MCP speed (normally V2) plus 20 knots." THEN "If an engine failure occurs during takeoff, the pitch command target speed is:•V2, if airspeed is below V2•existing speed, if airspeed is between V2 and V2 + 20•V2 + 20, if airspeed is above V2 + 20."

Thanks for getting back so quick. You have a point there with the first part, definitely seems to be correct about the very first phase after liftoff. On a side note though, is that "sufficient climb rate" a fixed number? Does it depend on anything? Currently however, I still don't see the FD command any other speed than V₂ with an engine out condition, even after "sufficient climb rate" is acquired. I just reflew the whole thing (with engine out) and the speed bug first went to V₂+20 as mentioned above (climb rate not yet sufficient), and I maintained exactly V₂+10 just for trying it out. This time however I was observing vertical speed, and at around 1000 fpm this very condition seems to have been met, and the speed bug jumped back to V₂ and FD commanded that speed. As per FCOM I would expect current speed to be commanded, as explained above. Again, might be picky here, one more very specific question about all that awkward "IAS<V₂ / V₂<IAS<V₂+20 / V₂+20<IAS" condition just to get this 100% straight for me: When exactly is this condition evaluated in the engine out case? If I understand it correctly, I would say at the very point where the "sufficient climb rate" is attained - right? So in my case, flying V₂+10 at the point where sufficient climb rate occurs, it should command V₂+10...!?

The speed bug should never move off V2 without manually moving it or selecting an AP mode - that's fixed in SP1.

Ok, so if I get this right, it SHOULD be like this in any case, with or without engine failure: PFD readout, speed bug and MCP speed STAY AT V₂ no matter what after liftoff. ONLY the FD will command V₂ (for the all-engine case) or evaluate current speed for the above condition in the engine out case - correct? Of course I'm looking forward to the SP to see if there will be any differences to what I see right now.

One last thing related to the TOGA stuff (for now, har har). Just noticed the FD popup mode doesn't seem to be modelled. Or I'm too dumb to try it out correctly, so anyways this is what I have done: Perform preflight duties, but switch off FDs (switches physically OFF). Take off manually. After liftoff, now don't wait until the 150 seconds are over or you have climbed above the 2000 feet, hit TOGA. To my understanding this should have brought up the FD bars, but nothing happens. If I'm doing anything wrong let me know. Again, tiny detail here, but because it has been brought up way before release by one of the tech team guys I thought it would have made it in, that's why I'm asking.

PRE HF4==============================================================POST HF4 Alright, this is about the hotfixed version now. Speed bug doesn't go to V₂+20 anymore, correct. The first time it changes is when either engaging AP out of TOGA pitch mode or engaging VNAV or similar, hence leaving TOGA as well. However, the engine failure behavior still seems odd. Or I'm testing it wrong, as always. To me it looks like FD commands V₂ regardless of your current speed. To further investigate this I'd actually like to know WHEN exactly the AFDS checks for your current speed in order to determine if it should command V₂, V₂+20 or any speed in between should you fly within that range. For testing purposes I programmed in a V₁ cut, then accelerated way past V₂+20 (on ground) to make sure it takes my high speed (in that case it should have commanded V₂+20 sooner or later), but it would again command V₂ only.

It looks like it checks the airspeed the moment the engine fails, which in your test case is below V2. Hence the command speed will be V2.

Then the whole concept doesn't make any sense at all. After all you are flying AFTER liftoff (learn something new every day ), so what speed the engine failure was when still on ground seems pretty much pointless to me. And failing the engine after takeoff is most likely beyond the scope of the intended scenarios. After all this is about what speed the FD commands after liftoff, so what happens after that can't be of any importance, I would think. But just to be sure, I tried it out the hard way. Stayed on ground veeery long and failed the engine past V₂+20. Guess what, FD commanded V₂. Go figure. LOL

And failing the engine after takeoff is most likely beyond the scope of the intended scenarios. After all this is about what speed the FD commands after liftoff, so what happens after that can't be of any importance, I would think.

This is a favorite trick of instructors during sim training: Hammer the students with V1 Cuts and just when they think they've got a normal takeoff, fail the engine at 100 feet. I believe the F/Ds will do the same thing as during a V1 Cut.

Alright, so:Takeoff normally. After liftoff, FD commands 15° until positive rate of climb is attained. Then FD commands V₂+20 (all engine case). Engine fails at 100R.Now, are you saying at this point the AFDS evaluates your current speed? So if you fly V₂+10 at this very point of engine failure, at ~100R, FD should command V₂+10? There needs to be a condition somewhere that will decide if current speed will be evaluated or not. Could be tied to TOGA, for example. But then this would work as long as TOGA is active. If you have a flap retraction or acceleration at, say, 1500 AFE or higher, your engine could also fail at 1450 and the speed should be evaluated that way.

per Boeing's FCTM:

If an engine fails at an airspeed between V2 and V2 + 20, climb at the airspeed at which the failure occurred. If engine failure occurs above V2 + 20, increase pitch to reduce airspeed to V2 + 20 and maintain V2 + 20 until reaching acceleration height.

I guess I've been more worried about coordination and track to worry about exactly what speed I was given by the F/D. To me it sounds like it looks at the speed when your engine has failed and then follows the rules we cited. However, I think I'm usually climbing at V2+20, but I'm not sure. I guess I need to do some more raw data flying.

Thanks for the hint. I should admit I was a bit focused on the FCOM Vol II 4.20.12 bottom of page part, and of course the observations in the sim. The FCOM states a bit vaguely:

if an engine failure occurs during takeoff, the pitch command target speed is: [... conditions]

What "during takeoff" means to AFDS isn't really said there. Sure enough, this is (again) more a theoretical discussion. Just because I'm a nerd, and I want to know the exact conditions, in case they can be given. But since someone coded this thing, that should be the case. And after all, we should expect this bird to behave as per the manuals, so I suspect any such discussion is valid.