Autothrottle Re-engages When Selecting VNAV and Arm Switch Off?

[lbarber 20]

Situation:
Flying manually with A/T disengaged (A/T ARM off), FDs on in HDG SEL/ALT, SPD window open to appropriate bug speed. Setting up for an RNAV approach.

I want to get VNAV PTH guidance on the approach, so I select VNAV on the MCP. Immediately A/T goes into SPD, window blanks, and target speed increases to DES.

The first time I noticed this I was flying level at bug speed, when the A/T kicked in back to 240k it really upset the apple cart.

Per FCOM 4.10.3, with the A/T Switch OFF, A/T activation is disabled. Is this something the real bird does?

[scandinavian13 5,192]

18 minutes ago, lbarber said:

Per FCOM 4.10.3, with the A/T Switch OFF, A/T activation is disabled. Is this something the real bird does?

From your pictures, it looks like only the FMA showing what it would be doing if AT were on, but I can't tell exactly because I can't see any evidence that your throttles are being commanded.

Did you let the speed bleed off excessively in the level out? If so, I believe the AT will wake up to save your [hind quarters] from stalling an aircraft that very much does not like to be stalled.

[lbarber 20]

13 minutes ago, scandinavian13 said:

From your pictures, it looks like only the FMA showing what it would be doing if AT were on, but I can't tell exactly because I can't see any evidence that your throttles are being commanded.

Did you let the speed bleed off excessively in the level out? If so, I believe the AT will wake up to save your [hind quarters] from stalling an aircraft that very much does not like to be stalled.

Screenshots were taken about 10 seconds apart, the first shot shows the conditions right before I hit VNAV with speed stable at the 5 bug. The engines respond as would be expected in the mode shown on the FMA unless the A/T is disconnected again (in this case I hit the A/T DISC right after the second shot was taken and carried on my merry way).

As far as I can tell, there isn't any envelope protection or A/T wakeup function with that switch off on the real thing, but it also certainly wouldn't be unheard of for this to be an undocumented "feature."

[scandinavian13 5,192]

30 minutes ago, lbarber said:

Screenshots were taken about 10 seconds apart, the first shot shows the conditions right before I hit VNAV with speed stable at the 5 bug. The engines respond as would be expected in the mode shown on the FMA unless the A/T is disconnected again (in this case I hit the A/T DISC right after the second shot was taken and carried on my merry way).

As far as I can tell, there isn't any envelope protection or A/T wakeup function with that switch off on the real thing, but it also certainly wouldn't be unheard of for this to be an undocumented "feature."

With the switch down like that, all power should be removed from AT, now that I think about it. Mind submitting a ticket? It'll get someone on it faster than it would if I were to do it when I get home. Link's in my sig.

[lbarber 20]

Because I have the day off and am bored, here's another set of screenshots.

Same deal, set up for the RNAV 22 at ROC, this time in CMD because I'm lazy.

Hit VNAV and...

As a side note, with the AT disconnected but armed, it appears that it will not wake up for low speed protection. On downwind with HDG SEL and ALT on, I disconnected the A/T, reduced thrust to idle. Speed bled off into the yellow dots, then ALT got a yellow line through it and the aircraft started descending to keep the speed above stall. Throttles never budged.

[lbarber 20]

2 minutes ago, scandinavian13 said:

With the switch down like that, all power should be removed from AT, now that I think about it. Mind submitting a ticket? It'll get someone on it faster than it would if I were to do it when I get home. Link's in my sig.

Wilco.

[scandinavian13 5,192]

2 minutes ago, lbarber said:

Wilco.

Thanks!

[Qavion2 39]

Quote

As a side note, with the AT disconnected but armed, it appears that it will not wake up for low speed protection. On downwind with HDG SEL and ALT on, I disconnected the A/T, reduced thrust to idle. Speed bled off into the yellow dots, then ALT got a yellow line through it and the aircraft started descending to keep the speed above stall. Throttles never budged.

Speed protection appears to be optional in the real world (and in some desktop sims)

Quote

The low speed protection acts at [Min MAN Speed - 8kts]

It engages the SPD mode of the A/THR in the following conditions:
-A/THR in ARM and fully serviceable (Active or not)
-Above 100ft Gnd for Approach / Above 400ft Gnd after Takeoff
-The AFDS vertical mode FLCH SPD, VNAV SPD, TOGA are not the active modes

However... posted by a 744 pilot....

Quote

Just did a quick test of this in big sim.
Autopilot engaged, Alt HOLD, AUTOTHROTTLE armed but disconnected (mode blank). Thrust levers closed.
Full stall ensued with no wake-up.

Cheers

JHW

 

 

[skelsey 765]

6 hours ago, lbarber said:

I want to get VNAV PTH guidance on the approach, so I select VNAV on the MCP. Immediately A/T goes into SPD, window blanks, and target speed increases to DES.

Yup. This is a gotcha when conducting a VNAV approach from ALT mode.

Firstly: before we do anything, in order for the "On Approach logic" to activate, you must have selected the approach from the database (and not modified it) and there must be a glidepath angle coded in the procedure and displayed on the LEGS page. This you should check in the first instance.

Secondly (and particularly relevant to your situation): if you press VNAV direct from ALT mode, you will get exactly the behaviour you are seeing because at this point the FMC speed target comes in to play. The solution is, before you press VNAV (on the MCP), press the VNAV key on the FMC once. This should (usually) open the VNAV DES page, but on rare occasions it may be CRZ or even (very rarely) CLB. Either way, whichever page comes up when you press the FMC VNAV key once, you should overtype the speed in there with your current, sensible speed (i.e. 180kt or whatever you are using to manage the intermediate approach). Once you have done this and you are near the FAF and at or close to the FAF platform altitude, you can then press VNAV on the MCP and the thrust levers will not go anywhere. Hopefully you will get VNAV PTH (if you are at the correct altitude). You may get VNAV ALT, in which case if you are level above the FAF altitude you will need to press the ALT INTV button in order to commence your descent, though if you are level at the FAF altitude it will start down automatically.

In both cases you must select a lower altitude (i.e. MDA rounded down to the nearest 100ft) in the altsel otherwise you will go nowhere.

Once you are in VNAV PTH and inside the FAF you should now be in "On Approach" mode and can now open the speed window and control the speed through the MCP whilst remaining in VNAV PTH.

Once you have done this, all that is left is to set the missed approach altitude once you are at least 300ft below it.

Hope that helps (incidentally -- I agree with Kyle that if the A/T arming switch is off then you should not get any automatic thrust lever movement. However, the points about FMC commanded speed etc all still stand).