Trim after AP disconnect

[jamarns]

So I've had a problem occur three or four times now, I'm approaching the runway and I disconnect autopilot to fly a manual approach, but whenever I try to trim the pitch down the pitch automatically pitches back up straight away, if I try to pitch down using the yoke it will also adjust the trim. It basically won't let my descend, I have even gone as far as to turn off FD and AT so there should be no automated systems running but it tries to keep me level. Is this a bug or am I doing something wrong here?

[mmerelles]

what you are experiencing is not normal. It seems the systems are still engaged.

How do you disengage AP? You should use a "custom command" for that, default xplane toggle will not work.

 

It could be very useful if you can attach some screen captures of your front panel and instruments while the problem occurs to see what systems are still engaged and why the aircraft refuses to descend.

[jamarns]

I normally disconnect using the disengage switch on the MCP, I will try to recreate the problem and get some video footage tomorrow as it's quite late here now.

[Jaessr666]

The disconnect button on the mcp is not for disengagement of the autopilot in normal operation. It's for emergency in case there is a failure. You disengage the autopilot via the knob on the left handle on the yoke!!

[Litjan]

On 26.8.2016 at 8:11 AM, jamarns said:

I normally disconnect using the disengage switch on the MCP, I will try to recreate the problem and get some video footage tomorrow as it's quite late here now.

Hi jamarns, I would be very interested to see a short video of this happening - even if you use the AP disengage bar on the MCP, the plane should be totally controllable. The only instance that you would see trimming while the AP is off would be when the speed trimming system kicks in (only during flaps not up and high power). But it should never render the aircraft uncontrollable.

Cheers, Jan

 

[jamarns]

I have tried to recreate this problem several times now to no avail (probably because the camera is running!)

Whilst I do understand now that the proper way to disengage autopilot is using the button on the yoke, as Jan mentioned above that still should not stop the auto trim from disengaging.

 

I've also run into another problem where the AT stops functioning during flight (in this case also on approach) the AT switch and light were on, and I had LNAV and VNAV engaged but I was able to move the throttle up and down (the indicator for hardware throttle and actual throttle position did not appear), I run very minimal plugins with X-Plane and I'm starting to think that I may have an issue with my IXEG install. Incidentally, I was able to fix the AT problem by disengaging everything on the MCP and re-engaging, once I did this AT operated as expected.

 

I'm going to re download the aircraft and see if any of these problems continue to occur.

[mmerelles]

2 hours ago, jamarns said:

I have tried to recreate this problem several times now to no avail (probably because the camera is running!)

Whilst I do understand now that the proper way to disengage autopilot is using the button on the yoke, as Jan mentioned above that still should not stop the auto trim from disengaging.

 

I've also run into another problem where the AT stops functioning during flight (in this case also on approach) the AT switch and light were on, and I had LNAV and VNAV engaged but I was able to move the throttle up and down (the indicator for hardware throttle and actual throttle position did not appear), I run very minimal plugins with X-Plane and I'm starting to think that I may have an issue with my IXEG install. Incidentally, I was able to fix the AT problem by disengaging everything on the MCP and re-engaging, once I did this AT operated as expected.

 

I'm going to re download the aircraft and see if any of these problems continue to occur.

 

A/P disconnect:

The problem using the yoke button is you have to rotate the camera down while you are aligned to the runway for landing and this could be distracting. On real life this is not a problem, because you are feeling the button with your thumb, no need to look at it for pushing it

My advice would be you map any button of your yoke (if you own one), or any key on the keyboard of your liking to the A/P disconnect button. So you can mimic you are pressing the yoke button but actually without rotating the camera to clic it with the mouse.

If you want to test it, map a button or key to "ixeg/733/autopilot/AP_disengage". Same as on the yoke you need 2 clics to disengage.

 

A/T:

I am not sure being understanding your issue. When you are approaching to land, you want to move your hardware lever to regain throttle control but it ignores you?

You need to clic A/T DISC first on the throttle levers, then you need to move your hardware lever to sync them with the aircraft lever and the aircraft will go to manual throttle mode.

You can also map a key or button for throttle disconnect. You have to map "ixeg/733/autopilot/at_disengage"

 

hope this helps

[jamarns]

Yes I understand this, and I do now have a button on my joystick mapped to AP disengage, but for the purposes of reproducing the bug I have been using the disconnect on the MCP to try capture the issue I was having on video.

Also the AT issue I am having is the opposite of what you describe and I'll link a video to it. Basically the AT is not working, but it's switched on on the MCP and AP and VNAV are engaged, the AT had been working normally up to this point.

 

[mmerelles]

36 minutes ago, jamarns said:

Yes I understand this, and I do now have a button on my joystick mapped to AP disengage, but for the purposes of reproducing the bug I have been using the disconnect on the MCP to try capture the issue I was having on video.

Also the AT issue I am having is the opposite of what you describe and I'll link a video to it. Basically the AT is not working, but it's switched on on the MCP and AP and VNAV are engaged, the AT had been working normally up to this point.

 

ok i see. This is my understanding of the situation.

 

1. You are in VNAV PATH mode, you are also experiencing tailwind which will push the aircraft forward more than anticipated for the FMC calculated descend, this means the aircraft will be prone to overspeeding during descend. You as the pilot in command should account for this situation and do proper planning. Maybe switching to VS or FLCH.

 

2. As soon as your video starts you are already on A/T ARM (armed) mode, look at the display were the artificial horizon is, top left corner of the display. It says ARM in white letters.

This means the aircraft is overspeeding the target speed (your target speed is 210 knots and you are 233 knots), A/T goes fully retarded and enters ARM mode awaiting for the aircraft to reduce speed for applying throttle back automatically when required to maintain target speed. While the aircraft is in retarded-arm mode it will not apply more thrust by itself, it makes things even worst! Quite smart system.

 

3. At 00:53 you decide to turn off the rebel A/T entirely by flipping the A/T switch, then you reengage A/T then you re-engage VNAV

But if you look closely your displays, you engage everything back A/T + VNAV and A/T goes FMC SPEED MODE, but immediately it senses you are overspeeding the target speed and A/T goes back to ARM mode, video ends. So, you are basically back to the video starting point

 

I am unsure what you are trying to do here, the aircraft is behaving properly.

-The aircraft will not command more thrust, you are overspeeding already

-You may switch to FLCH or VS and command thrust manually or via SPEED dial.

 

hope this helps

[Litjan]

mmerelles is correct, the aircraft is behaving as intended.

While you are in a VNAV PTH descent, the correct behaviour for the A/T is to go to RETARD first, then to revert to ARM. While in RETARD, you can not grab control of the throttles, only while in ARM.

The intention is for the plane to have idle thrust and glide down the pre-calculated path.

The autothrottle is "watching" the speed, though - if you are too fast, you will eventually get the "DRAG REQUIRED" message, if you get too slow, the autothrottle will revert to FMC SPD and add thrust until you are back on speed - then the cycle will go RETARD-ARM again.

Jan

 

[jamarns]

Ok, thanks, I obviously misunderstood how the AT works, I'm still trying to reproduce the bug I actually originally posted about. If I can reproduce it I will post a video in here.

Thankyou.

[Litjan]

3 minutes ago, jamarns said:

Ok, thanks, I obviously misunderstood how the AT works, I'm still trying to reproduce the bug I actually originally posted about. If I can reproduce it I will post a video in here.

Thankyou.

 

Excellent - and don´t worry, the "combined modes" of A/T and AP pitch channel are probably the most complex systems in the 737.

Still looking forward to clearing up that original bug - keep me posted!

Cheers, Jan

 

Edited September 6, 2016 by Litjan