ISSUE WITH PITCH TRIM DRIFTING ON ICLS APPROACH

[ML1872]

I keep finding in the later part of an ICLS approach that pitch trim moves from the datum set at a stable 11 degrees and steady on speed, down to 8 and up to as much as 15 with no trim or pitch input, towards the end of IMC appraoches.  Can anyone help addrss this issue? 

[Hulkbust44]

Well are you still on speed and flying fine? There is no trim position indicator in the cockpit (aside from rudder trim knob) so you are most likely seeing the stabilator position which should change as the FCS controls the aircraft, and not you.

Mobius708

[ML1872]

Mobius708 - Thanks for the reply. I am refering to the FCS sub-menu page - having stabilised on-speed with confortable RoD (700fpm), with little throttle input to change the RoD, the trim (STAB) then wanders significantly causing the speed to increase or decrease and the E bracket subsequently follows. In answer to your question - No, on-speed is not maintained, the speed increases or decreases depending on the pitch/stab runaway, which makes the approach very difficult to fly in IMC!

I have just flown a straight in approach using the ICLS in the Mission Qual CAT 1 mission and had no diffiuclty at all with the STAB.

Could this be a configuration issue - where the gear and flaps go down quite quickly which then causes the FCS to chase itself?

 

Any further thoughts?   

ML

[maxTRX]

17 minutes ago, ML1872 said:

Mobius708 - Thanks for the reply. I am refering to the FCS sub-menu page - having stabilised on-speed with confortable RoD (700fpm), with little throttle input to change the RoD, the trim (STAB) then wanders significantly causing the speed to increase or decrease and the E bracket subsequently follows. In answer to your question - No, on-speed is not maintained, the speed increases or decreases depending on the pitch/stab runaway, which makes the approach very difficult to fly in IMC!

I have just flown a straight in approach using the ICLS in the Mission Qual CAT 1 mission and had no diffiuclty at all with the STAB.

Could this be a configuration issue - where the gear and flaps go down quite quickly which then causes the FCS to chase itself?

 

Any further thoughts?   

ML

 

There is a thread in the bugs forum related to a very sensitive trim input. I only experienced this when I entered landing configuration with auto pilot on. (ATTH but it might be any of them causing problems).  It would manifest itself as soon as the flaps and gear are lowered, not in the final part of the approach, as you described.

Any turbulent air setup at the ground level?  Also, I hope you're not using the auto throttle.

[ML1872]

Thanks Mobius708. The thread in the bugs forum is describing a very similar experience to mine.  I only use auto throttle in the cruise. Yes, there is some turbulence, but a U tube video of the same approach dosent seem to have the same problem. I have downloaded the mission, where the problem occurs at the back end of the approach - but this has happened on other missions.  ML
 

 

[Swift.]

Would you mind posting a track of the issue, describing a problem like this is very challenging to do as everyone expects different nominals.

[CBStu]

I have had a similar problem on the end of some of the supplied Caucuses missions. I haven't gotten to where I understand the marshall stack etc so I just do a straight-in approach. Adjusting trim and throttle to get to proper aol and suddenly trim moves up and down the E bracket. Sometimes it is pretty stable but maybe at the bottom of the E. I quick hit and release the trim button expecting a slight upward movement of the indicator and bam, it goes way to the top of the E. Since I have no idea how DCS upgrades and FA-18 upgrades are applied to older missions I just figured it is the new 2.7 (I have only had for 2 weeks) working a mission that came w/ DCS when I first got it 3 yrs ago. Sorry I am no help other than to say that the OP isn't the only one noticing some strange ICLS and trim problems lately.

[Cruizzzzer]

On 7/6/2021 at 11:27 PM, CBStu said:

I haven't gotten to where I understand the marshall stack etc so I just do a straight-in approach.

maybe this helps the issue of the "marshal stack"

 

greets

[ML1872]

Following Swiftwin9s' suggestion, I have made a video that demonstrates the issue with the trim/stab divergence (apologies for the poor quality but you can see the problem).
This appears to be related to prior use of the autopilot before going ‘on speed’ during the ICLS approach.  I flew the full approach 3 times:
 

A.      Without any use of the autopilot = no issues with time/stab.

B.      With use of the heading and barometric height holds, disengaged with the Paddle Switch prior to going ‘on speed’ = no issues with trim/stab.

C.      With use of the heading and barometric height holds, disengaged in turn with the respective Option Selection Buttons prior to going ‘on speed’ = significant issues with trim/stab diverging from a steady ↑12 degrees. These divergences occurred at least 6 times on the final approach as the power was reduced to increase RoD, as follows:
 

1.       15 secs/4.7 miles pre-descent = ↑16 degrees

2.       46 Secs/4 miles pre-descent = ↑16 degrees

3.       1 min 5 secs/3.5 miles entering the descent on the ICLS = ↑15 degrees

4.       1 min 26 secs/3 miles in the descent = ↑15 degrees

5.       2 mins 27 secs/1.6 miles = ↑16 degrees

6.       3 mins 13 secs/.7 miles = ↑19 degrees – which makes the approach extremely challenging!
 

With each event I needed to add at least 2 blips of trim the counter the divergence and return 'on speed'.  Whilst there was a little turbulence it did not unduly affect the approach.

I welcome any thoughts or comments.
 

ML

 

PS. excellent diagrams provided above by Cruizzzzer.

 

[CBStu]

Cruizzzzer thank you. This is one of those things I am going to have to dedicate a lot of time to. Your diagrams will be a big help.

[Hulkbust44]

Well are you still on speed and flying fine? There is no trim position indicator in the cockpit (aside from rudder trim knob) so you are most likely seeing the stabilator position which should change as the FCS controls the aircraft and not you.

Mobius708

Mobius708

[Cruizzzzer]

On 7/12/2021 at 5:28 PM, CBStu said:

Cruizzzzer thank you. This is one of those things I am going to have to dedicate a lot of time to. Your diagrams will be a big help.

Hey ML1872 & CBStu, you are welcome, and I surely appreciate...

 

In the thread of the "flight manual chapter-1" I pointed out some bits concerning the symbology and usage especially of the XTRK distance plus turn radius, you might want to check out.

These are of great help during entry of the marshal stack...

Maybe it helps.

 

Greets

Edited July 18, 2021 by Cruizzzzer

[Svend_Dellepude]

On 7/10/2021 at 10:42 AM, ML1872 said:

Following Swiftwin9s' suggestion, I have made a video that demonstrates the issue with the trim/stab divergence (apologies for the poor quality but you can see the problem).
This appears to be related to prior use of the autopilot before going ‘on speed’ during the ICLS approach.  I flew the full approach 3 times:
 

A.      Without any use of the autopilot = no issues with time/stab.

B.      With use of the heading and barometric height holds, disengaged with the Paddle Switch prior to going ‘on speed’ = no issues with trim/stab.

C.      With use of the heading and barometric height holds, disengaged in turn with the respective Option Selection Buttons prior to going ‘on speed’ = significant issues with trim/stab diverging from a steady ↑12 degrees. These divergences occurred at least 6 times on the final approach as the power was reduced to increase RoD, as follows:
 

1.       15 secs/4.7 miles pre-descent = ↑16 degrees

2.       46 Secs/4 miles pre-descent = ↑16 degrees

3.       1 min 5 secs/3.5 miles entering the descent on the ICLS = ↑15 degrees

4.       1 min 26 secs/3 miles in the descent = ↑15 degrees

5.       2 mins 27 secs/1.6 miles = ↑16 degrees

6.       3 mins 13 secs/.7 miles = ↑19 degrees – which makes the approach extremely challenging!
 

With each event I needed to add at least 2 blips of trim the counter the divergence and return 'on speed'.  Whilst there was a little turbulence it did not unduly affect the approach.

I welcome any thoughts or comments.
 

ML

 

PS. excellent diagrams provided above by Cruizzzzer.

 

 

AFAIK you just deselected a particular mode on the third ( C ) run but did not turn off the autopilot. In that case you should have A/P in the ADV line on the left DDI. Press paddle to fix.

 

EDIT: At 3:26 in your video you can see that A/P is indeed displayed on the ADV line on left DDI.

Edited July 18, 2021 by Svend_Dellepude

[vCSG3 Bloat 203]

On 7/6/2021 at 3:38 PM, Hulkbust44 said:

Well are you still on speed and flying fine? There is no trim position indicator in the cockpit (aside from rudder trim knob) so you are most likely seeing the stabilator position which should change as the FCS controls the aircraft, and not you.

Mobius708
 

Both the FCS page (all trims) and the Checklist (elevator only) page provide effective trim information.

[Hulkbust44]

Both the FCS page (all trims) and the Checklist (elevator only) page provide effective trim information.

You're still just seeing the deflection of a given flight control surface. There is no indication of the amount of "trim" you've input to the FCS. So no, there is no trim indicator in flight.

Mobius708

[vCSG3 Bloat 203]

25 minutes ago, Hulkbust44 said:

You're still just seeing the deflection of a given flight control surface. There is no indication of the amount of "trim" you've input to the FCS. So no, there is no trim indicator in flight.

Mobius708
 

I see what you mean. NB: in the real jet, there is an engineering page you can turn on to see the current trim setting.

[ML1872]

Thank you Svende_Dellepude. 

 

Good spot.  It would seem that deselecting the modes does not deactivate the AP.  

 

On selecting the AP modes they each appear on the ADV line.  Once deselected, the AP mode appears on the ADV line (as you noted).

Thus, the only way to turn the AP off is through the Paddle Switch.

I wonder if this is the case in the aircraft itself?

 

Many thanks

 

ML

[Hulkbust44]

Thank you Svende_Dellepude. 
 
Good spot.  It would seem that deselecting the modes does not deactivate the AP.  
 
On selecting the AP modes they each appear on the ADV line.  Once deselected, the AP mode appears on the ADV line (as you noted).
Thus, the only way to turn the AP off is through the Paddle Switch.
I wonder if this is the case in the aircraft itself?
 
Many thanks
 
ML

Yes this is correct. You cancel A/P with the paddle switch. If you just deselect the A/P mode the aircraft will remain in CSS A/P.

Mobius708

[ML1872]

Thanks Mobius 708 - what is CSS A/P?

ML

[Tholozor]

Command Stick Steering Autopilot, basically just a 'pilot-relief' autopilot mode that doesn't hold aggressive parameters like the other autopilot modes.

Edited July 22, 2021 by Tholozor

[Hulkbust44]

Thanks Mobius 708 - what is CSS A/P?

ML

To add on to the previous response, with CSS you can use the trim hat to adjust pitch and roll. It will not affect the previous trim inputs once CSS is disabled.

Mobius708

[ML1872]

Got it - thank you both.

 

ML