Boeing 737-200 Autopilot - Elevator Axis Question

[biggiraffe]

Hello,

Thank you for making the 737-200 available!!  I'm having some trouble understanding the autopilot's elevator control using the recommended Tinmouse V1.25/1.26 panel.  Specifically, when the autopilot is first engaged during climb, exactly what is the elevator axis controlling?  It does not appear to be Rate of Climb, and I can't tell whether it is trying to control pitch angle.  I tried using pitch trim to change Rate of Climb, with the paddle switch still engaged, without too much success.

Thank you,
Kurt

[aerofoto - HJG Admin]

The recommended panel's AP is composed of 2 paddle switches.

The left paddle engages the basic AP functions and governs Aileron axis .... whilst the right paddle switch governs Elevator axis.

Elevator axis is "basically" pitch mode .... whilst Aileron axis is "basically" CWS mode control.

HOWEVER .... with both AP axis' engaged one needs to be particularly cautious in regard to how one applies power as this will influence the ROC (+ or -) even during AP controlled flight .... regardless of what specific ROC is established upon engaging the AP Elevator paddle. Too much power/airspeed will result in the ROC increasing above what was set when AP Elevator axis was engaged .... and insufficient power/airspeed will result in the ROC decreasing below what was set when AP Elevator axis was engaged. Maintaining the established ROC/ROD via full AP control is a delicate balance of power setting and airspeed and also requires constant subtle manual adjustment/s. This's a reflection of the type, and era, of AP equipment (SPERRY 77 in this particular panel/case) featured among early B737-200 aircraft (prior to the ADV series) .... which does not allow a specific ROC to be dialed into, and maintained, by the AP system. Again .... caution with the use of power combined with subtle manual adjustment/s are a constantly necessity .... BUT .... it's "fun" and easy to control once adapted to.

The supported TM B732 panel also has a another unusual (for FS) feature compiled into it and which can be used (although I don't like it) in order to control ROC/ROD via full AP control. In this case one "can" use one's controller device to "very gently" push/reduce the ROC .... or pull/increase the ROC .... and without disengaging either AP axis'. I find this feature "overly sensitive" though and therefore seldom ever use it, but, the ability to exploit this feature "IS" none-the-less complied into the panel's AP system.

Personally .... and for a number of years now .... I simply engage the left AP paddle (Aileron axis) only, but, avoid engaging the the right AP paddle (Elevator axis). Using this AP configuration (be it right or wrong) I then control the ROC/ROD by "very gently" tapping the "END" keyboard tab (for manual trim "up") .... and equally "very gently" tapping the "HOME" keyboard tab (for manual trim "down"). It works beautifully .... again once adapted to. Having said all this I do, sometimes, engage both AP paddles during banked climbing turns after TO (it tends to keep the ROC in check) .... whilst also continuing to gently tapping both "END" and "HOME" keys accordingly too. Maybe I've adapted to this method by now (?) .... but "for me" .... it works "PERFECTLY" and very smoothly too, but, the ROC/ROD does still require attention and constant adjustment. Once more .... it's just a matter of adapting to it/experience over time and after which its "a breeze"

I just wish I could make a video to demonstrate how well these recommendations each actually work.

ONE OTHER THING TOO ....

If using my procedure to climb to altitude with the AP Elevator axis disengaged, then be aware, both AP paddles need to be engaged together just prior to reaching one's intended cruising altitude .... or it won't be possible to engage AP ALT HLD .... since the actual altitude captures is governed by the AP Elevator axis.

ONE LAST THING ALSO ....

On the OH sub panel .... one must also ensure one has the LEFT GEN switch set to "BAT" position and it's corresponding RIGHT GEN switch set to either "GEN1" or "GEN2" position in order for the AP to continue working. If these switches remain set at their default positions, then, the AP will likely fail at some stage and flight control via the AP will then be lost.

PLEASE NOTE: I have made references to each of these details per NOTE #2 within "SECTION 1.17: KNOWN ISSUES" .... and within the entirety of "SECTION 3" of my following-linked forum based B737-200 manual ....


B737-200 MANUAL
tonymadgehjg.proboards.com/thread/10406/b737-200-installation-handling-notes


Mark C
AKL/NZ

[biggiraffe]

Good Morning!

Thank you very much for this explanation. It explains some things I experienced but did not comprehend. One thing I did was turned off the elevator axis, then engaged it when I reached my altitude and watched the plane go into a 3000 fpm ROC... Now I see I need to disengage the aileron switch and engage both together.

I'll be practicing both methods of control later today... I'll probably do with your method (disengage elevator AP) since yours always work the best for me.

Best regards,
Kurt

[aerofoto - HJG Admin]

YEP .... stick to my suggestion.

Engage AP Aileron axis only (left paddle)

Disengage AP Elevator Axis only (right paddle) .... then use gentle keyboard based trim inputs in order to set/adjust/maintain the desired ROC/ROD (follow my climb profiles within "SECTION 5" of the manual) .... whilst also being careful with power whilst you're at it.

As I said it does require constant monitoring and adjustment, but, it does also work beautifully if done properly/carefully.

Another way to do it is engage both AP paddles together .... then .... momentarily disengage the Elevator axis only in order to input trim adjustment/s and then re-engage it again .... whilst still being careful with power management.

After a few years of having been doing this I find I can do so with almost absolute perfection whilst making progressive and very small power adjustments (reductions) where necessary all the way up to cruising altitude. Part of the secret is trying to maintain a roughly 280-290 KTS airspeed throughout the climb above 10,000 FT.

Getting to altitude slightly below cruising speed .... engaging AP ALT HLD .... then letting the simulation slowly accelerate to between MACH 0.77 and MACH 0.78/0.79 (max) before reducing power to maintain the acquired velocity for the cruise.

Just wish I could make that video to prove my point .... because I anticipated this query would be posed.

Mark C
AKL/NZ

[biggiraffe]

Hi!
I followed what you said just fine, even without a video. I was able to take the aircraft up to a cruise altitude and level it off just fine.

Thank you again,
Kurt

[aerofoto - HJG Admin]

No trouble Kurt .... that's the way to go.

Maintain that procedure (you'll get even better over time and with practice) and before long you'll become "AN EXPERT" .... just like me

It's good to know at least someone listen to me/follows my advice .... for a change

Mark C
AKL/NZ