Altimatic IIIB GS Coupler

[Guest]

Hi all,

can anybody of you help with your experience on the glide slope coupler of the Altimatic IIIB?

In the first few minutes of the coupled approach the aircraft stays exactly on the glide. Even lowering the gear keeps it on GS. Having a positive coupling to the GS and no changes in flap or gear configuration after some minutes the aircraft wanders very slowly 2 - 3 dots above the glideslope and keeps it there. After some more minutes it has improved to only 2 dots above glideslope.
To have a longer time on the glide available I have tried the ILS several times from 9000ft down to minimums.
I have tried it at IAS 150mph down to 120 mph with no difference.
It looks to me that the sensitivity of the GS coupler is much to low.
Do you have any ideas?

How is your experince with more modern autopilots like the S-Tec 55. Does it keep the GS whilst changing the landing configuration?

Thanks in advance.

Friedrich

[Zach Grant L1011jock]

I will be in Muncie tomorrow, and I will ask when I pick my plane up. I'm sure Rockey will know. It strikes me as an adjustment issue. I have no issues with my Altimatic III on the Twinco when coupled to the GS or WAAS derived GS. Likewise the STec 60-2 that I also fly in several other aircraft as well as the Altimatic in an Aztec I fly are also very accurate on the GS. Lets see what the experts say. Stand By!
-Zach

[Friedrich Rehkopf]

Thanks, Zach, I look forward for your specialist's opinion
best rgds

[Zach Grant L1011jock]

Freidrich,
When was the last time you had the pitch servo out and cleaned/greased/checked? Rockey said that if the startup voltage and resistance was too high, you would notice this first on the GS coupling, and most likely you would go high on the slope. Also, an oscilation would likely be noticed when the aircraft was loaded at aft CG and any pitch mode was used. Another thing he said was that the clutch tension may be under spec, and you may be slipping at the pitch servo, and without cable tension, no trim will be introduced and the slipping will continue. His comment was that about 80% of the time the servo was at fault somehow, but it could also be the coupler, but if that was the case it would probably be a consistent displacement, like it would capture and fly exactly 1 dot high or low, all the way down, and not drift.

I hope this helps. Advise if I understood anything wrong, and I will get some more "free" advise if necessary (I spend so much money there, I should have a lifetime membership to the help line, for all my friends too!).

-Zach

[Friedrich Rehkopf]

Thank you very much for your reply, Zach

I didn't think of the start-up voltage on the pitch servo and possibly its internal mechanical restistance. I will check that first.

However, I would exclude the clutch torque not set properly since the electrical trim is working fine and immediate.

Many thanks for now.

[Zach Grant L1011jock]

Friedrich,
The trim servo and the pitch servo are two separate entities. One runs the trim cable and one the elevator, so one could have a clutch issue, and the other not.
-Zach

[Friedrich Rehkopf]

[Zach Grant L1011jock]

No problem. You should be shooting for 1 to 1.5 volt max start voltage on the servo. Good luck.
-Zach