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MillyD

Perplexing Autopilot Discrepancy

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We have a 74 model H1 with AN/AWY-1 single system autopilot, (Air Force Standard Flight Director, C12 Compass, single INU) that has had a perplexing discrepancy for a few years.  With the FDMS switches in SCNS, AP2 selected on the FCP and LNAV/NLOC selected on the mode select panel the write up reads as follows "AP2 will not capture beam with SCNS selected".  During the ops check with switches in the aforementioned positions the "N/L ARM" display on both pilot and copilot FPWU annunciators remain on and they never go to "N/L CAP".  When AP1 is selected and all switches as listed, they system works as advertised and immediately the N/L CAP annunciator comes on.  With that being said, the technicians have R2 the following components:  AFCP (a couple of times), copilot HSI, ILS and TACAN receivers and TACAN adapters, Flight Director Computers, C12 Amps/gyros, the ENTIRE NIRP relay panel under the crew bunk, BICU, LEDEX relays on the SCNS relay panel in the hell hole, FDMS switch on the copilots side, shot multitudes of wiring that checks good to the FDMS, FCP, MSP, SCNS NIRP panel, flight directors computer #2, AFCP and even wires that didn't make sense but was directed by an engineer to shot them and all checked good.  Very frustrating to say the least!!  One of the biggest questions we have is this:  Exactly what signal is it that the processor is looking for that enables the autopilot to go from L/ARM to L/CAP?  Where is the signal coming from?  We posed those questions to the engineer and never received an answer - he went off in left field and left us stumped...….If anyone on the forum has ever experienced something like this and can provide some tips and help, we would welcome any information you have.  Thank You.

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Well, to start with, I can't find anything in any books that give an indication of how the Capture lamp is enabled. The N/L ARM light comes on just to show it has been selected. I am not sure if the deviation signals have to be sensed by the A/P processor or not. At this point, I think the Nav valid signals enable it. If you look at the lower right hand corner of Fig 3-6, sheet 4, you will see the Copilots's Nav/ILS Flag Driver. AYW1-107A22 goes from pin 6 up to the AP Processor Nav Valid #2. Looking at the diagram, I don't see a SCNS input.

In the 22GS, FO-5, sheet 9 doesn't show it either. BTW, these foldouts are pretty decent to see signal paths through all the different relays.

I am checking with someone that knows SCNS very well to see if he has any input, but I will keep looking. Please keep in mind I've been retired for 8 years, so it may take me longer than it used to.

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Thank you so much for your help TinyClark.  We have not been able to find anything either regarding how this is supposed to work.   Perhaps that is why the engineer could not answer our question.  I will double check our troubleshooting diagrams that we used a long time ago, we may have already shot that wire, been a long time since we worked the issue.  It's been sitting in the "K" write ups for almost 2 years.  I too, am retiring the end of this year - and looking forward to it!!  38 years of service is LONG enough- ha ha!!  Keep me posted if your contact has any further info.  Thanks again, and look forward to hearing from you.

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So let me ask this Milly, are you guys just changing parts then letting it fly again? Are there any stored faults in the system? The processor does monitor Track Angle Error and Cross Track from SCNS. I would think it would flag it if it lacks one of those signals.

You could try swapping out K300E, C/P TKE Course Datum Relay shown on  sheet 6 of F0-5 in the 22GS-00-3 and K261MLS, C/P AZ Relay as shown on sheet 8. At least those could be ruled out.

What base are you all at, anyway?

You guys were lucky with swapping out the whole NIRP panel. I've seen back things happen doing that. 

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Thank you Tiny.  Over the years we have swapped out several parts and shot a lot of wiring with assistance from an engineer.  It fails on the ground all the time no matter what we have done - copilots side with AP2 selected and NAV/LOV selected on the mode select panel always stays in N/L ARM - it never goes to capture.  We used the IFR4000 and tested the glideslope, ILS and TACAN and all checks are good.  We are in Great Falls, MT.  We changed out the SCNS relay panel after swapping out a few NIRP relay's, just to rule out the panel after all else failed.  As for stored faults - we don't see anything related on the TOPs panel for this issue and I'm not sure where else we would find a stored fault.  The FI for the autopilot system is horrible to say the least.  Even during the operational check it does not have the technician check this function of the system so it's pretty frustrating!!  I sure appreciate your time and help, we will give those relays a try - can't hurt!! 

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I'm curious as to hpw you check the SCNS on the ground and get a capture on the pilot's side. I could fool the C-130E AWADS airplanes, but they had a CADC that had a minimum 100Kt output. How do you run a SCNS route on the ground?

 

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Hi Tiny,

What we basically do on the ground is have SCNS on and we turn on TACAN and ILS/VOR and just tune them to a local station.  With A/P engaged, we select LNAV/NLOC on the mode select panel - as soon as we depress that button we get a annunciator light on the Flight Progress Warning Unit Panel that goes from N/L ARM and immediately to N/L CAP.  We have checked several of our other aircraft on pilots and copilots sides - all perform correctly.  The plane I'm talking about is the only one that will not go into capture when we perform the aforementioned steps on the copilots side.  Again - we do don't have an actual tech data driven test to follow.  In the FI for the AN/AWY-1 autopilot it does have a fault tree for this scenario and it asks if the pilots AND copilots sides N/L CAP illuminate when we select LNAV/NLOC - we have to answer that as a NO.  Then it asks if the pilots side annunciator illuminates - we answer that as a YES and it ends right there.………..it does not go into further info if the copilots light does not illuminate...……….whether or not we are actually capturing a signal is a mystery to us.  When the capture light illuminates on the pilots side you can see and feel the control wheel move and stay on track.  The copilots side doesn't have this result.

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That seems like it is using the valid signal to turn on the the Capture light, and not deviation. When you do the TACAN and VOR, do you have the HSI course bar centered before you hit the L/NAV switch? If it wasn't< iwould think the A/P would drive the yoke hard one way or another. I will send you a message and give you my e-mail address. We can converse that way. Maybe if it gets solved we can post what the problem was.

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