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Digital Autopilot Porpoising


GACFaja
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Good morning!

We have a P model here that has a recurring porpoising issue with its AN/AYW-1 Autopilot. This aircraft has had a history of this for several years and in that time the elevator servo has been changed several times, numerous processors, air data sensor has been changed a couple of times, cable tensions checked/adjusted to the elevator servo, and at least two elevator boost packs. We have wrung the wiring out multiple times and also checked all the grounds for corrosion/paint, since another unit had a similar problem caused by painted-over grounds. The elevator trim relays were changed before the aircraft went into depot last year and the aircraft flew good for a few months and then the problem came back prior to its depot input. We got the aircraft back from depot a few months ago and the problem is still there. The TOPS panel shows no faults associated with the porpoising and we have also checked the wiring between the servo and processor, checked the ADS signals/wiring, and the MBIT 2 (ADS #1) and MBIT 19 (Elevator Sero Hyd) pass every time along with the full system operational checkout.

Has anyone seen anything like this? I'd welcome any suggestions. Thanks!

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I saw this many, many moons ago on 60221. It was doing the same thing, only with the E-4 autopilot, and as well had been doing it for many years.

The fix was found to be the elevator trim tab actuators (jackscrews). The maintenance books say to ops check them you run them up and down. What they wern't saying (and was figured out by another crew chief, Tony Jeffrey) was there needs to be an air load applied to the trim tabs whiole running them. Do this by allpying a small amount of pressure to the tab just aft of where the actuator attaches to it while running the tabs. What you will most likley hear is a banging coming from inside the elevator, at the forward end of the actuator. This is the internals of the actuators slipping. The actuators have no time change (we asked for this several years ago, late 1980's), and are a real "mother" to remove. I'd recommend removing the elevator and soaking the bolts with penetrating oil. We did this on 60223 in around 2001 when the banging was discovered on it. After a week of "whenever anyone walks by, squirt oil on the bolts", it still took using a rivet gun and angled rivet set to remove the bolts, as they were siezed in the rod end that rotates. Also, keep in mind, that the trim tab cables used to be a 6000 time change, don't know if they still are, and are usually "forgotten" due to the fun of replacing them, so this would be a great time to replace them, too.

We asked for changes to the books several years ago, to add in the simulated air load and a time change for the actuators, but were ignored.

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  • 2 weeks later...
  • 2 weeks later...

I work with GACfaja. We tried applying pressure to the trim tab with it moving in both directions and we didn't hear a thing. A/R came out for a rig check and said everything worked as advertised. Got any more suggestions? We're trying to get in touch with some people at Honeywell too.

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Zak,

If memory serves me correctly, it was happening during ILS approaches, and they changed the large filter caps that are mounted on the back of the NIRP. The caps smooth out the Loc and G/S signals.

GACFaja,

Does the aircraft porpoise in all modes, and at all times?

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  • 4 weeks later...

Zak,

GACFaja,

Does the aircraft porpoise in all modes, and at all times?

Yep. AP1/AP2, basic autopilot, and Altitude Hold. She's been doing this since she came to us from Patrick in '02 or early '03. It starts oscillating, small at first, then worse...almost like it's missing the servo feedback. No associated bit faults, wiring Ohm's out great, op checks great on the ground. We write it up for an inflight check and she fails every time. We've gotten it to go away for as much as 6 months by changing the elevator pack. A 7-lvl flew with the aircraft--when the aircraft would porpoise the elevator trim indication would follow the porpoising, which it should since the autopilot uses the elevator trim to assist the surface. If the crew put an OPPOSITE input to the flight controls the porpoising would null out and stop for a while.

Getting to the point of calling for a depot team. Or putting a grenade in the drybay.:D

Edited by GACFaja
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I assume the trim tab is trying to drive the aircraft back and not actually causing the condition.

If I'm thinking right, if the aircraft starts to pitch up, the trim tab should go up, trying to force the elevator down. Is that what's going on?

I never did see anything after I posted something about the servo clutch. Have you changed that out, and not just swapped the servo?

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I assume the trim tab is trying to drive the aircraft back and not actually causing the condition.

If I'm thinking right, if the aircraft starts to pitch up, the trim tab should go up, trying to force the elevator down. Is that what's going on?

I never did see anything after I posted something about the servo clutch. Have you changed that out, and not just swapped the servo?

The drum/bracket assy was changed out, but didn't help matters. I'm trying to get hold of the 7 lvl who flew with the acft (he's in ISO now) but I doubt he'd remember which way the trim drove. Maybe I can get the FE to watch the trim indication on the next inflight op check.

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  • 7 months later...

Did this problem get fixed? If so, what did it?

We put in for a 107 about a month ago--they had us check attitude indications, rigging, trim tabs jackscrews, and sim in flight with a 205 and jumpering the touchdown relay, then change out an elevator pack. The plane porpoised on the next flight and they had us change another pack. The aircraft flew up to Nellis last night--haven't heard about whether it porpoised.

More later, I'm sure--we didn't find anything wrong on all the other checks and I don't think the pack is going to fix it either.

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Amazing how much info is lost in 4 years being away from it. Does the Radar Altimeter interface with the autopilot or is it all with pressure altimeters? Can you do a 205 leak check for the hoses at the air data hoses? Does this problem happen at all altitudes or is there a specific range that this occurs?

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Amazing how much info is lost in 4 years being away from it. Does the Radar Altimeter interface with the autopilot or is it all with pressure altimeters? Can you do a 205 leak check for the hoses at the air data hoses? Does this problem happen at all altitudes or is there a specific range that this occurs?

The AYW-1A (E4 replacement) uses its own digital altimeter encoder for ALT Hold.

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