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Digital Autopilot Porpoising


GACFaja
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Over the years, we've done *everything* to it, short of sacrificing a goat to the Maintenance Gods. Changing the elevator pack seems to be the only reliable way to get the problem to go away...and that only lasts about 6 months.

The aircraft just returned to station and the autopilot in-flight checked good. One day down, 179 to go. :)

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  • 2 weeks later...

Over the years, we've done *everything* to it, short of sacrificing a goat to the Maintenance Gods. Changing the elevator pack seems to be the only reliable way to get the problem to go away...and that only lasts about 6 months.

The aircraft just returned to station and the autopilot in-flight checked good. One day down, 179 to go. :)

Aaand, that didn't last long. It porpoised again. Once the current exercise is done, we're putting in another -107 request. I hope they don't tell us to change another pack. :rolleyes:

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the 317th down at Dyess had the same thing back in 2007. If you know anyone that is down there give them a call. I'm stationed there but currently deployed.

G-Man (Mr. Gemarino) is the AFETS there at Dyess. If there was a problem like that, he would know about it. 461-4692

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Aaand, that didn't last long. It porpoised again. Once the current exercise is done, we're putting in another -107 request. I hope they don't tell us to change another pack. :rolleyes:

We've got our own aircraft that has the same problem now. Ours is doing it without ALT HLD selected as well. Haven't changed out the pack yet though.

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We've got our own aircraft that has the same problem now. Ours is doing it without ALT HLD selected as well. Haven't changed out the pack yet though.

If it helps, here's a list of the stuff they asked us to do in the -107 dispositions:

1. Check elevator drum and bracket assembly for worn teeth and worn/splayed cable. Compare wear to the rudder and

aileron drum and bracket assembly. Replace if worn.

2. Run the following MBITs on the TOP per TO 1C-130H-2-22FI-10-3-5:

MBIT 01 - AC REFERENCE

MBIT 02 - ADS #1

MBIT 03 - ADS #2

MBIT 04 - Pilot ADI

MBIT 05 - Copilot ADI

MBIT 19 - Elevator Servo Hydraulics

MBIT 20 - Flight Control Panel

MBIT 28 - AFCP IBIT

3. Check the configuration pins on the back of the AP processor tray for broken or crossed wires.

4. Ensure the autopilot processor is seated in the mount correctly. The devices for holding the processor in place have a

clutch that is set to slip when the proper torque is reached.

5. Check all autopilot servo motors for like part numbers and manufacturer. It has been known to cause similar issues if they

were different.

6. Check all bonds/grounds on the autopilot processor and servos.

7. Replace Air Data Sensor. This was last done in Dec. 2007.

8. Check the elev. trim motor wiring and relays for possible issues. If it is a trim cycle problem the cuttler hammer relay should

be able to be heard cycling in the aft of the A/C over the cargo door. Only clicking you should hear under operation is when

the relay is being manually driven by the switches on the control wheel.

9. Check the elevator boost pack for physical mounting tightness. (It is believed that it would be noticed during all stages of

flight if the pack was loose but it may be being compensated for by pilots without knowledge.)

10. Replace elevator boost pack.

And the second one:

1. Have the flight crews reported instability of the ADI attitude indications while on the ground or during smooth level flight?

2. Check elevator rigging per procedures in 1C-130H-2-27JG-30-1.

3. Check elevator trim tab actuators (jackscrews). There have been reports of bolts seizing in the rod end that rotates.

4. Recheck all elevator servo to processor wiring. Check resistance of wiring, inspect TB105A for corrosion and loose wires, and CAREFULLY check all wiring for chaffing. Ensure there are no splices in these wires. Notes: there have been other aircraft that experienced similar autopilot problems traced to chaffing of this wiring. Splices were found a few years back in these wires above the right troop door on a Davis Monthan aircraft.

5. Swap suspect elevator servo on 64-14860 with the aileron or rudder servo on 64-14860. See if problem follows the suspect elevator servo.

6. If all items above check good then an alternate suggestion is to put the aircraft in flight mode using the TTU-205 Air Data Test. Override the weight-on-wheels, and set the TTU-205 to put the aircraft above 10,000 feet and 250 knots indicated airspeed.

a. Set the elevator trim tab to 0 degrees

b. Pull the yoke midway through the travel range

c. Engage the autopilot

d. Let the yoke go until it stops moving

e. Watch the trim tab indicator for either continuous movement or pulsed movement. What we are attempting to see is if the trim tab command is pulsing above 220 knots as designed.

f. Set the TTU-205 to different airspeeds and altitudes and attempt to duplicate the autopilot porpoising problem

At 250 knots the trim tabs should show a pulsed movement, if it's running continuously it more than likely indicates a problem in what the Air Data Sensor is reporting to the Autopilot. That could indicate a bad ADS or possible problem in the pitot system.

7. Using the TTU-205 as described in step 6 above, set the TTU-205 to different airspeeds and altitudes and attempt to duplicate the autopilot porpoising problem

8. Replace elevator boost pack. Realize this was performed during the first week of March 2010, but there have been incidences of 2 or more boost pack replacements required to correct autopilot porpoising.

Sorry about the formatting issues--I was copy/pasting from the -107s.

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Yea, they pretty much went thru the stuff in your 107 reply.

It flies tomorrow, hopefully with a maintainer on board. The last flight it didn't start until 2 hours after takeoff. I was wondering if they had done anything at all when it began, like using the HF, running the Aux pump, etc. Any slight movements in hydraulic pressure gauges?

I think the TTU-205 idea is OK if the problem is just in ALT HLD, but since it's doing it all the time??

I want them to try it in Speed on Pitch mode too. I doubt that they use that mode very much.

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Yea, they pretty much went thru the stuff in your 107 reply.

It flies tomorrow, hopefully with a maintainer on board. The last flight it didn't start until 2 hours after takeoff. I was wondering if they had done anything at all when it began, like using the HF, running the Aux pump, etc. Any slight movements in hydraulic pressure gauges?

I think the TTU-205 idea is OK if the problem is just in ALT HLD, but since it's doing it all the time??

I want them to try it in Speed on Pitch mode too. I doubt that they use that mode very much.

Ours is doing it all the time--I think the -205 check was looking for proper trim tab response at higher airspeed. Reminds me of the Elevator Feel from C-141s.

Weirdly, ours seems to be influenced by fuel load. The lighter the aircraft, the more likely it is to porpoise.

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  • 3 weeks later...

Ours is doing it all the time--I think the -205 check was looking for proper trim tab response at higher airspeed. Reminds me of the Elevator Feel from C-141s.

Weirdly, ours seems to be influenced by fuel load. The lighter the aircraft, the more likely it is to porpoise.

Does this thing have the new VOR antennas on it?

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Just to add to the list to look at; Elevator torque tube bolts at flange BL 20 under the beaver tail access panels, for looseness, worn bolt holes. If my memory serves me correctly, we corrected a simular issue finding loose bolts and elongated bolt holes on the torque tube collar. I believe we had a slight roll to go with this. Take a look at the trim indications at various speeds with and without the auto-pilot engaged and coupled and uncoupled approaches.

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  • 3 weeks later...

years ago American had an aircraft with chronic porpoising. They finally took it out of service no matter what! til they found out why. Turns out - some 2inch or so bundle of wires had a splice in one of the elevator/ A/P wires. To the casual observation it looked ok but someone had used a wire splice one size too big. Apparantly, during temp changes, the wire would make and break.

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  • 1 month later...

Not that I know of. I'll ask Comm/Nav. Does it affect airflow?

GACFaja, just wondering, is it fixed? I have one E model that's got me bald after pulling all my hair...(not really, I shave what little hair grows) This one is intermittent and as far as I know it only porpoises on alt hld. I flew in it a few times this last two days and saw it abruptly porpoising on alt hld. Basic A/P was ok, as soon as alt/hld was engaged it would start a rodeo party. Could not see the elev trim trying to correct it, probably because the porpoising is so abrupt it does not calculate a correction.

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Has anyone checked the out put voltages from the AP processor in the differnt modes?

We did a mod to three herks with this same autopilot and the voltage outputs had to be adjusted to match the system(software). you may have to contact Honeywell Teterboro to get support.

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Check your static system. Had a similar problem recently with the a/c porpoising 200ft with number 1 system coupled to the autopilot. A small leak from the hose to number one ADC was found.

Will do, although I know the ADS was replaced and a leak check performed, I will double check for leaks. The problem here is intermittent, I'm leaning towards wiring faults, but will look at any possible culprit!

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