GFC500 problem

[PeytonM]

Happened to me, too. If someone with less Mooney time had flown that first flight, it would have been “interesting!” (MCAS!) Many (most) of these shops don’t have access to the trim boards anyway, so I think they are guesstimating. 

[Ragsf15e]

2 hours ago, PeytonM said:

Happened to me, too. If someone with less Mooney time had flown that first flight, it would have been “interesting!” (MCAS!) Many (most) of these shops don’t have access to the trim boards anyway, so I think they are guesstimating. 

I had that happen after an annual at a well known msc.  They did install a new trim torque tube, so maybe understandable?  I noticed on the preflight that that paint line at the tail looked different- the tail was at a different angle.  They disagreed.  Luckily i was ready for it at rotation because she tried really hard to nose up like a Concorde!

[rbp]

18 hours ago, EricJ said:

Appendix A of the G3X Experimental Installation Manual https://static.garmin.com/pumac/190-01115-01_aq.pdf has the flight test procedure for setting the gains. I am NOT advocating messing with the gains, but flying the specified test profiles while recording the data might give insights to the engineers into what's going on. 

 

I went through this last night. it was very informative! The testing procedure is quite straight forward, and I am going to try it this weekend

>>>Place the autopilot in IAS/ROL mode commanding a bank angle of zero and an IAS speed that roughly allows the aircraft to hold altitude for the given power setting. Remember to stay below the maneuvering speed of the aircraft (VA).

>>>To tune the airspeed gain, allow the aircraft to settle in its steady state, then use the control stick to over-power the servo and displace the aircraft from the desired airspeed. The pilot should pull or push the stick to up or down for roughly one-two seconds to establish a new airspeed, and then release the stick.

>>>After releasing the control stick see the AP response by watching the control stick and airspeed indicator. The aircraft should return to the desired airspeed without being too aggressive or lazy.

[PT20J]

3 hours ago, Ragsf15e said:

I had that happen after an annual at a well known msc.  They did install a new trim torque tube, so maybe understandable?  I noticed on the preflight that that paint line at the tail looked different- the tail was at a different angle.  They disagreed.  Luckily i was ready for it at rotation because she tried really hard to nose up like a Concorde!

I was worried about that, so I discussed it with my installer beforehand. I suggested he run the trim all the way to the down limit before disconnecting and then just not rotate the jack screw while it was disconnected though he could rotate the rest of the trim system as much as necessary for the installation making sure to run it all the way down again before reconnecting to the jack screw.   He said that worked great and my trim was fine after the installation.

[Mac80]

On 5/4/2022 at 12:48 PM, Davidv said:

No, they didn’t know, only that there was something internal with the clutches that showed there were some defective pieces.

 

On 5/1/2022 at 5:59 AM, Davidv said:

I’ve said this in some other threads, but I spent 6 months chasing a similar only to found out it was bad servos.  Once the Servos were replaced it was 95% better.

I believe he said servos were replaced.  Our present C 2000 had excessive oscillation in pitch. New pitch servo and other work on pitch control stopped oscillations.

[ArtVandelay]

I did try the trick of setting the VS and getting it stabilized and then switching to IAS. because there was so much lift, i kept having to set the ALT bug higher and climb out at 130 (instead of 120). I established a climb at in VS  @ 130, and then switch to IAS, which it held +/- 3 knots. then i rolled it down to 125 and 120, and the airspeed oscillations increased both times.

Today I tried both increasing and decreasing VS by 10, other than the normal +/-3 knots fluctuations I didn’t see any oscillations FWIW.

[rbp]

1 hour ago, ArtVandelay said:

Today I tried both increasing and decreasing VS by 10, other than the normal +/-3 knots fluctuations I didn’t see any oscillations FWIW.

Thank you 

[PT20J]

On 3/16/2023 at 5:57 PM, Mac80 said:

 

I believe he said servos were replaced.  Our present C 2000 had excessive oscillation in pitch. New pitch servo and other work on pitch control stopped oscillations.

What other work?

[Mac80]

I can look it up and be more specific from the repair logs. I think the trim servo was the one that had excessive wear ? They were all tested and adjusted as needed. I had previous posts after the service in Mineral  Wells Century shop.

What I recall is a AP control board ,maybe pitch, also had been fried, the technicians description, Shorted out maybe.

Also the trim switch on yoke stopped working and it was replaced and all the wiring from the switch to AP was replaced.

I thought maybe the switch or wiring had shorted out and caused the control board to short circuit.

Works fine for what I need. No oscillations in cruise, climb, bank, or descent. I can’t tell the plane has any rough spots in yoke. I try to keep them clean and have used silicone sparingly thinking it might be helping but I have been reading some of posts, yours ?, that it is no longer recommended.

I am very pleased with it now but maybe I don’t know how good others can be. Had a flight today from KJEF to KEZZ. Had some issues with wind changes so it was not very straight but was level. On flight aware N981SM.

 

 

 

[hais]

On 3/16/2023 at 10:06 AM, ArtVandelay said:

 

I had same problem as well but they fixed it before my first test flight.

 

How did you identify the problem during pre-flight?

[hais]

On 2/18/2023 at 7:32 AM, donkaye said:

It's purpose is to damp Yaw.  I use it in an extended climb to do just that.  On level off I turn it off, trim the rudder with the rudder trim switch, then turn it back on.

What happens if you adjust rudder trim without disengaging YD? 

[ArtVandelay]

How did you identify the problem during pre-flight?

I know my plane, when I set the trim to takeoff the elevators weren’t in the right position.

[AdamJD]

Interesting observation.  I had my gfc500 installed in Nov.  However, they finished the installation just as the autotrim service alert came out.  So I flew it for several months without autotrim.  I didn't notice any significant oscillation issues.  Autopilot was still enabled, but I had to trim by hand.

I recently had the software update completed.  I picked up the plane last week and have flown two 2.5+ hour cross country flights since having the autotrim turned on.  Now, the plane oscillates slightly in cruise.  Both 2.5+ hour flights had +/- 7kt airspeed (and groundspeed) osciallations.

BEFORE autotrim was enabled

AFTER autotrim was enabled

[ArtVandelay]

I recently had the software update completed.  I picked up the plane last week and have flown two 2.5+ hour cross country flights since having the autotrim turned on.  Now, the plane oscillates slightly in cruise.  Both 2.5+ hour flights had +/- 7kt airspeed (and groundspeed) osciallations.

If you have a G3X and Garmin Pilot, you can upload your flight data and get better data instead of just ground speeds which can vary depending on winds:



While the graph makes it look uneven, the variability is small, IAS +/- 2 knots at most.

Interesting the J points downward to fly level.

[AdamJD]

10 hours ago, ArtVandelay said:

If you have a G3X and Garmin Pilot, you can upload your flight data and get better data instead of just ground speeds which can vary depending on winds:



While the graph makes it look uneven, the variability is small, IAS +/- 2 knots at most.

Interesting the J points downward to fly level.

Yeah.  I watched the IAS swing about 5-10kts - my TAS on the 275 was slowly fluctuating between 135-145kts.  I usually see ~143 at that altitude and config.  

I *think* I can download the flight data from the 275, but haven't tried that yet.  But I could see it on both the analog ASI and the 275.

[PT20J]

14 hours ago, ArtVandelay said:

Interesting the J points downward to fly level.

Was the ADAHRS pitch offset calibrated with the airplane levelled? A M20J sits about 4-5 degrees nose up on the ground. My Pitch from the G3X log shows around -0.2 deg on average in cruise.

[ArtVandelay]

Was the ADAHRS pitch offset calibrated with the airplane levelled? A M20J sits about 4-5 degrees nose up on the ground. My Pitch from the G3X log shows around -0.2 deg on average in cruise.

Probably not, I will have to track down the configuration setting.

[hais]

On 3/19/2023 at 4:48 PM, ArtVandelay said:

I know my plane, when I set the trim to takeoff the elevators weren’t in the right position.

Thanks. I wish I could say the same. I will try full movements and see whether anything stands out.

[PT20J]

1 hour ago, hais said:

Thanks. I wish I could say the same. I will try full movements and see whether anything stands out.

It's easy with the M20Js and before that have the trim assist bungees because the elevator should be in trail with stabilizer when the trim is set to takeoff, In later models with the bob weight and down spring, the elevator sits full nose down on the ground and it's not so easy to find a problem without going through the rigging procedure.

[rbp]

Not much to add today, except that on climb out today, I switched from VS to IAS and didn't touch the UP/DOWN thumbwheel. it was pretty steady at +/- 2 knots. As soon as a moved the thumbwheel to adjust the target IAS, the oscillations began. 

 

[ArtVandelay]

Not much to add today, except that on climb out today, I switched from VS to IAS and didn't touch the UP/DOWN thumbwheel. it was pretty steady at +/- 2 knots. As soon as a moved the thumbwheel to adjust the target IAS, the oscillations began. 
 

I would look at the logs and see if you can see the any differences other than the pitch commands.

[rbp]

8 hours ago, ArtVandelay said:

I would look at the logs and see if you can see the any differences other than the pitch commands.

Good suggestion. Which specific parameters?

[PT20J]

16 hours ago, rbp said:

Not much to add today, except that on climb out today, I switched from VS to IAS and didn't touch the UP/DOWN thumbwheel. it was pretty steady at +/- 2 knots. As soon as a moved the thumbwheel to adjust the target IAS, the oscillations began. 

 

If I understand correctly, you have had the airframe and installation thoroughly checked and you only get oscillations now in IAS and then only when you change airspeed. Mine did the same thing. I could get it to oscillate and if I moved the thumbwheel more than about 5 kts at a time or if I excited the phugoid by applying and releasing elevator input for about a 5 degree pitch change. Tightening the elevator servo bridle cable fixed it. I know you had that checked, but it's a little funky getting in there with the tension meter. As a diagnostic test, I might try tightening the turnbuckle a half turn (because of safety clips, you can only adjust in half turns which is one thread -- half a thread for each end) and see if it makes a difference.

Skip

[ArtVandelay]

Good suggestion. Which specific parameters?

Obviously anything speed or servo (pitch) related or errors.

[rbp]

51 minutes ago, PT20J said:

If I understand correctly, you have had the airframe and installation thoroughly checked and you only get oscillations now in IAS and then only when you change airspeed. Mine did the same thing. I could get it to oscillate and if I moved the thumbwheel more than about 5 kts at a time or if I excited the phugoid by applying and releasing elevator input for about a 5 degree pitch change. Tightening the elevator servo bridle cable fixed it. I know you had that checked, but it's a little funky getting in there with the tension meter. As a diagnostic test, I might try tightening the turnbuckle a half turn (because of safety clips, you can only adjust in half turns which is one thread -- half a thread for each end) and see if it makes a difference.

Skip

i’ll suggest it.