seeking mx advice on mysterious sound heard in plane after landing

[faridaamar]

Hello! I own a 1976 Mooney M20F named Loreto. This is my first post on a Mooneyspace forum, so please forgive me if I do it wrong. ◡̈

On 2/17 I heard a slow, deep, repetitive grinding noise coming from my plane after landing that I had never heard before. It was very loud and coming from within the airframe somewhere below and slightly behind the front left seat. The green GEAR DOWN indicator light on the panel was flickering a little. When I turned the master switch off, it stopped. When I turned the master switch back on, the sound and flickering returned. If I sat very still, I could feel my seat vibrating a little in sync with the sound.

The DPE sitting next to me in the plane when this occurred suggested it may be an issue with my electric landing gear motor so I took the plane to the only maintenance facility on the airfield, Foothill Aircraft at Cable Airport. The mechanics at Foothill are hypothesizing... "a spring is snapping back and hitting a switch to tell the motor to continue to push the landing gear down even though it is already down and every time the spring extends the problem continues to repeat until the power is cut off." 

Foothill put the plane on jacks and has still not been able to recreate the problem. This makes sense because during our flight the noise only began after we were on the ground, not in the air. Worth mentioning is that my last flight was an IFR checkride where we shot three approaches in a row and extended and retracted the gear in the air twice with no issues before our final landing which was also totally fine and the landing gear came out and locked correctly. It was only after taxiing and sitting idle during shutdown that the noise began.

The sound was so alarming that I do not feel safe to fly my plane again until the issue is properly identified and corrected. I am not a mechanic so my personal knowledge is limited when it comes to understanding this issue. I’m not even entirely sure it is related to the landing gear actuator or motor; or if either of these service documents (SI M20-112 or SB M20-190B) are even connected to this issue since I recently had it removed and shipped to LASAR for a complete overhaul (March 2021) and my actuator is a ITT LA11C2114 which is technically exempt from these service announcements.

Have any of you heard this sound / had this problem before? Any feedback/advice would be greatly appreciated. Thank you.

[PeteMc]

No idea without hearing it, but in addition to the gear motor, it may also be a left wheel bearing or brake issue.  But that would need to be pretty loud and you'd probably hear something if you pushed the plane back into a hangar or tiedown spot.

 

[M20F]

8 minutes ago, PeteMc said:

No idea without hearing it, but in addition to the gear motor, it may also be a left wheel bearing or brake issue.  But that would need to be pretty loud and you'd probably hear something if you pushed the plane back into a hangar or tiedown spot.

 

Simple and cheap first.  Expensive and hard last.  Good advice. 

[outermarker]

Rotating beacon

 

[ArtVandelay]

There’s limit switches for flaps, gear…if they failed the motors don’t stop and continue to run. Easy to check, pull individual circuit breakers until the sound stops.
The problem can be intermittent, so you need to wait till you can recreate it.

[rickseeman]

I do like the rotating beacon idea.

[sarih919]

Yeah


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[Fly Boomer]

46 minutes ago, rickseeman said:

I do like the rotating beacon idea.

Seems like you might hear that while in the air?

[Bartman]

If the landing gear actuator was recently serviced I would start by asking questions about that. Why was it removed and serviced ? Was it installed properly ? Are the limit switches adjusted properly ?  If nothing else I would have the mechanic squirt cleaner in and around the limit switches.

Last annual while the belly pans were removed and on jack stands I located the limit switches. I squirted cleaner into and around each of them both in the extended and retracted positions multiple times and over multiple days.  My bird leaked a lot of oil before 2020 rebuild and a bit of oil mixed with dirt made for sticky switches, but no sticking flaps since they were cleaned  

 

[takair]

If the green light is flickering and you hear a noise, it seems like the gear motor is trying to run. Perhaps isolate by pulling the gear breaker when it happens. If it can’t be duplicated, I think a gear tension check is in order. It is required at 100 hour and annual anyways. You are right not to fly until understood. It can get expensive fast. 

[EricJ]

The rotating beacon probably wouldn't make the Gear Down lamp flicker.

It could be that the down limit switch is dirty/sticky and maybe not adjusted properly, or perhaps just has a loose wire somewhere.   This might make the gear motor try to continue driving the gear down and could also explain the flickering Gear Down lamp.

Cycling the gear with the airplane on jacks, if they haven't already, might help isolate the problem.   The first response from the shop isn't overly confidence-inspiring, though.   There are probably other maintainers in the area that you could fly to if that helps, and you could just fly it with the gear down and pull the circuit breaker for the gear motor to keep it from doing whatever it is doing.    That'd also be a good way to tell if the motor is the source of the noise, by pulling the gear actuator breaker the next time it does it.

[Guest]

5 hours ago, faridaamar said:

Hello! I own a 1976 Mooney M20F named Loreto. This is my first post on a Mooneyspace forum, so please forgive me if I do it wrong. ◡̈

On 2/17 I heard a slow, deep, repetitive grinding noise coming from my plane after landing that I had never heard before. It was very loud and coming from within the airframe somewhere below and slightly behind the front left seat. The green GEAR DOWN indicator light on the panel was flickering a little. When I turned the master switch off, it stopped. When I turned the master switch back on, the sound and flickering returned. If I sat very still, I could feel my seat vibrating a little in sync with the sound.

The DPE sitting next to me in the plane when this occurred suggested it may be an issue with my electric landing gear motor so I took the plane to the only maintenance facility on the airfield, Foothill Aircraft at Cable Airport. The mechanics at Foothill are hypothesizing... "a spring is snapping back and hitting a switch to tell the motor to continue to push the landing gear down even though it is already down and every time the spring extends the problem continues to repeat until the power is cut off." 

Foothill put the plane on jacks and has still not been able to recreate the problem. This makes sense because during our flight the noise only began after we were on the ground, not in the air. Worth mentioning is that my last flight was an IFR checkride where we shot three approaches in a row and extended and retracted the gear in the air twice with no issues before our final landing which was also totally fine and the landing gear came out and locked correctly. It was only after taxiing and sitting idle during shutdown that the noise began.

The sound was so alarming that I do not feel safe to fly my plane again until the issue is properly identified and corrected. I am not a mechanic so my personal knowledge is limited when it comes to understanding this issue. I’m not even entirely sure it is related to the landing gear actuator or motor; or if either of these service documents (SI M20-112 or SB M20-190B) are even connected to this issue since I recently had it removed and shipped to LASAR for a complete overhaul (March 2021) and my actuator is a ITT LA11C2114 which is technically exempt from these service announcements.

Have any of you heard this sound / had this problem before? Any feedback/advice would be greatly appreciated. Thank you.

Welcome to Mooneyspace!

Everything you’re describing says landing gear issues.  You’re correct to not fly the plane until it is resolved.  Mooney landing gear systems are unique in that both the primary and backup manual system use the same gear set and mechanisms.  There are a number of service bulletins on landing gear actuators.  Go to the Mooney website and search for anything related to landing gear.

 

[A64Pilot]

Assuming nothings broken, the motor can’t turn if a limit switch is messed up allowing power to stay on, on the motor.

The motor would stall, no noise, but the CB would trip.

The fact that there is a noise is troubling as it seems to indicate somethings broken, or the noise hopefully isn’t gear related.

I wouldn’t fly until I found the noise, you should be able to find it by pulling all the CB’s with the master off, then turn the master on, and one at a time push the breakers back in.

Hopefully it is an old Grimes “grinder” rotating beacon.

[Guest]

I’ve seen the landing gear actuator back up, open the down limit switch and cycle off and on randomly.  But I sure haven’t seen it all yet.

[AIREMATT]

If it’s the beacon, maybe it is coincidence that it failed at this exact time when the gear was activated several times on a checkride . Otherwise, I would think that noise would be heard and recognized as a usual sound to the OP. They are noisy, but still hard to hear over engine noise and ANR headsets normally.

[cliffy]

Question-

Was your electric fuel pump on at the same time?

Did the shop turn the fuel pump on while looking for the noise?

Are there any drips or streaks coming out of the fuel pump area?

Edited February 26, 2023 by cliffy

[PT20J]

Your post is great. You provided a lot of detail and clues. Based on your description, it sounds like it is definitely a problem with the electric gear. The flickering gear down light and your description of the location of the vibration imply that for some reason the motor is cycling on and off rapidly at the end of the gear down travel. There is a gear down limit switch that shuts off the motor and turns on the green light when the gear is fully down. Suppose something is rigged too tight so that something is under tension when the switch stops the motor and then the mechanism springs back slightly unseating the switch causing the motor to run again and the process repeats. I’m not sure this is the problem, but it would explain the issue you are seeing.

I would lower the plane back onto the ground so that the weight is on the wheels but keep jacks under the wings and an engine hoist attached to the engine mount just as a safety measure and see if you can get it to happen again which might provide further clues.

In any event, I would have someone familiar with Mooneys check the gear rigging. Maybe something wasn’t set quite right when the actuator was reinstalled.

I might also call the factory service center and see if they have run into this condition.

Skip

[faridaamar]

On 2/25/2023 at 3:49 PM, Bartman said:

If the landing gear actuator was recently serviced I would start by asking questions about that. Why was it removed and serviced ? Was it installed properly ? Are the limit switches adjusted properly ?  If nothing else I would have the mechanic squirt cleaner in and around the limit switches.

Last annual while the belly pans were removed and on jack stands I located the limit switches. I squirted cleaner into and around each of them both in the extended and retracted positions multiple times and over multiple days.  My bird leaked a lot of oil before 2020 rebuild and a bit of oil mixed with dirt made for sticky switches, but no sticking flaps since they were cleaned  

 

The gear was removed and overhauled because the circuit breaker kept popping out on its own all the time and the gear wouldn't retract (was stuck in the down position). I have not experienced this problem again since the overhaul.

[faridaamar]

On 2/25/2023 at 4:39 PM, EricJ said:

The rotating beacon probably wouldn't make the Gear Down lamp flicker.

It could be that the down limit switch is dirty/sticky and maybe not adjusted properly, or perhaps just has a loose wire somewhere.   This might make the gear motor try to continue driving the gear down and could also explain the flickering Gear Down lamp.

Cycling the gear with the airplane on jacks, if they haven't already, might help isolate the problem.   The first response from the shop isn't overly confidence-inspiring, though.   There are probably other maintainers in the area that you could fly to if that helps, and you could just fly it with the gear down and pull the circuit breaker for the gear motor to keep it from doing whatever it is doing.    That'd also be a good way to tell if the motor is the source of the noise, by pulling the gear actuator breaker the next time it does it.

thank you! do you know anyone near KCCB who you would recommend / trust to deal with this kind of issue? if I fly it, pull the circuit breaker, and do not retract the gear ... how can I recreate the problem? do you think it will happen again if the motor is not receiving any power?

[faridaamar]

On 2/25/2023 at 5:36 PM, A64Pilot said:

Assuming nothings broken, the motor can’t turn if a limit switch is messed up allowing power to stay on, on the motor.

The motor would stall, no noise, but the CB would trip.

The fact that there is a noise is troubling as it seems to indicate somethings broken, or the noise hopefully isn’t gear related.

I wouldn’t fly until I found the noise, you should be able to find it by pulling all the CB’s with the master off, then turn the master on, and one at a time push the breakers back in.

Hopefully it is an old Grimes “grinder” rotating beacon.

could the opposite be possible? maybe the switch is being held "on"?

[faridaamar]

On 2/25/2023 at 6:13 PM, AIREMATT said:

If it’s the beacon, maybe it is coincidence that it failed at this exact time when the gear was activated several times on a checkride . Otherwise, I would think that noise would be heard and recognized as a usual sound to the OP. They are noisy, but still hard to hear over engine noise and ANR headsets normally.

agreed. I feel VERY familiar with my beacon and all it's grumpy noises and this sound was not that. I understand why people might suggest this without having heard the mysterious noise I am reporting, but it was so wildly different from any normal sound I am used to hearing that I literally wouldn't fly home.

[faridaamar]

On 2/25/2023 at 7:31 PM, cliffy said:

Question-

Was your electric fuel pump on at the same time?

Did the shop turn the fuel pump on while looking for the noise?

Are there any drips or streaks coming out of the fuel pump area?

my fuel pump was off before we heard the noise. I turn it off in my after landing checks after clearing the runway. the sound didn't start until after we were done taxing and starting the shutdown.

[A64Pilot]

2 hours ago, faridaamar said:

could the opposite be possible? maybe the switch is being held "on"?

Doesn’t matter, if the motor is powered for whatever reason it will run until it stops, then stall. Then there will be no real noise, it will be pulling excessive current though and the CB will pop, but that’s apparently not happening, cause the CB isn’t popping or there is no fire.

Now as a couple have suggested IF it ran until close to stall which is further than it should, then the limit switch removed power it could back up from the tension, back up enough to where the out of adjustment limit switch closed running the motor again, until it bound up and the switch opened, then repeat. A limit switch that’s slipped could I guess cause that. I’ve not seen it, but then I don’t have much Mooney experience, but it’s logical.

IF that’s it, it should be real easy to see once the belly panel is removed. IF that’s it, you really, really want to fix it as if it keeps doing that, odds are somethings going to break.

Off the top of my head there are only a few motors that could possibly make noise, Flap motor if equipped, Rotating beacon if equipped, boost pump and gear motor. Maybe Avionics blower, but I wouldn’t think you could feel it in the floor under your seat, that sure sounds like gear motor.

If you can get it to make that noise, pulling the gear CB should stop the noise and pretty much isolate it to gear motor. If the noise comes and goes with the gear CB, then it’s very likely gear related.

[Guest]

The clue to it being landing gear actuator:

 I could feel my seat vibrating a little in sync with the sound.
 

Hard to imagine that a beacon or fuel pump could make the seat vibrate.

[Bartman]

If you have not already done it, I would take Loreto to a reputable Mooney Service Center. Get a ferry permit if you need and just fly it there with the gear down and CB pulled. Something is out of adjustment.