Noise upon master switch off (M20J)

[brianucsf]

I have a very new to me 1980 M20J. When I shut it off at the self serve fuel pump yesterday, right as I switched off the master, there was a fairly loud clunk sound. It almost sounded like the clunk/thud when the gears lock in place and it seemed as if I even felt a bit of a shock/vibration when it happend. I hadn't heard that sound before, but when I then fired it up again, taxied to the tie down, and shut it down again, there was the clunk again (maybe even two in rapid succession). I've only shut this particular plane down a half a dozen times before that (which were all uneventful), and I've been in a few other M20Js and I have never experienced this sort of clunk. Any thoughts about what might be the issue?

For what it is worth, the plane has speed brakes, but the pre-buy suggested there might be issues with retraction of one of them, so I haven't touched them yet. Also, the vacuum system has been removed from this plane.

PS I have learned a huge amount from MooneySpace as I have lurked and looked for my plane - I've joined and donated now but thanks to all who have contributed a great deal of knowledge to this site.

Edited November 14, 2023 by brianucsf
Typo fix

[ArtVandelay]

Try turning the master on and off without starting the engine.

There is a solenoid that will make a pronounced noise back by the battery.

[LANCECASPER]

7 hours ago, brianucsf said:

I have a very new to me 1980 M20J. When I shut it off at the self serve fuel pump yesterday, right as I switched off the master, there was a fairly loud clunk sound. It almost sounded like the clunk/thud when the gears lock in place and it seemed as if I even felt a bit of a shock/vibration when it happend. I hadn't heard that sound before, but when I then fired it up again, taxied to the tie down, and shut it down again, there was the clunk again (maybe even two in rapid succession). I've only shut this particular plane down a half a dozen times before that (which were all uneventful), and I've been in a few other M20Js and I have never experienced this sort of clunk. Any thoughts about what might be the issue?

For what it is worth, the plane has speed brakes, but the pre-buy suggested there might be issues with retraction of one of them, so I haven't touched them yet. Also, the vacuum system has been removed from this plane.

PS I have learned a huge amount from MooneySpace as I have lurked and looked for my plane - I've joined and donated now but thanks to all who have contributed a great deal of knowledge to this site.

Do you have electric speed brakes?

[brianucsf]

ArtVandelay, thanks, I will check for the sound without engine start up today or tomorrow.

LANCESCASPER, yes, the speed brakes are electric.

[PT20J]

If you have the yoke-mounted speed brake switch, it is very easy to brush against it with your left arm when you reach around the yoke and this will extend the speed brakes without you knowing it. When you turn off the master, the brakes free-fall with a loud clunk. Scared the heck out of me the first time it happened.

Skip

[brianucsf]

PT20J, that makes a lot of sense. I will see if I can repeat the sound with that process.

[PT20J]

I once brushed that switch while reaching to change the heading bug while making a 180 to extricate myself from icing. I immediately retracted them, but the left brake had collected enough ice to not fully retract. I now pull the speed brake CB if there is any chance of entering icing conditions.

[M20F-1968]

You likely shut off the master with the speed brakes deployed.

John Breda

[Beard]

I have an 84J with the Turbo Bullet conversion, so it has speed brakes too. If I shut off the master before I retract them they will collapse quickly and make a distinctive clunk. I do extend my speed brakes from time to time after I land to slow me down so I can use less brakes.

May I ask why you have speed brakes on your 80J? What STC does it have?

[ArtVandelay]

May I ask why you have speed brakes on your 80J? What STC does it have?

I have speed brakes on my 78J, was a Mod Works upgrade.

[PT20J]

All speed brake installations are by STC. When Mooney installs them at the factory, they still use the STC.

[brianucsf]

PT20J, Beard, and M20F-1968 were spot on. The speed brakes must have gotten bumped and deployed. One of them doesn't appear to full retract every time, so the circuit breaker is pulled so as to not have another accidental deployment until I get that sorted out.

Beard, the brakes are there because there were put in by a previous owner.

Thanks to you all.

[LANCECASPER]

11 hours ago, brianucsf said:

PT20J, Beard, and M20F-1968 were spot on. The speed brakes must have gotten bumped and deployed. One of them doesn't appear to full retract every time, so the circuit breaker is pulled so as to not have another accidental deployment until I get that sorted out.

Beard, the brakes are there because there were put in by a previous owner.

Thanks to you all.

This is something that is missed on almost every annual. The Instructions for continued airworthiness on the speed brakes say that the cartridge needs to come out (not difficult) and that the worm drive (not the clutch) needs to be lubricated with Aeroshell 22 every year (do NOT use any spray lubricant any where near the speed brakes). When the grease gets old and hard the mechanism becomes very slow until it fails. Cleaning it out and re-greasing with Aeroshell 22 and exercising the brakes will quiet them down and often get you many more years of service. 

You will learn to appreciate why the previous owner spent $5000+ to install them - when used properly they are a handy tool to have in the toolbox when flying your Mooney.