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252 Vacuum Speedbrakes question


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Question for 252 owners and maintainers that have the vacuum speedbrake system. 
What engine rpm do you need to get normal extension of the speedbrakes? 
I have never paid attention before, but I recently noticed that my speedbrakes don’t deploy at idle power on the ground. Maybe that’s normal as the vacuum pump will be turning pretty slowly. They work fine in the air, which is normally the only place I activate them.

Just trying to determine if I have a vacuum leak or if all is normal. I have no vacuum instruments so the pump is only running the speedbrakes.

Thanks

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Mine used to deploy on the ground or anytime the engine was running, even at idle. But they don't anymore. It has gotten to the point where they won't deploy on final with the engine at idle power either. 

It could be a weak vacuum pump or leaky vacuum line. It could also be one of the boots on the speed brakes as well. I'm pretty sure my issue is with the vacuum system and not the speed brakes themselves. I have to get up to about 22" to get the vacuum gauge to wake up.

My vacuum brakes are being replace with electric brakes right now during the Encore conversion.

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Thanks Paul. My servo in the belly is in excellent shape. Rubber is not stiff or brittle. So I am guessing I have a vacuum leak from my recent panel work. Finding it may be a challenge however.

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If you have a backup vacuum system installed, you could try to extend the brakes at idle with the backup pump on to see if that changes anything. I check mine just before starting the run up so c. 1000rpm.

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Check the conduits running in the wheal well for cracking, which adds quite a bit of resistance to extending the brakes. Grab it with your hand and see if it brittle. It doesn't last 35 years! The system only needs idle vacuum. The vacuum line is pretty strong plastic/nylon. Leak is unlikely if the cannister is in good shape.


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Mine are slow to deploy and retract in the cold weather in my 231 so that could be a factor.  They may also need some maintenance to reduce that effect as it’s really annoying when they take their time to retract in flight. 
 

At one point, I had a cracked vacuum line and they were really slow to deploy as well. 
 

 

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  • squeaky.stow changed the title to 252 Vacuum Speedbrakes question

@J Bird that is surely one way to service them...

There is a discussion around here about the best way to clean and lube them...

Slow, is a typical dirty, lack of lubricant issue...

No longer attached to the drive... that is a worn out clutch syndrome...

 

I sent my for service when the clutches were worn... I hardly ever used them.  So I assume the prior owner must have liked them... :)

Mine get tested prior to flight. Not much activity after that...

Best regards,

-a-

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One should not confuse service requirements of electric brakes with Vacuum brakes. They are entirely different. Precise does not service Vacuum brakes as any A&P can replace parts that is causing the issue. Vacuum brakes utilize a sealed bearing that Precise says not to lubricate since you can't get lubrication in past the seal, but by trying too one enables the bearing to attract more dirt.

  • The bearing is replaceable, but the brake has to be de-riveted to get at the bearing inside and then riveted back together. 
  • The spring is replaceable that pulls the brake back down. Easiest fix but if the spring hasn't broken its probably okay.
  • The conduit and cable are replaceable when it becomes brittle and begins to crack. This will cause the brake to extend slowly and also hang while retracting -  making many think the vacuum is weak. The system doesn't need more than ordinary vacuum, including idle vacuum.
  • The cannister that pulls the cable when vacuum is applied used to be replaceable but not currently - Brittain was about to supply them when they went out of business.
  • Of course a vacuum leak from the pump to the cannister can be an issue, but there will probably be other symptoms of a vacuum leak as well.
  • The electrical relay parts, switches etc are replaceable but rarely cause a problem. 
Bottom line is they are very simple system. But after 20+ years they may need some maintenance to replace some parts
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We did a thorough inspection of all components during my annual this past week. From the vacuum canister to the brakes themselves everything was in remarkably good condition for a 35 year old airplane. No stiff cables or bellows or broken springs.
During my recent panel upgrade I had the last remaining components of the vacuum system behind the panel tidied up and redundant hoses removed, along with a ton of extra wiring from removed systems. There were a lot of extra hoses that were capped and left when all of the vacuum instruments were removed some years ago. 
Since the speedbrakes were working well prior to this upgrade, my strong suspicion is that something got left open in the vacuum system behind the panel during this work.

I am hunting down an automotive smoke generator used for testing car vacuum systems for leaks. Hopefully that will help us pinpoint the leak. Should have an answer late next week.

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  • 2 weeks later...

Call Don Maxwell-he may be able to rebuild the cartridge.  My vacuum operated brakes finally would not deploy.  Service tech siliconed the holes in the boots up,  lasted about 4 flights.  Rebuilt the cartridges and about 3 pounds or less will deploy the brakes.  Much cheaper than the electric, although I could deep six the vacuum pump if I had converted to the electric-my panel is all glass now.

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On 1/21/2021 at 9:09 PM, J Bird said:

I have the electric speed brakes and from this link I guess the new ALC unit as my left brake is slowly going up and gets stuck to which it slams down and the right side sympathetically drops too. I thought this link would show how to lubricate but all this says is to remove both speed brakes and ALC and send them in for overhaul starting at 2amu’s. Does anybody know of a maintenance guide for where to lubricate areas and what type of lubricate to use? Mouse milk, 10w-40, grease?  

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On 1/20/2021 at 8:16 AM, gsxrpilot said:

Mine used to deploy on the ground or anytime the engine was running, even at idle. But they don't anymore. It has gotten to the point where they won't deploy on final with the engine at idle power either. 

It could be a weak vacuum pump or leaky vacuum line. It could also be one of the boots on the speed brakes as well. I'm pretty sure my issue is with the vacuum system and not the speed brakes themselves. I have to get up to about 22" to get the vacuum gauge to wake up.

My vacuum brakes are being replace with electric brakes right now during the Encore conversion.

 

Hi Paul, @gsxrpilot, do you have an estimate on what your vacuum to electric speed brake conversion will cost? Feel free to PM if you'd like. Issues seem to keep popping up with my vacuum system and I think it's time to bite the bullet during the next annual. My speed brakes are the last component still operated by vacuum.

 

Thanks,

Brett

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1 minute ago, Florg said:

 

Hi Paul, @gsxrpilot, do you have an estimate on what your vacuum to electric speed brake conversion will cost? Feel free to PM if you'd like. Issues seem to keep popping up with my vacuum system and I think it's time to bite the bullet during the next annual. My speed brakes are the last component still operated by vacuum.

Hi Brett, I think my speed brake conversion will come in around $3K all in. But this is a bit of a unicorn deal. 

  • I was able to buy a second-hand set of electric speed brakes for about $2K. 
  • A new set of speed brakes from Precise Flight are $6600. 
  • The used speed brakes I found, are an earlier version of the Precise Flight speed brakes and fit into the wing much like the vacuum brakes I'm taking out. 
  • The current version of brakes available from Precise, evidently takes a bit more modification to install. This means the labor costs would probably be higher.

I'd probably budget $10K for a full, brand new, electric speed brake conversion.

*It only took me 5 years of looking to come across the used set of brakes that I was able to acquire.

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3 minutes ago, gsxrpilot said:

Hi Brett, I think my speed brake conversion will come in around $3K all in. But this is a bit of a unicorn deal. 

  • I was able to buy a second-hand set of electric speed brakes for about $2K. 
  • A new set of speed brakes from Precise Flight are $6600. 
  • The used speed brakes I found, are an earlier version of the Precise Flight speed brakes and fit into the wing much like the vacuum brakes I'm taking out. 
  • The current version of brakes available from Precise, evidently takes a bit more modification to install. This means the labor costs would probably be higher.

I'd probably budget $10K for a full, brand new, electric speed brake conversion.

*It only took me 5 years of looking to come across the used set of brakes that I was able to acquire.

Great, thanks for the quick response, Paul. $10K is about what I've heard before which makes it a hard pill to swallow but I think it's time. Well done on finding your set of speed brakes and I hope you enjoy your Encore conversion when it's all done!

Thanks!

Brett

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