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Brake Failure in Yak

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jorgen.nielsen(at)mweb.co
Guest

Posted: Wed Dec 20, 2006 12:21 pm Post subject: Brake Failure in Yak

2 weeks back went out in the Yak52 - air was a bit low after the start and taxi to fuel bay, then starting again, but still plenty enough based on normal operation for the taxi, t/o and retraction. During the taxi I kept an eye on the air, and just before needing to turn 90 degrees towards 29 (the taxiway heads directly for runway centre) I heard air hissing (over the noise of the engine with canopy open), glanced down and saw the air down from 25 to 15 (this in about 10 secs since my last glance), tried the brakes and there were no brakes, hence also no directional control. I thought the brake reducing valve had failed, booted left rudder to avoid the runway signs in front of me, flipped off the mags and immediately told ATC I had no brakes nor control and was heading for the rwy.  There was an aircraft on short final. Went onto the grass running towards 11/29 and was busy opening the emergency air as I rolled to a stop. 

 

 I could possibly have deployed the emergency air a bit sooner but thought the brakes were gone anyway (as I still had 15 bars) and had my hands were full with first trying to miss the rwy sign / comms with ATC / shutting down engine etc. It all took just a few seconds. 

 

 So, no harm done other than a few go-arounds whilst I pushed the aircraft clear. The point of all this, is on subsequent investigation we found a weird failure mode: When the air is full or above 35, no leaks. As the pressure drops a leak starts and become worse as the main air pressure reduces, leading to the situation above. Air is leaking from the emergency air pop off valve in front cockpit on RHS (just in front of brake valve), which is not supposed to be in circuit with the main air anyway due to the non-return valves separating them. So the culprit appears to be a sticky/dirty non-return valve somewhere, which is kept closed when pressure is high, but then opens as the pressure drops. The pop off valve is keeping in the emergency air, but thats where the main air is blowing out from.  

   

ATC suggested I take up some extra flying or farming lessons...  

   

Any ideas, thoughts on problem?  

       [quote][b]

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Rob Rowe

Joined: 10 Jan 2006
Posts: 124
Location: Berkshire, UK

Posted: Thu Dec 21, 2006 5:55 am Post subject: Re: Brake Failure in Yak

 	  Quote: 	
		  

Air is leaking from the emergency air pop off valve in front cockpit on RHS (just in front of brake valve), which is not supposed to be in circuit with the main air anyway due to the non-return valves separating them

	




Jorgen,



Following on from Mark & Dennis's comments ... if air is leaking from;



1 - the emergency air protection valve (that will shut (at) >5bar pressure) then;



... one or more of the actuator shuttle valves (pea sized rubber balls) is a likely suspect.



... much less likely but possible is a faulty emergency air NRV that feeds into the brake valve. Try putting the gear controls in neutral, to vent any residual air in the actuators, & see if air still escapes from the protection valve. 



2 - the cockpit gear selector control then one or more of the gear actuator seals is a likely suspect (or even the gear selector control itself)



If it helps there are some system schematics (YAK-52) at ...



http://wiki.matronics.com/wiki/index.php/Yak



Cheers, Rob R

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JBernier(at)dart.org
Guest

Posted: Thu Dec 21, 2006 7:27 am Post subject: Brake Failure in Yak

A word of caustion. I first noticed that the actuation valves for the flap was stiff. Then the gear actuation valve started to leak. The compressor kept up with the leak. Minor problem, I thought. The front gear selector valve started to leak and the compressor was not able to keep up with it. I took the system apart, air filter and all valves. Everything was filled with this gray stuff, that if it came out of my nose I'd have to have surgery. The short of it is, big problems start with small signs. Don't wait. Look into it at the first sign of a sticking actuation valve or lose of pressure. When its working right its a great system. When it starts to go wrong, it cascades.

With the help of Doug, I fixed it.

Jim B   



 	  Quote: 	
		  >> yak-list(at)robrowe.plus.com 12/21/2006 7:55 AM >>>

	


 	  Quote: 	
		   

 Air is leaking from the emergency air pop off valve in front cockpit on RHS (just in front of brake valve), which is not supposed to be in circuit with the main air anyway due to the non-return valves separating them

 


	




Jorgen,



Following on from Mark & Dennis's comments ... if air is leaking from;



1 - the emergency air protection valve (that will shut (at) >5bar pressure) then;



.. one or more of the actuator shuttle valves (pea sized rubber balls) is a likely suspect.



.. much less likely but possible is a faulty emergency air NRV that feeds into the brake valve. Try putting the gear controls in neutral, to vent any residual air in the actuators, & see if air still escapes from the protection valve. 



2 - the cockpit gear selector control then one or more of the gear actuator seals is a likely suspect (or even the gear selector control itself)



If it helps there are some system schematics (YAK-52) at ...



http://wiki.matronics.com/wiki/index.php/Yak 



Cheers, Rob R





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=82830#82830

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dsavarese(at)elmore.rr.co
Guest

Posted: Thu Dec 21, 2006 8:32 am Post subject: Brake Failure in Yak

No truer words were ever spoken.

Dennis



---

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aihuabao(at)yahoo.com
Guest

Posted: Thu Dec 21, 2006 9:28 am Post subject: Brake Failure in Yak

 	  Quote: 	
		  >....Everything was filled with this gray stuff<<


	


Please, whats the gray stuff?





--- "A. Dennis Savarese" <dsavarese(at)elmore.rr.com>

wrote:



[quote] 

 <dsavarese(at)elmore.rr.com>

 

 No truer words were ever spoken.

 Dennis

 

 ---

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