Locked Brake

[coppercity]

This has likely happened to a few folks on the forum.  

One of our students set the parking brake during startup, but was unable to taxi due to the right brake not releasing.  As you can see in the photo the caliper puck ended up uneven and was unable to retract after pressure was released.  The brake pads were about halfway worn, still within limits but allowed the puck to move out further.  I suspect the student pulled some significant pressure while setting the brake and since the brake pad plate only covers half the puck, the lower uncovered portion was forced out a little further.  

Keep an eye on brake pad wear, and don't use gorilla force when setting the parking brake.

 

[FlyingMonkey]

Yup, I had the same thing.  Mine was definitely caused by pad wear -- the pad was just at the end of the wear indicator.  I now change pads when there is about 20-25% of the indicator left.  If you can no longer see a distinct curve in the "U" of the indicator, and just see a notch there, it's past time for sure.  The Matco pads are cheap enough per set that this is not really a cost issue.  I fly a couple of times a week and I'm not afraid to get on the brakes, so I go through pads about a set every 12-18 months.  If you have the tool to remove and set the shoe rivets, you can change a set in about an hour.

It's critical to change pads before this happens, if you land with a locked brake, or it locks when you first touch the brakes on landing, you *will* be off the runway and in a ditch in a hurry.

[FlyingMonkey]

BTW, I now keep a small vicegrip pliers in my airplane.  If a brake locks on a taxiway, you can clamp the puck on the "popped out" side and get it to straighten out so you can taxi back to the ramp. 

[Duane Jefts]

Had something similar.  After landing on 6L at Ryan field in Tucson, I was holding short of 6R for traffic.  When cleared to cross 6R the plane did not want to taxi.  Both brakes were locked up and required two dollies under the mains and a push to the hangar.  My hangar is not far from Roger Lee's hangar so a couple pucks later - back on the road again. 240 hours on the plane since new.

Thanks Roger.

 

[FlyingMonkey]

Just now, Duane Jefts said:

Had something similar.  After landing on 6L at Ryan field in Tucson, I was holding short of 6R for traffic.  When cleared to cross 6R the plane did not want to taxi.  Both brakes were locked up and required two dollies under the mains and a push to the hangar.  My hangar is not far from Roger Lee's hangar so a couple pucks later - back on the road again. 240 hours on the plane since new.

Thanks Roger.

 

Wow, both brakes at the same time...that almost sounds like a problem with the master cylinder not releasing pressure instead of a pad wear problem, but from your last sentence it sounds like maybe they just both gave up at the same time...?

[Duane Jefts]

We started trouble shooting master cylinder and brake lever first as that seemed logical but in the end, just replacing the pucks fixed the problem. 

[FlyingMonkey]

8 minutes ago, Duane Jefts said:

We started trouble shooting master cylinder and brake lever first as that seemed logical but in the end, just replacing the pucks fixed the problem. 

Interesting!  I wonder if they just had a bad lot of pucks that were machined a few thousandths too small.

[Anticept]

The pistons are not long enough, and are relatively soft. There's a small o-ring just out of view in OP's photo which further reduces the contact surface when it's extended.

To be frank, while I like matco wheels, the off center thrust on these pistons is terrible.

Finally: brake disc thickness is quite important and is a significant factor! If you're having to replace liners more frequently, even when there's quite a bit of wear left, you need to do the thickness measurement as prescribed in the matco technical manual. It's wearing from BOTH sides, so while it doesn't look significant, just think what you see for wear x2.

[coppercity]

Another consideration is the spacing of the wheel/disc from the caliper, there are two thickness washer/spacer on the CTLS wheel, the thinner one on the inside will place the disc closer to the caliper and keep the puck further in.