lucky333
Well Known Member
I was reading the 'Not necessarily life threatening but could ruin your day' thread and its a good one. GDU000 relates what happens when a brake fails on a Cirrus. On the -A RVs, it can be catastrophic, as a friend of mine learned the hard way. Ironflight wondered what's different about the RV brake system so I thought I would relate what we found out after the right brake failed suddenly during taxi. He pressed both brakes to slow down. One worked, the other did not with the subsequent loss of control. While there was damage to the plane, he did a very good job of realizing the problem quickly, securing the engine and minimizing it. Good show there, but what happened?
There have been several instances of brake failures in the RVs reported on the boards, most without damage. My friend was not so lucky but that's beside the point. Picking up the pieces, we investigated the causes and here is what we found.
Looking at the parts:
The brake:
There was no real obvious reason that the brake failed, but all of the fluid ran past the single 'O' ring on the caliper piston, causing failure. The 'O' ring looked OK and was still pliable (and the puck was in the right way). There was evidence of the brake assembly getting hot (wrinkly label) but not too much different than the solid left one, which also had a hot looking label.
The pedals:
Seemed to be OK with no obvious friction in the linkage.
Braking action:
Seemed to be OK too but as we will see later, may not have been so good.
While waiting for new parts, we decided to look into why the caliper seal failed. Like many avaiation accidents, it summed up to be a combination of things.
First stop: Kinetic energy requirements for aircraft brakes and found this:
http://www.infinityaerospace.com/Brake_Kinetic_Energy_Req.htm
It discusses the FAR Part 23 requirements for how much kinetic energy (essentially how hot they get) that brakes should be able to absorb. If you crunch the numbers ( http://www.groveaircraft.com/braketechinfo.html for a calculator) you might be suprised at the required Ke that your brakes have to absorb to get you stopped.
The ratings on the standard -7A brakes are in the 110K to 120K ft-lb range. I looked it up but don't have it handy..
Result for MY plane: the standard brakes are about 1/2 of this figure. Take an aborted takeoff in Amarillo in the summer (>9K density altitude) and we are way short..
Now, I know that pilot technique enters into this. Minimum speed landings and all that. But, if my engine chatters on takeoff from Amarillo on a summer's day, I want to be able to hit the brakes and get results.
Second stop:
Looking further into the brake system, we were told by some brake guys, some selling us stuff but others, too, that the standard Matco master cylinders do not have enough spring pressure to overcome friction in the pedals et al. I was also told that the Matco piston has to be fully extended to relieve the pressure on the calipers and that, combined with the low spring pressure can cause the brakes to drag. The RVator had an article on this too I think, with slots being milled into the shafts to get some slop. My friend thought this might have been happening on the right side but it failed before he could look into it. (He thought all it would cost him would be an early pad replacement..). MORAL: As in all things aviation, fix it NOW.
For us: We decided on the Grove 56-1A wheel/brake. Bolt right up. Minor changes to pants. Look at numbers and decide for yourself. Mine look good in the box. Buddy is flying with his. Very smooth. Easy to modulate. Lock tires at will. I blew off Van's AirHawk tires after seeing buddy's. Worn. AvConsumer rated Goodyear Flight Custom II tops. Bought em. Look good on wheel. 1yr before flight to tell. Robbie Grove knows brakes.
Regards,
RJO
There have been several instances of brake failures in the RVs reported on the boards, most without damage. My friend was not so lucky but that's beside the point. Picking up the pieces, we investigated the causes and here is what we found.
Looking at the parts:
The brake:
There was no real obvious reason that the brake failed, but all of the fluid ran past the single 'O' ring on the caliper piston, causing failure. The 'O' ring looked OK and was still pliable (and the puck was in the right way). There was evidence of the brake assembly getting hot (wrinkly label) but not too much different than the solid left one, which also had a hot looking label.
The pedals:
Seemed to be OK with no obvious friction in the linkage.
Braking action:
Seemed to be OK too but as we will see later, may not have been so good.
While waiting for new parts, we decided to look into why the caliper seal failed. Like many avaiation accidents, it summed up to be a combination of things.
First stop: Kinetic energy requirements for aircraft brakes and found this:
http://www.infinityaerospace.com/Brake_Kinetic_Energy_Req.htm
It discusses the FAR Part 23 requirements for how much kinetic energy (essentially how hot they get) that brakes should be able to absorb. If you crunch the numbers ( http://www.groveaircraft.com/braketechinfo.html for a calculator) you might be suprised at the required Ke that your brakes have to absorb to get you stopped.
The ratings on the standard -7A brakes are in the 110K to 120K ft-lb range. I looked it up but don't have it handy..
Result for MY plane: the standard brakes are about 1/2 of this figure. Take an aborted takeoff in Amarillo in the summer (>9K density altitude) and we are way short..
Now, I know that pilot technique enters into this. Minimum speed landings and all that. But, if my engine chatters on takeoff from Amarillo on a summer's day, I want to be able to hit the brakes and get results.
Second stop:
Looking further into the brake system, we were told by some brake guys, some selling us stuff but others, too, that the standard Matco master cylinders do not have enough spring pressure to overcome friction in the pedals et al. I was also told that the Matco piston has to be fully extended to relieve the pressure on the calipers and that, combined with the low spring pressure can cause the brakes to drag. The RVator had an article on this too I think, with slots being milled into the shafts to get some slop. My friend thought this might have been happening on the right side but it failed before he could look into it. (He thought all it would cost him would be an early pad replacement..). MORAL: As in all things aviation, fix it NOW.
For us: We decided on the Grove 56-1A wheel/brake. Bolt right up. Minor changes to pants. Look at numbers and decide for yourself. Mine look good in the box. Buddy is flying with his. Very smooth. Easy to modulate. Lock tires at will. I blew off Van's AirHawk tires after seeing buddy's. Worn. AvConsumer rated Goodyear Flight Custom II tops. Bought em. Look good on wheel. 1yr before flight to tell. Robbie Grove knows brakes.
Regards,
RJO
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