Uncommanded Parking Brake Activation

[rbowen]

This is a head's up to all. On roll-out after landing, I used left brake to turn around, and that brake set in the locked position. I found that the parking brake set even though the parking brake knob was not pulled out. What happened is the shaft that goes into the brake master cylinder (rudder pedal on one end; the brake master cylinder on the other) had gummy brake fluid on it, and it pulled the parking brake bar/arm up so that the brake set. the spring which is on this arm apparently wasn't sufficiently strong to keep the locking arm down.

I recommend all clean this master brake cylinder arm with solvent to assure no brake fluid is on it. I never use my parking brake, so I safetied both master cylinder parking brake arms in the "down" position so there is no chance this can happen again. Had I checked the brakes in the air to assure pressure was available (many believe this is a good practice) it could have locked that brake before landing.

The Cessna 170 maintenance forum suggests disconnecting the parking brake cable because several members have inadvertently activated the parking brake knob thus landing with the brakes set, with predictable results. Disconnecting the parking brake cable wouldn't have prevented my occurrence--it happened because the master cylinder rod had brake fluid on it and the parking brake activating arm stuck to it. That's why I safetied the arms down.

If you wish to retain the parking brake function, then I suggest you regularly clean the exposed areas of the brake master cylinder arm so that any gummy fluid residue is removed. It's not easy to access, but this is important.

[andy]

I've had it happen too. I pump the brakes 3 times as part of my pre-landing check to make sure they won't lock up on landing.

[Andy Young]

When I bought my M-6, the parking brake arms had already been completely removed from the aircraft, likely for this same reason. I've seen no reason to put them back on. I have contemplated trying to get a field approval to install the Americvan Champion-style parking brake. They use a hydraulic valve in the line between the masters and the calipers. As long as the knob is in, the valve is open, and allows flow through it in both directions. When the knob is pulled, it becomes a one-way valve, and will allow brake actuation, but not release. I've found it to be a fool-proof and maintenance free system. Not sure why Maule chose that funky system they use.

Andy

[maules.com]

Another culprit is the plastic plug, if not inserted correctly it will hold the locking lever upward.
If all is inspected each annual there should not be any sticky buildups however I have see LPS3 used for lube at the brake pedal pivot points and that is asking for trouble. LPS2 okay.
GUMPS. U standing for undercarriage 'brakes'. Always at least two brake activations prior to landing. First is to test for locked or free or pressure. Second is to recheck for spongy feel rather than hard, in case the lever is up and the brakes are inadvertantly locked in First press.
It is easy to mistake brakeknob with heat, ventilation or carb heat knobs.

[woodzi]

Reviving an old thread.

So today, as I was coming to a stop, I veered to the left. Not good with 8' corn stalks 3' off the wingtip. I got it stopped, applied some right brake to get back on the centre of the runway and it wouldn't move. Stomped on both brakes and away I went.

We lifted the wheel off the ground and played with the brakes and were able to reproduce the problem. With left rudder in, applying a bit of brake causes the parking brake actuator to touch the insulation on the firewall and it stays up, just like pulling the parking brake lever.

Obviously, removing the lever will solve the problem. Any other suggestions?

[andy]

You might already know this but I pump both brakes 3 times when I start the landing sequence to make sure the tab isn't stuck up on either side. At each annual I clean and lubricate the arm to reduce the possibility of any material on the arm preventing the tab from dropping down. It's a dirt magnet since it's so close to your feet. Most of us never use the parking brake due to the possibility of the tab sticking in the up position.

[plowboy]

if you pump the bakes before landing and they stick on, without the knob pulled, what are you going to do at that point

[TimB]

One needs to inspect and maintain the master cylinder shafts at every annual. This problem as lead to 2 damaged aircraft that I know of and one that frame tube at the lower master cylinder support tab ripped out- see SB #24 as the brakes jammed on and the owner continued to pump the brakes to get the parking brake off and ripped the mounting tab off. Maule has modified the Cleveland 10-35 master cylinder per drawing 4046B by adding the parking brake lever and the shaft was never designed to have a parking brake lever gouging the shaft. This lever scores and damages the shaft by "C" shaped gouges that enlarge the diameter of the main shaft and jam with the top bushing and as you said clean excess brake fluid from the top of the cylinder assembly. To remove the scores on the shaft, use a fine file and lift the lever spring and remove scoring damage.

[G-MAWL]

I fitted springs to keep the tabs down. Easily overcome when pulling the brake on but stops the taps lifting by themselves.
Also had the shaft of the brake cylinder case hardened. Tab still holds fine but doesn't dig in.

Derek

[AndrewK]

With left rudder in, applying a bit of brake causes the parking brake actuator to touch the insulation on the firewall...

If memory serves, you can gently turn the tab (or maybe it is the entire top lid of the master) away from the firewall. Maybe move it just enough to avoid contact with the insulation and monitor it in case it walks its way back?

[andy]

To answer plowboy's question, I did have the left brake stick one time during a 3-point landing after I had pumped the brakes. The airplane swerved violently to the left when I touched down. Since the tail was on the ground, it didn't nose over or even raise the tail. Later, I discovered that the piston shaft was gummed up with dirt. That made me much more diligent about inspecting the condition of the shafts and tabs. That incident is one of the reasons that I favor tail-low wheel landings or 3-point landings over tail-level wheel landings. I put a scale under the tail wheel one time with full main fuel and normal baggage. It came in at about 250 lbs. That's a lot of mass that will act like a big lever if the brakes are jammed on with the tail in the air. It's very hard to overcome movement of that much mass with elevators at low airspeed.

Keeping the shafts and tabs clean, lubricated, free of obstacles and not using the parking brake are strategies that minimize the possibility of a problem, but some owners have completely removed the tabs. I would do that too but for the occasional times that I've needed to use the parking brake temporarily to keep the airplane from rolling on a slope until I chock the wheels. You can't always find a level place to park in the back country.

[Andy Young]

I would do that too but for the occasional times that I've needed to use the parking brake temporarily to keep the airplane from rolling on a slope until I chock the wheels. You can't always find a level place to park in the back country.

True, though in those situations I’ve always been able to find something in the airplane that I could toss behind a wheel to serve as a temporary chock so I could let off the brakes and get out. Parking sideways on the hill is often a solution as well. Personally, I far prefer these options to leaving the parking brake installed. Too many of these stories of them being engaged inadvertently.

[crbnunit]

I have also looked into the American Champion hydraulic valve solution. Mine came to me with no parking brake and the tabs removed. I have only had one issue with no parking brake when a tail wind picked up and blew the Bushwheels over the chock rocks. The airplane started accelerating down hill with me watching from about 200 yards away... Thank God for tussocks!

[Andy Young]

I have also looked into the American Champion hydraulic valve solution. Mine came to me with no parking brake and the tabs removed. I have only had one issue with no parking brake when a tail wind picked up and blew the Bushwheels over the chock rocks. The airplane started accelerating down hill with me watching from about 200 yards away... Thank God for tussocks!

I’d forgotten about that option. We discussed it a while back in another thread. I remember finding and posting the Maule drawing number for doing the American Champion-style valve; with that drawing, the paperwork should be a slam-dunk.

Glad the tussocks caught your plane; similar thing happened to a friend a few weeks ago, with similar results.

[Andy Young]

By the way, that drawing number (for the hydraulic parking brake) is 9117E.