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Brake / tire issue

DavidHarris

Well Known Member
I had a wheel lock up while taxiing at Tuscon yesterday afternoon about 3:30 pm.

The weather forecast indicated 110 degrees. My OAT gauge read 120, and the temperature on the tarmac was probably 150. I had a 2-mile taxi from the FBO to the departure runway. I was behind a slow-moving airplane and had to apply occasional braking even at low thrust, and also some braking for differential steering in a modest crosswind. I was concerned about overheating brakes and mentioned it to my son.

Shortly before reaching runup area, the plane made a hard turn to the right and came to a complete stop. I tried starting again and the plane continued turning right, so I shut down the engine on the taxiway, notified ground control, and got out to investigate. I found the wheel fairing resting on the ground, with the tire completely flat. I was able to remove the fairing, and found the tire pulled off the rim, the tube flat, and the brake locked. The right wheel would not turn when I pushed the airplane. I removed the brake pad, pushed the airplane off the taxiway, and had a 2-hour adventure with the facility manager and three FBOs getting the plane to an FBO. I'm grateful to a bunch of kind people at TUS. This morning the mechanic replaced the tire and I reattached the brake pad and did a ground test, then ferried the aircraft back to KCCB in SoCal, thankfully with a short taxi to the crosswind runway and all before it got too hot.

I'm looking for ideas about the root cause. I have standard Vans RV7A Matco wheels and Cleveland brakes. I think the most likely reason is that the brake locked. I don't know too much about brake failure, but the temperature and length of taxi are both much higher than usual so I suspect it could be heat related. It is also conceivable that something such as unseen debris on the taxiway could have popped the tube, and then the tire deflated, and somehow this led to stresses that locked up the brake. In any event, the brake behaved normally when I reassembled it this morning.

As another possible clue, I've been dissatisfied with my right brake. The plane started flying 15 months ago and I have about 200 hours on it. The brake pad squealed frequently until I replaced it, and occasionally still squeals. The braking action is less effective than I'm accustomed to in a Cessna or Bonanza, and I usually brake gently and have a rollout of more than 1000' before coming to a complete stop. The brakes generate a lot of fine dust that needs cleaning. I'm also not wild about the tube design; I think I follow all the directions about talc powder and repeatedly inflating and deflating but I've had to replace two failed tubes so far on the airplane.

I'm also looking for feedback on a remedy. I've worked on the tires enough times already that I'm feeling fed up, as well as concerned about a ground loop if it fails on landing. It looks like the Beringer wheel and brake system is more reliable, and possibly has better braking action. It's fairly pricey, but I'm considering replacing all three wheels and two brakes with Beringer. Any thoughts or alternative suggestions?

Thank you,

David Harris
 
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My 2 cents

If brakes, why just one side? I suspect tube slipped under low pressure, tube was compromised, you got a flat. Flat Tire rolling on rim will not roll, thinking brake not locked. Continue enjoying your efforts…. don’t over think. Beringer good stuff, most have a version of your setup with no issues. Go fly !
 
David,

I suspect your right brake may have locked up because your right master cylinder did not fully retract. This will trap the fluid in the line, caliper, and master cylinder. When the fluid heats up, it has nowhere to go besides extending the piston in the caliper (thus applying the brake).

There are fixes in the archives including retract springs and one long brake pedal pivot bolt instead of two short ones. It's also possible to trap fluid by never completely taking one's toe off the brake pedal while compensating for a crosswind, for instance.

If it helps, consider yourself lucky that the brake locked up and ruined the tire rather that starting a brake fire.

Did you replace the right brake pads only, or both sides? How does the wear compare side to side?

Like gasman, I'm wondering about your tire pressures.
 
Trim the wheel pants?

First few years of flying my -6A I had some unexplained flats. Hole in tubes with nothing penetrating the tire. EAA friend recommended looking at clearance between wheel pants and main gear tires.

Recommendation was having a thumb-width clearance between tire and wheel pants. Got out a Drem**l tool and carved up some fiberglass to increase the clearance.

No more unexplained flats….

Carl
..
 
I have had a brake drag, get hot, and not release. It doesn't sound like what you described. In my case, the brake slowly dragged more and more, not suddenly like you experienced. Of I were to guess, could your tire have been low to begin with? And the heat of taxing, may have caused the tube to fail? The lock up may have been the tire jamming in the wheel pant or into the caliper and brake pad? How did the wheel turn after you reassembled things? I imagine the mechanic set the wheel bearing, spun the tire to check bearing drag, then torqued the brake pad, spun the tire again to check drag. .If the brake disk gets warped, I've made shims to space out the outer pad.

Another concern is that you mentioned previous tube failures. In almost 13,000 hours, I've had 2 flats. One on a tubeless Caravan tire, and one on a dry rotted Cessna tube that the valve stem fell off while in the hangar. None while in motion. Is it possible that you are pinching the tube between the wheel halfs? Try putting a tiny bit of air in the tube when you assemble the rim, check that the rim makes a metal to metal sound that indicates you don't have a tube getting pinched. Then do the 3 inflate/deflate cycles before you go to final pressure. What PSI do you run, and how often do you check them?
 
Thanks to all these ideas.

I've been running my main tires at about 25-30 psi and my nose wheel at 30-35 psi. I visually inspect the tires at each preflight but only have been removing the wheel pants to check pressure when the tire looks low. My last check would have been in March. I measured my left tire after the event and it was still reading 26 psi.

Last fall I did increase the clearance of my wheel pants to at least 3/4" all around.

I'm fairly confident the right brake was locked. When I tried pushing the plane off the taxiway, the right wheel wouldn't budge with three of us pushing. The airplane just wanted to go in circles. Once I removed the brake caliper, three people collectively were able to push the plane back, though the right wheel was of course more reluctant. I don't know what else would have explained that. It is conceivable that a flat occurred first and then something about the flat or sudden stop cause the brake to behave locked, but I can't explain exactly how that would happen.

After reinstalling the wheel, I spun it while the plane was jacked up. The disk doesn't seem significantly warped. There was a very light rubbing of the brake pad on the disk, which I think is normal, and the rubbing varied slightly as the wheel turned, but not much.

I haven't checked the left brake pad since March, but the right has a lot of life in it today and the left brake was in good condition in March.

I thought I was careful about inflating and deflating and avoiding pinches the first time I put the tires in, and I've reread the instructions carefully and been even more careful on my subsequent tires, but I realize I'm more than capable of error.

I know I was not riding the brake continuously; I was very aware of the risk of riding brakes in this situation and was tapping the right brake briefly every few seconds as required for directional control, then fully releasing.
 
I think you have your main tire pressure too low. You’d be surprised how much better it rolls with say 30-35lbs. Strange how the tube slips inside tire, and pin holes appear.
 
I run my mains at 50psi. 25 is pretty low, remember, they have to spin up from 0-60 in an instant. When you assemble the wheel, make sure the bead is clean of talc (you are using talc on the tube right? You want to tube to be able to move in the tire and the tire to not move on the rim. After you assemble the wheel inflate and deflate the tube a little a few times to make sure there is no twist in the tube.

If the brake did lock up then, as noted before, the pedal not retracting (master not extending) all the way is the most likely culprit. There is a mod to add springs on the brake master and improve the pedal pivot. These are easy to do and super worthwhile.
 
Loose bearings can create brake dragging. 25 psi is way too low IMHO. Possible that tube blew and the now deflated tube was causing an excessive bending moment on the wheel , due to the loose bearings, and that allowed that bending to cause pressure against the pad.

Larry
 
I agree with the above comments about tire pressure. Your brake may have locked up but it very likely could have done so after a deflated tire caused secondary problems.

25-35 is much too low. I run 45 on the mains, 50 on the nose. They should also be checked regularly with a gauge as it is difficult to tell the pressure level just by looking.
 
When I read your AOG thread my first inclination was that you actually had a tire failure, not a brake failure, but decided to wait for you to supply additional info before commenting.
I am even more sure now.

Your low air pressure is likely the cause.
 
Dave, thank you for the Desser table. I remember looking at it a year ago and finding it less than conclusive. The table for 5.00-5 6 ply says maximum unloaded inflation of 50 psi and says "Always consult with your owners manual for factory recommended pressures!" I'm not sure how to get from the maximum value to a recommended value.

I went through every thread I can find on VAF about tire pressures. The numbers come from a variety of aircraft, but the pressures in the threads don't seem to have any correlation with gross weight, and numbers for the RV6/7 vary from 20-50 psi. Here's a list of numbers various users have reported on the site. The mean is 36 psi and the standard deviation is 7.6. The range is extreme and the range of expertise of the posters on this site is also fairly broad.

Of the posters, Vic and Walt stand out as having an exceptional level of knowledge. Vic recommended 50. Walt recommended > 35 ps, checking pressure every 90 days, and using Beringer wheels.

It's interesting to see that the Vans website says 25-35 but Scott feels 26 is too low.

Looks like I'll try filling to 45 and refilling at 35 or at least every 90 days, and see if I have any shimmy issues. I'll put off the Beringers until I see if this makes a difference.

David

Jlitd 50
Vic Syracuse 50
XHEIAPErr 50
David-aviator 45
Special Delivery 45
DenKK90989 45
zilk 42
AlexPeterson 35-45
Dmadd 40
Marc DeGirolamo 40
db1yg 40
Marc DeGirolamo 40
flyboy1963 35-40
Bob Martin 35-40
Walt Aronow > 35 psi, check every 90 days, likes Beringer, finds flats come from low pressure
plehrke 35
steve 35
lostpilot28 35
hngrflyr 35
Svanarts 35
BendRV-4 32
robertahegy 30-35
Sam Buchanan 24-35
Kyle Boatright 24-35
Geico266 30
jhalrv4 30
smokyray 30
John_RV4 25-32
CraigH@KRPH 23-32
vac 24-27
n5lp 20-30
DeltaRomeo 20-30
randylervold 20-22
lhawkins 20
 
tire pressure

most cessna,piper 2 and 4 place aircraft,the POH, say 25 to 34 psi in the mains. rv's have less weight on the mains. so why should the psi in the RV need to be higher?
not trying to start an argument,just looking for explanation.

i used 32psi in the mains on my rv-6a for the 23 years i flew it. never had a flat in that time. i run 24 psi in the mains of my rv-3. works for me.
lawrence
 
Dave, thank you for the Desser table. I remember looking at it a year ago and finding it less than conclusive. The table for 5.00-5 6 ply says maximum unloaded inflation of 50 psi and says "Always consult with your owners manual for factory recommended pressures!" I'm not sure how to get from the maximum value to a recommended value.

I went through every thread I can find on VAF about tire pressures. The numbers come from a variety of aircraft, but the pressures in the threads don't seem to have any correlation with gross weight, and numbers for the RV6/7 vary from 20-50 psi. Here's a list of numbers various users have reported on the site. The mean is 36 psi and the standard deviation is 7.6. The range is extreme and the range of expertise of the posters on this site is also fairly broad.

Of the posters, Vic and Walt stand out as having an exceptional level of knowledge. Vic recommended 50. Walt recommended > 35 ps, checking pressure every 90 days, and using Beringer wheels.

It's interesting to see that the Vans website says 25-35 but Scott feels 26 is too low.

Looks like I'll try filling to 45 and refilling at 35 or at least every 90 days, and see if I have any shimmy issues. I'll put off the Beringers until I see if this makes a difference.

David

Jlitd 50
Vic Syracuse 50
XHEIAPErr 50
David-aviator 45
Special Delivery 45
DenKK90989 45
zilk 42
AlexPeterson 35-45
Dmadd 40
Marc DeGirolamo 40
db1yg 40
Marc DeGirolamo 40
flyboy1963 35-40
Bob Martin 35-40
Walt Aronow > 35 psi, check every 90 days, likes Beringer, finds flats come from low pressure
plehrke 35
steve 35
lostpilot28 35
hngrflyr 35
Svanarts 35
BendRV-4 32
robertahegy 30-35
Sam Buchanan 24-35
Kyle Boatright 24-35
Geico266 30
jhalrv4 30
smokyray 30
John_RV4 25-32
CraigH@KRPH 23-32
vac 24-27
n5lp 20-30
DeltaRomeo 20-30
randylervold 20-22
lhawkins 20

David,
I am making the assumption that you would have no objection to me publicly posting the PM you sent me, and the reply I sent back (pasted below), so that a broader audience would have access to the information.

One thing that should be kept in mind regarding the list of pressures that you have compiled is as mentioned in my PM, tire pressures are directly related to gross weight.
Your list contains values from people who owned RV-3's, all the way up to RV-7's (and maybe even a couple of RV-10's). Couple that with the fact that just because someone owns an RV and posts on line what they use for tire pressure, doesn't mean it is best practice.

Also keep in mind that even RV's that seem very similar like the RV-6 and RV-7, have a factory specified gross weight difference of 200 lbs. This may not seem like much but it is enough to be a factor for what an ideal tire pressure would be.

Scott,

Thank you for the feedback on low tire pressure. It's been so hard to find reliable information about tire pressure.

Do you have a specific recommendation on main wheel pressure for the RV7A?

David

Hi David,
With the gross weight of the RV-7A I suggest 35 PSI as the minimum. 40 - 45 is better if you operate near gross weight much and don't have any main gear leg shimmy at pressures higher than 35.

The shimmy document you referenced could probably be written a bit better because it generically lumps all RV models together without referencing them specifically.
Minimum tire pressure is directly related to aircraft gross weight.
That document relates to all RV models from the RV-3 (gross of 1100 lbs) to the RV-7A (gross of 1800 lbs)
There is also a huge difference in tire dynamics with just a 5 psi change. When adding air some time, look at how much the tire rises when adding just 5 psi (from 30-35).

Hope that helps

Scott
 
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most cessna,piper 2 and 4 place aircraft,the POH, say 25 to 34 psi in the mains. rv's have less weight on the mains. so why should the psi in the RV need to be higher?
not trying to start an argument,just looking for explanation.

i used 32psi in the mains on my rv-6a for the 23 years i flew it. never had a flat in that time. i run 24 psi in the mains of my rv-3. works for me.
lawrence
Ok, here is one data point for answering your “WHY, if 35 is the recommended pressure, do I inflate to 45-50?”

Well, reason one. These tires (tubes) do not retain air pressure as long as I would like (i.e. as in a car tire). So, I do not have a fancy door on my wheel pants to be able to check the pressure. I have to remove the wheel pant to do so. If I inflate to 45 for mains, 50 for nose, I can pretty much check at Walt’s 90 day recommendation without the tires going too low in between times I put a tire gauge on them.

Reason two. If the tire pressure drops into the 20-30 psi range there is a noticeable “wobble” or “shimmy” that starts when landing on hard surfaces.
 
Take a look at the tread area of the tire. It will give you an indication of how much of the tire is in contact when the tire is under load. If you run too much pressure only the center of the tire is on the ground, contact patch is small which you will get lower rolling resistance but also less braking traction. Very low pressures risks spinning the tire on the rim under hard breaking and ripping off the valve stem. My RV-4 has the 5" rims running 5.00x5 tires, Dresser Monsters at the moment. I wear out the outside because of camber and generally turn the tires over to wear the other side out. I run 25 psi and get pretty good contact across most of the width of the tire. I do maximum effort braking on occasion to practice short field landing and have never had a problem with tires rotating on the rim. I have run as low as 20 psi with no problems.
 
Cross country

Also remember at 10000 feet on a 4 hour cross country, the tires will get cold.
The tire pressure on landing is the most important, not the tire pressure when it has been sitting in a cozy hangar for a month.

Does anybody know how much the tire pressure will vary with temperature?
Edit: by my math looks like tire pressure varies by 20% from -10 F to 75F (-30C to 25C)
 
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Problems - No problems

I think it has been covered pretty well by now.

Here is my take on reading all the responses.

In my RV7A I used 45 psi in my mains and 35 psi in my nose.

I don't remember how I choose those numbers but I can tell you why I continue to use them. They work for me and I have had no problems with them. I have also noticed that no one (with an RV7A) has said that they have had any problems with 45 psi and had to use something lower.

So, my take ..... If you are having problems with your main tires at less than 30 psi, why do you not increase them to 45 psi?

Mic drop :)
 
Also remember at 10000 feet on a 4 hour cross country, the tires will get cold.
The tire pressure on landing is the most important, not the tire pressure when it has been sitting in a cozy hangar for a month.

Does anybody know how much the tire pressure will vary with temperature?
Edit: by my math looks like tire pressure varies by 20% from -10 F to 75F (-30C to 25C)

True, tires will be about 35 F colder but how much will they warm up during during decent. I did a gas pressure problem with 30 psi tires between -10 and 75 and they went from 30 to 36. Remember temp is kelvin.
 
actual temp? and pressure?

...sounds like a reason to run the little remote wireless tire pressure caps.
I can guess that a long taxi on 150 degree asphalt results in more than adequate tire pressure, if you started with at least 20 psi!
I know when I check mine, the sunny side is at least 5 lbs more than the shady side!
Perhaps instead of too LOW, the high temps resulted in the failure due to HIGH pressure!...and found the weak spot in the tube?
much to investigate further!
 
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