By Lisa Turner, EAA 509911
This article first appeared in the October 2018 issue of EAA Sport Aviation.
“We have a Super Cub being delivered today on a trailer.”
“On a trailer?” I asked. “What happened?”
“I’ll know more when we take it apart, but the pilot put it on its back right after a landing.”
“Low tailwheel experience?”
“Yes, but don’t assume that it was the cause of the accident. People jump to conclusions about tailwheel pilots and nosewheel pilots.”
“Of course,” I replied. “But it’s hard not to. What did the pilot say happened?”
“On a routine three-point landing the pilot applied gentle braking, and the plane suddenly flipped onto its back.”
“Gentle braking?” I asked. “Okay, I bet there was a little wind and a little speed. That, with some tailwheel inexperience, could do it.”
“The insurance company thinks so, too. But believe me, I’ve seen enough to tell you that you can’t jump to conclusions about this kind of thing.”
“Okay,” I replied, shaking my head. “I’ll suspend judgment.”
Tail Wheel Assemblies
There are as many tail wheel assembly designs as there are experimental airplanes, or at least it seems that way. In addition to homemade designs and generics, they include Matco, Alaskan Bushwheel, Maule, Airframes Alaska T3, Aircraft Products, Lang, and the ubiquitous Scott. Some tail wheel assemblies incorporate shock and spring struts. From full castering, nonsteering, and nonlocking to partial castering, steering, and locks in trail, the wheels all attempt to accomplish the same goal: to enable takeoffs and landings with maximum safety and provide directional control and ease of movement on the ground. Most tail wheels share the same problems, so I’ll talk about the top three trouble areas. You’ll find that these same problems are typical of both certified and experimental amateur-built tail wheels.
Shimmy
Tail wheels shimmy just like nose wheels, and for many of the same reasons. Here’s what to check if you have a shimmy problem.
Caster angle.
The caster, or caster angle, is the cause of more tail wheel shimmy than any other deficiency. To measure this angle, use a couple of straight edges. Align one edge parallel to the castering joint centerline, extend it to the floor, and mark that spot on the floor with chalk. Take the other straight edge and, moving it perpendicular to the floor, see where it intersects the other line at the midpoint of the joint (see diagram). Mark the floor. The angle formed by these two intersecting lines is your caster angle. A positive angle — what you want — is where the castering joint’s line extension hits the floor ahead of where the perpendicular line sits. If this sounds like gobbledygook, then visualize the top nut of the joint pointing aft or forward. If it points aft, then you have a positive caster angle, and if it points forward, you have a negative caster angle.
In theory, the larger the positive caster angle, the better. In practice, the larger this angle gets, the tougher it is to steer and turn. This measurement is complicated by the fact that the reading will be different when the aircraft is unloaded. When you take your reading, you should load the aircraft as if you were going to fly it with maximum people and baggage without violating the CG limits. Find out what the manufacturer recommends, look at the plans, and consult other builders and flyers.
When you take this measurement, you might find that the two lines actually line up — the top nut on the joint does not point either forward or aft. If it’s possible to achieve even a slight positive caster, you will reduce the chances for shimmy. If you don’t have a problem, don’t worry about it. If you do have a problem, you can add a wedge shim to the spring assembly, or replace the springs with another higher spring rate set.
A sagging main spring assembly can be caused by wear over time. Checking the spring bolts should be on your inspection checklist because it’s a high wear item and they are easy to replace. Bolts can bend and stretch, enlarging the hole they are in.
Tires and Wheels
Air pressure. Check air pressure if your tire is pneumatic. Low air pressure can cause the tire to slip on the rim on landing and tear the stem right out of the tube. Pressure that is too high will further reduce the gripping footprint of the tire, which is tiny to begin with. Your best action is to keep the pressure toward the high end on the recommended range and check with other flyers who have the same tailwheel assembly.
Wheel balance. Unload the gear by raising the tail; then rotate the wheel on the axle and observe what it does. A little bit of vertical out of round on the tire is normal, but severe variation is not. Look at lateral movement. You should not see or feel any wobble side to side. Check for play in the axle and re-adjust per your manual.
Bearings. When you rotate the wheel, what do you hear? Hopefully, nothing. If you hear something, then remove the wheel from the fork and inspect the bearings, including spacers and attached hardware. Does the hardware match what is in the assembly manual?
Trouble Tracking, Turning, and Castering
Free castering, won’t lock in trail. Look for worn and loose hardware, dirt, and grit in the pivot pin or pawl, which will cause it to stay retracted. Disassembly and cleaning should correct the problem. Check for wear, high spots, and burrs. For joints that have a Zerk fitting — it is really not adequate — disassembly, cleaning, and regreasing all parts of the shaft works much better.
Tracks to the left or right.
Check first for loose hardware and tire damage. Poor directional stability can also be caused by the tailwheel assembly being set up incorrectly (canted to one side or another), or by too much free play where the spring is attached to the tail wheel.
High effort turning or stuck
Hardware that has jammed, a caster angle that is too high, and components that have been over tightened, such as the castering joint bolt head, will cause high turning effort and jamming.
Steering linkage too tight.
A tight steering linkage (chain and spring, or cable) makes for smooth and positive maneuvering but has a high surprise quotient. Too tight, and breakage is inevitable; you’ll be along for an exciting ride. Linkage that is too tight can also bend the control arms. Follow your plans’ guidance and/or the manufacturer’s advice on chain/spring positioning and routing.
Steering linkage too loose
It’s better for the steering linkage to be too loose than too tight, but overly loose can mean less positive steering and the possibility of the linkage getting caught in another component and breaking. Then we’re back to a wild ride. Make sure nothing is dangling from the assembly, for example, extra chain links. Follow the exact directions for your particular tail wheel and make sure the springs are the ones specified in the plans.
Maintenance
A welcome feature of most tailwheel airplanes (Stearmans not included) is being able to see the assembly without removing a cover or pant. The downside is that the tail wheel collects all of the dust and grit from pavement and grass. The pre- and post-flight inspections should include a good look at the tail wheel, including sighting the caster angle. Make sure it’s not dramatically positive or negative. Grab the wheel and make sure there’s no wobble, and if it’s a pneumatic tire, check the pressure. Look for anomalies in the steering linkage. Look for cracks, bent components, and dislodged torque seal.
Finally, when you perform maintenance on your tail wheel, take a series of pictures before and during disassembly so you can go back and see where everything goes. Work with the plans and the aircraft maintenance manual to make sure geometries, clearances, and torque settings are correct.
Main Gear
Sometimes the trouble with tail wheels (or nose wheels) is not a problem with the tail gear or the nose gear, but with the main gear. If you find yourself with what seems to be an intractable shimmy or tracking condition, be sure to check your main gear, too. This would include removing the wheelpants if the airplane has them, and inspecting for leg cracks, attach point integrity, brake assembly condition, and especially tire wear and bearing condition. If all this looks great, then return to troubleshooting your “little wheel.”
Ruling Out Pilot Error
“Whoa. What is this?” Adam said in a loud voice, catching everyone’s attention in the shop. He was taking the Super Cub apart and inspecting and labeling the components prior to the assessment for repair.
Jerry and I walked over.
“Look at this,” Adam said, pointing to the brake assembly on the wheel. “The disc shouldn’t be moving. I haven’t even taken the assembly apart.”
“The whole unit is loose,” I said, and shook my head.
We watched Adam take the assembly apart. The bolts holding the wheel halves together had seven washers under the nuts, and the nuts were bottomed out on the bolt threads creating a gap in the wheel halves of more than 1/4 inch.
“Yikes,” I said. “Well that certainly explains how the brakes could have locked up. I take back everything I said, jumping to conclusions about pilot experience.”
“Amazing,” said Jerry. “I can’t wait to tell the owner that it was not his fault. He just bought the airplane. I’ll write up the finding. This accident was not pilot error.”
The story ended well but highlights the need to understand how landing gear works with the other systems on the airplane to provide smooth, safe, and consistent response. When you are checking your nose gear or tail wheel, don’t forget to look at the mains, and test the brakes once you are underway.
Lisa Turner, EAA 509911, is a manufacturing engineer, A&P, technical counselor, flight advisor, and former DAR. She built and flew a Pulsar XP and Kolb Mark III, and is currently restoring a Waco UPF-7 with her husband. Lisa is a member of the EAA Homebuilt Aircraft Council and Women in Aviation International. For more from Lisa, read her Airworthy column every month in EAA Sport Aviation.