Maintenance Matters

February 2017 Issue




Landing Gear Checks: Know Thy System

If you want to avoid a nasty inflight surprise, get down on all fours and inspect the gear the right way. Take your time and understand what youíre looking at.

While there are plenty of items that can be easily inspected to help prevent a landing gear malfunction, we’re convinced that too many pilots simply don’t spend the time to do so.

As we sift through the NTSB accident reports each month, we’re consistently finding landing gear mishaps that might have been avoided if the pilot spent a little extra time inspecting the gear during the preflight, or simply knew how to recognize potential problems areas before launching.

In this article, we’ll look at the things you and your mechanic should be inspecting—including items on fixed landing gear—in hopes that you won’t join the ranks of “those who have” landed gear up.

Mooney with flat tire

Aircraft that sit for an extended period require careful attention to the landing gear. We spotted a neglected Mooney with tires so flat the main landing gear doors were bent and driven into the pavement.

Get Jacked Up

We’re not suggesting you learn how to completely disassemble and reassemble the landing gear on the aircraft you fly, but instead, knowing what the system looks like when it’s stowed in the wheel wells and fully extended will give you a better mental picture and help you spot something that’s out of place during your preflight.

Since landing gear designs are all over the board—from stone simple to incredibly complex—it pays to spend some money on shop labor and have your mechanic put the aircraft on jacks so you can observe a gear retraction and extension. You know what it sounds like, but do you know what it looks like? Spend some time in the service manual looking at the component breakdown and be familiar with any ADs that apply to your system. Some aircraft have landing gear gotchas that are ready to happen.

For example, Piper Seneca operators should be familiar with AD 2005-13-16, which affects the nose-gear/retract rod and bolt. Also, take a close look at the nosegear shimmy dampener bolt. Even though it looks like it’s installed upside-down (with the head facing down), knowing that’s the way it’s supposed to be can prevent a nosegear-retracted landing if found the other way around.

Bonanza landing gear disassembled

There are many items to check on a Bonanzaís complex gear. Itís worth having a mechanic show you what to look for when itís disassembled.

Fly a Beech Bonanza? A broken uplock cable can result in a gear-up landing, but it’s an easy check on the ground. Savvy Mooney owners make it a habit to look at the nose-gear trunnion. Any dents indicating over-travel of the nosegear steering linkage are red flags, since this can lead to a gear collapse.

Pilots aren’t the only nuts behind the wheel. One thing every landing gear has in common is a critical nut (or nuts) for holding the wheel on. Considering the neglect wheel nuts receive, it’s amazing that it doesn’t rain rubber as wheels fall off. They deserve a hard look; perhaps not before every flight, but on a scheduled basis.

Keep a rag handy (wheel nuts are pretty greasy) and grab hold of it to see if you can turn it by hand. You shouldn’t be able to. If the nut is so dirty it’s hard to see, use that rag to wipe the dirt clean. Dirt should equate with excess wear in anyone’s mind, so keep that nut and the area around it clean. Cleaning will also allow you to check the condition of the cotter pin.

One of the most neglected bolts is the one that goes through the scissors, also called the torque link. It’s easy to overlook, even for mechanics. You should be looking for signs of wear around the head of the bolt and the nut on the other side.

aircraft brakes

While youíre preflighting the landing gear, look closely at the brakes for leaking and loose hardware.

Another critical nut-bolt combo is the one holding the shimmy dampener in place on the nosegear. These bolts are notorious for snapping, which in turn could cause the nosegear to fold from excessive shimmy—or you can lose control during takeoff or landing. The shimmy dampener bolts should be checked for signs of wear and excessive free play.

Grip the nosegear tire and give it a gentle wobble or have someone in the cockpit walk the rudder pedals back and forth. There should be no obvious free play, and the dampener should resist a wobbling motion. Beyond wobbling the gear side to side, try it fore and aft as well. Often, the actual attachment of the gear can start to work loose. Caught early, this can be relatively inexpensive to fix (relative to the cost when it pulls free entirely, that is). Try this on both the nose and main landing gear.

While you’re shaking things up, be sure to give the brakes a shake, too. Grab the caliper and see if it’s just slightly loose. If you don’t have either Cleveland or McCauley brakes, any looseness could mean trouble. Parker brakes should feel rock solid, so if they wiggle, look for loose bolts.

If you do have Clevelands or McCauleys, they should feel slightly loose. This shows that the calipers are still free to slide on the anchor pins, meaning both sides of the brakes are working. Also, be sure the anchor pins themselves are clean and free of rust (if not, make a note to clean them up). Hit those pins with a shot of LPS-1 or dry silicone lubricant. You want to use a dry lubricant so it doesn’t attract dirt, which WD-40 or plain oil will.

Landing Gear Doors

Like the landing gear itself, gear doors can be simple or complex. Regardless, they can tell you a lot about the health of the gear. This is particularly true of clamshell-type doors that are typically found on the nosegear, but may also be used on the mains.

Cessna Centurion landing gear failure

A landing gear failure on rollout in a Cessna Centurion can be especially ugly because of the side load it can create on the nosegear. Use caution when maneuvering the aircraft with a tug and inspect it carefully after ground crews move it.

You should be looking at the edges of the doors for scrape marks (possible misalignment of the doors), chafing or rubbing as indicated by bare metal (possible loose doors), crimped or crinkled door edges (again, misalignment) and whether the door as a whole seems true and fair. Try giving it a little wiggle to check for excess slop or looseness.

Whether you’ve got an electric retractable gear system, hydraulic system or manual system, they’re all actuated. In electric and manual systems, this will be by means of pushrods and bell cranks. Hydraulic actuators use a powerful electric motor to power the hydraulic pump.

Pushrods are easy enough to check. You’re looking for bending or breakage here. Breakage will usually be self-evident, but bending may not be. Here’s where having a good mental picture of what things are supposed to look like pays off. The pushrods in some aircrat take convoluted shapes to work around other gear components. You’ll either have to dive into the manual or know by memory what you’re looking at.

Hydraulic actuators all share some common problems, not least of which is leakage. Obviously, if you find one spewing hydraulic fluid, you’ve got a problem, but knowing where to look can be a challenge. Depending on the type of actuator you’ve got (single-acting or double-acting), it can leak from several places.

On some models, it’s easy to check the hydraulic fluid level, such as on the Cessna 172RG. On others, like the Piper Aztec, it’s impossible to check hydraulic fluid level because the airplane has to be on jacks and the gear has to be retracted.

Double-acting actuators, which are fairly common in hydraulic landing gear systems, have three possible leak points: the actuating rod and the two hydraulic lines attached to the cylinder. Run a finger around each line attach point and see what comes back. Also, make sure the rod is clean and not covered with grit or dirt, which may lead to later leaks. And while you’ve got your head up there, take a look at the hydraulic lines and fittings. Flexible lines should not be twisted or kinked (this can indicate improper installation and, eventually, could break). Solid lines should be smooth and free of dents. While you’re looking at all this stuff, pay attention to electrical components including limit switches, squat switches and indicator switches. You might be surprised to find kinked or frayed wires. These require immediate attention. We once caught an unsecured squat switch floating in the wheel well after the aircraft was released from annual inspection.

Baron and Bonanza owners should also take this opportunity to check the condition of the indicator cables that run along the top of the wheel well. Make sure they’re free and straight, have no cuts or dents, and are at least reasonably clean. Now is also a good time to check the condition of the uplock rollers and cable. Give it a spin (it should turn easily), look for signs of wear or gouging and that it’s lubricated. Make sure the protective boots are in good shape.

aircraft landing gear linkage damage

Exceeding tow limits can easily damage the linkage.

Down and Locked Forever

Even if you enjoy the simplicity of an airplane with fixed landing gear, there are some things you should be doing to avoid problems. One thing is to check for the early warning signs of corrosion. On Cessnas, closely examine the main gear leg itself for the telltale blistering or bumpiness in the finish that shows corrosion. For the Pipers and Beeches, check the tops of the fairings for the same, as well as excessive aluminum oxide buildup. Of course, make sure the scissors are free to move and not rusted. A careful look at the junction of the gear leg and the fuselage can detect evidence of looseness and excess play.

Bolts that hold the Cherokee gear to the wing protrude through the wing structure (usually slot-head bolts). Look specifically at these bolt heads as viewed from the top of the wing. They should not be “smoking” or show any evidence of working. Bends or wrinkles in the upper wing surface may indicate a hard landing.

Don’t forget the brakes. Check for even wear fore and aft on the pads. Wear mostly on one pad usually means the pads are not sliding free on the pins due to corrosion, lack of lubrication or bent pins. You also want to look for leaking brake fluid.

Real-World Gotchas

We remember a time when gear collapses on some models were common occurrences, including an issue on the Cessna 310 that was created on the maintenance floor. Cessna finally got the word out that you could not rig just one leg of the gear system as it will screw up the others. To rig the electromechanical gear on Cessna twins you have to follow the maintenance manual (which is very well illustrated), start in the center of the system and work outward progressively.

It takes two people who know what they’re doing about eight hours. In the mid-1970s Cessna built a full-size working mockup of that gear in a structure that could be disassembled and wheeled through a doorway. Back when Cessna dealers sent mechanics to Cessna for classes, that gear mockup was in constant use for training. The word finally got around that you had to maintain that gear by the book.

The Cessna single-engine gear system went through a bunch of iterations—three distinct ones on the Cardinal RG—and Cessna finally got it right on the third one. Those have to be maintained assertively. It’s always a little spooky to watch a gear swing because they’ve got to jack the airplane up high enough to allow for the mains to descend about 18 inches in the retraction cycle.

We mentioned the system on a Piper Aztec and the importance of checking the fluid level with the gear up. We remember when a shop checked the fluid level on one while it was standing on its gear. Of course it read low, so the tech filled it up. We took off, retracted the gear and flew 10 minutes to home base, landed and taxied in. When we got out of the airplane, we saw that most of the lower half was covered with hydraulic fluid. We don’t remember if any seals were blown out, but it sure was a mess to clean up.

Speaking of messes, even the best maintenance practices and most through preflight inspections won’t always fight off landing gear failure. Be ready for them by knowing the exact procedures for manually extending the gear in the airplane you fly. Have you ever tried to manually crank the gear down? Do it.

Last, don’t let a landing gear failure distract you from the most important task of all, which is flying the aircraft to a safe landing.