Give Me a Brake ... And Maybe a Tire and a Strut Too

Understanding Your Undercarriage

By Tom Hoffmann

Source: FAA Safety Briefing, Jan/Feb 2020

While its value is sometimes overlooked and underestimated, your aircraft’s landing gear is one of its hardest working parts. The stress of takeoffs and landings aside, your landing gear is also hard at work well after you have turned off the ignition and parked, supporting the weight of your airframe and powerplant around the clock. Let’s take a closer look at this perpetually purposeful part to further appreciate its valiant role.

Keeping It Wheel

A typical GA aircraft landing gear uses three wheels: two main wheels and a third either on the nose (tricycle gear), or at the rear of the aircraft (conventional gear).

Most modern GA aircraft employ the tricycle gear configuration with a steerable nosewheel. A tricycle gear airplane allows for more forceful braking without the risk of nosing over, and it offers pilots better visibility during taxi, takeoff, and landing. Its forward center of gravity (CG) also provides more directional stability.

Conventional or tailwheel gear aircraft have a more rearward CG, so directional control is a little more daunting. A ground loop can occur if the heavier tail swerves around to get ahead of the main gear during ground operations. Ground loops can cause serious damage to aircraft and in some extreme cases, have resulted in fatalities. Another operational impediment is a tailwheel’s limited forward visibility on or near the ground. These factors prompted the FAA to require specific training for a tailwheel endorsement. On the flip side, though, a tailwheel’s nose up ground attitude has more room for a larger propeller. It is also better suited for backcountry operation on rough, unimproved fields. (In my personal, non-objective opinion, they also look pretty darn cool.)

Don’t Steer Me Wrong

There are several different ways to steer an aircraft on the ground. Conventional gear aircraft with a free-moving tailwheel use just the rudder to steer, relying on airflow over the tail from the engine or forward motion. Taildraggers can also have steering linkage with the tailwheel in the cockpit, or rely on differential braking to turn the aircraft.

Aircraft like the Cirrus SR22 or Diamond DA-20 use a castering (free-moving) nosewheel that requires differential braking to maneuver on the ground. This setup is generally lighter and easier to maintain, but can be a bit tricky in windy conditions. It can also wear brakes down faster. Intermittent toe taps can help with preventing brake damage or overheating.

Squeaky Clean, or Down and Dirty

We can further categorize landing gear into two main camps: fixed or retractable. Fixed gear aficionados tout simplicity and minimal maintenance — “the fewer the moving parts, the better.” Today’s composite materials along with aerodynamic wheel pants and fairings can reduce the para-site drag traditionally caused by a fixed gear.

Retractable gear tidies up the aircraft’s profile and offers greater aerodynamic efficiency and speed. It also provides more gliding range if the engine fails. However, that 15-20 knot gain in cruise speed may be offset by the increased weight of a retractable system, which includes hydraulic pumps and actuators. Remember too that a faster aircraft will have more parasite drag. If you’re considering a retractable gear aircraft, run the numbers to see if saving 20 minutes on your next airport diner dash is worth the weight and hassle. Hassle includes higher insurance and maintenance costs, as well as the greater potential for things to break down.

Strut Your Stuff

Now let’s look at the part that helps transfer the load of landing and supports the aircraft’s weight on the ground. This important job belongs to the landing gear struts, which absorb the shock of everything from a greaser to a teeth-rattling runway strike.

Early designs made use of a rigid strut that was welded directly to the airframe. While simple, the design meant that the impact of a hard landing was transferred directly to the airframe and its occupants with only inflatable tires helping to cushion some of the shock load. You’ll still see rigid struts on helicopters, which can easily absorb the shock from their typically low-impact landings.

More common to GA is the spring steel strut, which is designed to flex upward and outward and safely transfer the impact load of your aircraft during landing. They can be designed with their namesake spring steel, or with aluminum and composite materials. You’ll see this type of maintenance-free strut on the majority of single-engine Cessnas.

Another common strut design uses bungee cords to transfer landing loads. These elastic bungee cords are positioned between the airframe and the gear and must be regularly inspected. Piper Cubs are a popular example of this strut style.

A strut design that you’ll likely see on a large transport aircraft as well as several GA aircraft, is the shock or oleo-pneumatic “oleo” strut. Similar in concept to automobile shock absorbers, oleo struts use compressed air or nitro-gen with hydraulic fluid inside two telescoping cylinders to absorb the landing load. This mechanism essentially transfers the kinetic energy of the hydraulic fluid being pushed through an orifice into thermal energy and can handle much higher loads (with less recoil) than a spring steel design. Yay science! A metering pin or tube in the oleo strut also adjusts the fluid flow to permit a softer initial compression of the strut that becomes firmer as the opening is further restricted.

Many tricycle gear aircraft use an oleo strut in the nosewheel, although it is by no means designed to bear the full weight of the aircraft. Its purpose is to smooth out the landing by reducing the recoil effect of the nose during touchdown.

Speaking of smooth landings, another gear variation with oleo struts is the trailing link gear, which uses a rear-facing L-shaped strut. In addition to the cushioning provided by the oleo strut, the L-shaped arm pivots to fur-ther help dissipate the upward energy of the touchdown.

According to pilot and aviation author William Dubois, whose Ercoupe uses this design, trailing link gear provides “buttery smooth Bob Hoover landings nearly every time.” Dubois also appreciates the maintenance benefits, as the oleo strut is out in the open for easy access. If you do have oleo struts, know the acceptable static load extension of the strut (i.e., how much of the inner cylinder, or piston, is visible during preflight). For example, the Piper Warrior should generally have 3.25 inches of the piston showing on the nose and 4.5 inches for the main gear struts. If it is below limits or collapsed, the strut needs servicing. Also note that if the strut seems too bouncy, it might need some fluid. The appearance of excessive hydraulic fluid during inspection might be an indication of a leaky seal or O-ring.

Brake It Down

That leaves the final two parts of the landing gear: tires and brakes. Tires provide several important functions. They support the weight of the aircraft and provide shock absorption along with traction and braking when landing. Aircraft tires are among the strongest and hardest working pneumatic tires made, but proper maintenance and inspection is essential.

Tires are usually constructed of rubber compound for durability. They rely on diagonal layers of rubber-coated nylon cord fabric to provide strength. Unlike automobile tires, aircraft tires are designed to flex more, sometimes twice as much. This flexing allows better traction, but it also builds up heat much faster. That’s why the hardest task for a tire is not necessarily that hard landing, but instead its ability to endure the rapid heat buildup from takeoffs and lengthy ground operations. The best safeguards against heat buildup include short ground rolls, slow taxi speeds, minimal braking, and proper inflation. Proper inflation assures the correct amount of flexing and keeps heat buildup to a minimum, increasing tire life and preventing excessive tread wear. When inspecting a tire for proper inflation, you can easily tell if the air pressure has been consistently high or low. Excessive wear in the shoulder area is an indication of under-inflation, while wear in the center suggests over-inflation. Also be sure to check for cuts, cracks, bald spots, and adequate tread depth.

Airplane brakes can help with slowing down after landing, as well as maneuvering and tight turns on the ground. Most GA aircraft have disc brakes, which use fluid to compress a brake pad against a metal disc attached to the wheel. The brakes are operated with foot pedals that work independently to allow for differential braking. On every preflight, be sure to look for hydraulic fluid leaks and ensure proper brake pad thickness. Look for excessive corrosion too, especially if it’s been some time since your last flight.

Getting It In Gear

So there you have it — landing gear 101. Although your undercarriage is sometimes underappreciated, hopefully this primer shines some new light on this hard working part of your plane.

Learn More

FAA Airframe Handbook, Chap. 13, Aircraft Landing Gear Systems - bit.ly/AircraftHandbooksChp13

Did You Know?

Did you know that 14 CFR part 43 Appendix A includes several owner-performed preventive maintenance tasks related to landing gear? They include:

• Removal, installation, and repair of landing gear tires
• Replacing elastic shock absorber cords on landing gear
• Servicing landing gear shock struts by adding oil, air, or both
• Servicing landing gear wheel bearings, such as cleaning and greasing

Before doing any of these tasks, be sure you understand all facets of the work you plan to perform, and consider any and all applicable regulations. This includes making certain you have all available tools, equipment, and test apparatus necessary, as well as reference materials or manuals.

Tom Hoffmann is the managing editor of FAA Safety Briefing. He is a commercial pilot and holds an A&P certificate.