Airframe and Powerplant

Figuring Those Folding Feet

August 1, 1994

The ins and outs of retracts

It's a satisfying thing, this retractable-gear experience. Select the gear handle up, sense the slight change in aerodynamics, hear the retraction mechanism working away, and, finally, feel that solid thunk as the wheels land home in the wells. As the airplane transitions from part tricycle to all bird, we seem to shed finally the equipment needed only on the ground.

Although the concept is simple — hide the gear away for better aerodynamics, just as birds do — in airplanes, manufacturers have concocted many methods putting this theory to practice. They range from the sublimely simple, such as the Johnson bar in early Mooney airplanes — one big yank and all three wheels retreat from the airflow — to the bewilderingly complex, as in some single-engine Cessnas, with motors and pumps and lines and valves, oh my.

Airplane design directs landing gear configuration. A low-wing airplane is generally easier to mount with retractable wheels, because they, the mains especially, have somewhere to go. Cessna in the late 1950s faced an uphill engineering battle to find a way of retracting the Centurion's mains; putting them in the wing just wasn't an option. Cessna engineers really didn't fret too much about where the mains would go — into the belly, aft of the cabin was really the only reasonable solution — but more so about how to implement the system with as few parts and subsystems as possible. It is interesting to note that through the years, the Cessna single-engine configuration became simpler and simpler, with fewer parts and generally less monkey motion. It also became gradually more durable and reliable.

Because we are dealing with relatively heavy components and typically stiff air loads, few manufacturers have chosen manual gear. Indeed, ask Mooney owners: Although they'll tell you that the manual gear offers a simple and durable mechanism, they'll concede that it still takes a hefty tug and good muscle isolation to bring the gear up without any drama. Watch someone getting checked out in an early Mooney, and you'll probably notice a slight pitch oscillation as they try to yank the gear handle with one hand and fly the airplane with the other. Longtime manual Mooney owners can do it without a thought, of course, but even the manufacturer decided that human-powered gear was a suboptimal solution. You won't see the new Ovation with a Johnson bar between the seats.

So the power to move the gear has to come from somewhere, and this leads us to the two main types, electric and hydraulic. (There is a subset system, which uses an electric motor to pressurize the hydraulics.) How these systems work and what you'll need to know about them are substantially different. What's more, the gear systems usually have some electric component that either allows the gear to come up or prevents it from doing so if the wheels are still on the ground.

Hydraulic systems, for example, offer some advantages in packaging and, occasionally, in weight. Lines can be routed nearly anywhere in the airplane, and the actuators can be quite small for their power. On the downside, high operating pressures and complex plumbing can be a mechanic's worst nightmare.

Piper's gear system in the Cherokee-based retractables is among the simplest. Here, a hydraulic power pack — basically an electric motor driving a small pump — supplies pressure to actuators at each wheel. The actuators are cylindrical bodies, inside which is a small disc called the piston. Attached to the piston is an actuator rod; it may protrude from the cylinder at one or both ends. Seals around the piston and actuator rod keep the hydraulic fluid from escaping. Two ports are drilled into the cylinder, usually at opposite ends. When fluid is pumped into one side, the cavity fills and the piston is pushed to the other end. In this way, the actuator can be made to move a device positively in two directions, by applying the appropriate pressure to one side of the cylinder or the other.

In the Pipers, the wheels are held in the extended position by over-center devices; the gear is not locked in place solely by hydraulic pressure. When you select gear up, the hydraulic actuator overcomes the over-center link and begins to retract the wheel and strut assembly. Once fully up, hydraulic pressure in the lines keeps the wheels retracted. That's why you will notice the power pack running occasionally in flight; as the pressure slowly bleeds on the up side of the circuit, a pressure- sensitive switch instructs the pack to run and reestablish operating pressure. That's usually between 1,000 and 1,500 psi.

Because the Pipers don't have a device called a mechanical uplock — whose sole job is to hold the gear in the wells using hooks that latch the gear legs in the up position — the emergency extension method is straightforward. In short, you simply release the pressure in the system, and the gear free-falls to the down position. Springs on the nose gear help force the strut forward against air pressure; this is partly why the handbook recommends a relatively low speed for emergency extension.

Single-engine Cessnas also use hydraulically actuated gear, but the various models represent typically more complicated ways of implementing it. Cessna employs down- and uplocks in most systems; in some cases, they are part of the hydraulic system, sequenced by valves to operate in the proper order, or they are electric solenoids. Naturally, these devices work to hold the gear up or down, relieving the hydraulic actuators of the task. In some models, separate gear doors are moved by their own small actuators, and they must naturally be sequenced correctly so the gear doesn't try to bust through the door or retract into a clamshell that hasn't yet opened.

Because the Cessnas must push the main gear forward against the airflow, emergency extension isn't just a matter of releasing pressure. Instead, the powered system must be bypassed and a small hand pump used to pressurize the lines. In the event of a massive hydraulic leak, it may be impossible to get the gear out at all. What's more, some of the early Cessnas, twins and singles, used engine-driven hydraulic pumps; in multis, a failure of the engine with the pump means pumping the gear down by hand. (This is, of course, true of Piper Apaches and Aztecs, as well.)

Some manufacturers believed that connecting the gear to an electric motor and eliminating a whole subsystem was the way to go. Bonanzas, modern Mooneys, and Piper Comanches are among those types using strictly electric gear. In these cases, the gear is connected with hefty rods or links to a motor and a gearbox. Flip the switch, the motor turns, pulling or pushing on the rods, and the gear comes up.

Such an electric system usually has fewer parts than a hydraulic arrangement, but that doesn't mean it's foolproof. For one thing, the gearbox and motor must be located where the runs to the gear legs are straight and as short as possible. This typically means that the apparatus roosts under the floor in the passenger compartment.

Alignment of the linkages, as well as their proper lubrication, is critical to the care and feeding of an electric system. And because the motor driving the gear must be stronger than one in a hydraulic power pack, there's the need for more electricity. Gearboxes have been known to jam, too, sometimes leaving the gear locked between up and down.

But if anything gives owners fits about retractable gear, it's usually the electric support systems attached to it. Most types have squat switches, a small, supposedly weatherproof, device that tells the system if there's weight on one or more of the wheels. If, for example, the switch fails in the "ground" position, it might not be possible to raise the gear after takeoff. (Some models provide a bypass, but in others, you'd be better off to come around, land, and diagnose on the ground.) The purpose of the squat switch is to keep someone from retracting the gear until the airplane is off the ground. Where it is not feasible to employ a squat switch, some systems use airspeed sensors.

Because there's so much more to a retractable system than for a fixed setup, preflight carries added significance. For the hydraulic systems, check for leaks of any kind; look for telltale red fluid stains. Any seepage should be investigated. At 1,500 psi, even a pinhole will allow a tremendous amount of fluid to be pumped overboard in a short time. Check the over-center links for complete engagement and alignment, and make certain that the squat switches are clean and the electrical connections tight.

You will want to check for cracks in the trunnions and retraction pivots. If the pivot is covered in grease, take a rag and clean it up; lubrication on the outside does no good. In airplanes with electric actuation, check the rod ends that move the gear for cracks and general security. The rods should not be bent or loose. Also, take an eyeball guess to see if the gear leg is perpendicular to the ground on low-wing airplanes — and don't let the wing dihedral fool you. For Cessnas, be sure to check that the main struts are firmly against the saddles, just above the pivot point where the legs meet the fuselage.

How do you know your gear is healthy? It depends on the airplane. At the annual, when you swing the gear, it should move freely, without a lot of cranking or groaning. One simple test is to run the system on the manual extension scheme to see if it moves easily. Some airplanes have a published extension and retraction time — if it takes the gear longer than specified to move, there's a good chance you'll find binding parts, a lack of lubrication, or a weak motor or hydraulic pump.

While you're crawling around under the airplane, take a gander at the gear doors. They should fit tightly and not be warped. Pilots who tend to push the published gear speeds — and you know who you are — get to replace doors and associated hardware more often than the rest of us. And don't assume because a later model has a higher gear speed than your airplane that you too can throw the wheels out at will. Manufacturers frequently make subtle changes when designing for increased gear speeds.

A word about annunciation. Most airplanes have a three-light system. (Mooneys and Comanches and some Cessnas, on the other hand, have just one light due to the design. In the M20s and PA-24s, if one's down, they're all down, and you can see if the mains are out in the Cessnas.) Lights burn out. If you don't get all three green lights the first time, slow down and think about what's happening. Test the lights first. If you're on an instrument approach and the lights are not the obvious culprit, declare a miss and find some nice quiet holding pattern to figure it all out. Then dig out the check list and follow the procedures; it's not always wise to cycle the gear immediately upon seeing one green light out. A leak in the system could mean that you have just pumped the last bit of fluid overboard.

Dig out the maintenance manual — you have that directly at hand, right? — and learn your gear's specifics and procedures. Doing so could well make the difference between hearing the disheartening and expensive sound of propeller tips and flap edges imprinting the runway and the routine but still satisfying thunk of the wheels going where you want them.