The Tiedowns That Bind

Almost every pilot I know follows a practice that we never had to study or answer a single FAA test question about.

I was introduced to flying by a friend who had a Cessna Skyhawk that he kept at a suburban airport. Before he took me for my first ride, I watched as he went through a routine of untying a rope from attachment points under each wing and the tail. I didn't give any more thought to this than I would to watching someone untie a boat before leaving the dock. Obviously we weren't going anywhere if the wings were tied to the ground. After our flight, he parked the airplane, shut everything down, locked the doors, and reattached the ropes with what he described as a "locking slipknot."

A typical tiedown consists of three anchor rings that are screwed into the ground until the eye at the top is flush with the earth. One anchor is behind the tail, and the other two are at the outboard the edges of the wings. At my home base, the FBO supplies half-inch nylon line attached to the top of each anchor. When a typical high-wing single is tied down in one of these spots, the rope from each wing's tiedown ring will slope outboard and down to the ground anchor at about a 45-degree angle. The tail skid rope slopes down to the aft anchor at about 30 degrees. It's important to leave a little slack in each line, especially if you are expecting gusty wind conditions. Slack will allow the airplane to move a little. Without any slack, a strong gust could damage the airframe.

When aircraft parking is available on paved ramps, tiedown anchors may consist of a steel bar embedded in a shallow recess in the ramp surface. It is a good idea to keep three nylon lines of suitable length in your airplane for those times when you visit an airport with permanent anchors but no lines to lend to transient pilots.

Nylon line is preferred because it is stronger than rope of the same size made from natural fibers such as hemp or manila. It also is resistant to rot, and has the useful quality of controlled stretch. Nylon's elasticity allows it to stretch one-third or more under load, which makes it ideal for absorbing shocks.

There are alternatives to tiedown ropes. One commercial product is a set of nylon straps with a hook on the end that can be adjusted like a belt. A system like this eliminates the need for any knot tying. Steel chain can be used, but it may corrode, the length is not easily adjusted, and it will not absorb any shock. Still, this is a popular option in parts of the Midwest and other areas where the wind really howls.

Mooring your airplane properly accomplishes a couple of things. It will prevent a wing from becoming airborne when hit by a strong gust of wind. It will also prevent the airplane from being swiveled around or rolled fore or aft by wind. This is especially important if the airplane is parked on a smooth, paved ramp. Don't rely upon your hand brake to hold the airplane still. The brake pressure can easily bleed off over time. And, of course, knowing that the aircraft is secured provides the owner with peace of mind.

In some areas of the United States, tying down an airplane is not enough. Imagine the precautions pilots must take in Anchorage, Alaska, where they may see sustained winds of 60 mph. Not only are parked aircraft tied down well, but wing covers with built-in spoilers are laid across the tops of the wings to kill the lift. Anchorage pilots report that they change the worn tiedown ropes every year, and if they think the wind is going to be bad enough, they park a pickup truck next to their airplane to block some of it.

You don't have to be in Alaska to see what a strong wind gust will do to a typical single-engine airplane. Thunderstorms are common warm-weather events in much of the United States. In Wilmington, Delaware, a few years ago, a storm generated enough wind to pick up a Cessna 140 and flip it over onto its back. Winds associated with a thunderstorm could just as easily flip a Cherokee.

Amazingly, some pilots park their airplanes out in the open and do not bother tying them down at all. It is my observation that these are invariably owners of low-wing aircraft. Since a 10-knot breeze will set a Cessna 172 rocking a little, I guess the high-wing folks don't need as much convincing about the value of tiedowns. But I would not want to be the owner of the Mooney who had to explain to his insurance company how his aircraft rolled out of its parking space and into a neighboring airplane because it was not tied down during a storm.

The locking slipknot is the most popular knot used when aircraft are tied down with rope. Some pilots say this knot is popular because it was used to secure aircraft to the decks of Navy carriers in World War II. The name is a problem, however: If it is a slipknot, how can it be locked? Actually, the knot is correctly known as a tautline hitch, and although it appears to be locked when first tied, it can work loose. If you want a knot that won't slip, tie a bowline (pronounced BO-lin). Ask any sailor worth his salt to show you how.

Another part of knowing about tiedowns is avoiding the hazard of getting a loose rope wrapped around your prop when you taxi. Common sense dictates that you taxi at the minimum necessary rpm to minimize eddies that can pick up loose gravel, stones, or other debris. If you have a tiedown that allows you to taxi into it, over the tail tiedown rope, it is suggested that when you remove the tail rope, coil up about half of it, flatten the coil, and then tightly wrap the remaining line around it all the way down to the anchor. This should prevent any prop eddies from dislodging it when you taxi over it.

There's really nothing difficult about tying down an airplane properly, and it can save you a lot of money and heartache in the event of a storm or isolated wind gusts. But like anything else, it only works if you take the time to do it every time you fly.