Finding the centerline

Many of the more experienced instructors reading this know what I’m discussing—when our clients land a tricycle gear airplane that is slightly crooked to the runway, we’ll feel the airplane give a little jerk as the airplane aligns itself. This is because the center of gravity is ahead of the main gear and pulls the airplane into alignment with the direction of travel. This was so novel in the 1950s, when tailwheel aircraft were much more common, that Cessna marketed tricycle gear as “Land-O-Matic.” Of course, the issue for a tailwheel airplane can be much more dramatic, as the center of gravity is behind the main gear. Landing in a crab can cause the airplane’s center of gravity to pivot around the main gear in what is known as a ground loop. In either tricycle gear or tailwheel aircraft, landing in a crab puts unnecessary stress on the landing gear and the tires, along with compromising directional control.

I’ve had pilots swear to me that they are aligned with the runway while I was saying, “No you’re not—more left rudder.” (Or right, depending on which seat they’re in.) In cases where they clearly didn’t agree with me, I’ve had to use the pedals on my side of the airplane to salvage a situation. This can be frustrating to both the instructor and the student, especially when the latter still doesn’t perceive the problem.

So why is this an issue?

Before the advent of digital cameras, there was a style of camera called the twin lens reflex camera (TLR). The photographer looks down into the viewfinder and two lenses are stacked up on the front of the camera. The bottom lens is the primary lens of the camera, while the top one is the viewfinder lens. There were a lot of advantages to this system, the significant one being that the image in the viewfinder was brighter than the single lens reflex cameras of similar vintage.

Of course, there’s a disadvantage to this, called parallax error. As you can imagine, with the viewfinder lens atop the primary camera lens, the images captured were not precisely the same. At great distances, such as in landscape photography, the difference was infinitesimal. However, when composing close-ups, the different positions of the lenses became apparent. There was a real risk of cutting off the head or top of a subject because the main lens was below the viewfinder lens.

Similarly, pilots who are unaware that they are not aligning the aircraft are likely suffering from the same effect as the TLR. They are looking too close to the nose, sometimes using the nose itself to line up with the runway. For someone in the left seat, the tendency is to push right rudder to get the nose “straight” while landing slightly to the left of the centerline. For someone transitioning to the right seat, the opposite is true, even if they have fixed the issue when flying from the left seat.

The primary technique to overcome this is to ensure the pilot looks farther down the runway, reducing the parallax error described above. One technique is to show the student the sight picture when centered on a taxiway and tell them to “have the stripe come up between your feet” when looking into the distance. Similarly, we tell pilots to look at the other end of the runway when landing, which also helps with the flare. The difficulty is that the desire to look close at the centerline is a strong one and can be hard to overcome. I believe I’ve developed an effective technique that helps overcome this tendency by a practical demonstration of parallax error and correction.

This method is straightforward, and the pilot’s participation helps drive the lesson home. Have the pilot sit in the appropriate seat of a parked aircraft. For this example, we’ll assume a that they will be in the left seat. Ensure that there is sufficient room in front of the airplane to walk about 60 to 100 feet, with a clear view of something farther away, such as trees or a hangar. Make sure that the area is safe so you, the instructor, don’t walk in front of a vehicle or a moving airplane. At the start, stand close to the nose, ensuring you’re on the centerline of the airplane. With the window or door open, ask the student to tell you to move to their left or right to stay aligned with the centerline as you back away from the airplane. At about 50 or 60 feet from the airplane, stop and observe where you are standing. If the pilot is looking too closely to the nose, you will likely be aligned with the tip of the right stabilizer (the opposite will be true for someone in the right seat). Without moving, have the student join you, standing directly in front of you facing the aircraft. They will often be surprised that you’re both so far off the centerline of the airplane.

At this point, repeat the experiment. Have them sit in the same seat as before, with the instructions to have you move left or right as you back up. However, before you start, instruct them to look over your head at something in the distance. The object doesn’t have to be on the horizon, just a reasonable distance, say a hangar on the other side of the ramp. I’ve found that even an object only 200 feet away is good enough for this demonstration. To ensure the pilot isn’t anticipating a result, drift slightly left or right so they have to correct you. Once you’ve reached the same distance, you’ll likely find that you’re standing very close to the centerline of the airplane. As before, have the student come join you to see the results—they’re often amazed at how well this worked.

I’ve found that by using this simple technique, the pilot’s landings will improve dramatically. They’ve experienced a practical demonstration, with their own eyes, on how parallax can cause them to misperceive the alignment of the aircraft, and will start looking further down the runway. This also has the benefits of helping the student keep the nose on the horizon when flaring for landing. Another advantage is that this is something that can be done before or after a lesson, or on an otherwise “unflyable” day. It’s a great investment of about 10 minutes that will benefit your clients.

Robert V. Meder is chair of the National Association of Flight Instructors.