Friday, June 12, 2009

Stabilized Approach

Last summer I flew in the back of an MD-80 on my way to Florida by way of Dallas/Fort Worth International Airport. I was seated over the wing and I watched as the ailerons and roll spoilers flapped about rhythmically all the way down final until touchdown on DFW's Runway 17 Center. Like many other airline flights I've been on, I also noticed large pitch and power excursions which intensified as we progressed toward the runway. As I'm sure many of my readers have done, I've also watched many YouTube videos of airliner landings, recorded both from the passenger cabin and from the flight deck, and I've seen this happening in the majority of these videos. I enjoy the flight deck videos because I'm often able to observe the pilot flying pushing, pulling, turning, and tussling with the yoke all the way down final. Of course, many general aviation pilots fly approaches this way, too. But, there's a better way.

I feel that the approach and landing phase reveals a great deal about a pilot's ability to be master of his or her machine. It is the phase of flight which requires proper technique to be executed correctly with efficiency and grace, and any imperfections in flying technique are often revealed here. This is because while approaching a runway the pilot has a target: the aiming point on the runway. And during landing the pilot must contact a very fixed and very unforgiving runway gently and at the correct spot and speed. This kind of precision usually is not required during any other phase of flight. Landing is one of the most difficult things we pilots do, and the accident statistics concur. Approach and landing accidents represent a large chunk of total accidents; luckily, many are not fatal. Accident data also indicates that many approach and landing accidents are the result of unstabilized approaches.

A stabilized approach is defined as the airplane being established on the proper glide (airspeed, power setting[s], and descent angle) and track, and minimal, if any, control inputs from the pilot are necessary. That means the airplane is at the appropriate approach speed (or landing reference speed when closer in), at a constant power setting which produces the proper glide angle, on a constant heading that will keep the airplane aligned with the runway, and in the proper aircraft configuration. Wind correction should also be included depending on the airplane or pilot technique (crab or wing-low method). Once you're established and the airplane is on power and in trim, sit back and ride the invisible glide rails to the aiming point where the flare should begin. Very minimal control input should be needed until it's time to change things for the flare maneuver. If you find yourself wrestling with the yoke, rudder pedals, and throttle(s) just remember that somewhere nestled inside all that frantic movement is the proper control position. Usually, only one fixed magic spot on the yoke exists until the flare. The same is true for the throttle(s) and rudder pedals. When you slop the flight controls around in incessant movement all the way down final you're engaging in a series of over-correcting oscillations. This will still get you to the runway, but with much wasted energy, sloppy flying technique, a greater chance of a bad or dangerous landing, and uncomfortable passengers. You'll also waste fuel if you're constantly changing power settings because when you do this you're continuously slowing down then speeding back up. Hidden right in the middle of those oscillations is the proper value, and all you need to do is set the power there and leave it alone. If it were possible to mathematically average, for instance, pitch oscillations that occurred during a sloppy approach, we'd find the single proper pitch attitude that would've sent the airplane to exactly the same place in space but without all the ups and downs. Tiny power changes might be required every now and then to keep the airplane on the proper glide path, but they should be small and few and far between. Try watching an approach video recorded from the flight deck on YouTube in time lapse by dragging the video progress ball forward manually. When the video is sped up the pitch and roll excursions become easier to see, and it appears as though the airplane is "homing" on the runway like a missile gyrating toward its target. Things get a whole lot easier and more efficient if you just take the direct route to the runway instead of constantly making minute changes to heading, pitch attitude, airspeed, and power setting(s).

For a given set of circumstances (aircraft weight, air density, wind, required glide angle, aircraft configuration) there is always a singular exact proper position for the ailerons, elevator (or stabilator), rudder, and power setting(s). This is the essence of the stabilized approach. Dipping, swooping, banking, and deviating from proper glide path and airspeed is nothing other than sloppy flying, and it's unnecessary. If you need more convincing, I've got an experiment for you to try. If your airplane is equipped with an approach capable autopilot, take it out and let it fly an ILS approach to two hundred feet. You'll still have to set the power manually (unless you fly an auto-throttle equipped jet!), so set it at the appropriate value and do not change it for the rest of the approach. Observe as your autopilot flies the approach, perfectly stabilized with extremely little control movement. If your autopilot can do it, so can you.

The only exception to all of this, of course, is when gusty winds are about. During those conditions the wind direction and velocity may be constantly shifting, therefore renewed control application will be required on a second-by-second basis. There is nothing improper about that, and your autopilot would do it the same way. In smooth (or mostly smooth) air, though, there's no excuse for imposing a hand and feet spasm on the flight controls. The only time an approach calls for constantly changing control inputs is when the wind is also constantly changing. If the wind is calm, your hands and feet should be, too.

Most airlines have policies regarding stabilized approaches. For instance, a policy might require that the airplane be "in the slot" (stabilized on speed, on glide path, and properly configured) by five hundred feet above the surface. If the approach is not stabilized by five hundred feet the crew must execute a mandatory go-around. These policies are great and are aimed at reducing landing accidents. Pilots of general aviation airplanes should use these guidelines, too. Our go-around altitudes can be lower, perhaps two hundred feet, since we're not required to reverse as much inertia as jets, but the rest of the requirements should remain the same. If the airplane isn't in the slot at the minimum altitude, go-around and do a better job on the next approach.

Next time you find yourself harassing the flight controls take a moment to intentionally stop all yoke movement. You'll probably find that the approach continues in about the same way as it was with all the man-handling, so really all you were accomplishing was releasing some of your nervous energy, which, ironically, was probably being produced by your insecurity about landing an airplane. Good airmanship involves a gentle touch. A pilot should never "wrestle" his airplane, only the wind and weather. The airplane is on your side, so keep the communication lines open and listen to what it is telling you. Your approaches will become less stressful and more stabilized.