Safety Publications/Articles

Don't Fly Flat

Practicing Proper Approaches

Here's an interesting, safe experiment that any pilot can perform in a Cessna 172 (or similar training aircraft), starting each step of the experiment at least 2,500 hundred feet above ground level. While climbing to that altitude at full throttle (maximum power), note the rpm. It will be about 2,400 rpm in most Cessna 172s.

First, do some constant altitude slow flight at 65 knots with the flaps extended at 30 degrees. The rpm will probably be at about 2,000 rpm.

Second, do a descent at 65 kt, keeping the flaps at 30 degrees. Set the rpm at 1,500 rpm. The descent rate will be about 500 feet per minute (fpm).

What can we learn from this? Well, let's apply the above information to our landing technique. Remember that rpm is the indication of power on an airplane with a fixed-pitch propeller.

The descent described in our experiment is how the airplane should be flown on final approach. If we need power to counteract some unexpected sink rate, there is 900 rpm of additional power available (2,400-1,500). This 900 rpm represents surplus power, or power that is not needed to maintain the descent flight path in still air. In contrast, if the airplane is making a flat approach to the runway, only 400 rpm of surplus power is available (2,400-2,000). A normal approach has more than twice as much surplus power available to counteract any unexpected downdrafts. This greater surplus power could keep you in the air some day when you experience a downdraft on short final. This surplus power can also help you make a go-around if you discover an obstacle on the runway or some other problem on short final. To hammer home the point that flat approaches are not as safe as those using a normal descent, let's climb back to the experiment altitude. Fly in level flight at 65 kt with 30 degrees of flaps. Pretend that you are on final and you start to sink. Add all of the remaining power and climb while noting the climb performance. Repeat the approach but use 1,500 rpm, 65 kt, and 30 degrees of flaps while descending at about 500 fpm. Again, pretend that you are on final and you start to sink. Add power and climb. How do the two climbs compare? I am willing to bet that you prefer the climb from the normal descent because of the greater rate of climb it provides.

Pilots don't often make completely flat approaches to the runway, flying somewhere between the two cases of flat and normal angle of descent. But the in-between approach still does not allow for as much surplus power as could be available. Also, a shallow approach can end up becoming a flat approach if the pilot raises the nose of the airplane before adding power.

A good general rule for a safe approach and touchdown is this: Don't raise the nose during final approach before the threshold of the runway, and don't lower the nose after you are over the runway. There are exceptions to the general rule that can be practiced by the experienced, competent pilot. On final, you may raise the nose - usually no more than level - after power has been applied and airspeed has increased to safe climb airspeed. And over the runway you may slowly and carefully relax the back pressure on the control wheel slightly for a short time to stop the airplane from floating or to correct a slightly too-high flare. Generally a flat approach is made when the pilot has not mastered the flare and is afraid of the ground. Flying the airplane onto the runway with a flat approach is usually an attempt to make up for a lack of skill in flaring, resulting in a nose-high attitude and often a long float down the runway. This leads to the potential hazards of using a lot of runway, flying the airplane off the end of the runway, or dropping the airplane onto the runway in a stalled condition. A good approach and a good flare precede a gentle touchdown.

A strong wind straight down the run- way can create a hazard on any approach, but the danger is especially severe during a flat or low approach to a hilltop airport. I call these conditions "aircraft carriers." Imagine that, instead of the wind, a current of water is flowing along the length of the runway. At the landing end, the water turns into a waterfall. What would you expect the airplane to do if you flew into a waterfall? Exactly! The airplane would descend with the flow. The same thing happens if you approach such a runway with a headwind and the air is descending into the valley at the approach end of the runway. You should particularly avoid a low, flat approach in a landing situation like this one. Even with a proper descent angle, it may be wise to anticipate the sink at the end of the "aircraft carrier" and add a little power before the downdraft causes the sink. How much power? Too much is better than too little. If you use too much power, you can always go around. With too little, or none, you may not get a chance to go around.

A good landing includes a well-controlled approach using the proper descent angle and airspeed, a proper flare, and finally a gentle touchdown. You can't beat flying a correct approach. Try it - you'll like the professional-looking final approach to landing.

By Bob Downs

Back to the Index of Instructor Reports