Proficient Pilot

Why all the flap?

July 1, 2009

Barry Schiff has written two aviation novels, The Vatican Target and Flight 902 Is Down .

Anyone who has spent time watching landings from the sidelines probably has wondered why so many pilots land on all three wheels simultaneously in tricycle-gear airplanes. In many cases, it is because some aircraft are more difficult than others to land properly (nose high) when flown with a forward center of gravity (no rear-seat passengers or baggage). The use of full flaps contributes to this problem because this often diminishes the elevator authority needed for the pilot to hold off the nosewheel after main-gear touchdown.

This makes me wonder why landings are always made with full flaps in such aircraft. It is easier to make a two-point landing when only partial flaps are deployed. Depending on the airplane, this would mean landing with half, two-thirds, or three-quarter flaps. This makes it easier to hold off the nosewheel until well after the mains roll onto the runway. Such landings are typically smoother, too.

I used this technique when teaching students who had particular difficulty in making two-point landings. By using only partial flaps, aircraft attitude on final is not quite so nose-low, and this alone makes it easier to transition from glide attitude to flare and touchdown. With greater elevator authority, students find it easier to prevent touchdown until the aircraft has been flared to the desired nose-high attitude. Less muscle is required to achieve that attitude, too. Once these students got the hang of landing with partial flaps, I transitioned them to full-flap landings.

Using maximum flap extension is the optimum way to land, but this usually is not required by airframe manufacturers or the FAA. Pilot operating handbooks for Cessna singles, for example, simply advise pilots to extend flaps for landing “as desired.” The pilot has the option to use as much or as little flap as he desires. One advantage of using full flaps is that this results in the maximum stall-speed reduction. The last portion or “notch” of flap deployment, however, typically does not reduce stall speed by more than a knot or two. In some cases, the last 10 degrees of flap extension does not lower stall speed at all. The additional flap is provided mostly to add parasitic drag and allow steeper descents at given approach speeds. It is axiomatic that most stall protection is provided by the first half of flap deployment and most of the drag results from the second half.

To compensate for the slightly increased stall speed resulting from a partial-flap approach, a pilot should add the same number of knots to one’s approach and over-the-fence speeds. The use of less than full flaps does result in more floating in the flare; the airplane decelerates less quickly. This provides pilots more time to properly posture the airplane for touchdown. Partial-flap deployment obviously increases landing distance somewhat, but not significantly unless runway length is critical or marginal. At such times, maximum-available flap extension should be utilized.

If you have not tried a partial-flap landing in airplanes that tend to land in a three-point attitude, you should. In these and other airplanes, landings are typically improved, and you are afforded more of an opportunity to “fly” the nosewheel down to an equally smooth touchdown. Another advantage of partial-flap approaches is that this configuration allows an easier transition to a go-around (VFR) or missed approach (IFR). It is my practice to execute partial-flap instrument approaches when the possibility exists that I might reach minimums and not be able to see the runway. If the runway is found waiting for me, I then have the option of completing the approach and landing with partial flaps, or I can extend the remaining flaps once I have the runway in sight. (Pilots are cautioned not to change flap settings when less than a few hundred feet above the ground unless they are comfortable with the configuration change and proficient in coping with the pitch change this causes.)

Ordinarily, no-flap landings are made only when there is a failure of the flap system, and the pilot has no option other than to land with flaps retracted. Although every pilot should practice this procedure once in a while, few do, and they will be completely surprised by the relatively nose-high attitude required on final approach when the real thing occurs. Their tendency is to lower the nose to a more familiar and comfortable attitude, which results in excess speed. Airspeed on final should be increased only a knot or two more than the difference in the “clean” versus the “dirty” stall speeds as shown between the bottoms of the white and green airspeed arcs. The additional airspeed used to compensate for the higher stall speed should be 1.3 times this difference in stall speeds.

Pilots doing this for the first time are amazed at how much a “clean” airplane floats during the flare. The runway zips by at an alarming rate, which is why excess airspeed on final approach can be dangerous. This results in a tendency to plant the airplane on the ground prematurely, which in itself can be dangerous. Be patient during the flare, and do not touch down until in a substantially nose-high attitude.

Practice no-flap landings only on long runways. If uncomfortable doing this for the first time when alone, obtain an experienced instructor. Your first no-flap landing should not be during a genuine emergency.