Flaps

Sometimes, less is more

On the third (and what turned out to be the last) landing of his night checkout, John said, "I'll make this one a no-flap, too." This was our final flight in a VFR/IFR recurrency program of some 33.7 flying hours to bring him back up to speed after a 15-year absence from the cockpit.

I said, "Why don't we use full flaps?"

The reasons for my suggestion weren't consciously apparent to me at the time but, reflecting on it now, there was a definite rationale. First, John hadn't made a full-flap landing during the last three flights. Given his former-jet-fighter-pilot preference, I knew he would land without flaps every time. Second, there was about a 12-knot crosswind that night, and I wanted to see how he handled the situation using full flaps when it was dark.

He ended up with a sign-off for a great landing in challenging conditions at the end of a long, distracting day.

Flap settings make a difference

Does it really make any difference what degree of flaps you use on your landings?

Technically, there are several types of flaps typically used on general aviation airplanes: plain, split, slotted, and Fowler (named after its inventor, Harland Fowler, an Air Corps engineer). Each has a slightly different effect on airplane characteristics.

Flaps, regardless of the type installed on the airplane you fly, do make a difference in the way the airplane flies, especially on takeoff and landing. For any given type of flap installation, it also makes a difference how far you extend the flaps, depending on the situation you face for that takeoff or landing.

How much flap you use is strictly at pilot's discretion. Generally speaking, little (up to 10 degrees) or no flap is prescribed for takeoff; see the pilot's operating handbook (POH) for the airplane that you fly.

Landing flap settings are at the pilot's discretion, although this article will outline what a variety of sources say about typical situations. Most manufacturers recommend full flaps for landing, but many landing distance charts fail to reflect landing information without flaps.

Unimportant? How much flap you use for a given airplane has a lot to do with what happens after you get on the ground, especially where you get it on the ground.

What the performance charts show

In the Cessna 172S Skyhawk POH, figure 5-11, Short field landing distance, states: "If landing with flaps up, increase approach speeds by 9 KIAS and allow 35 percent longer landing distances." Thirty-five percent can be a lot more runway.

At 2,550 pounds and sea level, landing distance on a standard day (15 degrees Celsius/59 degrees Fahrenheit) is 575 feet. If you happen to have a 50-foot obstacle on final approach, landing distance then becomes 1,305 feet. If you land without flaps, it becomes 1,762 feet--more than three times the normal landing roll with full flaps.

Continuing this scenario, if you're landing on dry grass instead of a hard-surfaced runway, your landing distance is 834 feet. Add the same 50-foot obstacle, it becomes 1,892 feet--then, if you land without flaps, the landing distance is 2,555 feet. The British Civil Aviation Authority (CAA)--along with New Zealand and Australia--really take this stuff seriously because there are a lot of short, unpaved runways in those countries. CAA publications recommend that the computed landing distance equal no more than 70 percent of the distance actually available; to do that in the specific instance above, you'd need at least 3,650 feet of grass runway. That's a far cry from the 575 feet you find on quick inspection of the 172 POH.

On a tight final is not the place to have this thought occur for the first time.

Here's some data from the Piper Arrow IV POH (figures 5-35 and 37). This is from the short field landing distance performance chart, same standard-day, sea-level conditions, at maximum allowable gross weight. The landing distance is 620 feet. With a 50-foot obstacle it becomes 1,525 feet (nearly two and one half times the no-obstacle ground roll). But the Arrow POH gives no way to compute the no-flap landing distance.

Neither does the Cirrus SR20 POH. The normal ground roll using the procedures described in the handbook is 1,068 feet. Landing over a 50-foot obstacle nearly doubles that to 2,036 feet. No-flap distances are not provided.

As you read further, can you gain additional insights into takeoff and landing problems by applying some CAA planning factors listed below to your situation?

Do the little things count?

There's an old saw that says accidents are caused by a series of "little things," not one catastrophic mistake. The big deal can occur with respect to flaps when there's an obstacle, gusty crosswinds, and you're carrying extra airspeed. Then throw in a few more knots for the uncertainty you feel because you're out of practice and pretty soon your airspeed is faster than you'd like.

The airplane floats and floats before it touches down. If you try to put it on the ground before it's ready, it will just bounce and keep flying. And when you finally land, unexpectedly far down the runway, your judgment is further stressed by the uncertainty of whether you've got enough room in which to stop. It's easy to see how not knowing ahead of time can cause all kinds of problems. But the remedy is simple--just know!

So, how much runway do you have? Enough? What does your POH say--or is it silent on this point?

Remember, the airplane will land when it wants to land--not when you want it to land. It usually wants to do that at or very near stall speed. Note that the bottom of the green and white arcs on the airspeed indicator are stall speeds at max gross weight; if the airplane is lighter, the stall speed will be slightly less. Also notice that there is typically only eight to nine knots' difference between flaps-up stall speed and flaps-down stall speed--and that differential remains pretty constant as gross weight decreases. But that seemingly small spread is really something big to think about.

How far will your aircraft float while it's losing those eight or nine knots? Think about that next time you select landing flaps at a runway somewhat shorter than you're accustomed to. Make sure to check out what the book says--and try to fly your landing exactly the way it says. As a matter of fact, why not even do that before you go flying?

Now let's return to what the British CAA says. Start with the landing distance specified in your POH. Then add 10 percent to that if you have to land over a 50-foot obstacle. The CAA has even gone so far as to provide a chart listing various "add-on multipliers" to takeoff and landing distances because of such little things as an extra passenger (x 1.10), airport elevation (x 1.05 for each 1,000 feet above sea level), a wet runway (x 1.15), dry grass as opposed to hard surface (x 1.15), soft ground (x 1.25), wet grass (x 1.35), and a 2 percent downhill slope (x 1.10).

To that, you add another 20 percent in a tailwind for each 10 percent of your landing speed comprised by the tailwind. Personally, I never land in a tailwind unless I have miles of runway ahead of me.

Then, when it's all over, multiply your new computed landing distance by 1.43 as an additional standard safety factor. Is this getting tiresome? Ask yourself, "What's so critical here that I have to land in a tailwind?"

As you can see, landings are affected by several factors in addition to flaps. You should seriously consider planning for these factors in any marginal situation. Check your aircraft's POH for the corrections you should apply.

The CAA information comes from Safety Sense Leaflet 7C, titled Aeroplane Performance. Its real importance is that it makes us think about some of these "little" things to gain a sense of how they can affect us. To access the leaflet online, see AOPA Flight Training's online links page.

Use full flaps for landing, except in crosswinds

The general rule is to use full flaps for most landings--but it depends.

Using flaps will increase the camber of the wing, improve lift, reduce touchdown speed, give you a better view from the cockpit, and permit more precise management of your touchdown point than not using flaps.

But full flaps can also cause directional control problems on landing in a crosswind. That's why most manufacturers recommend "partial or no flaps" for crosswind landings. By far the most important reason for this is that crosswinds affect the upwind wing more because of the increased lift on that wing imparted by the extended flap, compared to the wind-sheltered wing, and the seeming stability imbalance.

Add to this a crosswind pushing against the vertical stabilizer, and the airplane can weathervane seriously into the wind. This combination of circumstances in a stiff crosswind, even one within demonstrated numbers, could produce trouble.

But what do you do when you're confronted with having to hit a specific landing point on a short runway, and a crosswind exists? There's not enough room here to go into the mechanics and permutations of the maneuver, but why not try a forward slip? Basically, a forward slip enables you to maintain airspeed while increasing the rate of descent to steepen your final glide slope over an approach obstacle--this is not the same as the sideslip you perform to keep the airplane aligned with the runway in a crosswind.

But exercise care when slipping with flaps extended, and pay attention to the guidance (or any prohibitions) in your POH. The POH for the Cessna 172S says, "Steep slips should be avoided with flap settings greater than 20 degrees due to a slight tendency for the elevator to oscillate under certain conditions of airspeed, sideslip angle, and center of gravity loadings."

Practice slips to become proficient. Don't be caught flat-footed in a situation where you have to employ a slip and you are unsure how to do it. Before airplanes had flaps, the only way to steepen a final descent was to execute a slip. Since that time, slips have received less attention than they deserve in flight training programs.

It's up to you

It is important to think about how you use the flaps on your airplane in different situations, understand that they can make a difference (a critical difference under some circumstances), and maintain landing proficiency using a variety of flap positions--not just one. Try triggering your next few landings with the thought, "What flaps does this situation dictate?" After all, how much flaps you use really do make a difference.

Wally Miller is president of an aviation training, consulting, and marketing firm in Monument, Colorado. He is a Gold Seal CFI who has been instructing for more than 30 years and flying for more than 40.