The Flap About Flaps

How much is enough?

August 1, 2004

As Shakespeare didn't say, "To flap or partial flap, that is the question." Flap usage becomes a matter of experience and, therefore, personal opinion. That may be why, over the years, pilots of single-engine airplanes have hotly debated the issue. Those flying aircraft that do not have flaps, including biplanes and many models of tailwheel aircraft, are laughing about the so-called debate now. They have learned to substitute a forward slip for flaps as needed.

Let's take stock of the current arguments to see where the debate stands and in the process, take a second look at advice you have read in AOPA Pilot over the past several years:

Does the FAA require full-flap landings?

It may have seemed that way in the 1970s, although it was never a regulation. Operational suggestions the FAA makes are usually consistent with manufacturer's recommendations, and in the mid-1970s manufacturers suggested vaguely that aircraft should contact the ground at the "minimum possible safe speed consistent with conditions," and defined a normal landing as one "where full flaps are used." A search on the Internet found a 1976 FAA Eastern Region bulletin suggesting full-flap landings whenever possible. But a check with today's FAA Eastern Region public affairs office turned up no evidence of any current bulletin offering the same argument.

The 1976 bulletin noted the dangers of using full flaps in crosswind landings, but appeared to stick with their use: "Since the flaps are located behind the main wheels, a crosswind acting on the flaps increases the weathervaning tendency and the wing will also tend to rise on the side from which the wind is blowing." But instead of suggesting partial flaps as a way to remedy the problem, the bulletin went on to say: "That is why it is very important to continue the follow-through crosswind correction technique during the landing roll."

Today the manufacturers' handbooks read differently — some of them, anyway. Many now suggest using partial flaps in strong crosswinds. Not unexpectedly, the FAA supports the manufacturers' recommendations.

The use of full flaps in crosswinds becomes a problem on the landing roll, not in the air. In fact, use of full flaps during the approach alters the airflow in such a way as to improve aileron effectiveness (roll control). They also offer a stabilizing effect. (At the same time, full flaps can impede the aircraft's ability to quickly accelerate out of a wind shear condition.) At first it might seem best to use full flaps in the air and raise them after touchdown, but doing so is a distraction from the main task — maintaining control in gusty crosswind conditions.

What if your runway, perhaps one located on your personal property, is so short you need full flaps just to avoid rolling off the end, no matter what the wind?

"If you need full flaps in a raging crosswind because of an extremely short runway, it is time to consider going to a different airport," said Bruce Landsberg, executive director of the AOPA Air Safety Foundation.

Using partial flaps offers the best of both worlds: They stabilize the approach but cause minimal problems on the runway because they are not fully deployed, and therefore don't need to be raised.

Not all manufacturers offer recommendations for flap usage in crosswinds. The handbook for the Beechcraft A36 Bonanza, for example, says nothing at all. To fill that information gap, a training center in Wichita was called that specializes in Bonanzas. The instructor said he teaches a stabilized approach, even in crosswinds, and that means partial flaps. Use of approach flaps (12 degrees) in a Bonanza stabilizes the airplane — that is, it reduces yaw tendencies. It also helps to stabilize airspeed at the correct value. What does he think about using full flaps to lower the stall speed? The difference in stall speed between partial and full flaps isn't significant, the instructor said. And in strong turbulence he uses no flaps at all. He wants the extra control that higher speeds provide, and the ability to accelerate quickly out of wind shear.

Do full-flap landings save the tires?

Not by much. The difference in touchdown speeds may make a millimeter of difference in the rubber used, but what really wears out tires is improper brake usage by the pilot.

An argument could even be made that greasers, the type of landings we crave, actually skim the tires along the surface and burn more rubber than a firmer landing, one that starts the tires rotating more quickly.

Full flaps allow touchdown at the minimum possible speed, but higher-speed landings are not unusual and are sometimes required. Every time there are gusty conditions proper technique calls for adding half the gust factor to the approach speed, and that often results in a higher touchdown speed than normal. It's part of normal wear and tear, and the tires are designed for it. Just don't skid them.

Do partial flaps result in prettier landings?

That's a valid and important consideration, considering a pilot's ego is involved, and one that begs for testing. In the interest of thorough research, I made partial- and full-flap landings in a Cessna 172, followed by landings in a Piper Archer. The only criteria for the test was whether, to my eye, the partial-flap landings were pretty — so much for boring aeronautical terminology.

The answer for both airplanes was yes. It seemed easier to react to the sink rate properly in the flare and to hold the nose off after the mains touched using a mid-deployment flap setting.

The most difficult landings were those with no flaps in the Archer because of the flatter approach. The nose blocked much of my view of the runway during the approach, and I found myself trying to grow an extra vertebrae for better visibility over the nose.

Full-flap landings required greater yoke pressures during the roundout in both aircraft, making minute adjustments in pitch more difficult. They were a second runner-up in the beauty contest.

Go-arounds were also easier with partial deployment of flaps. A go-around in the Cessna 172 with full flaps resulted in three separate trim adjustments as the flaps were raised. The first adjustment required four turns of the trim wheel, the second three, and the final adjustment two. That's a high pilot workload at a busy time when the pilot is also supposed to look outside to keep track of a departing aircraft that caused the go-around. With partial flaps there were only two trim adjustments — one requiring three turns and the other requiring two.

Use of partial flaps in a crosswind had to wait for another flight, this one in the Archer used earlier. I was able to stay on the centerline after touchdown without swerving, but there were other things to consider. The runway in northern Maryland was about 2,200 feet. I came in faster than normal, because of gusts, and with partial flaps. The result was a longer landing than normal — something to keep in mind when landing on shorter runways with partial flaps.

Engine failure raises another issue. On the day of my test a rough magneto had been reported on an earlier flight. It seemed to cure itself during the runup, but I couldn't be sure. I knew that a possible partial-flap deployment on final was safer than full flaps should the engine quit. As it turned out, however, a mag check on downwind revealed that the rough mag had returned and I left the flaps up on base and final until making the runway was assured.

What would a 747 pilot do?

Just for fun, does a Boeing 747 pilot ever make flapless landings? How about partial-flap landings?

Few airlines offer their pilots procedures for flaps-up landings in a 747, according to one pilot. Their manuals, however, provide data allowing crews to determine how much runway will be needed. They know what to expect: The aircraft will approach as slowly as possible, but will still be faster than the design limit for the tires. Most of the tires will fail during the touchdown. One airline has pilots practice making no-flaps approaches, but the training is never carried through to touchdown.

Given the available backup systems on board, it is unlikely a 747's flaps would ever fail to come down. For example, there are ways to substitute the use of electric motors for lost hydraulic power. In addition, when alternate flap-control measures are used, all leading edge flaps extend simultaneously as soon as the flap handle is moved from its Up detent. That lowers stall speeds by 20 knots and approach speeds, while still faster than normal, are then within tire limits.

It is possible to land a 747 with less than full flaps, or what 747 pilots call "25 flaps." But that is rarely done. (One airline, however, required it solely as a fuel-saving measure.) Pilots may elect to use 25 degrees of flaps in turbulent conditions because such a procedure assures better acceleration following a sudden airspeed loss. But it is more likely that 747 pilots will elect to leave 'em down (30-degree deployment is normal) even in nasty crosswinds. That's been done, and the 747 aircraft can easily handle a far greater crosswind than its official limit (36 knots on a dry runway). Even with flaps all the way out, the aircraft accelerates well in the event of a go-around. As a point of interest, 747 pilots use the slip method for controlling crosswinds up to about 15 knots, and a crab-and-slip method for faster winds; the low-slung outboard engine nacelle limits the bank angle used for the slip. Bank too steeply and the nacelle will strike the surface.

Are all airliners like the 747? No, the Boeing 737 can land safely without flaps.

So what is the answer?

It seems there is at least a consensus among those interviewed, including the FAA, on use of partial flaps in strong crosswinds. Procedures for light crosswinds depend on your personal experience. What may be light to a 2,000-hour pilot could be challenging to a 200-hour pilot. Do you want to use partial flaps in normal winds so that you look cool? The answer is yes, if a friend is watching.

If you are a newly minted pilot, use only the procedures you were taught and save experimentation for later in your flying career. Develop the ability to ad lib, rather than just putting down full flaps automatically after turning onto final. For example, do you really want to fly a five-mile straight-in approach with full flaps?

The debate isn't over. There are as many answers as there are pilots. Flaps were originally invented because of the large number of accidents involving stalls. They increase lift, decrease stall speed, and allow a steeper angle of descent over an obstacle without accelerating past the final approach airspeed. They keep you safe at slower speeds and when operating from shorter runways. Use 'em if you got 'em, but be aware there may be times when they hinder more than they help.

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