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Asymmetric Flap Failure? Here's How To Land Safely

Boldmethod

What happens if you need to fly your airplane in a split-flap condition? Here's what you should know...

How Asymmetric "Split" Flap Failures Affect Your Airplane

An asymmetric, or split, flap condition happens when one flap deploys or retracts while the other remains in place. While many GA aircraft are designed with physically interconnected flaps, meaning they can't extend or retract asymmetrically, there are plenty of aircraft where a split-flap situation can occur.

If you don't have advanced flap indications, you'll first notice an asymmetric flap deployment when there's a pronounced roll toward the wing with the least flap deflection.

This happens because the wing with the most flap deflection is producing a substantially higher amount of lift. At the same time, increased drag on the flaps-down side creates a pronounced adverse yaw situation, with the nose swinging toward the wing with the most flaps deflected.

Flying In A Cross-Controlled Condition

You'll counter the roll from asymmetric flaps with opposite aileron, and you'll counter the yaw with substantial opposite rudder. This results in your airplane flying in a cross-controlled condition.

Remember how you add opposite aileron to counter an overbanking tendency during a commercial steep-turn? The same principle holds true here too, except it results from a mechanical overbanking tendency. You'll be adding aileron input in the direction outside of the turn to prevent the roll from continuing. If you control the airplane effectively, you can balance the yaw and roll to keep the airplane aligned with the relative wind, which keeps your plane coordinated.

Fly A Faster Approach Speed To Maintain Roll Authority

At reduced airspeed on final approach, you may need nearly full aileron to maintain wings-level flight. With less airflow, the controls lose their effectiveness. Because of this, you should fly a faster-than-normal approach speed on final approach through landing. As you approach your roundout and flare, don't let airspeed dissipate to the point where a cross-controlled stall or a loss or roll authority could happen. Fly the airplane onto the runway with an airspeed that's a safe margin above flaps-up landing speed. And as always, make sure to follow any POH procedures for your airplane.

What About Crosswinds?

So what happens if you need to land with a crosswind? Do you want the crosswind coming from the down-flap side, or the up-flap side?

According to the FAA's Airplane Flying Handbook: "Avoid attempting to land in a crosswind from the side of the deployed flap because additional roll required to counteract the crosswind may not be available".

Should You Retract Flaps To The Last "Good" Position?

There are two trains of thought here. 1) If you bring the flaps back to their last known good position, you might eliminate the asymmetric flap situation. 2) If you bring the flaps back to their last known good position, you may do more damage to the plane, and affect other flight controls, like your ailerons.

You should always follow your POH's advice. But if your POH doesn't have a recommendation, and you can fly the plane safely in its current configuration, it's probably safest to leave your configuration "as is" and land the plane.

Have You Ever Had A Control Failure?

Flight control failures are a big deal. Have you experienced a split-flap condition or another flight control failure? Tell us about it in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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