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Of bounces, balloons, and porpoises

Of bounces, balloons, and porpoises

Recognizing a mistake and correcting it is often as important as preventing it

Recognizing a mistake and correcting it is often as important as preventing it

There isn’t a person in aviation who hasn’t experienced a healthy, often embarrassing—and sometimes dangerous—balloon or bounce on landing. Balloons and bounces are simple facts of aviation life and difficult to avoid, but it’s important to recognize which is which and know what to do when we find ourselves fully engaged in one or the other.

Defining balloons and bounces may help identify something in our landing technique that we can change to prevent them or make them less severe. One difficulty in defining balloons, bounces, and associated phenomena is that they often blend together at the edges. A balloon, for instance, can easily lead to a bounce, and one bounce can lead to a string of baby bounces. So, the way we initially react is critical as to what’s going to happen next.

Energy management: the common factor. When an airplane crosses the runway threshold, it possesses two types of energy: kinetic (borne of its speed) and potential (a function of its altitude). These are fundamentally intertwined. When the airplane is in the flare, we can easily trade one type of energy for the other unintentionally. This is especially true if there’s a surplus of either form of energy, e.g. the airplane is too fast, too high, or both. Too much speed can easily become too much altitude, and too much altitude can become too much speed.

There’s an endless number of combinations and permutations of speed and altitude that one can have in the flare (too high/low, too fast/slow, too abrupt on controls, gusts, thermal lift on hot runways, et cetera) so the subject can get incredibly complex. For that reason, let’s try to stick to the middle-of-the-envelope, most commonly occurring situations and explanations.

The balloon: excess energy and its tradeoffs. For this discussion we’re defining a balloon as that seemingly uncommanded, and definitely unwanted, gain of altitude that sometimes occurs when we pull the airplane level in ground effect. The airplane magically “balloons” upward and surprises us.

There are two primary reasons balloons happen: excess airspeed (kinetic energy) and too much pressure on the elevator in the flare for the speed at hand.

The faster the airplane is moving, the more effective the elevator will be, and the more lift the wing will be generating. So less pressure on the yoke is required to bring up the nose while trying for a mains-only touchdown. When we’re too fast and we try to flare, what we thought was a normal increase in pressure on the yoke results in the nose coming up the way we want it to—but the airplane gains altitude at the same time. This is almost guaranteed to be the result of too much airspeed. Not only is the elevator more effective, but with lift being a function of airspeed and angle of attack, increase both of those and the airplane naturally climbs.

Some believe balloons can be caused by “bouncing” off of ground effect, but that’s something of a misnomer. Ground effect is not so much a cushion as it is the result of decreased drag caused by, among other things, the ground interrupting the formation of the wingtip vortices. This aggravates the effect of being overly aggressive with the elevators.

In the moment, all we know is that when we should be slowly sinking toward the runway while increasing the nose attitude, just the opposite is happening: We’re going uphill. It’s at this point, when the airplane begins moving away from the runway, that we should immediately stop the process that’s in progress.

The airplane is gaining altitude. At the same time, what excess airspeed it had is rapidly deteriorating, and we’re painting ourselves into an aerial corner. The airplane will run out of speed while far too high and fall out of the air like a manhole cover, which sets up a serious bounce—or worse. So, how do we stop this before something ugly happens?

First, if an airplane balloons, it’s usually because of the aforementioned hard pull. However, a strong gust can sometimes do the same thing, causing a momentary increase in lift. Regardless, the first move is to arrest the upward movement immediately. The longer we wait to relax the back-pressure and get the nose back down, the more serious the situation becomes, and we’re talking a second or two. We’re also talking very small control movements. A change in stick pressure is all that’s needed to get rid of just enough pitch attitude (by lowering the angle of attack) to cap off the balloon and stop it from getting worse. If we slam the nose down, we’re likely to see the runway coming up to meet us much more forcibly than we’d like. If we wait too long and run out of speed, the same thing happens. However, just pitching the nose level doesn’t always solve the situation because at that moment, we’re high and slowing down.

Here is where judgment comes in: We have to decide whether we have enough speed left to fly back down from our too-high position. Or, should we add just a hint of power to soften the touchdown? Experienced pilots know their airplane and the situation well enough to recognize when they need power. When in doubt, feed in just a hint of power to soften the touchdown.

The real goal is to be at the proper threshold speed in the first place. Failing that, if you’re fast, be patient with the airplane. Just skate along on ground effect, and enjoy the ride while waiting for the speed to dissipate. Don’t try to rush things by squeezing the nose up. Wait until it starts to settle before introducing elevator. Then just barely pressure the yoke and do your best to keep the airplane from touching down. This is especially difficult in Cessnas with full flaps down. None of this is a problem, but life is much easier if the speed over the threshold is correct.

Or just forget all that and go around.

Bounces. A bounce occurs when the aircraft is dropped onto the runway, either because it’s out of airspeed or because we planted it hard enough that the gear springs us back toward the sky. The airplane hits the ground hard and compresses the gear, and the gear causes the airplane to rebound. Airspeed, the abrupt change of pitch attitude when the main wheels contact the ground, and back-elevator pressure applied too late all will affect the severity of the bounce. The corrective action can run from almost nothing to a mad scramble to add some power, depending on how hard the airplane hits.

Different landing gear designs rebound in different ways. A Piper-type oleo gear heavily damps the compression and decompression of the gear legs, so the tendency to rebound is limited. Cessna-type spring/tube gears, on the other hand, are undamped. They’re happy to give back energy immediately. They will produce much heartier bounces from a given runway hit than a comparable oleo gear.

Regardless of the gear type, a pilot’s reactions to a bounce will be governed by the severity of the bounce and the knowledge that the airplane will return to Earth in a dynamic, not aerodynamic mode: The wings are still developing a little lift, but not enough lift to fly the airplane. So, the pilot can’t depend on the wings to solve the situation, which can range from embarrassing to dangerous.

The majority of bounces are harmless, if somewhat ugly. The airplane bounces a foot or two off the ground and almost immediately comes down and dribbles down the runway in a short series of smaller bounces. The pilot rides it out, keeping the airplane straight and, if needed, a wing down to fight a crosswind. Sometimes, however, the airplane really rebounds (often helped by the excessive “up” elevator the pilot desperately applied at the last second), and the pilot is in for a real ride. In those situations, the pilot needs to stop the coming series of possibly destructive bounces before they start by adding as much power as is necessary to get the airplane under control—and back on the runway as gently as possible.

Also, don’t forget the always-available go-around. Upon initial touchdown, the airplane is only a few miles per hour below flying speed and may actually still be above stall. Adding full throttle lets us stagger away to have another go at it.

A word of warning on go-arounds like these: The airplane will be trimmed for approach speed with little or no power. When the power comes in to go around, the trim will make the nose try to leap skyward. Also, staying in ground effect allows the pilot to take advantage of the free jolt it affords and to convert available energy to acceleration, not climb. As the power goes forward, the yoke should move in the same direction to keep the nose down. This digs us out of the hole much faster and with more stability.

The porpoise. A porpoise usually occurs when the airplane hits nose gear first, the nose rebounds, and the mains then come down abruptly. They then rebound, and the cycle repeats itself. In this situation, one can potentially damage the airplane. Porpoises usually result from carrying too much speed into the flare, losing patience, and trying to force the airplane onto the runway.

Once a porpoise is started, it is almost always aggravated by the pilot trying to control the pitch attitude with massive amounts of elevator in both directions. Unfortunately, it only takes a split-second to get out of sync with what’s going on, and zig when one should be zagging. It’s a classic pilot-induced oscillation. So, the elevator should be held back enough to attempt a landing on the main gear. Some pilots will add a little power at that point or simply go around. The go-around decision is strongly dependent on how much the airplane has decelerated. After the first bounce or two, the aircraft has generally decelerated to the point that trying to fly it off may cause more problems.

While bounces are impossible to avoid because we’re working in an unstable, fluid medium, balloons and porpoises are the result of ignoring the basics. We all were taught to come over the threshold at a given speed, which lets us make a normal flare to touchdown. No instructor has students smoking over the runway numbers at warp speed and then purposely working with excessive speed. So, if bounces, balloons, and porpoises are regular features of our flying, perhaps it’s time to go back and get a little CFI time. Revisiting the basics is never a mistake.

Budd Davisson

Budd Davisson is an aviation writer/photographer and magazine editor. A CFI since 1967, he teaches about 30 hours a month in his Pitts S–2A.

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