Safety Publications/Articles

The Asymmetrical Traffic Pattern

Minimizing The Stall/Spin Threat

One of the most dangerous phases of a typical general aviation flight is in the traffic pattern, on the turn from base to final. The airplane is vulnerable; there is little altitude to recover from errors. In a typical accident scenario, a relatively new pilot experiences a strong tailwind on the base leg. He doesn't realize how strongly the wind is pushing him until after he has started the turn from base to final. Looking at the runway, he sees himself overshooting the extended centerline. If he has been well trained and is disciplined, he corrects it on final by flying into the crosswind and turning to align himself with the runway once re-established on the centerline. If the overshoot is so severe that he cannot do this, he adds power and goes around.

In a distressingly frequent number of cases, however, the pilot tries to correct the overshoot while still in the turn. He does this in one of two ways. In the first instance, he increases the bank to command a more rapid rate of turn, hoping to catch the extended centerline before being blown too far downwind. The problem with this approach is that the aircraft is already slowed to well below cruise speed and we all know what happens to stall speed as bank increases, right? (It goes up.) The margin between the pilot's present airspeed and the aircraft's stall speed in this configuration becomes very slim. At this point, the aircraft is buffeted by a strong gust. The load factor increases, and anyone who has done accelerated stalls knows what happens next. Four hundred feet of altitude is precious little to recover from an unanticipated accelerated stall.

The second instance is worse. The pilot feels uncomfortable increasing the bank, so he starts to skid the aircraft¿s nose toward the runway with rudder. Of course skidding increases the speed of the outside wing, increasing its lift. This causes the aircraft to bank more steeply, and the pilot unconsciously compensates by applying opposite aileron. The aircraft is now in the classic cross-controlled configuration. If the aircraft's speed decays, a stall with a spin in the direction of the lowered wing is possible. Recovery from the spin is unlikely so close to the ground.

Jet pilots don't even chance this scenario developing. Watch the pros at a commercial airport some day. You will note that they almost always are lined up with the runway at least five miles out. That gives them plenty of time to correct an overshoot to the final approach course; any maneuvering they do to remain on the extended centerline can be accomplished with shallow banks. Take a lesson! If these experienced, highly trained professionals don't allow themselves to get into situations where they may be tempted to bank steeply close to the ground, why should you?

Of course, it's not exactly practical nor is it an especially good idea to turn final five miles out in a light aircraft--remember that you want to be in gliding distance of the runway. But there is a way you can emulate the big boys. First and foremost, foster discipline. Make a rock-solid determination to abandon any approach that has gotten out of hand. Another solution is to plan your base far enough out to avoid the pressure to tighten the turn to final--this doesn't mean five miles out.

What else can be done? Consider the asymmetrical traffic pattern. Compare the patterns in the figures to the right. Most pilots are taught to fly relatively crisp patterns such as that depicted in Figure 1. Bank angles should be steep enough (30 degrees is commonly taught) to minimize the time you can¿t see runway (high-wing aircraft) or the outside of the traffic pattern (high- and low-wing aircraft) where other aircraft may be entering at a nonstandard point. Additionally, the moderately steep bank increases your aircraft's visibility to others in the pattern by exposing more of the wing.

I am in agreement with teaching 30-degree banked turns in the pattern, for all the reasons mentioned. But I make an exception for the turn from base to final. Refer to Figure 2. The pilot flying this traffic pattern has begun his turn early, using a much shallower bank--perhaps 15 to 20 degrees. The result is a somewhat sloppier-looking, but safer, pattern. It gives the pilot more time to assess the effect of any crosswind and adjust his turn to smoothly intercept the final approach course. Because he starts the turn to final with a shallow bank angle, he can safely increase his bank (within limits) to counter an overshoot. Likewise, there should be little reason to tempt the pilot to skid. Even if he does, at the shallower bank this is much less likely to result in disaster. Check out the stall speed versus bank chart in your aircraft's flight manual. Although a pilot who increases his bank from 30 degrees to 45 degrees and one who increases it from 15 to 30 are both increasing by the same number of degrees; the effect on stall speed is much more dramatic in the first case.

Fifty years ago, a generation of flight instructors whose background was largely military taught that crisp turns were the mark of a precise pilot. Today the catchword is safety. The asymmetrical pattern may not look as pretty, but if it makes you safer, who really cares?

By Robert I. Snow

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