February 2003Departments

Accident Analysis

How flaps cause flops


Avoid that sinking feeling

Here is a slow-flight drill that will give you a good feel for your airplane, but more important, could keep you from having an accident some day. Start out with a time-honored coordination drill as a warm-up exercise - a good opener on any training flight: Established in level flight at a safe altitude, decelerate in 10-knot increments to minimum controllable airspeed. At each step, spend a few moments performing a coordination exercise by waggling the ailerons back and forth lightly to induce aileron drag, while using rudder to offset the adverse yaw and hold a constant heading. After you have loosened up with this drill, spend some time on level-altitude slow flight, both on a constant heading and in shallow banked turns.

Now here's the part you may not have tried before: While you are flying a constant heading at various speeds within the white arc on the airspeed indicator, have your flight instructor extend and retract your aircraft's flaps, in no particular order. At some times during the drill you will be flying with full flaps; at other times, clean. Sometimes the CFI will retract the flaps an increment at a time; at other times, all at once. Your task will be to maintain course and speed during these configuration changes.

What will you have to do to keep the airplane flying at the assigned speed and altitude? And, how can this drill possibly prevent an accident some day? It turns out that numerous accidents, in and out of the training environment, occur when pilots begin a go-around with the wrong step. Either they retract the flaps before adding power, or they add power but then retract all the flaps at once instead of climbing with an intermediate flap setting, as recommended in most pilot's operating handbooks. As the aforementioned drill will clearly demonstrate - and as you may have experienced already - flap retraction (absent any other inputs such as power) will cause an airplane flying level to sink or will cause a descending airplane to descend at a higher rate as the retracting flaps lower the wing's angle of attack, reducing its lift.

Why this happens may well go back to what, and how, a pilot first learned. Flight instructor candidates must study and pass a written test on the learning process. In the FAA's venerable Aviation Instructor Handbook, a reader encounters a passage discussing "levels of learning." There are four such levels: rote, understanding, application, and correlation. Correlation means knowing the cause-and-effect relationship between control inputs and aircraft response and making the right choices when the need suddenly arises, say, to perform a go-around. The accident pilot has failed to correlate his management of the flaps with the expected behavior of the aircraft, because he has no such expectation. His actions reflect the rote level of learning - he reaches for the flaps because he thinks he remembers someone telling him to reach for the flaps.

An occasional accident of this type might seem like a random occurrence. But flaps-before-power mishaps, or too-much-too-soon accidents in go-arounds, occur too often. It is an error that flight instructors observe on a regular basis - and not only with new trainees. Many certificated pilots don't practice much on pleasure flights (hence the name), and this is one of the common errors that surface on flight reviews. A related problem is the tendency to lunge for a radio microphone and bellow one's intentions to abort the landing in lieu of adding power at the commencement of a go-around. Aviate, then communicate - but at least a microphone has no adverse effect on lift.

Why are so many pilots flops at managing flaps? Habit may also be to blame. For various reasons - time and money foremost - we do a lot of touch-and-go landings during training. You land with some amount of flaps deflected, and while rolling down the runway, retract the flaps, remove carb heat, add power, and become airborne again. Note that sequence: touch down, retract flaps, carb heat off, power up, rotate. Until the student learns to relax and take all this in stride, the process can be hectic and produce some nervousness.

The generic go-around procedure is the exact opposite. Because the maneuver commences with an airplane that is already airborne, the goal is not to get into the air, but to stay there. You add power, shut off carb heat, and retract flaps to the recommended setting for a balked-landing climb. Once the airplane is established in a positive rate of climb - but only then - is flap retraction completed.

So much for theory and checklists. Real-life go-arounds are prompted by adversity such as runway incursions, sudden gusts, or problems with directional control, also accompanied by some amount of nervousness. So it is reasonable to assume, and this CFI's observation supports, that an unwary pilot facing this scenario suffers a moment's confusion and selects the familiar if incorrect procedure. But a pilot who has performed the drill described above and has come to associate flap retraction with sink rate will not commit either of the errors described here.

An accident that occurred in Carefree, Arizona, on February 20, 1995, was attributed by the NTSB to "the pilot's premature retraction of the flaps and failure to maintain an adequate airspeed during a go-around, which resulted in an inadvertent stall/mush." It was a familiar set of particulars. The Cessna 150 flown by a student pilot/owner was high on the approach to a 1,000-foot airstrip. He "decided to execute a go-around after floating about two-thirds of the way down the runway. The pilot said he applied full power and retracted the flaps. He reported that the engine 'sputtered' momentarily then 'caught'; however, he let the airspeed get too slow and the aircraft stalled. The aircraft settled into the ground about one-quarter mile beyond the runway." Winds were calm and weather good at the time of the accident, which occurred an hour and 15 minutes into the flight.

Not to pick on Arizona, but 17 days before the Carefree accident, a Cessna 152 with a pilot and passenger aboard was approaching an airport situated atop a mesa in Sedona where strong winds were being reported. Mesa is the Spanish word for table. Imagine the downdraft action of strong winds blowing over the surface of a table. And in this case, the threshold of the runway in use was only 150 feet from the mesa's edge. Quoting the NTSB report: "A Federal Aviation Administration (FAA) inspector reported that the pilot said he encountered a 'severe downdraft' while on final approach. The pilot could not arrest the downdraft, and the airplane collided with the down-sloping terrain short of the runway threshold. The pilot repeated his statement in the aircraft accident report. He added, however, that while on final approach, the flaps were fully extended (30 degrees). When the airplane encountered the downdraft, he raised the flaps to 20 degrees, applied full power, and then turned off the carburetor heat. The pilot raised the nose to attain the best angle of climb (VX) airspeed, but without success; the airplane continued descending. He continued to raise the nose to arrest the descent until the stall warning horn sounded. At this point, the pilot lowered the airplane nose and the airplane collided with the terrain."

Could simply reversing the steps have caused the accident - especially under the conditions that prevailed? Even if retracting the flaps first did not cause the initial sink, it could be expected to make it worse, whereas adding power first would not have aggravated the existing problem. Other aircraft had landed successfully before and after the mishap, noted the NTSB report, which concluded that causal factors were "the pilot's improper compensation for the prevailing wind conditions and his improper remedial action by raising the flaps before applying the power. The downdraft was a factor."

These are seemingly small technical missteps, but they could certainly bust a checkride, and they illustrate how adherence to procedural precision is imperative if a pilot is going to fly successfully out of a crisis. They are also avoidable - both in the teaching and in the subsequent retention of flying skills. Flight instructors are exhorted by the FAA to apply several "laws of learning" to their work with student pilots. One of these, the "law of exercise," reminds us that "those things most often repeated are best remembered." (Do more go-arounds when flying on your own.) Another is the "law of primacy," which deals with first impressions. Were you correctly taught how to perform balked landings in the first place? It is difficult to "unteach" something incorrectly presented.

There is also a "law of intensity." Applied to this discussion, it states that flying a go-around is more effective than listening to someone tell about it. The law of intensity is also the best argument for learning how your airplane will react to a flap-retraction drill at a safe altitude, rather than experimenting with flap responses in a crisis - when the ground is within striking distance.

Dan Namowitz is an aviation writer and flight instructor. A pilot for 18 years and an instructor for 12, he enjoys learning to fly "anything new and different."


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