Good at Slowing Down

AC 90-109A is quite long and may tax spans of attention weaned on television, but it is full of pithy observations and good advice and is well worth reading. (It can be readily downloaded; just search for the name.) After a discussion of various kinds of flying characteristics and types of stability and instability, it supplies a multipage list of amateur-built airplane types and the broad categories to which they belong. Some of these categories are self-evident; for example, it is quite obvious that a VariEze has a nonconventional configuration. But one category stands out for the number of types in it, particular challenges that it presents, and the fact that it might have never occurred to you that it is a category at all: the “low-inertia, high-drag” airplane.

What brought me to this publication was its mention in an accident report on the summer 2019 crash of a Rans S-12—a two-seat, side-by-side, pod-and-boom pusher—in Indiana. The accident was notable mainly for its lack of distinguishing characteristics. It took place on a clear evening half an hour before sunset at the pilot’s private grass strip, where he had been practicing landings. No one saw the crash, but a surveillance camera captured the airplane’s spiraling descent, impact and ensuing fire. The airplane came to rest in a nearly vertical position, and the pilot, 42 years old, must have died instantly.

A neighbor later told a relative of the pilot that he had heard the airplane fly over his barn. He said the motor had “died,” and he went outside to look. The engine restarted, and the airplane proceeded toward the runway. He turned away, but then heard the crash and saw the fire.

Accident investigators did not tear down the engine, a two-cylinder, two-stroke Rotax 582 of 65 horsepower, but the witness’s report implied that it was running at the time of the crash. It is unclear what would have caused it to “die” and then return to life. Perhaps the pilot merely pulled back the throttle abruptly and then pushed it forward again. Suppose, however, that without any action on the pilot’s part, the engine had done something unexpected: given some sign that it might, at any moment, quit running for good.

Such a signal coming from an engine heightens the attention of any pilot. But different pilots react in different ways. Experienced pilots try to analyze the situation and remind themselves not to be distracted from flying the airplane. Inexperienced pilots may shed what little finesse they have. This pilot had around 20 hours toward a sport-pilot license and a solo signoff. He was hardly likely to react to engine trouble with icy calm.

This is where the low-inertia, high-drag idea comes in. As the National Transportation Safety Board’s analysis of the accident explains, airplanes of this type have a narrow margin between cruise speed and stall speed; they are always in that “coffin corner.” They “rapidly lose airspeed when there is a loss or reduction of power.” They may also experience significant airspeed decay with increased load factor, for instance, during turns and so are “particularly susceptible to unintentional stalls.” (This last characterization may be a little misleading. The drag increase in turns is due to induced, or lift-dependent, drag and is a function of the wingspan, not the dragginess of the airplane as a whole. It is true, however, that when an airplane has little excess power to start with, the effect of increased induced drag is more evident.)

The Rans S-12 is included in the circular’s list of such airplanes, but it’s in good company; the Piper Cub, Aeronca Champ, Quicksilver, Pietenpol Air Camper and Piel Emeraude—all classic types—are there, along with many others.

We often speak of an airplane’s wing and power loading, which are easily calculated, but we never mention its drag loading, which can be equally important. Drag loading could be expressed in various ways. We could divide the weight of the airplane by its equivalent flat-plate area, or “drag area,” in square feet; this would give us a pounds-per-square-foot result resembling wing loading. Or we could give the airplane’s terminal velocity—the speed at which it would stabilize in a vertical, power-off dive. Either of these numbers would provide a metric for what the phrase “low-inertia, high-drag” implies: an airplane whose drag is sufficiently high, and weight sufficiently low, that it slows down rapidly when not under power. Think of a parachute.

The NTSB’s probable cause cites the pilot’s “exceedance of the airplane’s critical angle of attack while maneuvering toward the runway, which resulted in an aerodynamic stall and a loss of control.” The engine problem reported by the neighbor is not included as a contributing factor, but its mention in the analysis portion of the report suggests that the pilot could have been hastening to return to the runway. The wreckage was located near the middle of the 1,000-foot strip, a location that could require some sort of tight turn to get into position for an approach. Presumably, the airplane was already at a low altitude. If its nose dropped, the pilot’s instinctive reaction to pull back on the stick could only lead to disaster.

Pilots who are accustomed to flying the opposite type of airplane—ones having low drag and high inertia—are familiar with the difficulty of getting them to come down. They lose touch with the other type, which seem to fly in thicker air and upon which gravity seems to exert a more irresistible influence. This is not to say that this category of airplane is evil or characteristically dangerous. For decades, in the early years of aviation, this was the only type of airplane there was. Still, once you have flown the other type—the slick, fast type—low-inertia, high-drag airplanes must be approached with particular care. Airplanes, like people, have their personalities and must be treated considerately. Otherwise, they take offense.

A professional pilot, 61, who was a multiengine ATP with glider and instructor ratings and 18,870 hours, asked to fly his friend’s “trike,” which had been in storage for 10 years. The owner refurbished the aircraft thoroughly and flew it several hours before turning it over to his friend, who had logged 13 hours of instruction in this category of aircraft and awaited only a check ride to get an official type endorsement. A trike is a weight-shift aircraft. It consists of a fabric wing from which dangles an open frame with a seat and, attached to the back, an engine. The pilot controls the angle of attack of the pivoting wing by pushing or pulling on a control bar. It is the very quintessence of low-inertia, high-drag aircraft. The pilot took off, climbed above the tree tops, began a turn to the left, stalled and spiraled in. The crash was fatal. It is not clear whether the aircraft was still climbing as it began the turn, but what does seem clear is that a narrow speed range is a trap into which even a highly experienced pilot may fall.

This story appeared in the March 2021 issue of Flying Magazine