It's Possible to Roll This Airplane

"It's a one-G maneuver. It's absolutely nonhazardous, but it's very impressive."

So Tex Johnson explained himself to Boeing president Bill Allen after performing an impromptu barrel-roll in the 707 prototype before a Seattle gathering of IATA bigwigs. Equally impressive, but more subtle, was Bob Hoover's demonstration, watchable on YouTube, that he could smoothly pour water from a pitcher into a glass while rolling an Aero Commander, and people sitting back in the cabin might be unaware that anything unusual had occurred.

Almost any airplane can be rolled, but few can be rolled legally. Normal and utility category airplanes are not permitted to exceed a bank angle of 60 degrees. Only acrobatic category airplanes may be rolled, and then only when the occupants are equipped with parachutes. Nevertheless, the enforceability of such regulations at several thousand feet and in remote places being somewhat problematic, a good many Cessnas, Cherokees, Cirruses and their ilk have, in fact, been rolled-or so I would surmise, never having seen it done myself.

There is something not only impressive but viscerally delightful about turning an airplane on its back and bringing it round upright again; it is the same delight children feel when they turn somersaults in swimming pools. We are not surprised, therefore, to read of a man who, after seeing a Beech 18 and a Baron rolled, wanted to try it in his own newly purchased Baron. I will call him Joe. Joe had over 1,000 hours, acquired over a three-year period, and an instrument rating; he had recently gotten his multiengine rating as well.

A friend of Joe's reported having been with him on the return flight from Sun 'n Fun, where they had seen the Beech 18 rolled in an airshow. As they cruised at 9,500 feet, Joe said, "I want to try something." He banked left and right and then said, "I believe it's possible to roll this airplane." With that, he entered a shallow dive, banked left, pulled back and rolled to the right. The friend in the right seat, displaying an assertiveness and independence of spirit rare among right-seat occupants, grabbed the controls and leveled the airplane, telling Joe, "I cannot do this."

"I believe it's possible to roll this airplane," Joe repeated. He then descended to 7,500 feet, leveled off, inexplicably caged the right engine-nothing was wrong with it, but it did have more hours since overhaul than the left-and continued to Griffin, Georgia, where he restarted the right engine for the landing.

Another pilot, who knew Joe and had flown with him, considered Joe's flying skills to be "below his standards." He "was known for overstressing the planes he flew," said the pilot, who had himself predicted, with remarkable prescience, that Joe "would probably crash an airplane within the next year."

Another friend, when Joe told him that he thought he could roll the Baron, replied that he, Joe, "was stupid," and cautioned him "not to do anything in the airplane that could get him hurt."

Two days later, Joe, together with two other men and the 13-year-old twin sons of one of them, was returning in the afternoon from a fishing trip to Gulf Shores, Alabama. They were cruising in mild and clear weather at 9,700 feet with a groundspeed of 191 knots. A witness, fishing in a boat on a lake near Hamilton, Georgia, heard the approaching Baron and judged from its engine sounds that it was performing some kind of aerobatic maneuvers. He looked up, but could not spot the airplane. As the engine sounds increased in intensity, he looked again and this time saw the Baron, high and descending rapidly in a steep dive. As he watched, he saw a part separate from the airplane. The Baron disintegrated in flight, raining pieces down over a path almost a quarter-mile long.

When an airplane breaks up in flight, the sequence of events can usually be inferred from the order in which parts lie along the so-called "debris path." The National Transportation Safety Board's report on the accident devotes seven single-spaced pages to a minutely detailed description of the Baron's debris path. The breakup began at the tail and progressed forward. The first item was the rudder, which had torn away from the fin. The tip cap and balance weight had broken off and the rudder showed marks of overtravel in both directions. Next were the left horizontal stabilizer and elevator, which were heavily fragmented, followed by the right stabilizer and elevator. Then came the vertical fin and portions of the aft fuselage structure, followed by the cabin door, left wing, right wing, engines and so on.

Investigators found no indication of engine trouble or other mechanical difficulty, and concluded that the probable cause of the accident was "the pilot's exceeding the design stress limits of the airplane while performing aerobatics in a nonaerobatic airplane." The final record downloaded from the airplane's Garmin 496 showed a maximum groundspeed of 266 knots at 8,500 feet; but GPS records contain no information about the airplane's attitude. The broad sequence of failures, starting with the tail and followed by downward failure of the wings, is typical of what happens when a panicked pilot pulls up too rapidly and at too high a speed. The horizontal stabilizers fail downward; without them to hold the tail down, the airplane pitches over to a negative angle of attack, overstressing the wings in a downward direction. The vertical and horizontal empennage components were close enough to one another along the debris path to have been reordered by the random motions of their descent; most likely, it was the non-simultaneous failure of the horizontal surfaces that twisted the fuselage first one way, then the other, shattering the rudder.

Although the pilot was apparently known for his rough handling of airplanes and had repeatedly declared his interest in rolling the Baron, there was no direct evidence that the breakup was the result of a roll. Given the mild weather, however, the absence of severe turbulence, the reputation and statements of the pilot and the report of the witness, the NTSB concluded that some sort of aerobatics had been involved in the accident, and a barrel roll seemed the most likely suspect.

If, as Tex Johnson said, the roll is "absolutely nonhazardous," why did this accident happen?

The roll that Johnson performed in the 707, and that Bob Hoover would perform while pouring water, is a barrel roll, so called because the airplane's flight path is not along a straight line, as in a pure aileron roll or slow roll, but along a helical path, as if it were spiraling along the surface of an invisible cylinder or barrel. The centrifugal acceleration generated by the curved flight path keeps the G-load positive throughout the maneuver. In a slow roll (which is so called not because it is necessarily rotationally slow, but because it does not require the initial acceleration needed for the barrel roll) rudder and elevators are used to keep the airplane's flight path straight while it rotates; the airplane should experience one G negative halfway through the roll. (A third type of roll, the snap roll, is performed at relatively low speed, and is actually more like a spin performed along a horizontal axis.)

When the airplane is inverted at the top of the barrel roll, it must be accelerating downward with twice the acceleration of gravity in order for the occupants to experience one G positive, but the flight path should be horizontal. One must therefore have a good head of steam and a fair amount of vertical speed during the entry; this gets neutralized during the inverted portion-which is brief-and the maneuver ends with a descent back to the original altitude and a roll out on the original heading.

At least that's what is supposed to happen, and in an airplane with a rapid roll rate there is not much time for things to go wrong. In a slowly rolling airplane, however, or one whose control forces in roll are high enough to make it difficult to deflect the ailerons fully at high speed, there is room for error. A pilot unskilled in the maneuver is likely to find his nose coming down while he is still partially inverted, and to recover well below the entry altitude and pointed well off the entry heading-left after a right roll, and vice versa. It is also possible for a pilot to become so distracted by the incipient dive that he unconsciously eases up on the ailerons, actually prolonging the maneuver and allowing the airplane more time to pick up speed. In a heavy, powerful airplane like the Baron, speed builds rapidly in a dive, and with it the forces to which control surface are subjected during a pull-out.

It should be noted, by the way, that Joe evidently understood the entry procedure for a barrel roll. When he attempted the earlier roll that was stopped by his right-seat companion, he began correctly, with a dive to gain speed and a turn to the left prior to beginning the right roll.

Airshow performances by professionals in airplanes that are not normally considered acrobatic, like the Beech 18, encourage pilots to think that any airplane can be rolled, and in fact this is generally true. But while any airplane can be rolled, any roll can also be botched, occasionally with disastrous results. Pilots performing a barrel roll at low altitude have hit the ground while "dishing out" of the maneuver. Joe, at least, gave himself plenty of altitude. He would have done better to confine himself to aerobatics in airplanes actually designed for them; and he should never, above all, have performed this unfamiliar maneuver in the company of four people who trusted him to keep them safe, and who lacked the knowledge or confidence to grab the controls and say "No!"

This article is based on the National Transportation Safety Board's report of the accident and is intended to bring the issues raised to the attention of our readers. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.