Aftermath: Prelude to a Wake

It's better not to tickle a dragon's tail.

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The pilot in the left seat of the Piper Arrow was a 33,000-hour ATP; his right-seat companion had a private ticket and less than 200 hours. The two took off from Racine, Wisconsin, on a midsummer afternoon and headed northward, just off the western shore of Lake Michigan. There were scattered clouds at 3,400 feet and an overcast layer at 4,000; the surface wind was 300 degrees at 13 knots.

The flight passed at 1,500 feet under the outer shelf of Milwaukee Class C — Milwaukee is 12 nm north of Racine — and entered the inner ring, in contact with approach control, 5 miles south of Milwaukee's Mitchell International.

In the meantime, an airliner that had flown a right-hand pattern north of Mitchell was now on final approach to Runway 25. The approach controller asked the Arrow pilot to turn eastward, and he did so. Shortly after, the controller identified the traffic as "12 o'clock and about 2 miles descending out of 2,300, an MD-80."

The Arrow pilot replied, "All right, I can go down lower if you like."

"Negative," said the controller. "I just need you to turn out of there; then I'll get you northbound as soon as I can."

"OK, I've got them in sight," the pilot replied.

"Thank you. Just pass behind that traffic and then you can proceed northbound as requested."

The Arrow proceeded east about a mile and then turned north again. As it crossed the Runway 25 final approach course, its transponder return disappeared. At that moment the MD-80 was at the shoreline, about 1.4 nm west of the Arrow.

The wreckage of the Arrow was recovered from 46 feet of water. Primary radar returns and the condition of the wreckage indicated that the Arrow had broken up in flight before hitting the water. The probable cause, the National Transportation Safety Board found, was "an encounter with wake turbulence, which resulted in the pilot's loss of control of the airplane and its subsequent in-flight breakup." The controller's failure to issue an explicit wake-turbulence advisory to the pilot was deemed a contributing factor, though it is unlikely that the pilot, a lifelong flight and ground school instructor, needed to be informed about wake vortices.

From radar plots included in the accident's online docket it is possible to deduce a little more information than the accident report itself provides. The MD-80 appears to have been approaching at a groundspeed of about 135 knots; the Arrow was doing about 110. After the Arrow turned eastward at the controller's request, the projected tracks of the two airplanes were about 1.8 miles apart and roughly parallel. The jet was 50 degrees off the Arrow's nose when the pilot of the Arrow began to turn back northward. It was at 1,800 feet when it crossed the point where the two airplanes' tracks would intersect. The Arrow reached that same point 36 seconds later. If the wake vortices moved downward at 300 fpm, they would have sunk to 1,600 feet, almost exactly the altitude at which the Arrow was cruising. The vortices might not have dropped quite that far, however, because they were being carried eastward by the wind, and so the Arrow encountered a somewhat "younger" portion of the MD-80's wake than it would have in still air.

The NTSB determined that the breakup of the Arrow was due to the pilot's actions and not to the wake turbulence itself. In an analogous instance in Kansas City, Missouri, in 2006, the board attributed the in-flight breakup of a Piper Saratoga to "loss of aircraft control and the in-flight separation of [the stabilator]" after an encounter with the wake of a 737 that had passed nearly two minutes earlier. In that case, the Saratoga's stabilator appeared initially to have failed in upward bending, followed by the wing in downward bending. (The downward overload of the wing is typical when the horizontal stabilizer's balancing down-force is lost and the airplane abruptly pitches nose-down.) In its analysis of that accident, the board emphasized that the Saratoga was traveling at 183 ktas, far above its maneuvering speed. The information seemed to imply that the pilot had reacted to the wake encounter by suddenly pushing the yoke forward, but that was not explicitly stated.

It is difficult to find information about the velocities present in the decaying wake vortices of large aircraft. One source gives tangential velocities — that is, those at a right angle to the direction of flight — of about 35 feet per second, or 21 knots, for a DC-9. This is not an excessive gust velocity for an Arrow traveling at 110 knots (but it could impose a nearly 5 G acceleration on a Saratoga traveling at 183 knots). At any rate, the danger in wake turbulence encounters is not that the vortex velocities themselves can break an airplane, but that the pilot's hasty reaction to a sudden acceleration or upset may.

None of this can have been news to the 79-year-old, 33,000-hour pilot in the left seat. So it is natural to wonder why the Arrow followed the flight path that it did.

The pilot operated an FBO at Racine and was certainly very familiar with the Runway 25 LOC approach at Mitchell Field. His initial offer to "go down lower" to avoid the MD-80 was logical: If he continued straight ahead he would cross the localizer in about 50 seconds, and in that interval the Arrow could not have climbed to a comfortable altitude above the glidepath.

The Arrow turned northward after only a brief deviation to the east. At that time the MD-80 was still outside the Arrow's intended track; in other words, the pilot made a conscious decision to pass quite close behind the jet. The Arrow's altitude was still between 1,500 and 1,600 feet, below the glidepath height and right in the zone of particular danger from settling wake vortices.

Milwaukee's radar interrogated the Arrow's transponder at intervals of about 4.6 seconds. The last three replies, all from the vicinity of the wake encounter, reported altitudes of 1,500 to 1,600 feet, zero, and 1,800 feet respectively. The anomalous zero may represent an unusual attitude in which the Arrow's transponder antenna was shielded from the view of radar. The sudden increase in altitude to 1,800 feet may have been due to uplift from the wake vortex. After transponder replies ceased, radar picked up four primary returns that were probably parts of the airplane falling separately.

Unlike most NTSB reports on in-flight breakups, including the one on the 2006 Saratoga accident, the report on the Arrow breakup says nothing about the exact character of the structural failures. In principle, it should not have been possible to break the Arrow by any control movement, however abrupt or ill-advised, because at a sedate speed of 110 knots, or even 120, the wing or stabilator would have stalled before reaching its ultimate loading. It is difficult to predict what may happen in an uncontrolled situation, however, and there is no way to know which pilot, the old-timer or the relative novice, was handling the controls.

Although the decision to vector the Arrow behind the MD-80 was the controller's, the Arrow's subsequent track was the responsibility of the pilot in command. Controllers are supposed to alert pilots to possible wake turbulence, but pilots are supposed to decide what to do about it. Ironically, if the Arrow had never been vectored to pass behind the MD-80 it would have crossed the final approach course 3,000 feet in front of the jet and several hundred feet below it. As it was, retrospect — always the clearest way of seeing — suggests that, if the Arrow had made a right climbing 270 from its eastbound heading in order to kill a little more time and had then passed well above the MD-80's path, the outcome would likely have been better.

This article is based on the NTSB's report of the accident and is intended to bring the issues raised to our readers' attention. 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.

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