Technique: ’Round the patch

Each circuit is traditionally divided into five legs—departure/upwind, crosswind, downwind, base, and final—separated by 90-degree turns. The origin of this technique is obscure, but it’s gone virtually unchallenged for years. Still, the airman certification standards don’t define what the traffic pattern should look like, nor does any other FAA publication or regulation explicitly state that it must be a rectangle. (Occasional calls from tower controllers to “square your base” also imply some pilot discretion.) A brief scan of the accident data suggests that we may just be doing this wrong.

The AOPA Air Safety Institute’s analysis of accidents involving unintended stalls found that half took place in the traffic pattern, and more than two-thirds of those during the turns to base or final were fatal.

Except for the experience of the pilots involved—an 82-year-old, 18,000-hour commercial pilot and a 62-year-old, 8,000-hour CFI—the May 3, 2012, crash of a Beechcraft Bonanza was fairly typical. A witness saw a steep bank during the base-to-final turn develop into an uncontrolled descent that took the lives of these two highly experienced aviators. The NTSB found the probable cause to be “the pilot’s excessive bank angle…which resulted in an inadvertent aerodynamic stall and spin” and noted that wind conditions were “conducive to a runway overshoot,” providing support to the belief that the pilot had made an aggressive effort to realign the aircraft with the runway after overshooting the base-to-final turn.

The origin of the traditional traffic pattern is obscure, but it’s gone virtually unchallenged for years.For its part, the NTSB has identified preventing “loss of control in flight” (a frustratingly broad description) in general aviation as one of its “most wanted” safety improvements for the past three years. In the 2016 version of this list, the NTSB identified “losses of control”—i.e., stalls—during approach to landing, maneuvering, and initial climb as the deadliest because of the limited time and altitude available for recovery. Possible remedies include greater emphasis of angle-of-attack awareness during training and flight reviews and the addition of AOA indicators to cockpit instrumentation, but one of the simplest and most effective could be a slight modification to the traffic pattern itself.

An attractive alternative to the traditional rectangular pattern is the so-called “rounded-base turn” pattern, which replaces the squared base leg with a constant, gentle turn from the end of downwind onto final. This is similar to the technique often used by pilots conducting “power-off 180” maneuvers during a simulated (or real) engine failure in the traffic pattern—as well as to the overhead pattern entries taught to military pilots for decades. The potential benefits include lower and more stable bank angles and reduced likelihood of being misaligned with the runway upon completion of the turn, decreasing the risk of the kind of overshoot accident described earlier.

The first steps toward assessing those benefits were taken at the University of North Dakota in the winter of 2016-2017. Flight instructors in the aerospace program flying Garmin G1000-equipped Cessna 172S Skyhawks found that the rounded-base turn pattern did not differ greatly from the standard prescribed in UND’s procedures manual. The spacing between the downwind leg and the runway, and the point at which the turn from downwind began, were the same as used in the rectangular pattern. Target power settings and airspeeds also were essentially identical. While the rectangular track calls for 10 degrees of flaps at the abeam point and 20 degrees once established on base, the rounded-base turn method calls for extending the flaps to 10 degrees early in the downwind, then to 20 degrees just before the turn. A final configuration change to full flaps was permitted once the turn was complete and the aircraft was established wings level on final. The primary difference was the lack of a wings-level rollout between leaving the downwind and entering final.

An attractive alternative to the traditional rectangular pattern is to replace the squared base leg with a constant, gentle turn from the end of downwind onto final.One of the lesser-known features of the G1000 is its flight data recording capability, which allows almost every measurable aspect of the flight to be recorded approximately once per second. This study focused on airspeed, bank angle, pitch attitude, and vertical speed. An additional variable, which required some deeper analysis, was the alignment of the aircraft with the runway upon completion of the turn to final. This was determined based on the frequency and intensity of changes to bank angle that occurred after the aircraft rolled wings level on final.

This project was considered a pilot study, a potentially confusing term in the field of aviation research. It means that the study was primarily intended to serve as proof of concept for a future full-scale version. The results clearly demonstrated the feasibility of using onboard data logging to measure the stability of an approach path and detect misalignment with the runway on final. Analysis of 16 rounded-base turn patterns flown by UND instructors showed impressive consistency of airspeed, pitch, bank angle, and descent rate, while data from pattern work on 16 randomly selected training flights were more variable. Although the presumed differences in skill between students and instructors make a direct comparison uninformative, prospects of obtaining conclusive results from a properly randomized comparison using this methodology are strong.

Reactions to the idea of the rounded-base pattern within the community have been varied. One common criticism is the assumed degradation of safety caused by a supposed inability to check for traffic on final while banked during the downwind-to-final turn. A further concern is the wisdom of making configuration changes while the aircraft banks. The visibility of traffic on final will need to be investigated in a full-scale research project, but instructor comment from the UND study suggests that the shallow bank angle used in the turn combined with the practice of checking for traffic before beginning the turn largely avoids this problem. Unease regarding configuration changes while banked is easily dismissed, as the procedure tested in this study restricts configuration changes to before and after the turn, with none made while banking. Additional aspects of the rounded-base pattern that merit investigation include integration into tower-controlled airspace, evaluation of the procedure using a variety of aircraft, and analysis of a constant upwind-to-downwind turn.

The evidence is not yet sufficient to establish the rounded-base pattern as preferable to the traditional rectangular pattern. It is safe to say, however, that it deserves additional investigation in a study large and robust enough to reach definitive conclusions.

Lewis Archer is a professor at the University of North Dakota and was the lead researcher on the rounded base-to-final research. David Jack Kenny is a freelance aviation writer and former statistician for the AOPA Air Safety Institute.

David Jack Kenny is a freelance aviation writer and former statistician for the AOPA Air Safety Institute.