Whip Stall and Tumble Awareness

As discussed in chapter 2, the WSC aircraft does not have a tail with a vertical stabilizer similar to an airplane, and there is the possibility of the wing tucking and tumbling. If a WSC tumbles, this will most likely result in a structural failure of the WSC and serious injury or death to the pilot and/or passenger. It is most important for the pilot to understand tumble awareness and use all means to avoid such an occurrence. The pilot can avoid a tuck and tumble by:

  • Flying within the manufacturer’s limitations.
  • Flying in conditions that are not conducive to tucks and tumbles.
  • Obtaining the proper training in pitch stability for the WSC.

Flying within the manufacturer’s pitch and airspeed limitations is simply adhering to the POH/AFM limitations. Depending on the manufacturer, this could mean no full power stalls, not exceeding pitch limits of ± 40 pitch angle, not flying below the safe flying speed in turbulence, etc. Manufacturer’s limitations are provided for the specific aircraft to avoid tucks and tumbles.

Preflight preparation is the first step to avoid the possibility of a tuck/tumble to avoid flying in strong weather conditions. This could be strong winds that create wind shear or strong convective thermals that create updrafts and downdrafts. This weather analysis is part of the preflight preparation weather analysis. The second pilot decision regarding appropriate weather while flying is to look at the environment during flight to understand and evaluate the situation. Weather conditions should always be evaluated as the flight progresses with ADM used to determine the best outcome for the situation. This could be turning back or landing depending on the situation.

As a student or pilot progresses, turbulence will be encountered. Use the procedures for flying straight and level as shown in Figure 6-8. Use this exercise as a foundation for developing pitch control awareness to keep the wing managed with proper control bar pitch and throttle control.

Figure 6-8. Thermal updraft and downdraft sequence.

Figure 6-8. Thermal updraft and downdraft sequence.

For high pitch angles, the POH may have specific procedures that should be followed for the particular WSC aircraft, but the following general guidelines are provided. After reviewing the aerodynamic aspects of the tuck/tumble in chapter 2, refer to the following tuck/tumble awareness and avoidance procedures.

As defined in the aerodynamics section, a whip stall is a high pitch angle when the tips stall because they exceed the critical angle of attack. This can be the result of strong turbulence or power-on stall, pilot induced, or any combination of these factors. A pilot must avoid all of these factors to avoid the possibility of a whip stall resulting in a tumble, but the following procedures are provided for tumble avoidance in case a whip stall or a nose rotating down below the manufacturer’s limitations is encountered.

The aircraft rotates nose down. [Figure 6-23, Whip Stall to Phase 1] Push the control bar out to the front tube and level wings while increasing to full power and keeping control bar full out to reduce overpitching. [Figure 6-23, Phase 1 to Phase 2] If rotation is so severe that it progresses to phase 4 and the WSC aircraft is tumbling, the ballistic parachute (if so equipped) should be deployed.

Figure 6-23. Whip stall/tuck/tumble sequence.

Figure 6-23. Whip stall/tuck/tumble sequence.

There are other weather situations in which the nose is not at a high pitch attitude, where the back of the wing can get pushed up and enter phase 1 without an unusually high pitch attitude or whip stall. If pitched nose low, increase to full power while pushing the control bar full out to reduce nosedown pitching rotation. Generally, the control bar full out and full-throttle create a nose-up moment.

It takes extremely strong weather conditions and/or pilot error to tuck/tumble a WSC aircraft. Experienced pilots fly all day in moderate turbulence, but building experience flying in turbulence should be approached slowly and cautiously to determine the pilot and aircraft capabilities and limitations.

A Scenario

The following is one example of a scenario that could lead to a tuck/tumble. It is based on a viable training program in one location but lack of experience in another location.

A student obtains his or her pilot’s license with the minimum number of hours for the pilot certificate. The new pilot trained, soloed, and obtained his or her license only in conditions near the ocean where there was typically an inverted midday sea breeze with little to no convective turbulence (thermals). This developed confidence for flying in winds up to 15 knots but no experience was gained in thermals. In fact, the pilot was not aware that strong thermals could be hazardous.

Now, with a new license, the pilot visits his parents in the middle of the high desert of Colorado. Unfamiliar with the local conditions, the new pilot gets a weather report of winds to 15 knots, something the pilot has experienced before. By the time the pilot arrives at the airport, discusses the situation with the airport officials, and sets up the WSC aircraft, it is 2:00 in the afternoon. The wind is generally calm but increasing to 15 knots occasionally. There are towering cumulus clouds in the sky surrounding the current airport similar to clouds that the pilot had seen far inland from where he or she took instruction and soloed.

The pilot takes off in relatively calm winds, but it is unusually bumpy air. Without any experience in the high desert or with thermal conditions, the pilot has misjudged the conditions and is flying in strong thermal convection. The new pilot climbs out trying to get above the turbulence, which usually works near the beach because of the mechanical turbulence near the ground. However, the turbulence increases.

As the pilot is climbing to a pattern altitude of 1,000 feet AGL at full throttle, the aircraft is pitched nose up while the pilot lets the force of the updraft raise the nose. Never has the pilot felt the nose rise with this type of force before. The pilot is shocked and disoriented at this high pitch attitude, but eventually lets up on the throttle. But now at an unusually high pitch angle, the WSC nose flies into the downdraft of the thermal. At the same time, the updraft is still pushing up on the tips of the wing while the downdraft is pushing down on the nose creating a forward rotation with a weightless sensation. Before the pilot knows it, the wing is rotating pitch down for a vertical dive. [Phase 1 in Figure 6-23] The student remembers from training that “in a nose down rotation into a steep dive the control bar is pushed full forward and full throttle applied” and initiates this corrective action. The pilot reaches the vertical dive, but because of the corrective action the WSC aircraft recovers from the dive and proceeds back to land safely.

What went wrong? What were the errors? How could this near catastrophe have been avoided?

  • In a new area and unfamiliar with the conditions, the new pilot should have asked the local instructor or other pilots about the conditions for the day. Local WSC pilots are a great resource for flying the local conditions, but pilots of any category aircraft are knowledgeable of the conditions and could have provided advice for the new pilot. This might have prevented the new pilot from attempting this flight.
  • Flying in a new environment and not understanding the power of midday thermals in the high desert should have forced the new pilot to scrap this midday flight. The pilot should have started flying in the morning when there is little thermal convection and gained experience and understanding about the weather in this new area.
  • Better preflight planning should have been accomplished, especially in a new location. The pilot should have known to obtain convective information and realize it was going to be too bumpy for his or her limited experience. The pilot was accustomed to seeing towering cumulus clouds where he or she trained, but they were way inland and not in the normal flying area. Here clouds were observed all around.
  • Site observations indicated strong thermal activity. Observation of winds picking up to 15 knots and then becoming calm normally indicates thermal activity. The pilot was familiar with steady 15 knot winds, but did not understand that calm wind increasing cyclically to 15 knots indicates thermal activity.
  • The pilot did not initially react to the updraft and resultant high pitch angle properly because pitch management habits had not been developed. The pilot hit the updraft and allowed the force of the updraft to move the control bar forward, increasing the pitch angle while not letting up on the throttle immediately. Both the control bar forward and full throttle forced the nose too high, creating the high pitch angle and whip stall condition. At the same time, the WSC aircraft flew into the downdraft, starting the nosedown rotation. 
  • If the pilot had reacted quickly, pulled in the bar while letting up on the throttle and immediately going into the strong thermal, the high pitch angle would not have been achieved and the strong forward rotation would not have happened so abruptly.

After the series of errors occurred, the pilot finally performed the preventive action to avoid a tumble—from the basic training of “If the WSC is at a high pitch angle and the nose starts to rotate down to a low pitch angle, increase to full power while pushing the control bar full out to avoid a tumble.”

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