A stall is an aerodynamic condition which occurs when smooth airflow over the airplane’s wings is disrupted, resulting in loss of lift. Specifically, a stall occurs when the AOA—the angle between the chord line of the wing and the relative wind—exceeds the wing’s critical AOA. It is possible to exceed the critical AOA at any airspeed, at any attitude, and at any power setting. [Figure 1]
|Figure 1. Critical angle of attack and stall|
For these reasons, it is important to understand factors and situations that can lead to a stall, and develop proficiency in stall recognition and recovery. Performing intentional stalls will familiarize the pilot with the conditions that result in a stall, assist in recognition of an impending stall, and develop the proper corrective response if a stall occurs. Stalls are practiced to two different levels:
- Impending Stall—an impending stall occurs when the AOA causes a stall warning, but has not yet reached the critical AOA. Indications of an impending stall can include buffeting, stick shaker, or aural warning.
- Full Stall—a full stall occurs when the critical AOA is exceeded. Indications of a full stall are typically that an uncommanded nose-down pitch cannot be readily arrested, and this may be accompanied by an uncommanded rolling motion. For airplanes equipped with stick pushers, its activation is also a full stall indication.
Although it depends on the degree to which a stall has progressed, some loss of altitude is expected during recovery. The longer it takes for the pilot to recognize an impending stall, the more likely it is that a full stall will result. Intentional stalls should therefore be performed at an altitude that provides adequate height above the ground for recovery and return to normal level flight.
A pilot must recognize the flight conditions that are conducive to stalls and know how to apply the necessary corrective action. This level of proficiency requires learning to recognize an impending stall by sight, sound, and feel.
Stalls are usually accompanied by a continuous stall warning for airplanes equipped with stall warning devices. These devices may include an aural alert, lights, or a stick shaker all which alert the pilot when approaching the critical AOA. Certification standards permit manufacturers to provide the required stall warning either through the inherent aerodynamic qualities of the airplane or through a stall warning device that gives a clear indication of the impending stall. However, most vintage airplanes, and many types of light sport and experimental airplanes, do not have stall warning devices installed.
Other sensory cues for the pilot include:
- Feel—the pilot will feel control pressures change as speed is reduced. With progressively less resistance on the control surfaces, the pilot must use larger control movements to get the desired airplane response. The pilot will notice the airplane’s reaction time to control movement increases. Just before the stall occurs, buffeting, uncommanded rolling, or vibrations may begin to occur.
- Vision—since the airplane can be stalled in any attitude, vision is not a foolproof indicator of an impending stall. However, maintaining pitch awareness is important.
- Hearing—as speed decreases, the pilot should notice a change in sound made by the air flowing along the airplane structure.
- Kinesthesia—the physical sensation (sometimes referred to as “seat of the pants” sensations) of changes in direction or speed is an important indicator to the trained and experienced pilot in visual flight. If this sensitivity is properly developed, it can warn the pilot of an impending stall.
Pilots in training must remember that a level-flight 1G stalling speed is valid only:
- In unaccelerated 1G flight
- In coordinated flight (slip-skid indicator centered)
- At one weight (typically maximum gross weight)
- At a particular center of gravity (CG) (typically maximum forward CG)
Angle of Attack Indicators
Learning to recognize stalls without relying on stall warning devices is important. However, airplanes can be equipped with AOA indicators that can provide a visual indication of the airplane’s proximity to the critical AOA. There are several different kinds of AOA indicators with varying methods for calculating AOA, therefore proper installation and training on the use of these devices is important. AOA indicators measure several parameters simultaneously, determine the current AOA, and provide a visual image of the proximity to the critical AOA. [Figure 2] Some AOA indicators also provide aural indications, which can provide awareness to a change in AOA that is trending towards the critical AOA prior to installed stall warning systems. It’s important to note that some indicators take flap position into consideration, but not all do.
|Figure 2. A conceptual representation of an AOA indicator. It is important to become familiar with the equipment installed in a specific airplane|
Understanding what type of AOA indicator is installed on an airplane, how the particular device determines AOA, what the display is indicating and when the critical AOA is reached, and what the appropriate response is to those indications are all important components to AOA indicator training. It is also encouraged to conduct in-flight training to see the indications throughout various maneuvers, like slow flight, stalls, takeoffs, and landings, and to practice the appropriate responses to those indications. It is also important to note that some items may limit the effectiveness of an AOA indicator (e.g., calibration techniques, wing contamination, unheated probes/vanes). Pilots flying an airplane equipped with an AOA indicator should refer to the pilot handbook information or contact the manufacturer for specific limitations applicable to that indicator type.