Upon mastering slow flight, the next maneuvers introduced to most student pilots are the power-on and power-off stall. The connotation of the word “stall” often puts fear into people’s minds. However, this can be overcome after a quick explanation regarding the aerodynamics of stalls as well as a demonstration of the maneuvers.
Basic Aerodynamics of Stalls
Before we get into stall’s, let’s define a few items to make sure we’re all on the same page.
- Relative Wind: The relative wind is the air that “flows in a direction parallel with and opposite to the direction of flight.” (PHAK G-25)
- Chord Line: The chord line is “an imaginary straight line drawn through an airfoil from the leading edge to the trailing edge.” (PHAK G-6)
- Angle of Attack: The angle of attack is “the angle that is formed by the chord of the airfoil and the direction of the relative wind or between the chord line and the flight path.” (PHAK G-3)
- Critical Angle of Attack: The critical angle of attack is the “angle of attack at which a wing stalls regardless of airspeed, flight attitude, or weight.” (PHAK G-9)
Now that we’ve got those out of the way, let’s dive into the aerodynamics of a stall a little bit more. One of the most common questions asked during training is “when does an airplane stall?”. Well, using a little bit of brain power and the definitions above, we can determine that an airplane will stall once the angle of attack exceeds its critical angle of attack. It’s at this point where most people start to worry. The lift that the airplane is generating is not able to overcome its opposing force, weight. This results in a rapid loss of lift with any further increase of AOA (see PHAK Figure 5-5), however, the important thing is that we are still generating lift. The airplane is NOT falling out of the sky, like most people think. Also, thanks to the smart people who design airplanes, due to the location of the Center of Gravity vs. Center of Lift, the airplane would actually recover itself from a stall and the airplane would start flying again if the pilot were to not intervene. Hopefully this eases your mind a little bit. There are a few things we do as pilots to help ensure the airplane recovers promptly and correctly, which we will discuss a little later on.
Power-On Stall Explained
The power-on stall, commonly referred to as the “departure” stall, is a stall that would most generally occur after take-off as a result of an excessively high angle of attack. You may be thinking to yourself, “all my attention is on the takeoff, I’ve configured the airplane accordingly for takeoff, how would this happen?” Well, there are a few things that could result in the power-on stall. Aside from being spatially disoriented (if departing into IMC) or over-rotating (for one reason or another), another practical reason may be because your seat wasn’t secured into a detent and upon rotation you slide back and instinctually grab the yoke pulling the nose of the airplane up. Or, maybe you have some cargo in the back not properly secured and as you rotate all of that weight slides towards the back of the airplane causing the nose to go up again. Unlikely? Maybe, but, as pilots we need to be prepared for all situations.
Power-Off Stall Explained
Up next is the power-off stall, which can also be referred to as the “approach to landing” stall. Again, you may be asking yourself how I truly get myself into a power-off stalling situation when I’m coming in to land, so let’s discuss the reality of it. In VFR conditions we know that we should be spending ~90% of our time looking outside and only ~10% looking inside the airplane, especially when we are coming in to land. However, if we are on final, focused on our aiming point outside and we are unaware of what the airspeed indicator is reading, we may actually be pulling back on the yoke to try and “stretch” the airplane to get to its aiming point without adding any power. As we know from slow flight, we need to pitch for airspeed and use power for altitude. However, if we’re trying to “stretch” the airplane we may be increasing pitch, therefore we are decreasing airspeed which would eventually result in a stall. Another example would be during a simulated, or real, engine failure. As we are gliding to our touchdown point, we may realize that we are going to come up short of our aiming point and again we will instinctually try to pull back on the yoke to try and stretch the glide. This is where we run the risk of getting too slow and stalling.
Importance of Stalls
You are probably saying to yourself, “Ok, I know how these happen, I know when and where they would happen, so why do I need to go practice them repeatedly?” The reason we practice these stalls is three-fold. First, we need to know how the airplane sounds, feels, and responds as we approach a stall in these two very different configurations. If we are unaware of what the stall horn sounds like or what an aerodynamic buffet feels like, we may not know how to react if we experience those situations. Second, we have to know the proper recovery procedures if we do happen to find ourselves in these situations. We don’t want to keep pulling back on the yoke when in reality the only way to overcome the stall is to decrease the angle of attack to regain sufficient lift. Finally, like we talked about in slow flight, we need to be able to maintain coordination in various configurations and doing so during a stall is even more important.
If we take what we know about when the airplane will stall, the logical way to “un-stall” the airplane is to reduce the airfoil below its critical angle of attack. How do we do that? Well, we need to release back pressure on the yoke and let the nose come down enough so that we can regain sufficient lift. We also know the importance of coordination and, just like in slow flight, we maintain that by using the rudders. What about the ailerons? We know that our ailerons rely upon airflow over the wing to be effective and as we pass the critical angle of attack, the airflow over the top of the wing separates and begins to drastically decrease. Therefore, the ailerons are quite ineffective during a stall. However, you may have heard that the airplane will stall from wing root outward to wing tip to allow more aileron authority. Why is that? The reason behind this is due to a design in the wings called washout. Reference the article “How Wing Washout Makes Your Airplane More Stable” by Boldmethod to learn more. Last but not least, if we are in a power-off stall with flaps extended we need to reduce flaps in accordance with the POH. That way we can begin to reduce drag and allow the airplane to build up airspeed and climb away from the ground at a faster rate.
Tips & Tricks
- Use rudders to maintain coordination – Again, “Step on the High Wing”
- Release back pressure during the power-off stall, don’t push the nose down excessively
- Use audible cues to help determine you’re approaching one of these situations
- When practicing, bug your entry heading and maintain heading with the rudders