A fairly new private pilot recently mentioned that hes read about stalls occurring while turning from base to final but doesnt remember hearing much discussion about it during his private pilot training.
The pilot had done all the approach-to-stall, stall recognition and stall recovery training required to perform the tasks required by his certificate. However, he had no idea why he was doing them – other than to pass the checkride – nor how he might apply this training in the real world.
The central concept never presented to him is that the main reason for stall training, including the spin discussions (which replace the mandatory spin training of the old hairy-chested days), is to impress on the student how important it is not to stall or spin the plane when its too low to recover. The rest is just learning the symptoms of an impending stall, the control inputs that are conducive to spins, and the mechanics of preventing or recovering from an impending stall or incipient spin.
Its clear that many pilots, old and new, have little understanding of why learning about stall and spin prevention is important. They may know the rote answers to the questions and have the mechanical skills to get out of a stall theyre prepared for, but they have little understanding of the aerodynamics and pilot tendencies that push pilots into that danger zone in the first place.
This student also asked: Why is it that in accident reports, inadvertent stall/spins always seem to occur in the landing pattern or otherwise at low altitude? Does this mean people dont stall at higher altitudes?
This is one of those cases of the dog that didnt bark. The reason is that inadvertent stall/spins that occur at higher altitudes never get reported because they dont end in an accident. The pilot recovers, brings the plane back and vows to be more careful next time. Its only when the events occur at altitudes too low to make a recovery that they turn into accident reports – and thats why the accidents almost always involve either a landing pattern or a buzz job.
Setting the Scene
Stall/spin accidents involve more than just a case of improper airspeed and the ball not being centered.
The ball being out of center is relevant only to whether you spin out of the stall. Airspeed is also not directly relevant, as a stall can occur at a much higher than book speeds if youre pulling some g-load.
The stall will happen any time the critical angle of attack is exceeded, which can occur at any indicated airspeed and at any flight attitude. The spin occurs when the airplane has a significant yawing moment at the stall.
Typical scenarios for the stall in the base-to-final turn usually involve a crosswind from the base leg side. The pilot sees that the flight path will shoot through the final approach course and recognizes the need to increase the rate of turn.
What happens next might be:
• The pilot pulls the nose around with back stick, increasing the angle of attack and causing a stall. Since the airplane is turning, the down wing is moving slower. The pilot is maintaining bank with a bit of opposite aileron – normal, acceptable technique to counter the normal overbanking tendency.
The down wing has a slightly higher angle of attack, due to the downward deflection of the aileron trailing edge and an airspeed thats lower than the indicated speed because its on the inside of the turn.
The down wing stalls first and the wing drops in the direction of the turn at the stall, followed by a rapid nose drop. However, if the ball is centered, this will not result in an actual spin entry. Recovering from the stall requires the least altitude and the simplest technique, making it the most recoverable situation.
• The pilot steepens the bank to increase turn rate. This causes the nose to fall a bit as more of the wings lift is turned from vertical to horizontal component, and the pilot incorrectly uses opposite, or top, rudder to yaw the nose up rather than back stick to pitch the nose up. (Better yet would be to recognize the mistake, add power, roll the wings level and come around for another try.)
In this case, as angle of attack increases and speed decays, the yaw toward the top side (outside) causes the top wing to stall first and the airplane snaps over to the outside of the turn in what is very nearly a basic snap roll entry. The rapidity of this maneuver, coupled with its multi-axis rotation, makes it somewhat disorienting. That can delay recognition and the initiation of proper recovery procedures – a delay that is usually fatal at 400 feet agl.
• The pilot stomps bottom/inside rudder to increase the turn rate by yawing the plane. This accelerates the top wing and decelerates the bottom wing. As a result, top wing lift increases and bottom wing lift decreases, and the airplane rolls more into the turn. The pilot then corrects with opposite aileron to stop the roll.
If the airspeed decays, the airplane stalls with a lot of rudder into the turn and the ailerons away from the turn, which causes a large induced drag on the bottom wing that doubles the yaw into the turn at the stall. This results in a hard snap down and into the turn.
Unfortunately, the natural reaction in this case is to pull back on the stick as the nose drops, exacerbating the situation into a spin entry. At 400 feet, there just isnt enough room to recover from a spin even if proper recovery technique is immediate.
If you get some spin training, you will discover that the altitude loss in a recovery from a spin entry isnt much less than this, and you were prepared for the spin and ready to make the proper control inputs for recovery. In a surprise situation, its most unlikely that youll recognize, react to and recover from one of these base-to-final spin entries.
A fully developed spin takes even more altitude for recovery.
Thats why its so important that pilots learn how to recognize the situations that are conducive to spins – and how to recognize all the signs of impending stall so the full stall never happens.
The two main things pilots need to be aware of are how to tell when the plane is reaching a stalled condition and the amount of altitude it takes to recover. This is an important area that is frequently overlooked by some less experienced instructors. They may be great at teaching a PTS-perfect stall/recovery demonstration at 2,000 feet agl, but either dont know or cant communicate why theyre doing it or what practical application that training task has.
The task is taught, but the reason for learning it is lost and, with it, the opportunity for complete understanding. Unfortunately, the result is that another new pilot learns procedures but not their meaning, and 250 hours and an initial CFI checkride later, perpetuates the same upon his or her own students.
Why It Happens
In vintage airplanes, one of the first indications of the impending stall is the loss of aileron effectiveness due to the beginning of the stall at the outer portions of the wing, followed by a progression inward. The ailerons, mounted near the wing tips, are the first part of the trailing edge of the wing to experience the turbulent flow.
So when the ailerons get sloppy and unresponsive, the pilot knows the outer portion of the wing is stalled. The problem with this is that pilots often respond by increasing aileron deflection in an effort to control roll, with the unfortunate effect of merely increasing drag on one wing without increasing lift. The result is a sharp yaw away from the stick deflection and a spin entry.
Newer aircraft solve this problem with either wing twist or stall strips.
Twist, used in Cessnas and tapered-wing Pipers, can be seen by looking inward from the wing tip – you can see that the outboard end of the wing is twisted to a higher incidence angle than the inner portion, ensuring that it always has a higher angle of attack and will stall first.
Stall strips (as on Grummans and Hershey bar-wing Pipers) are those little strips with triangular cross-sections on the leading edge, which disrupt the airflow over that portion of the wing before it would otherwise stall.
In either case, the stall now occurs on the inboard portions of the wing first, retaining roll control until almost the whole wing is stalled. If the pilot uses ailerons near the stall, the airplane will still roll in the commanded direction. Adverse yaw will still occur, but it is easily controlled by rudder.
By the time the nose or a wing drops, youve already stalled or very nearly stalled that wing. Your best indication will be the buffet, or burble – that sort of thrumming feeling through the controls and the airframe. It feels a bit like a washing machine on spin cycle with the load a bit out of balance.
You are feeling the turbulent air aft of the flow separation point on the top of the wing beating against the wing and other surfaces. That is the one true indication of a stall – stall warning systems can fail, but the burble is an aerodynamic element of the stall and will occur before the stall in any airfoil certified for light aircraft.
Keep in mind also the amount of altitude it takes to recover.
Student pilots are quick to learn that you develop a big sink rate well before the stall break actually occurs because the drag curve rises steeply at high angles of attack that are still short of a stall. The sink rate does not indicate the stall. The stall has not actually fully occurred until the wing completely loses its lift, resulting in a pitch down in most light planes.
This pitch down occurs because of the change in airflow over the horizontal stabilizer when the wings lift disappears at the stall. The airflow over the tail changes to an angle much more from below the plane than it was.
Remember the horizontal stabilizer produces a downforce to balance the weight of the plane thats in front of the center of lift – a teeter-totter arrangement with the center of lift as the fulcrum. Remove the fulcrum, and the whole assembly falls.
Because the horizontal stabilizer has to produce a downforce rather than an upforce like the wing, it is in essence an upside-down wing. Its angle of attack is normally slightly negative with respect to the whole airplane in order to produce a positive AOA for its inverted airfoil.
As the airplane falls, this AOA with respect to the horizontal tail reverses and the downforce is lost, causing the tail to go up and the nose to pitch down. Until the break occurs, the wing is still producing enough lift to avoid this reversal.
One way to teach students how much altitude is required to recover is simply to set the airplane in level slow flight and smoothly reduce power to idle. The student then tries to hold altitude by moving the yoke back and increasing pitch attitude.
This works until the airplane reaches the peak of the lift curve, at which point further back stick results in a lower coefficient of lift. The sink begins, followed quickly by the break. The student recovers and notes the lowest altitude during the recovery.
This can also be done with the power on, but in planes like a Cessna 172 it results in an extreme nose-up attitude before the stall occurs, especially at full takeoff power.
Once youre comfortable with wings-level stalls, both clean and in landing configuration, move to stalls in banked flight, and then with the controls mispositioned as described in the typical base-to-final turn stall/spin entries. Not that there is a rapid increase in altitude loss once you get away from the most basic stall entry/recovery combinations.
The important thing is that, by studying the progression, you learn a great deal about the many cues to approaching stalls, and that stalls are entirely controllable with simple procedures. You also learn that there is a price to be paid in the form of altitude. If you dont have the price of admission (enough altitude), you aint gonna see the show (make a successful recovery).
The base-to-final turn is lethal when it happens, but its also easily preventable if pilots are taught the basic cues of impending stall and the inability to recover from cross-controlled turning stalls at low altitude.
An important thing to understand is that there are times when an approach cannot be salvaged, and the only thing to do is go around before the situation gets past the point where a successful recovery can be made.
To fly safely, recognize that the corners of the flight envelope in cross-controlled stalls are a serious concern and learn the many signals of the impending stall. That way, when you overshoot a turn to final, you can be extra alert to the danger signals and ready to cob the throttle, level the wings, reduce back pressure, and take it around for another try.
Also With This Article
Click here to view “Taming the Wild Ride.”
-by Ron Levy
Ron Levy, an ATP and CFI, is an assistant chief flight instructor at American Eagle Flight Academy.