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Flight At Minimum Controllable Airspeed
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Every properly executed takeoff and landing you make requires you to operate the airplane at low airspeed. During training, students are taught "flight at minimum controllable airspeed" so they may learn the effect that airspeed has on airplane performance and controllability. Information evaluated by the Data Analysis Section of the Federal Aviation Administration reveals that about 65% of the reportable airplane accidents occur during the takeoff and landing phases of flight. This indicates flight at low airspeed has a high accident potential. Pilots who are skillful and confident in operating an airplane at low speeds may prevent many of these accidents. In getting acquainted with a new or different airplane, experienced pilots can and do use "flight at minimum controllable airspeed." Listed below are some of the objectives for teaching flight at minimum controllable airspeed: |
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1. The student will be able to recognize that the airplane is approaching or has attained a critically low airspeed.
2. The student will be able to control the airplane at airspeeds just above the stall.
3. To increase the confidence of the pilot in his ability to operate the airplane throughout its full range of controllability.
There is more training value to the maneuver than just showing how slow the airplane can be flown. For example, the following items should be demonstrated and taught:
1. Airplane attitude at minimum controllable airspeed.
Power required versus airspeed produced.
Trim needed.
Control effectiveness.
5. Turns and rate of turn compared to degree of bank.
6. Stall as a result of level turn.
7. Adverse yaw.
8. Effect of flap extension.
9. Effect of flap retraction.
10. Descents and descending turns.
11. Climbs and climbing turns.
12. Attempted operation "behind the power curve.
13. Go-around procedures.
The instructor needs to describe "Minimum Controllable Airspeed." This is not a set figure. It will vary with loading, configuration, power setting, and pilot technique. It is best described as a speed just above stall or a point at which a further reduction in airspeed, or an increase in angle of attack or load factor will cause an immediate physical indication of a stall. The physical indication would be shuddering, pitch down of the nose, rolling to right or left, or reaching the limit of up elevator travel. The goal for proficiency will be to have the student hold within 100 feet of prescribed altitude and airspeed within the range between minimum controllable airspeed and that speed +5 knots.
The instructor should provide a basis for comparison of control pressures and rates of response. While in cruise flight at cruising airspeed, have the student use rudder, aileron and elevator and note the pressure applied and the response rate.
Then, while maintaining heading and altitude, reduce power and increase angle of attack, slowing the airplane to minimum controllable airspeed. As speed is reduced, point out a change in pitch. A change in pitch attitude is needed in order to maintain altitude. There will be a point at which pitch change alone does not increase lift to the point that altitude can be maintained. Power must be added, and point out, he is now operating "behind the power curve." The angle of attack must be decreased before altitude can be gained, even when maximum power has been applied.
Another way of saying it is, he must add power to go slower while maintaining altitude.
When all set at minimum controllable airspeed, demonstrate how to recognize that the airplane is close to operating limits: sight, sound, and feel give the clues. The pitch attitude of the nose, the angle of the wing tips in reference to the horizon, the sound of the engine compared to the lack of wind noise, the lessened resistance to control pressures and the need of elevator and rudder trim all tell that the airplane is at a low airspeed. now have the student apply aileron, elevator, and rudder pressures and note the response. Everything still affects the airplane the same way except that greater control movement is needed to produce the same rates of response that were obtained at cruise speed.
Now roll smoothly into a medium banked turn. This is done to show the student the airplane is maneuverable even at low airspeed. The medium bank results in a high rate of turn at this low airspeed. It will seem that the airplane is almost pivoting over a point on the ground. Point out that a steep bank is not needed in order to obtain a high rate of turn when operating at low airspeed.
The turn made at medium bank is also used to demonstrate that a level turn does increase stall speed and, unless power is added, a stall will occur soon after the turn is established. when the first indication of stall is felt, point out the stall indication to the student and gently recover by simultaneously reducing the angle of attack, adding power, and rolling out of the turn. (if the instructor doesn't appear nervous or apprehensive about the stall and recovery, the student will be favorably impresses and his confidence will be increased.)
Return to straight and level flight and set up at minimum controllable airspeed, demonstrate the reason for proper coordination of rudder and aileron in turn entry and recovery. In other words, demonstrate adverse yaw. While the student watches the nose of the airplane to compare its movement against outside references, use aileron only to establish a banked attitude. He will note that application of left aileron causes the nose to show yaw. As left aileron is used, the right wing tip would appear to move aft. The amount of yaw obtained will be affected by the degree of aileron applied and the design of the system. Some ailerons are rigged for differential travel to minimize adverse yaw. In this case the yaw is most easily seen by sighting across the wing tip.
Now, demonstrate properly coordinated turn entries and recoveries so the student can see and feel the difference.
Next on the list is the demonstration of the effect of flaps on minimum controllable airspeed and airplane attitude. Extending full flaps will cause the airplane to "balloon" (to climb above desired altitude). Ballooning is caused by the combination of airspeed and the increase in coefficient of lift which occurs when the flaps are extended. Drag also results from the flap extension. As the airplane decelerates, lift is reduced. When all forces (lift, thrust, weight, and drag) are again stabilized, the airplane will have a new, lower minimum controllable airspeed and a different pitch attitude for level flight. The power setting may be the same as before the flaps were added, or if a power change is needed to maintain level flight, it will be a small one.
When the airplane is established in straight and level flight with the flaps down, turns can be demonstrated, again noting response rates to control pressures and high rate of turn produced by medium banked turns. Descents can be performed by reducing power while maintaining airspeed. While descending, turns to the right and left should be practiced. Now, power can be applied to climb. At this time, if the airspeed has been allowed to get excessively low, it may be impossible to climb even with full power. By reducing the amount of flap extension, we can also reduce drag and should now be able to climb.
If the flaps are manually actuated, rapid retraction can result in a stall. This point should be demonstrated. If the flaps are electrically or hydraulically actuated, retraction may be slow enough that the airplane will accelerate so that flaps-up stall speed is attained before the flaps are fully retracted. In the case of manual flap operation, slower, smooth retraction will also permit acceleration as drag is reduced and a stall will be avoided. All that is needed is a pitch change in order to maintain level flight or prevent sinking as the flaps come up. In other words, the pitch change compensates for the change of camber and coefficient of lift of the wing.
Climbs and descents, in straight flight and turns, with and without flaps should be practiced.
To complete the demonstration, attempt a simulated go-around with flaps fully extended. As in attempting a climb, conditions of power, load, and configuration may make acceleration and climb impossible. It is suggested that the go-around procedure be:
1. Power - smoothly increase to full or climb power, as appropriate.
2. Flaps up - (to 1/2 position, complete retraction after flaps up stall speed is reached.)
3. Carburetor heat "OFF" if used.
4. Gear up when climb is established and touchdown is unlikely.
If the Owner's Manual suggests a specific procedure, we suggest following that recommendation.
Climb to and level off at the altitude specified by the instructor should follow the go-around.
If the Owner's Manual suggest a specific procedure, we suggest following that recommendation.
Climb to and level off at the altitude specified by the instructor should follow the go-around.
This suggested procedure for demonstrating and practicing "Flight at Minimum Controllable Airspeed" does take a few minutes to go through. Keep alert for indications of engine overheating as indicated by cylinder head or oil temperatures. If the airplane is equipped with cowl flaps, teach the student how to use them to keep temperatures within limits. I no cowl flaps are e installed, it may be necessary to increase speed for cooling.
All throughout the demonstrations, point out the relationship between pitch attitude, power setting, and the results obtained. The results would be climb, descent, or level flight. Performance is a function of angle of attack. Angle of attack is controlled through the combined use of pitch attitude and power setting.
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