Contents:
Ground Instruction about Stalls and Spins
Stall Defined:
A stall is a loss of lift and increase in drag that occurs when an aircraft is flown at an angle of attack greater than the angle for maximum lift. Failure to recover from a stall may result in a spin or secondary stall. All spins are preceded by at least a partial stall of one wing.
Stall Speed:
This is the speed at which the critical angle of the relative wind is exceeded. Stall speed is the basis on which other AFM or POH operational speeds are calculated.
Angle of Attack:
AOA is the angle at which the chord line of the wing meets the relative wind. Exceeding the critical AOA of an airfoil section will always result in a stall of that section. If the required AOA for flight exceeds the ability of the air to maintain a flow over the wing, the result is a separation of airflow, loss of lift, a large increase in drag and eventually a stall. The angle of attack where a wing produces the most lift is also the point where it stalls.
Airspeed:
Stalls come from excessive AOA for the airspeed and not from the airspeed itself. A stall can occur at any airspeed, in any attitude, at any power setting.
Configuration:
Configuration affects the stall speed. The landing configuration will reduce the stall speed. The lower airspeed limit of the white arc is the power-off stall speed in landing configuration. The lower airspeed limit of the green arc is the power-off stall speed in the clean configuration.
Load Factor:
Load factor is the ratio of the lifting ability of the wings to the weight of the aircraft and its contents. The stall speed increases proportional to the square root of the load factor. An inadvertent stall is more likely when preceded by an increased load factor. If the stall occurs at a speed exceeding the Va maneuvering speed the aircraft may warp, spindle or mutilate. All recoveries from stall and spins require a loss of altitude, increase in airspeed and the load factor in the pull up.
Center of Gravity:
The CG affects stability and stall or spin recovery. An aft CG reduces the control forces required to stall the aircraft these light forces can generate higher and destructive load forces. Extreme aft CGs lead to inadvertent stalls that can result in an unrecoverable flat spin. A forward CG will require a higher speed before a stall occurs. Higher elevator forces are required to raise the nose with a forward CG.
Weight:
The stability of the aircraft is directly affected by weight distribution. As the weight of an airplane increases so does the stall speed. Higher weight results in higher stall speeds. This is because a higher AOA is also required. Maneuvering speed, Va, is lower at lower weight. The rule of thumb, for aircraft where speeds are not available for all weights, is to find the percentage of weight difference between the maximum and any reduced actual weight. Reduce your calibrated approach speed a percentage that is half the percentage of weight difference to figure Va(ref)..
Altitude and Temperature:
Altitude does not appreciably affect the indicated stall speed. Recommended indicated airspeeds should be used regardless of the elevation or density altitude.
Turbulence:
Turbulence can cause stalls at much higher than normal speeds. A vertical gust or wind shear that changes the relative wind will increase the AOA. Pilots should fly in turbulence at a speed well above the indicated stall speed but below the maneuvering speed.
Distractions:
The pilot who is subjected to distractions is put in a situation of greatly increased risk of an inadvertent stall.
VIII. AREA OF OPERATION - SLOW FLIGHT AND STALLS
D. TASK:
MANEUVERING DURING SLOW FLIGHT
REFERENCES: AC 61-21, OPERATING HANDBOOK, and FLIGHT MANUAL
P 1. Knows specifics of entering and flying in slow flight. Aware of reasons for slow flight competence and hazards existing.
P 2. Selects entry altitude that will preclude descent no lower than 1500' or as assigned.
P 3. Stabilizes the airspeed at 1.2 Vs1, + 5 knots.
P 4. Makes coordinated straight and level flight and level turns, at bank angles and in configurations, as specified by the examiner.
P 5. Accomplishes coordinated climbs and descents, straight and turning, at bank angles and in configurations as specified by the examiner.
P 6. Divides attention between airplane control and orientation.
P 7. Maintains specified altitude +100', specified heading + 10 degrees, and specified airspeed +10/-5 knots.
P 8. Maintains the specified angle of bank, 30 degrees; maximum in level flight, +0/-10 degrees, maintains the specified angle of bank, not to exceed 20 degrees; in climbing or descending flight, +0/-10 degrees;, rolls out on the specified heading, + 10 degrees and levels off from climbs and descents within + 100 feet.
EX This constitutes flight where any inattention to bank, airspeed, or attitude will produce a stall. A minimum safe altitude might be 3000'. Climbs, descents, and level flight with turns may be directed with altitudes to be held within 100'. The examiner may ask for either or both explanation and performance. Not to be recovered from below 1500 feet. AGL, 1.2 Vso +5 knots airspeed, altitude + 100 feet, heading + 10 degrees. Maintains specified bank, not to exceed 30 degrees, +0 degrees and -10 degrees, climbing or descending flight not to exceed a 20-degree bank at + 0 degrees and -10 degrees. Rolls out on headings + 10 degrees, levels off in climbs, descent within 100 feet.
Initiate the configuration by using Carb Heat, reducing power, holding heading and altitude, and trimming for minimum control pressure. Any bank may require power to maintain altitude. Be smooth and careful. Full flaps may require full power.
It will be next to impossible to fly this configuration without full use of trim and knowledge of aircraft performance. The most important element is the smoothness of entry. You must be able to get to the desired speed while holding heading and altitude, hands off. Once attained the speed can be held throughout any turn or maneuver if the bank is 30 degrees or less and power is applied in anticipation of loss of lift. Slow flight is an exercise in the use of rudder. The aircraft requires much greater control deflection than in normal cruise flight but the smoothness of application must be maintained. Power and pitch changes must be in anticipation of actual requirements. We are flying with a narrower margin of safety above the stall
C-150
No flap minimum controllable: Full flap minimum controllable
Carb Heat ..............................Carb Heat
1500 RPM ............................1500 RPM
Hold Hdg/Alt .........................Hold Hdg/Alt
Trim Down 4 ..........................Full Flaps
2000 RPM/Rudder .................Full Pwr/Rudder
Power as required ...................Trim up 1
55 knots or slower ..................Power as required @ 40 knots or slower
If a steeper than 20-degree bank is called for then power should be used to compensate for loss of lift. Constant bank angle requires careful attention. You will be constantly making small changes in the yoke and rudder. More often than not you will be flying in a cross-control condition. You might find that the examiner will accept a greater speed" like 60 knots without flaps and 45 knots with full flaps. Remember any change in power will affect the trim and the airspeed unless trim is corrected. Discuss with the examiner what he feels to be desired before you get into the plane. You might even ask him to demonstrate. Can't hurt; might help. Listen, the sound will be your first indication of things going wrong.
Stall Avoidance Practice at Slow airspeeds
1. Assign altitude and heading to be maintained by student using trim until stall warning activated.
2. Demonstrate elevator and rudder trim effects along with left turning tendency and need for right rudder.
3. Demonstrate turns in this configuration without rudder.
4. Practice turns, climbs and descents.
5. Demonstrate flap use procedures in slow flight to avoid stall during application and removal.
6. Slow flight, with airspeed indicator covered, with change in flap configuration plus distractions.
7. Distractions:
--Dropped pencil
--Chart use
--Resetting clock
--Retrieve from back seat
--Read temperature
--Call FSS
--Use E 6-B
--Locate ground targets
--Find emergency airports
--Perform 200 foot vertical Ss
Special considerations apply to twin-engine stalls. (Covered at length in AC 61-67C)
See instructional material on slow flight.
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