CFI Topics

Stall Speed

Lesson 3

The 1.3 Vso is the calculated stall speed times 1.3. Why is this important? Because it gives you, the pilot, a cushion of a few knots of lift before stall occurs. The problem we run into is the absolute belief that what the airspeed indicator is showing is the gospel. It isn’t. For one thing the indicated airspeed varies from the calibrated airspeed (called instrument error). Therefore what you are seeing is not essentially what the real airspeed is. The margin of error is capped by 14CFR part 23, paragraph 23.1323 at 3 percent or 5 knots whichever is greater. So, the instrument certification is based on this known instrument error. But do we keep that in mind in our day-to-day flying? Probably not. Knowing this can alter the “flight by the numbers” thinking. The errors between book values and real values must be determined and the real values flown (a mix of flight-by-the-seat-of-your-pants and by-the-numbers). A power off stall (Vs) may not occur at the designated indicated airspeed or the converse, it could happen before that specified airspeed. An aircraft will fly or, put it another way, the wing will continue to produce lift as long as the airspeed is above the stall speed or Vs. To keep that comfort zone, therefore, an approach to landing is done best at 1.3 times Vso. This gives the pilot the few extra knots till the aircraft is close to the ground and ready to touch-down. As you can now begin to understand, if this indicated airspeed is not telling the real tale then the stall speed times 1.3 will also be inaccurate and the margin of comfort could be less or more.

Also critical to this understanding is that we are talking about the stall speed (Vs) in a straight and level mode. It is here where the maximum coefficient of lift and lift itself equals to weight. But it’s important for pilots to remember that the wing can stall at any speed. A stall is the result of the loss of lift when the angle of attack exceeds the critical angle.

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