What minimum upward current must a glider encounter to maintain altitude?

• A. The same as the glider's sink rate
• B. Twice the glider's sink rate
• C. Half the glider's sink rate
• D. The square of the glider's sink rate

Answer: (A)

To stay at the same altitude, the upward movement of the air must counter the glider’s own tendency to lose height in still air. The glider’s sink rate is how fast it would descend with no lift. If the updraft pushes upward at the same rate, the downward descent is exactly balanced and your altitude stays constant. If the updraft is weaker, you descend; if stronger, you climb. So the minimum upward current needed to maintain altitude is equal to the glider’s sink rate. For example, if the sink rate is 400 ft per minute, an updraft of 400 ft per minute keeps you level.