At the threshold, the airspeed should be approximately how many knots below final airspeed?

At the threshold, the ideal approach is to maintain an airspeed that is about 10 knots below the final approach speed. This approach speed, often referred to as V-ref, provides a buffer that allows for safety margins during the critical phase of landing. By being 10 knots slower than the final airspeed, pilots can initiate a controlled descent and accommodate any slight increases in drag or changes in aircraft configuration, such as extending landing gear or deploying flaps, without risking a stall.

Maintaining this specific difference ensures that pilots have adequate control and can make necessary adjustments to stabilize the aircraft for landing. Additionally, this practice helps in managing the energy state of the aircraft as it approaches the runway, thus improving overall safety during landing operations.