What does the vertical line from point E to point F on an airspeed indicator represent?

The vertical line from point E to point F on an airspeed indicator represents the upper limit of the yellow arc. This upper limit is significant because the yellow arc indicates a caution range where the aircraft is not fully in the safe operating zone, and pilots should exercise caution to avoid approaching critical speeds. This range typically denotes airspeeds that allow for some margin of safety but are not advisable for continuous operation.

The importance of this distinction in the airspeed indicator is crucial for pilots as it informs them about the operational limits of the aircraft. For example, flying within the yellow arc may be acceptable in certain conditions, but it suggests that the pilot should be prepared for potential issues that could arise from flying at these speeds, such as turbulence or limited control effectiveness.

Understanding where this upper limit lies enables pilots to make informed decisions about airspeed management during operations, ensuring safety and adherence to proper flight practices.