When is it mandatory for pilots to use IFR clearances?

Pilots are required to use IFR clearances when operating in controlled airspace under specific conditions. Controlled airspace is designated airspace where air traffic control (ATC) provides oversight and separation for aircraft. In this environment, ATC issues IFR clearances to ensure safe traffic management, particularly for aircraft operating along specific routes and altitudes.

When flying under instrument flight rules (IFR), pilots must file an IFR flight plan and receive a clearance before entering controlled airspace. This procedure is vital for maintaining safety and efficiency in busy airspace where other aircraft may also be operating below, at, or above the IFR altitudes.

The requirement to utilize IFR clearances in controlled airspace helps manage air traffic and prevents potential mid-air collisions, as ATC can monitor the positions and intentions of multiple aircraft. This is especially critical on specific routes designed for IFR traffic, further underscoring the necessity of flight clearances in these circumstances.