Which requirement is necessary for pilots when operating under IFR?

When operating under Instrument Flight Rules (IFR), the most critical requirement for pilots is possessing an instrument rating. This rating indicates that a pilot has completed the necessary training and demonstrated proficiency in flying an aircraft solely by reference to instruments in the cockpit, rather than by visual references outside the aircraft. This is essential because IFR operations often occur in conditions where visibility is poor or when flying through clouds, necessitating reliance on instruments for navigation and control.

An instrument rating ensures that pilots understand the principles behind instrument navigation, how to interpret various instruments, and how to deal with the challenges of flying in such conditions. This requirement is set forth by the Federal Aviation Administration (FAA) to ensure safety and competency in IFR operations.

While familiarity with VFR regulations, having a second pilot on board, and basic flight training are all important aspects of flying, they are not specific requirements for operating under IFR. The instrument rating is what specifically equips pilots with the necessary skills and knowledge for safe IFR flight.