Final answer:
A civil aircraft pilot must have an instrument rating when flying under instrument flight rules (IFR), which is typically during poor visibility or in controlled airspace. The FAA regulates distances between aircraft, and such operations usually require instrument ratings. Height restrictions for pilots can affect who is eligible to fly.
Step-by-step explanation:
The pilot in command of a civil aircraft must have an instrument rating when operating under certain conditions that require the ability to fly the aircraft by referencing the instruments in the cockpit, rather than by visual references on the ground. This typically includes scenarios where visibility outside the aircraft is poor, such as in clouds, heavy rain, fog, or at night (when visual flight rules, or VFR, are not sufficient for safe navigation). An instrument rating is also required when flying in controlled airspace under instrument flight rules (IFR), which might be the case during most commercial airline operations.
Considering the referenced information, commercial airliners under the regulation of the Federal Aviation Administration (FAA) are required to maintain specific distances from one another, which is part of the broader set of rules and regulations governing air traffic. Operating within these parameters often necessitates an instrument rating, as commercial flights frequently encounter conditions that are not conducive to VFR.
Moreover, rules such as physical requirements for pilots can impact who has the opportunity to fly commercially. For instance, height restrictions in the US Air Force historically affected the composition of pilot candidates but have since been adjusted for greater inclusivity.