How often must a pilot log instrument approaches to remain eligible to operate under IFR?

To remain eligible to operate under Instrument Flight Rules (IFR), a pilot must log at least six instrument approaches within the preceding six months. This requirement is stipulated by the Federal Aviation Administration (FAA) to ensure that pilots maintain their proficiency in flying under instrument conditions, which can be significantly different from visual flight.

Completing these approaches helps reinforce key skills, such as navigation using instruments, understanding aircraft systems, and executing approaches. By requiring pilots to perform these approaches regularly, the FAA aims to enhance safety and operational competency, particularly as instrument conditions can present significant challenges.

The other timeframes provided do not align with the FAA's specific requirement, as they do not ensure that pilots keep their IFR skills current within the appropriate interval. Therefore, the correct answer regarding how often a pilot must log instrument approaches to maintain IFR eligibility is six months.