What is one requirement for an aircraft operating under IFR in controlled airspace?

When an aircraft operates under Instrument Flight Rules (IFR) in controlled airspace, a primary requirement is that it must adhere to the altitude assignments given by Air Traffic Control (ATC). This requirement ensures that all aircraft are safely separated from one another to prevent collisions and maintain orderly traffic flow within the airspace.

ATC provides altitude assignments to ensure that aircraft are flying at designated flight levels that promote safety and efficiency. This is particularly important in controlled airspace, where many aircraft may be operating simultaneously, often at similar altitudes. By following ATC's instructions, pilots contribute to the safety of all parties involved and maintain the integrity of the air traffic management process.

Other options, while related to aircraft operation, do not represent basic requirements for IFR operation in controlled airspace. For instance, a fully functioning radar system is not a requirement for all aircraft under IFR; rather, radar service may be provided by ATC where available. Filing a flight plan in advance is not strictly required 24 hours before the flight, as plans can often be filed closer to departure time. Lastly, the requirement to be at least 3,000 feet above ground level is not a universal IFR requirement, as specific altitude constraints can vary based on the type of air