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Airspace is divided into Controlled, Uncontrolled, Special Use and Other. Airspace designation is determined by considering the density of aircraft, the types of operations, the level of safety required, and the national and public interest. Controlled airspace is generally where ATC service is provided. There are different requirements if the flight is operated under Instrument Flight Rules (IFR) or Visual Flight Rules (VFR). For example, a pilot flying IFR in controlled airspace will have to file an IFR flight plan and receive an ATC clearance. For VFR flight, controlled airspace means increased cloud clearance and visibility requirements.  For the VFR pilot flying in B, C, and D airspace, there are communication and/or clearance requirements.

Mnemonic acronyms are wonderful mechanisms for condensing complex terms, phrases or lists into something more manageable to commit to memory.  Aviation has its fair share of clever—and sometimes silly—recall devices, which stands to reason when there is critically important information that should be committed to memory.

Speaking of clever and silly, "ATOMATOFLAMES" is a mnemonic acronym used to help pilots remember the critical components required for aircraft airworthiness for daytime flight in visual (VFR) conditions. Each letter in ATOMATOFLAMES corresponds to a specific item that must be operational before takeoff.  This equipment list can be found in § 91.205.