91.245 Operations in controlled airspace

• (a) Except as provided in paragraphs (e) and (f), a pilot-in-command of an aircraft must not enter a control area or control zone designated under Part 71 unless the pilot-in-command obtains an ATC clearance to enter the control area or control zone.
• (b) A pilot-in-command of an aircraft operating in Class A airspace must--
  • (1) operate the aircraft under IFR; and
  • (2) unless otherwise authorised by the ATC unit responsible for the class A airspace, maintain two-way communications with that ATC unit on the appropriate frequency.
• (c) A pilot-in-command of an aircraft that operates in the following classes of airspace must maintain two-way radio communications with the ATC unit responsible for the airspace concerned on the appropriate frequency unless otherwise authorised by the ATC unit:
  • (1) Class B, C, or D airspace:
  • (2) Class E airspace under IFR.
• (d) If different classes of airspace adjoin one above the other, a pilot operating at the common level may comply with the requirements of the less restrictive class of airspace.
• (e) A pilot of an aircraft operating under VFR does not require an ATC clearance to enter a control area that is classified under Part 71 as class E airspace.
• (f) Airspace within a control area and a control zone becomes uncontrolled class G airspace during those times when an air traffic control service is not being provided within that control area or control zone.