You are flying along a sunny day outside of Class B, C, or D airspace. Like many pilots, you’re taking advantage of VFR radar advisory services, commonly known as “flight following.”
(For details about these services, see AIM 4-1-15. Radar Traffic Information Service and BruceAir’s Guide to ATC Services for VFR Pilots.)
Out of the blue, the controller directs you to fly a new heading that will take you out of your way. Perhaps you’re instructed to stop a climb or descent, frustrating your attempts to fly most efficiently.
Must you comply with such instructions? Remember, you’re operating under VFR in Class E airspace, well away from strictly controlled areas such as Class B and Class C airspace. (For a good review of airspace, start with Airspace for Everyone [PDF] from AOPA Air Safety Institute.)
The short answer is “yes.” See 14 CFR §91.123 Compliance with ATC clearances and instructions, which states in part:
…(b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.
Many pilots argue about the applicability of that rule in the situation described here, and they quibble about distinctions between clearances and instructions.
But the FAA has made its position clear. See, for example, the Karas letter from 2013, issued by the office of chief counsel. That letter notes in part:
Section 91.123 deals with compliance with ATC clearances and instructions. Section 91.123(b) states: "Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised."
Pilots flying in controlled airspace must comply with all ATC instructions, regardless of whether the pilot is flying VFR or IFR, in accordance with § 91.123(b). ATC instructions include headings, turns, altitude instructions and general directions. The Pilot/Controller Glossary of the Aeronautical Information Manual (AIM) defines ATC instructions as "[d]irectives issued by air traffic control for the purpose of requiring a pilot to take specific actions; e.g., ‘Tum left heading two five zero,’ ‘Go around,’ ‘Clear the runway. ‘" See AIM, Pilot/Controller Glossary. In contrast, the Glossary defines advisory as "[a]dvice and information provided to assist pilots in the safe conduct of flight and aircraft movement." Id.
A pilot flying VFR in Class E airspace, which is controlled airspace, is not required to communicate with ATC; however, if a pilot is communicating with ATC and ATC issues an instruction, the pilot must comply with that instruction.
The last sentence is key, and straightforward:
…if a pilot is communicating with ATC and ATC issues an instruction, the pilot must comply with that instruction.
That statement is also general, in that it applies even if you’re talking to a control tower, not a radar facility.
Of course, as pilot in command, you have the authority—and the responsibility—to refuse instructions that would, for example, send you into the clouds while operating under VFR. Such situations require you to tell ATC that you’re unable to comply with instructions and to request an alternative directive. See, for example, this statement from AIM 4-1-18:
e. PILOT RESPONSIBILITY. THESE SERVICES ARE NOT TO BE INTERPRETED AS RELIEVING PILOTS OF THEIR RESPONSIBILITIES TO SEE AND AVOID OTHER TRAFFIC OPERATING IN BASIC VFR WEATHER CONDITIONS, TO ADJUST THEIR OPERATIONS AND FLIGHT PATH AS NECESSARY TO PRECLUDE SERIOUS WAKE ENCOUNTERS, TO MAINTAIN APPROPRIATE TERRAIN AND OBSTRUCTION CLEARANCE, OR TO REMAIN IN WEATHER CONDITIONS EQUAL TO OR BETTER THAN THE MINIMUMS REQUIRED BY 14 CFR SECTION 91.155. WHENEVER COMPLIANCE WITH AN ASSIGNED ROUTE, HEADING AND/OR ALTITUDE IS LIKELY TO COMPROMISE PILOT RESPONSIBILITY RESPECTING TERRAIN AND OBSTRUCTION CLEARANCE, VORTEX EXPOSURE, AND WEATHER MINIMUMS, APPROACH CONTROL SHOULD BE SO ADVISED AND A REVISED CLEARANCE OR INSTRUCTION OBTAINED.