This situation was presented to the FAA's Office of Chief Counsel in a request for a legal interpretation. Specifically, an individual requested an interpretation of the phrase "necessary for takeoff or landing" as used in 14 C.F.R. § 135.183(b). Apparently the individual operated single-engine Cessna Caravan aircraft in Part 135 operations between the Bahamas and Fort Lauderdale, Florida, along the FAA's published DEKAL TWO arrival route. When the flight reached the DEKAL fix, 30 miles from shore, air traffic control (ATC) usually instructed the flight to descend to 4,000 feet to separate turboprop traffic from jet traffic.
In response, the Office of Chief Counsel issued a Legal Interpretation which initially observed that Section 135.183 prohibits a single engine aircraft, when carrying passengers, from operating over water unless the aircraft is within power-off gliding distance from land, or when it is necessary for take off or landing. It also noted that to determine whether an altitude is "necessary for takeoff or landing" you have to look at "whether that portion of the flight is necessary to permit the pilot to transition between the surface and the en route or pattern altitude in connection with a takeoff or landing."
Applying the facts it was provided, the FAA explained that descent to 4,000 feet at the DEKAL fix would not be necessary for landing because the altitude was assigned for traffic separation, and the Caravan's performance would not require it to be at the assigned altitude for approach into the destination airport. In response to the individual's concern regarding compliance with 14 C.F.R. § 91.123 (requiring compliance with ATC clearances and instructions), the Interpretation cited Chapter 4-4-1(a) of the Aeronautical Information Manual for the proposition that "an ATC clearance 'is not authorization for a pilot to deviate from any rule, regulation, or minimum altitude.'"
It then concluded that, rather than accepting a clearance that would put the Caravan beyond power-off glide distance from shore, and violate Section 135.183, "the operator would be required to select another route or request a different clearance in order to maintain an altitude that keeps the aircraft within power off glide distance from shore."
This Interpretation is a good reminder that the pilot is ultimately responsible for compliance with the regulations applicable to his or her flight. Yes, you need to comply with ATC instructions to avoid violating Section 91.123. However, if ATC's instructions would result in violation(s) of the regulation(s), the pilot has a duty to reject those instructions. Not an easy decision, I know. Hopefully you won't find yourself in that position.