Which of the following is NOT a method to start a helicopter?

The manual crank start is not a recognized method to initiate the start-up of a helicopter. Helicopters are typically designed with advanced starting systems that ensure the safe and efficient operation of the aircraft.

External air sources, ground start methods, auxiliary power units (APUs), and cross-bleed or pneumatic starts are all standard procedures that take advantage of the helicopter's systems to provide the necessary power and airflow to the engine for starting.

The use of a manual crank is generally associated with much older aircraft or certain types of engines that do not have modern starter systems. In contemporary helicopter design, manual cranking is impractical and not implemented, as it would require significant physical effort and could potentially pose safety hazards. Thus, it stands apart from the other methods which are integral components in the operation and start-up procedures of current helicopter technology.