What are the primary reference instruments for maintaining straight flight?

The primary reference instruments for maintaining straight flight are the Attitude Indicator, Heading Indicator, and Turn Coordinator. The Attitude Indicator provides critical information on the aircraft's orientation with respect to the horizon, showing whether the wings are level or if the aircraft is climbing or descending. This information is essential for maintaining straight and level flight.

The Heading Indicator allows the pilot to understand the aircraft’s direction, which is important for straight flight along a chosen heading. If an aircraft deviates from the intended course, the Heading Indicator assists the pilot in correcting the flight path back to the desired heading.

The Turn Coordinator plays a crucial role in assessing whether the aircraft is in a turn or if it is maintaining straight flight. It indicates the rate of turn and helps the pilot maintain straight flight by ensuring that the aircraft is neither turning nor in uncoordinated flight.

Together, these instruments provide the necessary information to maintain stable, straight flight without deviation, making them the primary tools for achieving this objective.