What is the primary reference for attitude when looking inside the airplane?

The primary reference for attitude when looking inside the airplane is the position of the miniature airplane of the attitude indicator in relation to the horizon bar. The attitude indicator is specifically designed to visually display the aircraft's orientation relative to the horizon, showing whether the wings are level or if the airplane is climbing, descending, or in a turn.

By observing the relationship between the miniature airplane symbol and the horizon bar, a pilot can quickly and accurately determine the aircraft's pitch and roll attitudes. This is crucial for maintaining proper control of the aircraft, particularly during flight maneuvers, transitions, and when flying in conditions where external visual references are limited.

While other instruments like the airspeed indicator, turn coordinator, and altimeter provide valuable information regarding the aircraft's performance and status, they do not directly convey the aircraft's attitude in the same way that the attitude indicator does. The airspeed indicator shows the speed of the aircraft, the turn coordinator indicates rate of turn and coordination, and the altimeter provides altitude information—none of which directly informs the pilot about the aircraft’s orientation concerning the horizon.