Gyroscopic instruments, a convenience for VFR flight, are an absolute necessity for IFR flight and provide a direct indication of an aircraft's attitude. Every aircraft is equipped with three gyroscopic instruments and comprise the attitude indicator, heading indicator, and turn coordinator. On most small aircraft, the vacuum system powers the attitude and heading indicators, while the electrical system powers the turn coordinator. This is done for safety reasons since the turn coordinator can function as a backup in case of a vacuum system failure. Gyroscopic instrument operation is based on two fundamental concepts that apply to gyroscopes, namely; rigidity in space and precession.