Backup instrument for an aircraft

The instrument (10) has an inertial sensor (1) and a pressure sensor (2) connected to a calculation unit (3) that determines critical flight information for an aircraft. The calculation unit includes an output connected to actuators (15) of autopilot of the aircraft and an input connected to a full authority digital engine control (5) enabling the instrument to display information on a display unit (4) about utilization limit for one or more engines of the aircraft and about available power or torque margin relative to the limit in the event of a failure of limitation instrument. An independent claim is also included for a method for operating a standby instrument for an aircraft.