A standby instrument (10) for an aircraft, the instrument comprising at least one inertial sensor (1), at least one pressure sensor (2), calculation means (3) connected to said inertial and pressure sensors (1, 2), a display unit (4). Said calculation means (3) are suitable for determining critical flight information for said aircraft, and for displaying said critical flight information on the display unit (4) in the event of a main information system of said aircraft failing. In addition, said standby instrument (10) also incorporates stabilization relationships enabling said calculation means (3) to determine control relationships in order to control the actuators (15) of an autopilot of said aircraft in the event of said autopilot failing. Finally, said calculation means (3) are connected to at least one engine operation computer (5) enabling said instrument (10) to display information about a first limit of the engine on said display unit (4).