SimSoft Working Group

Rev:03ARINC 610 Analogy Article9. Altitude ChangeAltitude Change flags that the altitude of the aircraft has been changed artificially, so notdue to normal flight; it signifies a step-change in altitude.So, consider Analogy Vehicle: Let’s assume it is being driven normally along a highwayand that Altitude Change is asserted.• The vehicle speed [altitude] is suddenly different;• The driver sees a jump in the reading on the speedometer;• The cruise control [autopilot] starts to bring the speed back to its previous setting.The last item, above, illustrates an important concept: Whilst a system may be affected byand have to take account of a simulator function, altitude change not due to normal flight inthis case, it does not always hold that the system is expected to be stable after the change.Altitude change is so far-reaching in its effect on virtually all aircraft systems that completestability after it cannot be expected. Indeed, some effects cannot be determined withoutcrew intervention: Consider an aircraft flying under autopilot control with the autopilot inAlt Hold mode. After an altitude change, should the autopilot try to regain the previousaltitude at which it was holding, or should it hold at the new altitude? Without crewintervention the answer cannot be known. In cases such as these it is acceptable to force acrew input – in this case disconnecting the autopilot after the altitude slew forces the crewto take control and resolve the anomaly.10 July 08 Page 13 of 28