FAIL PASSIVE AUTOMATIC LANDING SYSTEM
An automatic landing system is fail passive if, in the event of a failure of its redundant systems, there is no significant out of trim condition or deviation of flight path or attitude but the landing is not completed automatically.
What this definition implies is that, if the automatic landing system of an aircraft is fail passive, a single failure could jeopardize an automatic landing. To give you an example, an automatic landing carried out with a single autopilot channel engaged is a fail passive system. Why because if that autopilot fails, the aircraft can no longer fly the approach automatically and the pilot has to take over manual control and either fly the approach or commence a go around, depending on the circumstances.
Fail passive automatic landings generally have a higher approach visibility minima compared to a fail operational automatic landing.
FAIL OPERATIONAL AUTOMATIC LANDING SYSTEM
An automatic landing is fail operational if, in the event of a failure of its redundant systems, the approach, flare and the landing can be completed by the remaining part of the automatic system.
This tells us that a fail operational system can continue an automatic landing if a failure occurs in a part of its redundant systems. An example of this is an airplane with 3 autopilots where a failure of 1 autopilot will still leave 2 of them, keeping the system fail operational. An aircraft with 2 autopilots on the other hand requires 2 of them serviceable to maintain fail operational status. If 1 of them fails, the system goes fail passive. Most Boeing aircraft have 3 autopilots while Airbus aircraft have 2 autopilots. So, Boeings are more resilient when it comes to maintaining fail operational capability.
Three autopilots of a Boeing 767.
When we talk of fail passive or fail operational automatic landing systems we have to realise that it is not always about the autopilot. There are also many other system failures that can affect the fail passive/ fail operational status of an aircraft. An example can be the failure of an electrical generator or a hydraulic system which can degrade the aircraft from a fail operational to a fail passive system.
Author – Anas Maaz
