  Safety is always the priority in aviation. However, current state-of-the-art
passive fault-tolerant control is too conservative to use; current
state-of-the-art active fault-tolerant control requires time to perform fault
detection and diagnosis, and control switching. But it may be later to recover
impaired aircraft. Most designs depend on failures determined as a priori and
cannot deal with fault, causing the original system's state to be
uncontrollable. However, experienced human pilots can save a serve impaired
aircraft as far as they can. Motivated by this, this paper develops a principle
to try to explain human pilot behavior behind, coined the
gravity-compensation-first principle. This further supports reliable flight
control for aircraft such as quadcopters and tail-sitter unmanned aerial
vehicles.
