  Enabling vertical take-off and landing while providing the ability to fly
long ranges opens the door to a wide range of new real-world aircraft
applications while improving many existing tasks. Tiltrotor vertical take-off
and landing (VTOL) unmanned aerial vehicles (UAVs) are a better choice than
fixed-wing and multirotor aircraft for such applications. Prior works on these
aircraft have addressed aerodynamic performance, design, modeling, and control.
However, a less explored area is the study of their potential fault tolerance
due to their inherent redundancy, which allows them to tolerate some degree of
actuation failure. This paper introduces tolerance to several types of actuator
failures in a tiltrotor VTOL aircraft. We discuss the design and modeling of a
custom tiltrotor VTOL UAV, which is a combination of a fixed-wing aircraft and
a quadrotor with tilting rotors, where the four propellers can be rotated
individually. Then, we analyze the feasible wrench space the vehicle can
generate and design the dynamic control allocation so that the system can adapt
to actuator failures, benefiting from the configuration redundancy. The
proposed approach is lightweight and is implemented as an extension to an
already-existing flight control stack. Extensive experiments validate that the
system can maintain the controlled flight under different actuator failures. To
the best of our knowledge, this work is the first study of the tiltrotor VTOL's
fault-tolerance that exploits the configuration redundancy. The source code and
simulation can be accessed at https://theairlab.org/vtol.
