1. A method of assessing the health of a pneumatic valve controlled system onboard an aircraft, comprising: monitoring the states of a first process controlled by a first pneumatic valve; monitoring the states of a second process controlled by a second pneumatic valve; comparing the monitored states of the first process with the monitored states of the second process to derive at least one comparison result; and ascertaining at least one parameter associated with the health of at least one of the first pneumatic valve and the second pneumatic valve in response at least in part to said comparison result.