Agree about the dangers of ash and erring on the side of safety - however, airplanes "falling out of the sky" is a common misconception. Most multiengine airplanes can land safely with 1 engine running, and even if all fail, pilots practice engine out approaches in basic pilot training - at high altitude, a jetliner can glide up to 100 miles if the optimum lift/drag airspeed (different for different airplane models and one of the important emergency numbers for pilots to know) is maintained. This is one reason that most airline accidents occur during takeoff/landing - stalls, etc (while more likely due to slower speeds) have less room to recover. Unfortunately, the presence of the cloud over the oceanic portions of the airline routes will cause problems even with the glide range, since an expert ditching in the North Sea still results in a lot of cold wet passengers.
True, but then God said "Neitzsche is dead".
As in certain cults it is possible to kill a process if you know its true name. -- Ken Thompson and Dennis M. Ritchie