Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!
Emergency Decent to 10,000ft, that will most likely be a problem with the pilot's Oxygen supply. At 40,000ft without Oxygen they would loose conciousness very quickly, hence the need for a rapid decent into thicker air. In a situation like that, as long as the maximum safe speed for the plane isn't broken, breaking the sound barrier won't be a consideration.