Phillipzu wrote:B2707SST, as a person with no real knowledge about aviation, can you explain to me what a cold air pocket does to an aircraft?
This seems like a simple question, but it's a good one since there were actually several things going on at once in this case:
- Cold air is denser than warm air, so it provides more lift. In this case, when Concorde flew into a colder air pocket, the wings suddenly generated more lift than they had been previously.
- Denser cold air also produces more engine thrust than less dense warm air, so the engines would have suddenly generated extra power.
- Most importantly, the speed of sound varies directly with air temperature (and only weakly with air pressure). Sound travels slower in cold air and faster in warm air. This is why the speed of sound at standard conditions is about 760 mph at sea level but only about 660 mph at high altitudes, where air temperatures are much colder.
The first two effects are not negligible but probably much less significant than the third point. This is because Concorde had a "Mach Hold" autopilot mode that left the engines at full dry thrust and maintained cruising speed (Mach 2 to 2.04 in normal conditions) by varying the pitch of the aircraft, and thus its climb rate. When outside air temperature suddenly decreases, the speed of sound falls while the airspeed does not, so the aircraft's Mach number is suddenly higher. The autopilot responds to the situation by pitching up to bleed off the excess Mach number. Apparently the temperature drop in this case was so large that the pitch-up maneuver resulted in Trubshaw's "barely controllable climb."
As Trubshaw mentions, the autopilot logic was modified after this incident. I am not certain but suspect the result was the "Max Cruise" mode, which acted like Mach Hold except that it WOULD use the auto-throttle system to reduce engine thrust rather than simply pitch up in overspeed situations. In commercial service, Max Cruise was typically used for the majority of the flight.
See http://www.concordesst.com/autopilot.html for more technical details on Concorde's various autopilot modes.