Why are aircraft considered to be less manoeuverable at high altitude?
Some potential reasons:
1. Inverse square law. Every time an aircraft doubles its distance from a ground observer, it gets half as big. Pretty soon you can't even see it from the ground. Although its weight also halves every time the distance doubles, the weight-related distance is calculated from the center of the earth, not the point of view of an observer standing on the surface of the earth. At about 18,000 feet above the surface, the area of the control surfaces on the aircraft is no longer appropriate for the aircraft weight, and it becomes more difficult to maneuver. Although pilots have, for years, attempted to counter this effect by setting their altimeters to 29.92 as they pass through this critical altitude, the effects of the change are minimal and performance degrades, none-the-less.
2. As you know, an aircraft in flight is supported on a column (a cushion, if you will) of highly compressed air. Actually, "column" is probably the wrong word here, because the cushion of air supporting the aircraft is actually pyramid-shaped. From the edges of the aircraft wings the cushion extends outward and downward at a 45 degree angle until it is stopped by the surface of the earth. (This is why the areas near the ends of runways are clear of everything except for the highly reinforced structures supporting lights and the localizer. Anything in that area which is NOT built from incredibly strong materials will be crushed by the weight of aircraft passing directly overhead). At low altitudes, the atmosphere below the aircraft is quite compressed, because the weight of the aircraft is concentrated in a lower volume of air. This compression allows the aircraft to operate in an incredibly stable environment where control surfaces need only turn, bank, climb, and descend the aircraft. In contrast, at high altitude the ratio describing the relationship between the weight of the aircraft and the volume of compressed air supporting it is such that there is very little support provided. Therefore, aircraft at high altitude must devote the majority of their control surfaces to maintaining the stability of the aircraft. This leaves very little surface area to be used in maneuvering. Aircraft at altitude always run the risk of departing from controlled flight whenever a pilot touches the controls, because the aircraft is "concentrating" on just staying in the air. The "coffin corner" refers to the actual corner of the control surface that is left for use by the pilot in maneuvering. As the aircraft flies higher, the "coffin corner" gets smaller. At some point (depending upon complexities which I will not go into here) the ability to maneuver the aircraft at all disappears, and ANY control inputs are met with certain disaster. Because the aircraft cannot turn, climb, bank, descend, slow down, or speed up without being destroyed, it is a "coffin" waiting to run out of "gas."
3. It is not the aircraft, per se, which is less maneuverable. Rather, it has to do with the fact that there is less oxygen at high altitude and the ability of a pilot to function in this environment is impeded. Every few years we hear of an adventurous soul who tries to take his "bird" above 12,000 feet and does not live to tell the tale. Oxygen deprivation is a serious issue in aviation, and the extreme cold at altitude just makes things worse. Perhaps someone will find an answer for this scourge some day, but for now, we would all be safer if pilots stuck to the rules.
There are other reasons as well, but let's just start with these.
Dave