At the top of its service ceiling, the amount of air over the wings makes any turbulence a little more troublesome, either pushing the plane into its critical high-speed regime or perhaps nudging it back to stall speeds. The coffin corner is the flight region where these two speeds are very close. Fortunately, many airliners are unable to get to this altitude at normal operating weights because the engines just don't have the poop up there.
The service ceiling is defined as the maximum altitude at which the aircraft can still generate a positive rate of climb of 100 feet per minute. The absolute ceiling is the maximum height the aircraft can reach, beyond its service ceiling, where the air density is extremely low so that the climb rate of the aircraft drops to zero. Normally service ceiling of a typical passenger aircraft ranges from 41000 to 43000 feet, though it can be 51000 feet for private jets.
The heavier the plane, the faster it had to go. So, the heavier the plane, the lower its maximum altitude, because when you go higher, the minimum stall speed exceeds the maximum speed of the plane. At high altitude, you have to go faster to stay above the stall speed, and the faster you go, the closer you get to the high speed limit where Mach buffet stall occurs.
The FAA has over the years tightened up the certification process for new aircraft and about twenty years ago placed a strict requirement that subsonic commercial aircraft are not to exceed altitudes of 40,000 feet unless the structure is certified to not have any type of decompression. NOTE: FAA sets a maximum certification altitude of 51,000 feet. Each aircraft is certified to fly at a maximum altitude, which for commercial aircraft is around 40,000 feet. Some business jets are certified to around 45,000 feet but any higher requires special equipment and also increases the amount of solar radiation that a person receives.
Concorde had a maximum cruise altitude of 18,300 meters (60,039 ft) and an average cruise speed of Mach 2.02, about 1155 knots. The current airliners rarely travel above 41,000 feet.