The service ceiling of an aircraft defines how high it can fly under standard loads. Most modern aircraft have a service ceiling ranging between 37,000 ft to 44,000 ft (11,300 m to 13,400 m). The calculation of a service ceiling, or certified maximum operating altitude, considers the difference between the outside and the inside (cabin) pressure. Additionally, the structural allowance of the emergency descent rate is considered in case of a depressurization event.
While most aircraft do not test the ceiling under standard operating conditions, composite-based structures are likely to fly closer to their service ceilings. This is because of their lightweight design, lower pressure differential, and efficient engines providing the necessary thrust at higher altitudes.
Irrespective of the service ceiling by design, the aircraft’s cruising altitude depends on several factors. The aircraft’s weight significantly affects how high the aircraft can and must fly. The weather (commonly air temperature and wind direction) also determine the aircraft’s altitude on a given day.
Air density
As the aircraft climbs higher, the density of air becomes lower. As such, the lift generated for the same angle of attack and velocity is reduced. Aircraft typically continue to gain speed as they climb to compensate for the lowering air density and lift. Moreover, the engines produce less thrust with lower density. This is due to reduced air mass flow passing through the engines.
Photo: Vincenzo Pace | Simple Flying
This relationship means that at some altitude, the thrust will equal the total drag generated by the aircraft. This is where the aircraft is supposed to have reached its service ceiling, and the climb must stop. As a rule of thumb, the aircraft is nearing the service ceiling if it cannot climb at least 500 ft (150 m) per minute.
Aircraft speed
For most subsonic aircraft, the aircraft speed also becomes a limiting factor in how high the aircraft can climb. As more lift is generated to climb higher, the air flowing over the top of the wing is accelerated. At some point, shock waves begin to form over the wing.
As a result, the lift is significantly reduced, and the drag is increased. Subsequently, the aircraft enters what is called a coffin corner. Pilots must always avoid this state because it can lead to engine flameout, aircraft stall, and loss of control.
The coffin corner
To prevent shockwaves from forming, aircraft speed must be reduced. However, in doing so, more than the lift produced by the aircraft will be required to maintain altitude. Therefore, the aircraft must descend to a lower altitude to prevent stall conditions.
Photo: Vincenzo Pace | Simple Flying
Increasing the aircraft speed, the maximum Mach number can be exceeded, which may result in extreme vibrations and loss of control. In that state, the aircraft roll movements may increase wing loading, thereby increasing the minimum speed required for necessary lift. Moreover, the inner portion of the wing may experience stall while the outer portion exceeds the maximum speed, resulting in an uncontrollable spin.
Based on the myriad of factors, including aircraft structural properties, operating conditions, and engine performance, it is difficult to derive the maximum altitude an aircraft can fly.
What do you think about the determination of cruising altitudes for aircraft? Tell us in the comments section.
Source: simpleflying.com
