Service Ceiling
Aircraft Service Ceiling
The term service ceiling appears in reports related to Malaysia Airlines flight MH370, and related media articles.
This article explains what a service ceiling is; how a service ceiling is defined; and provides the data for the Boeing 777 aircraft.
The altitude to which any specific aircraft can climb is limited by the aerodynamic design of the airframe and the thrust developed by the engines. Air density (and atmospheric pressure) is greatest at sea level and reduces with altitude. Jet engines need oxygen to burn the liquid jet fuel. Fuel consumption is generally optimised for a cruising altitude and cruising air speed. As fuel is consumed the weight of the aircraft decreases and the optimum altitude may increase. However, if the aircraft climbs above the normal cruising altitudes the lower density air provides less lift and the engines can struggle to get enough oxygen to burn the fuel required to climb or even maintain airspeed. This defines the absolute ceiling for that aircraft - it simply cannot climb any higher.
The rate at which an aircraft can climb is generally stated as the change in altitude per minute. For example, at departure a Boeing 777-200 will typically climb to 5000 feet at a rate of 3000 feet per minute. From an altitude of 5000 feet to 15,000 feet the rate of climb is reduced to 2500 feet per minute; from 15,000 feet to 24,000 ft the Rate of Climb (ROC) reduces to 2000 ft/min; and as the aircraft climbs to a cruising altitude and a typical cruise air speed of 480 knots, the ROC drops to 1500 ft/min.[1]
If the pilots tried to keep climbing, eventually the Rate of Climb would reduce to only 100 feet per minute. The altitude at which this occurred is called the service ceiling and for the Boeing 777-200 the Service Ceiling is an altitude of 43,100 feet above sea level.[2]
The absolute ceiling for the Boeing 777 is not publicised, but if the aircraft reached the service ceiling of 43,100 feet and the pilots attempted to climb higher, at an indicated air speed of 480 knots the plane would be moving forward about 48,600 feet every minute but climbing at a reducing rate of less than 100 feet every minute - it's just not worth the fuel consumption.
Also, the external air pressure reduces with altitude so if the aircraft internal pressure is maintained as though it were still at 10,000 feet, the pressure difference can become too great so it would be best to treat the service ceiling as the maximum altitude from a safety perspective.
More Precise terminology
The following terms and definitions have been extracted from the Pilot's Handbook of Aeronautical Knowledge, U.S. Department of Federal Aviation Administration (FAA).
Altitude
- True Altitude
- The vertical distance of the airplane above sea level - the actual altitude. It is often expressed as feet above mean sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.
- Pressure Altitude
- The altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92. This is the altitude above the standard datum plane, which is a theoretical plane where air pressure (corrected to 15°C) equals 29.92 in. Hg. Pressure altitude is used to compute density altitude, true altitude, true airspeed, and other performance data.
- Density Altitude
- This altitude is pressure altitude corrected for variations from standard temperature.
When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the airplane’s performance.
- Rate of Climb
- The rate of climb or descent is indicated in feet per minute. (Measured by the Vertical Speed Indicator (VSI)).
- The rate of climb is the vertical component of the flightpath velocity.
- Absolute Ceiling
- An increase in altitude also will increase the power required and decrease the power available. Therefore, the climb performance of an airplane diminishes with altitude. The speeds for maximum rate of climb, maximum angle of climb, and maximum and minimum level flight airspeeds vary with altitude. As altitude is increased, these various speeds finally converge at the absolute ceiling of the airplane. At the absolute ceiling, there is no excess of power and only one speed will allow steady, level flight. Consequently, the absolute ceiling of the airplane produces zero rate of climb.
- Service Ceiling
- The maximum density altitude where the best rate-of-climb airspeed will produce a 100 feet-per-minute climb at maximum weight while in a clean configuration with maximum continuous power.
References and Notes
- ↑ Source: Aircraft Performance Database, B777 https://contentzone.eurocontrol.int/aircraftperformance/details.aspx?ICAO=B772&
- ↑ Many websites cite the Boeing 777 service ceiling as 43,100 feet. However, attempts to locate a document from Boeing which validates that have been unsuccessful.
The Safety Investigation Report (2018) Section 1.6.8 Aircraft Systems Description, includes this statement at 14/Auxiliary Power Unit: "The APU can start at all altitudes up to the service ceiling of the aircraft (43,100 ft/13,100 m)."