When you think of a ceiling, it is usually above your head, and can vary in height depending on the room you find yourself in, but if you were to jump up really high, you would not be able to go higher than this. Similarly, ceiling applies to how high an aircraft can climb, but this is further divided into an absolute ceiling, and a service ceiling. Let’s have a look at the difference and what they mean…
Starting with absolute ceiling, this can be defined as the altitude where the aircraft can no longer produce an increase in altitude even though it is at maximum power. In other words, a rate of climb is no longer being produced. Absolute ceiling is determined by test pilots, when checking the performance of an aircraft, but there are factors which change from day to day, such as density altitude and aircraft mass, which will define how high this will be. In the climb, the drag produced is considerable – the greater the angle of the climb, the more drag it will produce, mostly because the weight of the aircraft, which tends to act more in a rearwards direction, which ‘adds’ to the effect of drag What this means, is that the aircraft engine will have to produce enough power (or thrust in the case of a jet engine) to not only balance the drag, but to produce an excess that will allow the aircraft to climb, and gain in altitude. Similarly, forward speed also allows for air flow over the wings which generates lift. The lift also needs to exceed the effect of weight for the aircraft to continue climbing. When excess power or thrust is no longer being produced, then the rate of climb will be zero and the aircraft will no longer climb.
As absolute ceiling is a pretty unrealistic altitude to achieve, service ceiling is defined. Service ceiling is the density altitude where the aircraft is still able to produce a specific rate of climb (100ft per minute for piston engines and 500ft per minute for jet aircraft), when at full maximum continuous power.
What does service ceiling mean for you? Well, depending on the day, it may be higher or lower than what you expect, so choosing a cruising altitude close to the service ceiling is not a good idea, especially if you have high terrain to cross over on your route. Just because your aircraft supposedly has a certain absolute ceiling, definitely does not mean that it will climb to this altitude. Even though the service ceiling is more realistic, you need to be aware that if you have a heavily loaded aircraft on a hot day then this ceiling is likely to be a lot lower.