Expert: Paul Soderman - 7/29/2008

Question
best angle of climb ceiling (absolute)is always lower than best rate of climb
ceiling for a commercial jet isn't it?
how do you calculate that best angle ceiling or flight level?
can you take an average flight level like FL250 as average reference?

Answer
Patrick
By definition, at the aircraft ceiling it can climb no further.  It doesn't matter if it got to that altitude by best climb angle or best rate of climb.  In other words, the ceiling for best climb angle and best rate of climb is the same.  The following website shows the equations for aircraft climb and show how the two paths converge to the same altitude:

http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page10.html

To calculate ceiling of a jet aircraft you need to know the power required and power available as a function of velocity and altitude.  At ceiling, excess power (power available - power required) is zero.  Similarly, excess thrust goes to zero at ceiling.

Excess power and thrust go to zero because as altitude increases, density and pressure go down so the jet loses thrust ability at the same time as the aircraft induced drag increases.  Induced drag increases because the lift coefficient must increase to maintain lift as density goes down.  So you also need to know the aircraft lift to drag ratio as a function of velocity and altitude.  

All this leads me to believe that no aircraft can be an average reference because they all have different power plants and aerodynamic performance.
Paul