(a) Climb Gradients.
For each design standard (eg FAR 25), there are minimum airworthiness gradients in different configurations. These are usually referred to a WAT-limits (as the main parameters affecting them are aircraft Weight, Altitude, and outside air Temperature). These limits serve to provide some minimum likelihood that the aircraft will, in fact, be able to climb at a modest rate.
The four segment WAT limits (sometimes five) apply to heavy turbine designs - jet and turboprop.
If the actual weight is less than the WAT-limited weight, then the aircraft's climb capability will improve. This information is available in the flight manual takeoff chart section.
(b) Obstacle Clearance.
For determining optimum payload (meaning maximum payload) one matches the aircraft climb capability against the actual runway, after takeoff obstacles etc.
Supplementary TakeOff Distance Available - otherwise known as effective operational length in some quarters.
The TODA (takeoff distance available) is measured from the end of the runway (clearway if declared) and has no regard to obstacles - as a result the gradient from end TODA to the critical obstacle (ie that which gives the highest gradient) can be sufficiently high as to render the TODA not much use to the heavy operator.
If the optimised process (b) is not practicable for some reason, the operator may look at the declared STODA values (reduced length TODA values which result in a specific gradient which clears all obstacles within the declared trapezoid). Provided the available gradient from the flight manual is not less than the STODA, then the weight is fine. This is complicated somewhat when we consider the level third segment but that is only a complication which can be addressed in various ways.
(d) Aircraft Weight
more = decreased climb gradient
less = increased climb gradient
The relationship is a matter of iterative analysis to find the maximum weight which meets both WAT and obstacle clearance requirements
Last edited by john_tullamarine; 12th Jul 2004 at 06:54.