FL-PERF

The vertical distance of a level, a point or an object considered as a point, measured from mean sea level (MSL)

The vertical distance of a level, a point or an object considered as a point, measured from a specific datum

Pressure Altitude is the altitude in the international standard atmosphere whose pressure value is equal to the actual ambient pressure at the place of observation.
Pressure altitude is the indicated altitude when an altimeter is set to 1013.25 hPa

Density Altitude is the altitude in the international standard atmosphere whose density value is equal to the actual ambient density at the place of observation.
Density Altitude is the pressure altitude adjusted for non-standard temperature.

Humidity, when used in aeroplane performance, is always related to relative humidity.
CS 25.101 states:
The performance, as affected by engine power or thrust, must be based on the following relative humidities:
(1) 80%, at and below standard temperatures; and
(2) 34%, and above standard temperatures plus 10ºC (50ºF).
Between these two temperatures, the relative humidity must vary linearly.

It represents the average performance that a fleet of aeroplanes can be expected to achieve if they are satisfactorily maintained. In other words it is the level of performance that an average pilot flying an average aeroplane could be reasonably expected to attain.

To account for further variations in performance caused by manoeuvring, piloting technique and temporary below average aircraft performance the gross performance values are further diminished for each phase of flight, as specified by the EASA CS 25.115 (b). This is the safety factorization. It is based on an incident probability rate of one in one million  flights, making it a remote probability that the aircraft will not achieve the specified performance level.

Runway slope for the purposes of take-off and landing performance is assumed to be uniform over the entire runway length. The range of slopes included in the Flight Manual is from +2% to —2% and the effective runway gradient is to be used for takeoff computations. CS 25.105(d)(2).

If slope varies along the runway length then the average slope is the one which is promulgated. This slope should be used for all field lengths used in take-off performance planning calculations. For Class 'A' (CS-25) aeroplane scheduled landing performance computations, the average slope must be used if it exceeds 2% and may be used if so desired for lower values. The runway slope formula is:

\(Slope = {Change \, of \, height \over Runway \, length} \cdot 100\)