How to read Aero Data calculations
FAR 121.191 requires the airline to plan each flight with consideration given to an engine failure enroute. Compliance with this regulation is primarily a pre-flight/dispatching function. This regulation specifies two sets of requirements, of which, the airline must comply with at least one on each flight.
We will refer to these two sets of requirements as Method 1 and Method 2.
Both methods measure performance based on aircraft net flight path (or net altitude). For a two engine airplane, net altitude is the altitude at which the aircraft can maintain a 1.1% climb gradient based on its current weight. The required climb gradients for three and four engine airplanes is 1.4% and 1.7%, respectively. The airplane’s actual altitude capability (service ceiling or 0% climb gradient) is approximately 5,000 feet higher than the net altitude capability.
The difference between the net and actual altitude is the operating safety margin. This operating safety margin accounts for
factors such as not being on the planned route of flight, ice accumulation on unheated surfaces, the use of ice protection systems when it was not planned, the unavailability of an APU when it was planned, flying at other than optimum speeds, and enroute temperatures greater than forecast. AFM altitude capability is based on enroute climb speed or driftdown speed.