FLIGHT PLANNING
EMB-145 A1 LB FAA
AIRPLANE
OPERATIONS
MANUAL
1-05-45
Page
4
Code
06
REVISION 39
AOM-145/1114
FLIGHT OVER MOUNTAINOUS AREA
Two situations must be considered when flying over mountainous
areas, in order to clear route obstacles:
Engine Failure
For most normal cruise weights and altitudes, an airplane will not be
able to maintain its cruise altitude following an engine failure, and will
begin to descend (driftdown). In order to remain as high as possible,
the pilot will use maximum continuous thrust on the remaining engine
and slow down to the optimum driftdown speed, which is the speed
that results in the lower descent gradient. The airplane will then
descend along what is called the optimum driftdown profile. The
optimum driftdown profile will keep the airplane as high as possible
during the descent.
Regulations require that the actual airplane performance be calculated
in the most conservative airplane configuration and then further
decreased by a 1.1% climb gradient for two-engine airplanes. This
reduced gradient path is called the en-route net flight path and is used
toensure en-route obstacle clearance (FAR 25.123 / CS 25.123).
During a driftdown, the available thrust increases as the airplane
descends. Eventually, at a certain altitude the available thrust will
become equal to the airplane drag, and the airplane will level off. This
altitude is called the gross level off altitude. The gross level-off altitude,
when corrected by the 1.1% gradient margin, is called the net level-off
altitude and will depend on the atmospheric temperature and the
airplane weight.
The airplane actual climb gradient (gross gradient) at the net level-off
altitude will be 1.1%. The net gradient is the gross gradient subtracted
by 1.1%. Obviously, the net gradient is zero at the net level off altitude,
and the gross gradient is zero at the gross level off altitude.
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