(3)
Descent.
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(a)
The descent mode controls the airplane in descending flight from end-of-cruise to a selected clearance altitude at or above 2500 feet radio altitude. Airplane descent is accomplished by elevator (pitch) control only, with the throttles retarded and in 'throttle hold'. During the actual descent, the PMS controls only one parameter (speed or path) at a time.
(b)
To activate the descent mode, a clearance altitude must be selected below present cruise altitude. When a descent mode is entered the PMS retards the throttles. When the throttles are retarded to the required power setting the PMS enters the 'throttle hold' mode (PMS ceases to control throttles).
(c)
The pilot is responsible for throttle settings while in 'throttle hold'. During this period the throttle levers remain at their last setting until manually moved to a new setting. All descents are programmed to continue until the clearance altitude is reached. When the clearance altitude is reached the PMS regains active control of the throttles, and transitions back to the manual cruise mode at the descent speed.
(d)
Six descent sub-modes are provided, these modes are: 1) Speed Arm 2) Speed Select 3) Path Arm 4) Path Select 5) Manual (MAN) 6) Flight Path Angle (FPA)
(e)
For descent purposes altitude restrictions are added to PMS waypoints (addition of altitude creates a three-dimensional waypoint). The resulting waypoints are called hard-loaded bottom of descent (BOD), or descent waypoints. Speed limits may also be added to descent waypoints. When altitude and speed restrictions are added to waypoints, a descent may be split into two or more separate legs.
(f)
PMS descents can be accurately oriented to bottom of descent (BOD) waypoints. For this type of geographically oriented descent, the PMS provides three descent options: 1) Descent Speed-Armed Mode 2) Descent Path - Armed Mode 3) Descent Flight Path Angle Mode (FPA)
(g)
The descent speed-armed mode requires at least one BOD waypoint to be loaded. In this mode, the PMS controls the elevators to maintain the desired speed.
(h)
The descent path - armed mode requires at least one BOD waypoint to be loaded. In this mode the PMS controls the trajectory during descent so that the airplane remains on the ideal calculated path.
EFFECTIVITY
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(i) The descent flight path angle mode (FPA) is used in situations
where air traffic control requests immediate initiation of a
descent to a BOD waypoint. In this mode the PMS controls the
elevators to maintain a trajectory that has a constant angle
with respect to the local vertical.
(j) It is also possible to descend without regard to waypoints.
For these non-geographically oriented descents, the PMS
provides two descent options:
1) Descent Speed - Select Mode
2) Descent Manual Mode (MAN)
(k) The descent speed - select mode can be used without a defined
BOD waypoint. This mode is intended for terminal descents
where minimum fuel burn and maximum forward distance is desired
for efficiency.
(l) The descent manual mode (MAN) does not require hard-loaded
waypoints, and is intended basically for step descents. In
this mode the PMS controls the elevators to maintain the
selected speed during descent.
(m) If a BOD waypoint is loaded, the PMS continuously computes the
optimum top of descent (TOD) point. The computation is a
function of current cruise altitude, speed, and winds; and also
a function of selected descent speed, and BOD speed limit if
loaded.
NOTE: Top of descent (TOD) is the point in cruise at which the____
descent is initiated in order to acquire the defined
BOD waypoint.
(n) During cruise the PMS constructs three descent profile paths to
the next defined BOD waypoint, based on optimum (minimum fuel)
descent speed, maximum (Vmax) descent speed, and selected
descent speed. The PMS continuously computes distance from
present position to TOD, as well as flight path angles for
optimum and maximum speed descents, while taking into account
present cruise conditions, descent speed schedule, descent
path schedule, and lateral flight plan. The PMS also
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