2.1. Thrust Setting
The standard engine rating for descent is “Flight Idle Thrust”. For fly-by-wire
aircraft, the thrust throttle position doesn’t change when autothrust is engaged. The
throttles remain on the “CL” (climb) gate for the entire flight (Figure H6). The enginemonitoring
computer, or FADEC (Full Authority Digital Engine Control), adjusts the
thrust level to the required value.
In case of an altitude constraint or a repressurization segment (see ”Cabin
Descent”), the aircraft’s vertical speed may have to be limited during descent. This is
achieved at a thrust called “Adapted Thrust”. The adapted thrust may vary between
flight idle thrust and maximum cruise thrust. It is delivered by the engines, when
autothrust is engaged, as soon as the aircraft descent speed plus one of the two
descent parameters (gradient or rate) have to be maintained at fixed values.
2.4. Cabin Descent
The cabin pressure rate is optimized during descent, so that it reaches the
landing field pressure + 0.1 psi just prior to landing.
Depending on the initial cabin and destination airport altitudes, the FMGS
calculates the necessary cabin descent time. This time is obtained from the selected
cabin rate of descent, defaulted to –350 feet per minute in the FMGS, but which can
be modified up to a maximum of –750 feet per minute.
IN FLIGHT PERFORMANCE
DESCENT
3.05.30 P 2
SEQ 110 REV 31
DESCENT - M.78/300KT/250KT
IDLE THRUST ISA
NORMAL AIR CONDITIONING CG=33.0% MAXIMUM CABIN RATE OF DESCENT 350FT/MIN
ANTI-ICING OFF
WEIGHT
(1000KG) 45 65
FL
TIME FUEL DIST. N1 TIME FUEL DIST. N1 IAS
(MIN) (KG) (NM) (MIN) (KG) (NM) (KT)
390 16.1 181 102 73.0 17.0 125 103 IDLE 241
370 14.6 152 90 74.0 16.2 121 98 IDLE 252
350 12.9 115 77 75.9 15.6 118 93 IDLE 264
330 11.7 90 69 IDLE 14.9 114 88 IDLE 277
310 11.2 87 65 IDLE 14.3 110 83 IDLE 289
290 10.8 84 61 IDLE 13.7 107 79 IDLE 300
270 10.2 81 57 IDLE 12.9 103 73 IDLE 300
250 9.6 77 53 IDLE 12.1 98 67 IDLE 300
240 9.3 75 50 IDLE 11.7 95 64 IDLE 300
220 8.6 71 46 IDLE 10.9 90 59 IDLE 300
200 8.0 67 42 IDLE 10.1 84 53 IDLE 300
180 7.4 63 38 IDLE 9.3 79 48 IDLE 300
160 6.8 58 34 IDLE 8.5 73 43 IDLE 300
140 6.1 53 30 IDLE 7.6 66 37 IDLE 300
120 5.5 49 26 IDLE 6.8 60 32 IDLE 300
100 4.8 44 22 IDLE 5.9 53 27 IDLE 300
50 1.8 17 8 IDLE 2.2 21 9 IDLE 250
15 .0 0 0 IDLE .0 0 0 IDLE 250
CORRECTIONS LOW AIR
CONDITIONING
ENGINE
ANTI ICE ON
TOTAL
ANTI ICE ON
PER 1° ABOVE ISA
TIME − + 10 % + 10 % −
FUEL − 2 % + 60 % + 75 % + 0.60 %
DISTANCE − + 10 % + 10 % + 0.60 %
R
2.4. Cabin Descent
The cabin pressure rate is optimized during descent, so that it reaches the
landing field pressure + 0.1 psi just prior to landing.
Depending on the initial cabin and destination airport altitudes, the FMGS
calculates the necessary cabin descent time. This time is obtained from the selected
cabin rate of descent, defaulted to –350 feet per minute in the FMGS, but which can
be modified up to a maximum of –750 feet per minute.
As soon as the cabin descent time is longer than the aircraft descent time, a
repressurization segment is necessary, during which the aircraft vertical speed is
limited to permit cabin repressurization (Figure H9).
Figure H9: Cabin Repressurization Segment
The above A320 descent table (Figure H8) shows that to descend from FL390
at a weight of 45 tons, the N1 parameter must be maintained at 73%, from the start of
the descent, in order to limit aircraft vertical speed.
Note that, in some particular cases (landing at high altitude airports) , the cabin
pressure at cruise level is higher than the pressure at the landing airport. Therefore,
the cabin pressure has to decrease during descent, which means that the cabin’s
vertical speed is positive while the aircraft’s vertical speed is negative.