-Repressurization segment (Ref. Fig. 020) The pilot can enter a maximum cabin rate on the PERF CRZ page in ft/mn sea level (a default value of 350 ft/mn is used if no pilot entry exists). Once the predictions have determined a descent time based on the theoretical profile, the average rate of cabin repressurization is computed. If this rate is above the maximum cabin rate, a repressurization segment is added to the descent path so as to increase descent time and satisfy pilot requirement. This segment is computed based on the hold of a descent speed and a V/S computed from the maximum cabin rate. If even with this repressurization segment added to the descent path, the average rate of cabin repressurization is above the maximum cabin rate:
Theoretical Descent Profile Construction
Figure 019
Repressurization Segment
Figure 020
c
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d
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e
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f
_
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the message CABIN RATE EXCEEDED is displayed to alert the pilot
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the average rate of cabin repressurization of the descent path overwrites the maximum cabin rate on the PERF CRZ page.
Recomputation of the theoretical descent profile During Descent phase, the recomputation of the complete descent profile (altitude and speed) for more than a defined short time causes the loss of managed vertical control so that any unexpected change in guidance is avoided. Therefore cases of recomputation have to be limited. That is the reason why the causes for recomputation are differentiated whether the aircraft is in Descent phase or not.
A/C before transition to Descent phase The complete profile (altitude and speed) is recomputed if there is any modification:
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in the lateral F-PLN
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in the vertical F-PLN (including repressurization segment)
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in the cost index
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in the entered predicted atmospheric conditions (wind or temperature) related to descent
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in the preselected speed mode (Econ to auto speed or vice versa).
A/C in DESCENT phase The complete profile (altitude and speed) is recomputed if there is any modification:
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in the lateral F-PLN (except insertion or deletion of an HM downpath)
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in the vertical F-PLN
-wind or T. modified on PERF/APPR page.
Predictions Predictions assume the aircraft will fly the theoretical profile. And if the A/C is off that profile in altitude and/or speed, predictions suppose an immediate return to the theoretical profile from A/C present position to interception of the profile in altitude and speed. The way to return to the profile depends on the current position of the A/C versus alt profile as described below.
-A/C above altitude profile (A/C beyond T/D).
g
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Return to the profile is assumed to be performed with:
-idle thrust
-speedbrakes extended to mid travel
-hold of the ECON descent speed plus a margin, until the
interception of the profile.
(Ref. Fig. 021)
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A/C below altitude profile in DES phase and above DES SPD LIM altitude If A/C is above descent speed limit altitude, hold of V/S = -1000 ft/mn and ECON descent speed until either the altitude of a constraint or the interception of the profile.
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A/C below altitude profile in DES phase and below DES SPD LIM altitude If A/C is below descent speed limit altitude, hold of V/S = -500 ft/mn and ECON descent speed until either the altitude of a constraint or the interception of the profile. (Ref. Fig. 021)
-Consideration of HM leg As explained in para. C(1)(a), a few NM before entering the HM leg, the holding is supposed to be flown. From that time and as long as the aircraft is in the holding, predictions assume that the leg is flown at holding speed. When the HM leg is flown, predictions assume completion of the hold at the exit fix. If the pilot continues the hold upon the next crossing of the exit fix, predictions again assume one and only one flight of the hold. (Ref. Fig. 022)
Guidance
-Speed margin To allow smart guidance on the path during descent, a speed margin is defined. This margin is used for guidance and for display on the PFD speed scale. (Ref. Fig. 023, 024, 025, 026)
-Reference path altitude and vertical deviation To enable pilot to know the aircraft position versus theoretical descent profile, the altitude of profile precomputed at A/C current position is displayed on the PFD altitude scale. The difference between that reference altitude and aircraft altitude (named vertical deviation) is displayed on the PROG page. (Ref. Fig. 023, 024, 025, 026)
Return to Path Predictions
Figure 021
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