Introduction to the new DES Mode logic applicable to Honeywell FMS2 Release 2
Reference: 22.83.00012
Issue date: 27-FEB-2020
Last check date: 27-FEB-2020
Status: Open
A/C type/serie: A318, A319, A320, A321, A330
ATA: 22-83
Engine manufacturer:
Supplier:
Purpose / Reason for revision: ISI published in order to provide clarification on the new DES Mode applicable to Honeywell FMS2 Release 2
Flight Operations and Training
Status: Open
Modification on Operational Impact: NO
Introduction of a new DES Mode logic for Honeywell FMS2 Release 2
Applicability:
With the introduction of the Honeywell FMS2 Release 2 H3 (for A320 Family aircraft) and FMS2 Release 2 P6 (for A330 aircraft), some guidance logics
applicable to the DES mode have been changed.
This ISI article is applicable to all A320 Family aircraft or A330 aircraft embodied with a Honeywell FMS2 Release 2.
General Overview
With the introduction of the Honeywell FMS2 Release 2 H3 (for A320 Family aircraft) and FMS2 Release 2 P6 (for A330 aircraft), some guidance logics
applicable to the DES mode have been changed.
This article aims to provide Operators with an overview of the changes applicable to this new DES mode.
A. Why a new DES mode?
1) Many A320/A330 Operators provided Airbus with negative feedback on the current DES mode, specifically difficulties reported. These include all of the following:
- Numerous FMA changes (THR IDLE, SPEED, MACH, or THR DES)
- Non-explicit FMA messages (MORE DRAG (For A320 Family aircraft) or EXTEND SPD BRK / RETRACT SPD BRK (For A330 aircraft)) to satisfy constraints.
2) Further ATC perspectives (e.g. Trajectory Based Operations as supported by SESAR and NEXTGEN programs) lead to think that vertical profiles will take more and more
importance.
3) Operators provided Airbus with positive feedback on the guidance logics applied on the A350 aircraft below FL 100. On A350 aircraft, the FMS gives priority to the profile over
the speed when below FL 100.
For these three reasons, and to ensure that the FMS is consistent during descent, Airbus decided to change the philosophy of the DES mode. With the
new DES mode, the vertical profile has priority over the speed.
B. What are the Changes of this New DES Mode?
This section aims to provide a summary of the changes of the new DES mode. This section should be used to complement the information provided in
the Flight Operations Manuals (FCOM and FCTM).
With the NEW DES mode, depending on the position of the aircraft with regards to the FMS computed profile (on path, above path, below path) the
guidance logics and displays are different:
1) When the Aircraft is on the Descent Path Computed by the FMS
The aircraft is considered as “latched” on the profile (the latch symbol appears next to the VDEV symbol). In this situation, the following applies:
-
The elevators adjust the pitch to enable the aircraft to stay on the FMS-computed path
The A/THR orders a nominal engines thrust at IDLE
No speed margins are displayed:
o In the case of deceleration below the FMS speed target, the A/THR adjusts the thrust to maintain the FMS speed target
o In the case of acceleration above the FMS speed target, the A/THR keeps the thrust at IDLE and the aircraft remains on path. In such situation, once VMO minus
5kt or MMO minus 0.01 is reached, the aircraft will pitch up and leave the profile. The AP protections ensure that the aircraft does not exceed VMO/MMO.
When the aircraft is latched on the profile, the flight crew is responsible for the speed management. Therefore, the flight crew should use either of the
following to manage the aircraft speed:
-
Use of speed brakes (if already at IDLE and speed target cannot be maintained), or
Revert to another vertical mode (V/S, OPEN DES). In this situation, profile tracking will no longer be ensured and ALT CST may be missed.
The MORE DRAG or EXTEND SPD BRK FMA messages are displayed only if an altitude or speed constraint is predicted as missed by the FMS.
2) When the Aircraft is above the Descent Path Computed by the FMS
The aircraft converges towards the FMS-computed profile. In this situation, the following logics applies:
-
The speed margins are displayed
The descent rate of the aircraft increases until the aircraft reaches the upper limit of the speed margin
The elevators adjust the pitch to accelerate to the upper limit of the speed margins (THR IDLE appears on the FMA).
The MORE DRAG or EXTEND SPD BRK message will only be displayed if an altitude or speed constraint is predicted as missed by the FMS.
When the aircraft reaches the capture zone of the FMS-computed profile, the aircraft is latched to the profile (aircraft on the descent path).
3) When the Aircraft is below the Descent Path Computed by the FMS
The aircraft converges towards the FMS-computed profile at a constant descent rate:
-
This descent rate is equal to - 1 000 ft/min above the SPD LIM altitude
This descent rate is equal to - 500 ft/min below the SPD LIM altitude.
In addition, the following logic applies:
-
The speed margins are not displayed
The A/THR orders the required engine thrust to maintain the target speed with the constant descent rate (MACH or SPEED appears on the FMA).
When the aircraft reaches the capture zone of the FMS-computed profile, the aircraft is latched to the profile (aircraft on the descent path).
C. Operational Considerations Associated with the New DES Mode Logic
1. Considerations about the Descent Phase
The flight crew should consider the descent phase from both a strategic and tactical view.
The strategic part starts before the Top of Descent (T/D). During this phase, the FMS computes an optimized descent trajectory (including speed
profile), based on all available assumptions:
-
Lateral path to the runway
Speed (ECON speed or fixed speed profile)
Weather (e.g. wind profile).
A precise insertion of all the above inputs allows an accurate guidance on the FMS DES profile throughout descent, with almost no pilot intervention.
It is therefore important to update the descent hypothesis when changes in the corresponding parameters occur. This update should be done before
Top of Descent, but also during the descent phase. This allows the FMS to recompute the vertical profile, thus optimizing the descent in an iterative
way.
In real operations, some changes that occur during descent will require a tactical response from the flight crew (e.g. subsequent to ATC intervention).
In fact, ATC requests have priority over descent optimization. But once ATC requests are complied with, there is always a benefit to update the FMS
descent parameters and to try to join the new optimized profile.
There are many possibilities, and clearly not one single course of action for the flight crew to apply. However, the flight crew has a full set of solutions
to control of the speed (e.g. alternative vertical guidance modes: OP DES, V/S, and the speed brakes). Analysis of the situation (that includes ATC and
other traffic) is necessary to select the most appropriate solution.
For example, one significant new characteristic of the DES mode is that the vertical profile now has priority over the speed. This is indicated by the
“bracket symbol” on VDEV. This doesn’t mean that the pilot needs to stay in DES mode (on FMS profile) at all times, but he can elect to do so if
deemed necessary.
In addition to the above consideration, the below consideration should be highlighted with the new DES mode:
2. Overspeed Protection
The following applies when the aircraft is latched to the profile and the speed increases to reach the protection limits (VMO minus 5 kt or MMO minus
0.01). In this case, a reversion occurs:
-
The autopilot orders a pitch-up in order to decelerate and avoid an overspeed situation
The guidance logic changes (from “aircraft on the descent path” to “aircraft above the descent path” )
Once the aircraft speed reaches the upper limit of the speed margin (displayed if the aircraft is above the descent path) the aircraft resumes the descent without exceeding
the upper speed limit of the speed margins
When the aircraft reaches the profile, the guidance logic changes (aircraft is on the descent path computed by the FMS).
3. Constraints Management
With the new DES mode, the optimized descent path takes into account all the constraints of the descent.
If the FMS predicts an altitude or speed constraint to be missed, the FMA message MORE DRAG / EXTEND SPD BRK will be displayed.
In the case of an ATC speed restriction while in DES mode, if the flight crew sets the new speed target on the FCU and reverts to selected speed mode
the aircraft may not be able to decelerate without a flight crew intervention. The aircraft will remain latched to the profile. In this situation, the flight
crew has the following options:
a) If the flight crew assesses that the FMS profile is still meaningful (for example considering the sequence of aircraft ahead), then he may elect to stay in DES mode, and use
speed brakes as necessary to reach the new speed target.
b) If the crew assesses that the FMS profile is no more valid, he will probably elect to revert to OPEN DES mode (thus giving priority to speed), waiting for the next steps.
In this situation, once speed compliance with ATC is achieved, the flight crew should consider the update of the FMS parameters (in this case DES Speed), to compute a
new FMS profile taking into account the ATC speed.
He can then return to DES mode to converge to the new profile (from above or below).
4. Effect of QNH Changes during Descent
When the flight crew changes from the QNH STD to the QNH BARO setting, the FMS may order a decrease in pitch target (pitch down). This occurs if
the QNH BARO reference is above 1 013 hPa. This is due to the change of the QNH reference.
In this situation the flight crew may observe an increase in the speed of the aircraft due to the pitch down order transmitted by the FMS.
In this case, the flight crew is expected to manage the aircraft speed, in order to avoid an excessive acceleration of the aircraft.
5. IDLE Factor Considerations
For aircraft embodied with the FMS2 Release 2 H3 (for A320 Family) or P6 (for A330) and with the Descent Profile Optimization (DPO) the IDLE factor
should be set to 0 in order to avoid the computation of a steep descent path.
Operators should adjust the IDLE factor depending on the in-service monitoring.
Any negative value of this factor is prohibited.
Survey for the Flight Operations and Training section
Annex
General Information
Potential impact: Fuel Consumption / Saving, Specific Operations - ETOPS/EDTO/Others
Key information:
Solution benefit:
First issue date: 27-FEB-2020
Issue date: 27-FEB-2020
Last check date: 27-FEB-2020
Technical parameters
ATA: 22-83
A/C type/serie: A318, A319, A320, A321, A330
Engine:
Engine manufacturer:
Fault code/ECAM
warning:
FIN:
Part Number:
Supplier:
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