Control your Speed… During Descent, Approach and Landing

As for the previous flight phases, Green Dot, S and F speeds guide the flight crew during descent and approach phases.

Green Dot (GD) speed

• Definition

GD speed (fig.1) is the engine-out operating speed in clean configuration.
It provides an estimate of the speed for best lift-to-drag ratio.

GD speed is the managed speed target in CONF CLEAN when the FMS approach phase is activated. It is also the recommended speed to extend flaps to CONF 1 and for a holding in clean configuration.

• How is GD speed determined?

The Auto Flight System (AFS) computes GD speed using the aircraft weight, based on the Zero Fuel Weight (ZFW) entered in the FMS during flight preparation, and the pressure altitude. The GD formula has been set up so that the resulting airspeed provides the best lift-to-drag ratio for a given altitude and aircraft weight, in clean configuration with one engine out.

In some phases of flight, GD is computed to minimize drag and thus, the fuel consumption (for example during the HOLD phase).

S and F speeds

• Definition

S speed:
In approach phase, S speed is the managed speed target, when in CONF 1 or 1+F. It is the recommended speed to select CONF 2.

It is displayed as a green ‘‘S’’ on the PFD airspeed scale (fig. 2) and shown only when the Slats/Flaps control lever is on position 1 (CONF 1 or 1+F).

F speed:
In approach phase, F speed is the managed speed target, when in CONF 2 or 3. It is the recommended speed to select CONF 3 when in CONF 2, and to select CONF FULL when in CONF 3.

It is displayed as a green ‘‘F’’ on the PFD airspeed scale (fig. 3) and shown only when the Slats/Flaps control lever is in CONF 2 or 3 during the approach phase and go-around.

During descent, approach and landing, the operation of the aircraft is also framed within limit speeds. Their indication on the PFD or on a placard enables the flight crew to easily identify the aircraft speed envelope.

V_MAX: Maximum speed

• Definition

V_MAX is the maximum speed defining the aircraft’s flight envelope. V_MAX is equal to:

– V_MO/M_MO in clean configuration with landing gears up.
– V_FE in high lift configurations with landing gears up.
– V_LE/M_LE in clean configuration with landing gears down.
– The minimum of V_FE and V_LE/M_LE in high lift configurations with landing gears down.

On the PFD airspeed scale, it corresponds to the lower end of the red and black strip (fig.4).

V_MO/M_MO: Maximum Operating speed/Mach number

• Definition

In CONF CLEAN, V_MO/M_MO is the higher limit of the aircraft speed envelope.

• How is V_MO /M_MO determined?

V_MO/M_MO is derived from the design limit Mach/speed V_D/M_D by applying a margin related to aircraft dive characteristics. For more details on V_MO /M_MO determination, refer to the Safety first issue 21 dated January 2016.

VFE: Maximum speed with the Slats/Flaps extended

• Definition

VFE is the maximum speed with the slats or flaps extended.

There is one VFE per configuration.

The VFE is displayed on the airspeed scale of the PFD as the VMAX (fig. 5) when the Slats/Flaps are extended, based either on the Slats/Flaps lever position or the actual Slats/Flaps position (fig.6).

(fig.6)
Table showing source of information
for V_FE display on PFD

The V_FE of each Slats/Flaps configuration is also available on the speeds placard in the cockpit.

• How is V_FE determined?

The V_FE is based on the structural limit speed of the Slats/Flaps configuration plus a margin. It is a fixed value associated to the aircraft model.

VFE NEXT

• Definition

The aim of the VFE NEXT is to remind the flight crew the maximum speed at which they can extend the next Slats/Flaps configuration during approach.

VFE NEXT is displayed on the airspeed scale of the PFD (fig. 7).

VFE NEXT is displayed in flight, below FL200 (FL220 on A350).

• How is VFE determined?

VFE NEXT is the VFE of the next Slats/Flaps configuration.

V_LE /M_LE: Landing gear Extended speed/Mach

• Definition

V_LE/M_LE is the maximum speed/Mach at which the aircraft can fly with the landing gear extended.
V_LE /M_LE is displayed on the airspeed scale of the PFD as the V_MAX when the landing gear is extended as long as V_LE /M_LE is lower than V_FE. It is also available on the speeds placard in the cockpit (fig. 8).

• How is V_LE determined?

V_LE is determined to provide sufficient flight domain with landing gear extended, taking into account the structural limitation of the landing gear and landing gear doors.

VLO /MLO: Landing gear Operating speed/Mach

• Definition

VLO /MLO is the maximum speed/Mach to operate (both extend and retract) the landing gear.
VLO /MLO is not displayed on the PFD; it is available on the speeds placard in the cockpit (fig. 8).

• How is V_LO /M_LO determined?

V_LO/M_LO is determined to provide sufficient flight domain for landing gear extension/retraction, taking into account the structural limitation of the landing gear and landing gear doors.

VLS: Lowest Selectable Speed

• Definition

VLS is the lowest selectable speed for the autopilot and the autothrust. Even if the selected target speed is below VLS, the A/THR will maintain VLS as a minimum. VLS is indicated by the top of the amber strip on the PFD airspeed scale (fig. 9).

VLS (of selected landing configuration: CONF 3 or FULL), is also displayed on the FMS APPR page.

• How is V_LS determined in descent and approach?

For descent and approach flight phases, V_LS of Fly-By-Wire aircraft is obtained using the Stall speed demonstrated during flight tests (VS1G) of the corresponding configuration, multiplied by a factor of 1.23. On A320 family aircraft, the factor may be increased for some Slats/Flaps configurations for manoeuvrability improvement and/or to increase margins with protection speeds. V_LS is always greater or equal to the Minimum Control Speed at Landing (V_MCL).

Since Speedbrakes extension increases V_s1g, V_LS increases when the speedbrakes are extended.

ECON DES speed/Mach

• Definition

ECON DES speed/Mach is the optimum descent speed/Mach to lower the direct operating costs of the descent.

• How is ECON DES speed/Mach determined?

ECON DES speed/Mach is computed by the FMS based on the Cost Index (CI), cruise FL and on the aircraft weight.

VAPP: Approach speed

• Definition

VAPP is the final approach speed when the Slats/Flaps are in landing configuration and the landing gears are extended.

VAPP is displayed in the FMS PERF APPROACH page.

• How is VAPP determined?

The VAPP can be computed by the AFS or inserted manually by the pilot through the FMS PERF Page.

VAPP is based on the VLS of the landing configuration. For Airbus aircraft, in normal operations, the VAPP is defined by:

AFS Computation of V_APP

When computed by the AFS, the APPRoach CORrection (APPR COR) used by the AFS is:

Excepted on some older A320 aircraft where the APPR COR used by the AFS is 1/3 Headwind + 5kt, limited at 15kt.

VAPP Computation by the Flight Crew

The flight crew can chose to insert any V_APP by computing its own APPR CORR as follows:

During autoland or when A/THR is ON or in case of ice accretion or gusty crosswind greater than 20kt, V_APP must not be lower than V_LS + 5kt.

V_APP in the case of a system failure

In the case of a system failure during flight, the flight crew computes a new VAPP value:

∆V_REF is the speed increment related to the failure to counter associated handling qualities issues and/or increased stall speed.

APPR COR depends on the ∆V_REF, the ice accretion, the headwind value and the use of autothrust.

For more information on the determination of V_APP with failure by the flight crew, refer to the Flight Crew Techniques Manual (FCTM).

The descent profile computed by the Flight Management System (FMS) is a very efficient and useful tool to help the flight crew in managing the aircraft energy during the descent and approach phases.

The FMS can compute an accurate and optimized descent profile, provided the descent winds have been entered in the FMS during the descent preparation, and provided the PERF and IDLE factors are tuned according to the actual aircraft performance.

To locate the Top of Descent (T/D), the FMS computes the descent profile backwards from the Missed Approach Point (MAP), assuming the aircraft is stabilized at its VAPP 1000ft above the runway elevation, up to the T/D.

The FMS assumes the use of managed speed and accounts for all the speeds and altitude constraints coded on the FMS flight plan. Refer to (fig.10).

During the descent, approach and landing the managed speed is equal to either:

• ECON DES speed or the descent speed manually entered in the PERF DES page of the FMS, or
• The speed constraint, or
• The manoeuvring speed of the current aircraft configuration, or
• V_APP.

(fig.10)
Typical managed descent profile
(without Continuous Descent Approach (CDA) function)

The managed descent mode guides the aircraft along the FMS computed vertical flight path. The DES mode is preferred when conditions permit since it ensures the management of altitude constraints and reduces the operating cost when flying at ECON DES speed.

The DES mode is only available when the aircraft flies on the FMS lateral flight plan, i.e. when the aircraft uses the NAV horizontal guidance mode.

• On idle segment

In DES mode with managed speed the elevators adjust the pitch to enable the aircraft to stay on the computed path and the A/THR commands idle thrust.

The AFS allows the aircraft speed to vary in a range of +/- 20 knots around the managed speed target (+5 kt or -20 kt in the case of a speed constraint), limited to VMAX -5kt to stay on path:

 – If the speed decreases down to its lower limit, the A/THR will increase the thrust

 – If the speed reaches its upper limit, the aircraft will leave the path to maintain the upper limit speed.

• On geometric segment

On the geometric segment, the A/THR adapts thrust to maintain the managed speed target.

• Use of speedbrakes in DES mode

The use of speedbrakes in DES mode must be limited to the situation where there is either a strong tailwind or much less tailwind than expected, and the aircraft diverges from the profile. The flight crew should increase drag by extending the speed brakes (fig.12).

(fig.12)
Speed range principle during the idle segment of a managed descent.

As a visual clue the ND displays the intercept point at which the aircraft will reach the profile with half speed brakes extended. If the flight crew does not extend the speed brakes the interception point will continuously move forward along the flight plan. If the interception point gets closer to an altitude constraint, a ‘‘MORE DRAG’’ or EXTEND SPD BRK’’ message is displayed on the FMA and on the MCDU scratchpad/MFD.

Note: The speed range does not apply below FL 100 for A350 and A330 equipped with HONEYWELL P5 FMS 2 release 2. In this case, the aircraft stays on the path and the flight crew must monitor the speed and use speedbrakes when appropriate.

In OP DES mode, the AFS commands idle thrust and the elevators adjust the pitch to maintain the target speed (managed or selected).

The OP DES mode can be used to increase or reduce the descent slope. In OP DES, the flight crew adjusts the target speed to modify the descent path (fig.13).

(fig.13)
Adjustment of the selected speed to modify the descent path.

The flight crew can use the V/S mode during descent to get accurate guidance to recover the intended flight path by adjusting the V/S using the V/S selector.

In V/S mode, the AFS adjusts pitch and thrust to maintain the selected vertical speed and the target speed.

V/DEV Indication

When in NAV lateral mode, the flight crew uses the ‘‘yoyo’’ indication to estimate its position relative to the FMS computed path. The Vertical deviation (V/DEV) value is provided on the FMS PROG page (A320/A330/A340) (fig.14) or PERF DES page (A380/A350).

(fig.14)
example of V/DEV indication on the PFD and on the FMS PROG page (A320)

Energy Circle

When in HDG or TRK lateral mode, the ND displays the energy circle, and when the aircraft is within 180 NM of its destination. It provides a visual cue of the minimum required distance to land, i.e. the distance required to descend in a straight line from the current aircraft position at its current speed down to the altitude of the destination airport at approach speed. The descent profile used to compute the distance takes into account speed limits, the wind, a deceleration level off segment and a 3° final approach segment (fig.15). In other words, if the destination airport is inside the energy circle, the flight crew needs to lose some energy by extending the speed brakes and/or modifying the aircraft’s trajectory, and/or increasing speed during descent.

(fig.15)
Energy Circle Computation Principle

Level-off Arrow

Another useful tool to use during descent is the level-off arrow provided by the FMS. It provides an indication to the flight crew of where the aircraft will reach the altitude selected on the FCU (fig.16). A blending of actual wind conditions and the values for winds entered in the FMS are used to improve the accuracy of the computation. If in selected descent, the flight crew can adjust the speed of the aircraft to adapt the descent path or V/S to the situation.

(fig.16)
Level-off Arrow Computation Principle

Manual Flight at Crossover Altitude

When in descent close to MMO, if in manual flight (AP off), the risk of exceedance of the VMO at the crossover altitude is high. In this situation, the flight crew should know its crossover altitude and anticipate the switch to speed by reducing the aircraft pitch on approaching the crossover altitude.

Impact of Wind Direction

Flight crews should pay particular attention monitoring their speed in descent close to VMO/MMO and when flying close to the wind direction (fig.17). The impact of a wind gradient can be significant and bring the aircraft beyond VMO/MMO.

(fig.17)
Impact of wind direction

When reaching the Initial Approach Fix (IAF) the flight crew should have a defined approach strategy based on the selected type of approach: a choice of the guidance mode that will be used and the associated approach technique (decelerated approach or early stabilized approach). The flight crew is then ready to start the key phase of the approach in terms of speed management: the Intermediate Approach phase.

The Intermediate Approach phase starts at the deceleration point or earlier, if the flight crew activates manually the approach phase of the FMS.

The aircraft reduces speed from its last descent speed, generally 250kt, corresponding to the speed limit below FL100. The aircraft slows down to green dot speed and then slows further to the manoeuvring speed for the various Slats/Flaps configurations. It finally ends up at VAPP at or before the stabilization point (decelerated approach) or at or before the Final Descent Point (early stabilized approach) depending on the approach strategy.

Airbus recommends using A/THR in managed speed to reduce crew workload. If the flight crew needs to use selected speed, they should revert to managed speed when out of the ATC speed constraint because it will ease the deceleration handling.

The deceleration rate of the aircraft varies with its weight. A heavy aircraft will not decelerate as quickly as a lighter aircraft.

Whatever the Approach technique chosen by the flight crew (decelerated or early-stabilized approach), respecting stabilization criteria is key for a successful landing. Refer to the Flight Crew Operating Manual FCOM/PRO-NOR-SOP-18-A Stabilization Criteria.

When close to the ground, the wind can change, especially when in gusty conditions, and have a direct impact on the aircraft speed. As a consequence, monitoring of airspeed is crucial during final approach and landing to avoid:

• Runway undershoot, hard landing or tail strike if the aircraft speed becomes too low, or
• Runway overrun if the speed becomes too high.

If gusty conditions are expected at the destination airport, the flight crew can add an appropriate margin to the VAPP and manually enter the new VAPP in the FMS PERF APPR page.

Airbus recommends the use of autothrust during final approach to reduce crew workload and benefit from the Ground Speed Mini function (GS mini).

Manual Landing

In Normal or alternate law, the flight controls maintain the aircraft load factor demand (flight mode), if there is a wind change, the aircraft will maintain its path causing the speed to increase or decrease. This cannot be perceived by a pilot while looking outside, as the trajectory will not change (the aiming point will not move). Therefore, with autothrust disengaged, the flight crew must carefully monitor the speed as to detect any speed change. The role of the Pilot monitoring (PM) is key in this situation, especially when close to the ground.

Stabilisation criteria

Flight crews must respect the stabilisation criteria provided in the FCOM Standard Operating Procedures (SOPs). These criteria ensure a safe approach and landing. The aircraft must be at approach speed with stabilized thrust at the stabilisation height (1000 ft AGL in IMC, 500 ft in VMC or according to airline’s policy) If it is not the case, the PM should make a callout and a go around must be initiated if the flight crew assesses that the stabilisation can’t be obtained prior landing.