What is the airbus “soft go-around” function and how does it work?

Definition

A Take-off/Go Around (TO/GA) is an autopilot/autothrottle setting that activates the take-off or go-around thrust. Usually located on the thrust levers, they can be activated by depressing a switch or by manually moving the thrust levers to the appropriate position (depending on the aircraft type).

Representative | Source

The switch is the same for both take-off (TO) and go-around (GA) modes and by pressing the TO/GA switches when the autothrottle is engaged, the thrust lever servo-actuators are activated and they advance the thrust levers at a preset rate to reach the position for take-off or go-around.

Illustrative | Pinterest

Take-off mode (TO)

Advancing the thrust levers to the TO/GA position during take-off, commands the engines to increase their RPM to the calculated N1 or engine pressure ratio (EPR). Since most aircraft today can perform a Reduced Thrust Take-off in which the power needed by the engines for take-off is pre-determined by the crew based on several factors such as runway length, wind speed, temperature, and the weight of the aircraft, it additionally prevents wear and tears on the engines by only using as much power as is actually required to meet takeoff and climb obstacle clearance.

Illustrative | SKYbrary

Go-Around mode (GA)

Initiating the transition of the autopilot/autothrottle system from approach mode to go-around mode is yet another very important function of the TO/GA mode.

The “all engines” go-around is a standard procedure which is executed if the captain deems it necessary and it may be due to several reasons- unstable, unable to land in the touchdown zone, incorrect configuration, obstacle on the runway (aircraft, vehicle, animal), or aircraft controllability issues.

Illustrative | TheJournal.ie

A general rule of the thumb is that “if the aircraft isn’t on the ground in the first third of the runway” — go around.

Although a standard procedure, it tends to produce high accelerations due to TOGA thrust.

In-service experiments have shown that a lower thrust can still be sufficient to perform a safe go-around, provided both engines are running.

Go-Arounds(s) and Somatogravic Illusion (SI)

Go-arounds are usually performed when an aircraft’s weight is well below the Max Landing Weight and when flying at low speeds close to the Approach speed. These conditions are the recipe for unusually strong longitudinal acceleration which may ultimately lead to Spatial Disorientation (SD) of the flight crew caused by a Somatogravic Illusion (SI)- a leading factor suspected to have caused numerous fatal accidents.

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Airbus, in response, created a function which would allow the flight crew to perform a go-around with a reduced thrust, adapted to the aircraft’s weight, speed and altitude.

So, what is it all about?

SOFT GO-AROUND FUNCTION (SGA)

Principle

In simple words, the SGA function provides a lower than TOGA initial thrust level, thereby ensuring a reduced acceleration and requirement to pitch up and a lower but constant final rate of climb regardless of the aircraft weight, speed, altitude and Slat/Flaps configuration.

Airbus has meticulously designed the SGA climb capability to be able enough to deal with the world’s most demanding missed approaches. The target rate of climb is either 2000 or 2300 ft/min, depending on the aircraft model.

However, it is to be noted that the Soft Go-Around function is only available when all engines are operating:

  • TOGA thrust must be used in case the go-around is performed with one engine inoperative
  • In the case of an engine failure during a soft go-around, again, TOGA thrust is to be selected

Working

Based on the environmental conditions, the aircraft weight, altitude, speed and slats/flaps configuration, the Auto Flight System (AFS) via the PRIMs (A350/A380) or FMGECs (A330) or FMGCs (A320) computes a thrust target that will enable the aircraft to climb at 2000 (or 2300 ft/min).

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This thrust target is then sent to the engine’s FADEC which will apply the optimized thrust as soon as the function is activated via the thrust levers.

On aircraft equipped with the SGA function, SGA is now fully part of the Standard Operating Procedures (SOPs). The FCOM and QRH (quick reference handbook) are updated accordingly.

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On aircraft not fitted with the Soft Go-Around function, if the TOGA thrust is not required for a go-around, the flight crew can apply the procedure introduced in the FCOM/FCTM.

What about mixed fleets?

Due to its fleet-wide availability status and rather a recent introduction, operators will likely have to deal with aircraft that may or may not have SGA featured in them. In such cases, it becomes imperative to make sure that the flight crew is aware of the SGA / Non-SGA capability of the aircraft they are flying.

Communication is key here. Although in any case the Go Around initiation is always done by setting the thrust levers to the TOGA detent to engage the SRS guidance mode and the GO-AROUND phase of the FMS (Flight Management System). Subsequently, depending on the aircraft’s SGA capability and on the possibility to use a reduced go-around thrust, the SGA function may be used.

Representative | safetyfirst.airbus

SOURCE(s)

COVER: The Balance Careers

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