What is TOGA LK in Airbus 320? How to come out of TOGA LK? I see it comes in Windshear Escape Manoeuvre. How is it related to Alpha Floor.
TOGA LK is short for Takeoff Go-around (Power) Locked.
Which means, due to some kind of situation (e.g. reaching Alpha Floor), the aircraft decided it'd be a good idea to spool the engines up to maximum thrust.
It will remain at that power setting no matter what the thrust levers were set to before the TOGA LK state was applied by the aircraft, hence "locked". Only once you move the thrust levers after TOGA LK was activated and turn off the A/THR, the aircraft will reduce the thrust to whatever is set on the thrust levers.
TOGA LK (TOGA thrust locked) is displayed on the FMA after an activation of the ALPHA FLOOR protection when the activation conditions are no longer fulfilled (A FLOOR is displayed during the ALPHA FLOOR condition). The idea is that the thrust does not automatically reduce again after ALPHA FLOOR because this could result in a situation where the speed decreases again. The procedure to get out of TOGA LK is described in the FCTM (Flight Crew Training Manual):
When the aircraft’s angle-of-attack goes beyond the ALPHA FLOOR threshold, this means that the aircraft has decelerated significantly (below ALPHA PROT speed): A/THR activates automatically and orders TOGA thrust, regardless of the thrust lever position.
The example below illustrates that:
- The aircraft is in descent with the thrust levers manually set to IDLE.
- The aircraft decelerates, during manual flight with the FD off, as indicated on the FMA.
When the speed decreases, so that the angle-of-attack reaches the ALPHA FLOOR threshold, A/THR activates and orders TOGA thrust, despite the fact that the thrust levers are at IDLE.
When the aircraft accelerates again, the angle-of-attack drops below the ALPHA FLOOR threshold. TOGA thrust is maintained or locked. This enables the flight crew to reduce thrust, as necessary. TOGA LK appears on the FMA to indicate that TOGA thrust is locked. The desired thrust can only be recovered by setting A/THR to off, with the instinctive disconnect pushbutton.
ALPHA floor is available, when the flight controls are in NORMAL LAW, from liftoff to 100 ft RA at landing. It is inhibited in some cases of engine failure.
(Airbus A320 FCTM - Operational Philosophy - AP / FD / ATHR, emphasis mine)
The TOGA LK state is not necessarily linked to a Windshear, but since ALPHA FLOOR is part of the Windshear escape strategy, it can certainly appear in this context. The FCTM describes the following strategy to escape a Windshear:
There are three efficient tools to assist the flight crew to escape:
- The alpha floor protection
- the SRS AP/FD pitch law
- The high angle of attack protection
When the alpha floor protection is triggered, the A/THR triggers TOGA on all engines. The FMA displays A FLOOR, that changes to TOGA LK, when the aircraft angle-of-attack has decreased. TOGA/LK can only be deselected by turning the A/THR off.
The SRS pitch mode ensures the best aircraft climb performance. Therefore, the procedure requests following the SRS pitch bar and possibly full aft stick, in order to follow the SRS orders and minimize the loss of height.
The high angle-of-attack protection enables the PF to safely pull full aft stick, if needed, in order to follow the SRS pitch order, or to rapidly counteract a down movement. This provides maximum lift and minimum drag, by automatically retracting the speed brakes, if they are extended.
(A320 FCTM - Supplementary Information - Adverse Weather, emphasis mine)
A floor is a protection activated in Airbus when the speed drops below VA-prot. The FAC then sends signal to auto thrust system to set max thrust available irrespective of the thrust lever position. The thrust gets locked in this position.
To get out of A floor, you reduce angle of attack. The speeds increases now rapidly as max thrust is set.
If you don’t reduce thrust quickly, you may breach flap limiting speed or Vmo.
To get out of TOGA LOCK
- Set thrust lever to TOGA
- Disconnect auto thrust by using instinctive push buttons on thrust levers
- Set thrust levers to Climb
- Rearm autothrust system by pushing auto thrust push button on FCU.