and the thrust levers will advance
Are you talking about an Airbus?
If the A/T is disarmed, with insufficient thrust to maintain the approach path, then the jet will stall.
The Airbus can stall for example in severe turbulence but on the average weather day with ATHR off and the thrust levers at idle as the speed passes below Vls, towards Vs, Alpha prot will be met and Alpha Floor will kick in giving toga thrust.
What is T/H? By deselecting speed do you mean physically closing the thrust levers or going managed? What is the bar on the speed button? I don't think you are talking about Airbus.
Please excuse me if I am wrong.
Please read the following taken from Airbus FCOM:
ALPHA-FLOOR PROTECTION
Alpha-floor protection automatically sets the thrust at TOGA thrust, when the aircraft reaches a very high angle of attack.
The Flight Augmentation Computer (FAC) generates the signal that triggers the alpha-floor mode. This, in turn, sets TOGA thrust on the engines, regardless of the thrust lever positions
(1.22.30)
The FAC sends this signal when the angle of attack is above a predetermined threshold, that is a function of the configuration.
In CONF3 and CONF FULL, this threshold decreases as a function of the aircraft deceleration rate (down to - 3°).
Alpha-floor is available from lift-off until the aircraft reaches 100 feet RA in approach.
Note : - Alpha-floor is lost, when one of the following combinations of failures occurs :
SFCC1 and FAC2, or
SFCC2 and FAC1, or
Both FCU channels, or
1 EIU, or
Both FMGCs.
- Alpha-floor is lost under alternate or direct flight control law.
- Alpha-floor is lost in engine-out, when slats/flaps are extended.
A/THR ACTIVATION
The A/THR is active when it controls thrust or speed. The position of the thrust lever determines the maximum thrust that the A/THR system can command (except in α-floor condition).
The A/THR
being armed
, is activated :
- when the pilot sets both thrust levers between the CL and IDLE detents (two engines operative)
- when the pilot sets one thrust lever between the MCT and IDLE detents (one engine inoperative).
The A/THR being disconnected, is activated when the pilot pushes the FCU pushbutton while the thrust levers are within the active range, including IDLE position.
Note : When the pilot sets both thrust levers to IDLE position, the A/THR disconnects but, if the pilot pushes the A/THR pushbutton of the FCU, he will simultaneously arm and activate the autothrust. Due to the thrust levers position, IDLE thrust will be maintained.
-
when ALPHA FLOOR is activated, regardless of the initial status of A/THR and the position of the thrust levers.