Automatic Flight - Limit Modes

Automatic Flight Limit Modes

Autothrottle Limit Modes:

Pitch Limit Modes:

• FLAP LIM
• ALPHA
• SPD LIM

• FLAP LIM
• SPD LIM

AMM The minimum speed limit automatically becomes active when the A/T command permits the speed to go below the critical minimum speed (30% above stall speed [V2]). A reference AOA is computed from Mach and airplane configuration data. A load factor is added to compensate for the change in AOA due to body loads.

The TMC causes an ALPHA speed message when the airplanes speed is at or below Vref. The TMC calculates Vref as a function of AOA and then compares the calculated Vref with actual speed. This eliminates nuisance ALPHA speed messages when the airplane AOA is high.

Flap placards speeds, airplane maximum angle of attack and maximum speed limit are automatically monitored by the AFDS and TMC. The appropriate speed limit mode annunciation of FLAP LIM, ALPHA, or SPD LIM is displayed when a speed limit is approached and the MCP selected speed or FMC target speed is set to exceed a limit. When the limit mode is displayed, the limit speed becomes the reference speed for the Autothrottle and AFDS and the Fast/Slow Pointer.

When the AFDS is engaged in a speed mode (FLCH, GA), the speed limit monitoring is accomplished by the AFDS. When approaching a speed limit, the appropriate limit mode annunciation replaces the existing pitch mode.

On airplanes equipped with windshear guidance, the AFDS will not annunciate ALPHA when approaching maximum angle of attack speed, however, the alpha safe speed will be maintained by AFDS pitch and is the reference speed for the Fast/Slow pointer.

When the AFDS is not controlling speed, speed limit monitoring is accomplished by the TMC. When a speed limit is exceeded, the appropriate limit mode annunciation appears at the Autothrottle mode position. The speed limit mode annunciations may appear only when the autothrottles are engaged.

As installed

On airplanes with Fast/Slow indications, the speed limit mode annunciations may appear even when the autothrottles are not engaged.

The Fast/Slow Scale changes colour to amber and the Pointer flashes when a speed limit is exceeded (not applicable to airplanes equipped with speed tape).

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