The design intent of the autopilot/flight director and autothrottle Alpha Floor operation is to provide an automatic limit on the minimum speed available for airspeed control. Alpha floor protection is incorporated into the autopilot system which sets the minimum speed floors in the autothrottle and autopilot/flight director, which are active when flying a speed mode on the autothrottle (A/T) or, a speed mode on the autopilot/flight director (e.g. LVL CHG or VNAV SPEED). So, the autopilot/autothrottle Alpha Floor functionality is synonymous to minimum speed protection (see attached FCOM excerpts).
When the autopilot is controlling speed (such as when operating in level change or VNAV speed pitch modes) and if the autopilot detects that the airplanes airspeed is approaching the minimum speed (whose value is either the minimum maneuver speed provided by the stall management computer or the low buffet speed provided by the flight management computer) a minimum speed reversion may occur. If the minimum speed reversion does occur, there may be some visual cues that indicate this activity is present:
(a) A is shown next to the leftmost digit of the MCP speed, to indicate Alpha mode and a speed reversion is occurring.
(b) If the airplane was in a climb/descent, the airplane may be commanded by the autopilot to pitch the airplane in an opposing direction in order to recover airplane speed above the minimum speed. (This is referred to the minimum speed reversion)
When the autothrottle is controlling speed (such as in cruise), if the autothrottle detects the airplanes airspeed is approaching the minimum speed (whose value is the same as described above, with exception for airplanes that have the short field performance package and a 737-900ER airplanes, these airplanes use the greater of the stall management minimum maneuver speed and the FMC VREF speed), a minimum speed reversion may occur. If the reversion does occur, there may be some visual cues that indicate this activity is present:
(a) A is shown next to the leftmost digit of the MCP speed, to indicate Alpha mode and a speed reversion is occurring.
(b) The autothrottle will command advancement of the thrust levers in order to recover airplane speed above the minimum speed. (This is referred to the minimum speed reversion)
雷达卡
提升卡
置顶卡
沉默卡
喧嚣卡
变色卡
千斤顶
照妖镜
