Low Speed Awareness System

James Albright

Updated: 2020-07-15

What is it?

[G500 Ground and Flight Operations, p. 97] The low-speed awareness (LSA) display is designed to detect and prevent low speed conditions arising from a wide variety of causes. The LSA system is displayed, and is always active, except when on the ground below 100 KCAS, or when failures occur.

LSA Caution Speed (Yellow Band)

[G500 Ground and Flight Operations, p. 97] The top of the yellow band indicates the minimum speed at which a 30 or 40-degree bank turn (at a constant flight path angle) can be maintained using full aft stick, while in AOA limiting mode (0.95 NAOA). A 40-degree bank turn is normally used -- a 30-degree bank is used at FL350 and above, or in special cases such as below 50 feet AGL, or during takeoff with an engine failure. The caution speed is calculated in the FCCs using smoothed values of current speed, load factor, and NAOA. The caution speed therefore changes with Mach number, sideslip angle, flap position, and anti-ice. The caution speed does not change significantly with load factor or AOA because these two effects generally cancel each other out. This results in a stable caution speed while maneuvering vertically or holding flight path angle during turns. When the airspeed drops below the LSA caution speed (in the yellow band), the airspeed and AOA digits turn yellow and the HUD will display "AIRSPEED LOW" text with flashing airspeed and AOA indications. If the condition persists for more than 2 seconds or the airspeed drops more than 5 knots below the LSA caution speed, an "Airspeed Low!" voice prompt is initially repeated twice and then once every 5 seconds until the yellow region is exited.

[G500 Ground and Flight Operations, p. 98] The top of the yellow band will normally be about 5 knots below VREF. The yellow band may be useful to quickly estimate VREF without any reliance on pilot FMS inputs. It provides an independent verification which is helpful in detecting VREF errors (e.g. if the FMS weight is incorrect for any reason). The yellow band predicts where the red band will be (with a .02 AOA margin) during a 40 (30) degree bank constant FPA turn. . . . The yellow band indicates turn maneuver margin.

LSA Warning Speed (Red Band)

[G500 Ground and Flight Operations, p. 97] The margin between current speed and the LSA red warning speed indicates proximity to stick shaker and, thereby, proximity to stall (accelerated stall with increased load factor). The red bar moves dynamically with the PLI and provides similar information. The PLI and the top of the red band will both be "touched" when the AOA reaches 0.97 and the stick shaker activates. The top of the red band estimates the stick shaker speed (.97 NAOA) at the current conditions if only airspeed and AOA were to be changed. . . . The red band indicates accelerated stall speed margin.

[G500 Ground and Flight Operations, p. 98] The red band is normally below the yellow band, but is not necessarily so. When maneuvering requirements are beyond those required for normal operation, the red band provides immediate feedback of changes to the accelerated stall speed and proximity to it.

Pitch Limit Indicator (PLI)

[G500 Ground and Flight Operations, p. 97] The PLI (Pitch Limit Indicator) provides a conformal, dynamic display of AOA margin to stick shaker (0.97 AOA). The angle between the FPM and the PLI is AOA margin in degrees. The PLI is shown on the PFDs, HUD, and standby displays when the AOA is 0.75 or greater.

[G500 Ground and Flight Operations, p. 98] The PLI directly correlates to the Red Band and is normally not displayed. The most basic use of the PLI is noticing whether it is displayed or not. If the PLI is not displayed, the AOA is less than 0.75. Since 0.95 AOA is available with full aft stick, then at least 20% of AOA range is still available. Similarly, 20% higher load factor would typically be available at a given speed. When maximum performance is required, the PLI may be used to monitor AOA and proximity to stick shaker activation. . . . The PLI indicates AOA margin.