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Alpha Mach Indicator

The two alpha Mach indicators are located next to the attitude director indicators on panels F6 and F8. The AMIs consists of four tape meters displaying angle of attack ( alpha ), vehicle acceleration (accel), vehicle velocity ( M/vel ) and equivalent airspeed ( EAS ). The two units are driven independently but can have the same data source.

Alpha displays vehicle angle of attack, defined as the angle between the vehicle plus X axis and the wind-relative velocity vector (negative wind vector). Alpha is displayed by a combination moving scale and moving pointer. For angles between minus 4 degrees and plus 28 degrees, the scale remains stationary and the pointer moves to the correct reading. For angles less than minus 4 degrees or greater than plus 28 degrees, the pointer stops (at minus 4 or plus 28 degrees) and the scale moves so that the correct reading is adjacent to the pointer. The alpha tape ranges from minus 18 to plus 60 degrees with no scale changes. The negative scale numbers (below zero) have no minus signs; the actual tape has black markings on a white background on the negative side and white markings on a black background on the positive side.

The accel scale displays vehicle drag acceleration, which is the deceleration along the flight path. This is a moving tape upon which acceleration is read at the fixed lubber line. The tape range is minus 50 to plus 100 with a scale change at zero feet per second squared. Minus signs are assumed on the accel scale also; the negative region has a black background and the positive side has a white background.

The M/vel scale displays Mach number or relative velocity. Mach number is the ratio of vehicle airspeed to the speed of sound in the same medium. Relative velocity in this case is the vehicle airspeed. The actual parameter displayed is always Mach number; the tape is simply rescaled above Mach 4 to read relative velocity in thousands of feet per second (above 2,000 feet per second, Mach number = V REL /1,000). The M/vel scale is a moving tape from which Mach/velocity is read at the fixed lubber line. The scale ranges from zero to 27 with a scale change at Mach 4.

The EAS scale is used to display equivalent airspeed. On the moving-tape scale, equivalent airspeed is read at the fixed lubber line. The tape range is zero to 500 knots, and scaling is 1 inch per 10 knots.

Each scale on the AMI displays an off flag if the indicator malfunctions, invalid data are received at the DDU or a power failure occurs (all flags appear).

The air data source select switch on panel F6 for the commander and panel F8 for the pilot determines the source of data for the AMI and altitude/vertical velocity indicator. The nav position of the air data switch ensures that the alpha , Mach and EAS on the AMI are the same parameters sent to guidance, flight control, navigation and other software users; accel comes from navigation software.

The left, right position of the air data switch selects predetermined data from the left or right air data probe assembly after deployment of the left and right air data probes at Mach 3 for alpha, M/vel and EAS display. Accel is always derived from navigation software during entry. It is driven to zero during terminal area energy management and approach and landing.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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