What is this in the primary flight Display ?
It is a digital Angle of Attack Indicator. From the Boeing 737 FCOM:
Digital AOA Readout (white)
Indicates digital AOA value to the nearest 0.2 degrees. When on the ground and ground speed less than 80 knots, the readout is fixed at 0.0 degrees.
Stick Shaker Indicator (red)
Indicates point at which stick shaker activation occurs for existing flight conditions. Blank if AOA signal is invalid.
Analog Needle (white)
Indicates analog AOA value.
- needle travel is limited to a range of -6 degrees and +21 degrees
- fixed at 0.0 degrees when on the ground and ground speed is less than 80 knots.
Approach Reference Band (green)
Indicates appropriate range of approach AOA for a Vref(xx) + 5 approach.
- displayed when in normal or single engine landing flaps (15, 30, 40)
- moves with flap position
- inhibited on takeoff and initial climb.
Zero Degree Reference Line (white)
Indicates zero degrees angle of attack. Reference lines are displayed every 5 degrees from -5 degrees to +20 degrees.
(Boeing 737 NG FCOMv2 10.11.11 Flight Instruments, Displays - PFD/ND - Displays)
All Boeing aircraft have at least two alpha vanes to measure angle of attack, but the cockpit display shown above is an airline option, i.e. not every aircraft will display this dial:
There are two alpha vanes, one located on each side of the forward fuselage. The vanes measure airplane angle–of–attack relative to the air mass.
[Option - Angle of attack indicator]
The primary source of data for the AOA indicator on the PFD is supplied by the ADIRU, with the Stall Management Yaw Damper (SMYD) as the backup source. The source selection is automatic in the event of primary source failure. Slight differences between the Captain’s and FO’s indications may be noticed due to sideslip or vane installation errors. These differences could be as large as 2 degrees alpha.