Understanding Back Up Speed and Altitude Indication on the A320
- Sunil Rawat
- 35 minutes ago
- 2 min read
The backup speed indication relies on the adherence to an Sb of UAMM (Unreliable Airspeed Mitigation).
We will discuss both the pre and post SB indications.
Pre SB Modification Indications
When the air data is unreliable, meaning the ADR cannot provide accurate speed and altitude readings.
The ADR’s are switched off which displays the back up speed scale on the left side of the PDF in the following conditions
Back up speed function is installed
All ADR’s are off and BUSS ( Back Up Speed Scale) push button is not installed or BUSS pushbutton is installed and activated
Angle of Attack (AOA) is permitted
The back up speed scale gets inputs as follows
AOA valve from the IRS
Slat/Flap config from the SDAC
ADR’s OFF status from the FWC
The indications are as follows
Red lower area shows the stall speed with a white “slow” indication
An amber and red lower area between VLS and VSW
Green area between VLS and VFE with normal load margin
An amber and red area between VFE with normal load margin and VFE with decreased load margin
Red upper area above VEF with decreased load margin with the white “Fast” indications.
Current speed indicated by yellow line.
Post SB Modification Indications
Post the SB modifications when the ADR are not reliable in other words the ADR cannot give the speed and altitude data the DBUS(Digital BackUp Speed) scale come into view when you manually press the BKUP SPD/ALT pushbutton.
The FAC uses AOA, aircraft weight and aircraft configuration to calculate the back-up speed.
The DBUS is similar to the normal speed scale displayed on the left side of the pdf with the following condition
Last digit of the speed and mach number is shown by dashes because the value is not accurate.
When the crew pushes the BKUP SPD/ALT in air or on ground and the back-up speed is not available or not reliable a red SPD flag is shown at the location of the nominal speed scale.
Altitude scale
In both pre and post mod cases back up GOS altitude scale is shown when
when the three ADR are OFF and BUSS pushbutton is not installed or BUSS is manually activated
GSP is installed
BUSS is installed
GPS altitude is provide by IRS and ADR works in monitoring functions the back up altitude scale is shows with following characteristics
Last two digits are dashed with two thin amber lines to indicate loss of accuracy between GPS altitude and Barometric altitude.
White “GPS ALT” is displayed under the scale instead of QNH reference.
White GPS indication at the right side of the altitude drum.
No barometric reference are available but only GPS altitude and radio altimeter.
