The Airbus Back Up Speed Scale (BUSS)

[Remus T]

Regarding the unreliable airspeed on the newer Airbus model, FCOM says: If all ADRs provide unreliable data:
- Below FL250: Switch OFF the 3 ADRs to display the BUSS and fly the green area of the speed scale.
- Above FL250: Keep one ADR ON to prevent the display of the BUSS, and fly the aircraft using the Pitch/Thrust tables.

So what's the reason why we keep one ADR ON above a particular altitude (i.e. FL250) instead of displaying the BUSS?

Thanks.

[rudderrudderrat]

Hi Remus T,

Above FL 250, the Mach Number becomes significant and the BUSS doesn't know anything about Mach.

[nitpicker330]

Above FL 250 Airbus assume that the problem is most likely Icing and will go away below FL250 when the ice melts and the ADR's return to normal.

If below FL250 the problem still exists then its not icing and most likely not going to be fixed so turn them all of to show the BUSS.

[nitpicker330]

The BUSS doesn't know anything about IAS either, only AOA.

[rudderrudderrat]

Hi nitpicker330,
The BUSS doesn't know anything about IAS either

The Stall warning threshold Alpha changes with Mach Number.
The BUSS green area (safe Alpha) cannot be the same at sea level and FL 350. Therefore the recommended MAX FL for BUSS use is FL250.

See page 89 of http://www.bea.aero/docspa/2009/f-cp090601.en/pdf/f-cp090601.en.pdf

"The disabling of this warning was probably due to the fact that, between 13.4 and 15.5 and then between 17 and 19, and possibly at other times, the three Mach values were abnormally low (three Pitot probes iced up). The warning triggering threshold then suddenly increased to values of about 10°, much greater than the recorded angles of attack, which led to the warning stopping."

Edit.
See Page 7 of http://www.ukfsc.co.uk/files/Safety%20Briefings%20_%20Presentations/Airbus%20Safety%20First%20Mag%20-January%202011.pdf
"For a given configuration and at a given Mach number, a wing stalls at a given Angle of Attack (AoA) called AoA STALL. When the Mach number increases, the value of the AoA STALL decreases."

[Kimon]

Above FL250 Airbus says it does not "recommend" to use BUSS but that does not mean "do not use" and that it is non-functional.
If one is in an AF447 situation, you are better off using BUSS regardless of FL.
One thing less to worry about with pitch and thrust tables in moments of stress.
All you have to do is engage BUSS at the first sign of trouble, ram the ND to below FL250 and voilà!

[DozyWannabe]

That said, if one were being extra-cautious, would it not be prudent to use pitch and power to stabilise, then descend to FL250 and use BUSS if warranted?

[rudderrudderrat]

Hi DW,
I agree.
I'd rather be a passenger with you at the controls than someone like Kimon.

Words like "worry about" & "ram the ND to below FL 250" do not inspire confidence.

[nitpicker330]

If you are not able to remember 2.5 deg nose up and 78 to 80% N1 in Cruise then YOU SHOULDN'T BE FLYING A HEAVY JET.

Then if the problem still exists if you can't remember to set idle thrust and 1 deg nose down to descend then YOU SHOULDN'T BE FLYING A HEAVY JET.

The BUSS is designed for deceleration, configuring and landing the Aircraft, you shouldn't need to rush into using it early on if you are a half decent Pilot.:sad: