A30 Unreliable Speed & Flying the BUSS

[Boyington]

The new ADIRU Standard has the AOA sensors routed through the IRs.Therefore even if the ADRs are switched off we retain the AOA information. Why then is it required to keep one ADR On above F250. Thanks.

[vilas]

Because above 250 it is generally a temporary phenomenon due icing and inability of the deicing system to manage that. If you manage the situation with the procedure you may enter a region of higher temperature and recover your speeds. Switching to BUSS is irreversible so it is done below 250 because that is generally permanent phenomenon without possibility of speed recovery.

[Less Hair]

Are these lessons learned and implemented from the South Atlantic accident?

[Goldenrivett]

Switching to BUSS is irreversible
How so? Why couldn't you switch one (all) back on if the crew turned them off in error or when out of the condition which caused the UAS in the first place?

Why then is it required to keep one ADR On above F250
If you look at the QRH Unreliable Speed Indication tables for level flight in ABN - 34 Navigation, you will notice that above FL 250 the required pitch is 3 degs whilst below FL 250 it is 4 degs.

When clean, the middle of the green on the BUSS wants you to fly a pitch attitude equivalent to a speed slightly faster then GD. Above FL 250 the Mach No becomes more significant and I guess Airbus was uncomfortable with flying too slowly.

[JABBARA]

AOA measurement accuracy and its closeness to stall is all related to Mach no.
At increasing Mach no, max AOA decreases. AFAIK, above FL250, a Mach No equivalent to safe KIAS, may be too much giving a reliable AOA measurement. So as GR mentions, Airbus may not be comfortable flying simply as relying on AOA measurement (BUSS) above FL250.

In fact, this may be an explanation of Vls at high altitude. Although Vls leaves a margin to Vs1g at low level (1.23 x Vs1g at clean config for A 330), it changes to display margin (around 1.3 g) to buffet rather than to Vs1g at high altitude.

[vilas]

There was a discussion on this before. No where it is written that BUSS is only reliable or usable below 250 due to Mach effect etc. It is all guess work. The FCTM says: The BUSS enables to fly the aircraft in the entire flight domain while observing the design limits.
If stall warning is valid above 250 which also comes from AOA then BUSS display should be valid. All that airbus says about above and below 250 is what I said before. However the irreversibility part was quoted by some one in FCTM but I couldn't find it.
How so? Why couldn't you switch one (all) back on if the crew turned them off in error or when out of the condition which caused the UAS in the first place? If you switched off all ADRs there is no way of knowing that UAS has disappeared.

[Goldenrivett]

However the irreversibility part was quoted by some one...
Hi vilas,
You are correct.
According to FCOM DSC-31-40: Backup Speed/Altitude Scale:
"The BackUp Speed Scale (BUSS) enables to fly the aircraft when airspeed indications are unreliable.
When the BUSS is activated:
The BUSS replaces the normal speed,
The GPS altitude replaces the barometric altitude scales.
The BUSS is displayed on both PFDs when the flight crew turn off all ADRs.
The activation of this BUSS is not reversible.
The BUSS information is based on the angle of attack (AOA), and depends on the slat/flap configuration."

Thanks for the info as this behaviour is different to our sims.

there is no way of knowing that UAS has disappeared.
FCTM:
"At high altitude, typically above FL 250, the cases of unreliable speed situation are mostly a temporary phenomenon: They are usually due to contamination of the pitots, by water or ice, in particular meteorological conditions. In-service experience shows that such a contamination typically disappears after few minutes, allowing to recover normal speed indications."

[JABBARA]

It is all guess work.
Yes it is guess but,
"reliability" of AOA measurement at high Mach is not I guess
it as deep aerodynamic Fact.

[vilas]

At altitude the AOA threshold for stall warning is reduced to about 5 degrees from something like 13 degrees at lower altitude. That's why reliability is not an issue.If it was then why would you keep one ADR on? What purpose is served by a false stall warning?

[Goldenrivett]

At altitude the AOA threshold for stall warning is reduced to about 5 degrees
Hi vilas, please see post 4.
Above FL 250 Airbus recommend using pitch power tables and cruise pitch = 3 degs.
Below FL 250, cruise pitch = 4 degrees.

BUSS hasn't been designed for the required reduced pitch >FL250.

[vilas]

I am talking about angle of attack and not pitch. The bus shows you nothing but AOA. One ADR is kept above 250 and you are asked to respect stall warning at all times.

[JABBARA]

When I cheked AOA values from ALPHA CALL UP A 330-300, I had seen those approximate angle values at 3 degree final approach at Vref+5 or Vls+5. Here, Weight is not a factor because it increases Vref or Vls
CONF FULL: 6 degree (at Vref+5)
CONF 3 7.5 degree (at Vls)

The above angle values are big enough for stall at high altitude (high mach No). When cruising at high altitude, approximate AOA is 2.5-3.0 degrees. To trigger stall warning for slightly greater angles than 3.0 degree, ADC can bias mach Effect for this measured AOA PROBE/VANE values at high altitude.
Also, since the accuracy of AOA PROBE measurements decreases due to compressibility effect, when mathematical model of this error is set, accuracy of AOA can also be maintained at high Mach number.
These all sounds nice but as seen, only possible if Mach Number is calculated.. In case of UAS, the corret Mach number cannot be known for all above corrections. This means a kind of vicious Circle; in another word, if Speed/Mach cannot be measured correctly then AOA measurement cannot be corrected for use at high Mach Number. Means AOA measurement can not be independent from correct mach measurement at high mach number.

Boeing Aero Magazine Issue 12 (I am sure it can be downloaded in web) explains all details about AOA measurement and pros/ cons when displayed in cockpit. It explains why AOA display sometimes becomes useless; similiar to I have tried to explain with my poor English above