Unreliable Speed A320
 

Hello friends,

quick one on this. We had the scenario in January which was: ADR 1 totally inop, ADR 2 speed unreliable and altitude ok, ADR3 inop. So the checklist asks for identifying the affected ADR. Ok, all three are actually unreliable. BUT ADR2 has a valid Altitude which can be used and makes it much easier. So why does Airbus say you have to switch it of? Because of the warnings related to the unreliable speed on adr2? Otherwise you could keep it running?

Thanks so much

SW

Depending on aircraft capabilities (BUSS, DBUS), there are some differences. But typically you keep one Adr on to prevent flight control laws from using the bad ADR data, in which case you can keep ADR 2 on and then off 1 & 3. On aircraft with the BUSS you turn off all ADRs below FL250 and the altitude is replaced by the GPS altitude.

How did you confirm that altitude is correct?

X-Check with GPS Altitude plus radar readout. so static ports were not involved. So keep it on?

Switch off all three as suggested and get the BUSS. PFD on the BUSS shows GPS altitude.

And if it has no buss system? I would switch off a reliable alt indication…

You needed to provide following information in the original question:Whether aircraft had GPS?Whether aircraft had no BUSS, old BUSS or the new BUSS?since you have GPS but no BUSS you may keep the ADR2 but considering the complexity of the failure possibility of altitude becoming erroneous cannot be ruled out and CM1 doesn't have altitude on PFD anyway. So it's better to use GPS altitude from MCDU GPS page to fly the approach.

Get below 10,000', bleeds off, packs off, open the outflow valve and use cabin alt as an altimeter. Even get a VSI thrown in for nothing. Probably closer to reality than GPS alt. :E

Airbus should know better. They display GPS altitude on the BUSS. So can't be that bad. Better than depressurize and freeze passengers. Unreliable speed is bad as it is managing a descent to preserve passenger's eardrums, well! Possible in the sim may be.

Hi Speedwinner,

ATC radar cannot measure aircraft Altitude independently - they can only report the transponder altitude data transmitted from the aircraft - which will always agree with one of the onboard altimeters.

So there is no X-check there.

Yes! There was a fatal accident due to radar altitude.

I bet you’re thinking of Aeroperu Flight 603

”The pilots incorrectly believed that they could figure out the actual aircraft altitude by asking the controller, but neither the pilots nor the controller realized that the altitude information displayed on the controller's screen was sent from the aircraft's Mode C Transponder. As the transponder was receiving the same erroneous altitude information being displayed on the aircraft's altimeter, the altitude on the controller's display was also incorrect.”

Villas,
Old BUSS /New BUSS !!!!!
Can u pls elaborate

Airbuses come in four types.
1. Without Back Up Speed Scale.
2. With irreversible Back Up Speed Scale
3. With reversible Back Up Speed Scale.
4. With Digital Back Up Speed where alternate actual calculated speed replaces pitot static speed.

The problem with this is that all you know is that at that particular moment in time you have an altimeter that agrees with GPS altitude. Air data problems can be quite insidious in that a faulty system can provide seemingly accurate data at one moment in time, at a specific speed and altitude. This is why Airbus recommends that you switch off all three and rely on the BUSS if fitted.

In non BUSS Aircraft the stall warning was processed through ADR. So even with all ADRs unreliable it was recommended to keep one ADR on for stall warning. This was modified later by processing the stall warning through IRs. So when all three ADRs are switched off to get the BUSS the stall warning is still available. Even with BUSS equipped aircraft though stall warning is available through IR, when above FL250 you are still asked to keep one ADR on. That is for when you descend below 250 that lone ADR helps you check whether you have recovered valid speed. If it's still not OK then switch it off and change to BUSS. The ADR check procedure is only meant for speed indication and not for altitude.

Exactly, since it is completely probable, that while the data is matching at the instant, you never truly can trust it, especially under IMC/Night conditions. Essentially giving you a VERY false comfort.

Kinda off topic, but practically, what fault would tie these three systems together like this, apart from Static Port blockage?