A320 landing performance difference unreliable airspeed & dual adr fault

[redfly]

While having a close look at my performance calculations I have noticed that there is a difference between DUAL ADR FAULT (ADR 1+2, ADR 1+3 or ADR 2+3) and UNRELIABLE AIRSPEED.

I thought the scenario for UNRELIABLE AIRSPEED respective to Landing Performance is that you have a DUAL ADR FAULT leading you into ALTN LAW and CONF 3 for landing. Therefore your LD is slightly increased.

On the other side a DUAL ADR FAULT leads you also into ALTN LAW and CONF 3.

Comparing both calculations shows me a difference of about 100m in factored LD:
FACTORED LD without failure: 1750m,
with UNREALIABLE AIRSPEED: 2050m,
with DUAL ADR FAULT: 1950m (no matter wich 2 ADR I choose)

Where does this difference come from? In my opinion it should be rather the same distance.

Thanks for the advice!

[vilas]

redfly
The QRH I am looking at shows different delta Vrefs, for unreliable speed it is 16kts while dual/tripple ADR fail it is 10kts so the landing distance will vary.

[Citation2]

If you were in unreliable speed situation , you would be flying pitch and power , therefore you might be landing with excess of speed therefore landing distance is more conservative in that case.

As for the dual adr fault , you are not in unreliable speed as you still have one healthy instrument. you will still have one good remaining ADR which you will be using to fly your Vapp and have a proper Vref upon crossing threshold , landing distance is less affected.

[redfly]

Thank you very much for clarification. It is clear now. With unreliable speed your TARGET speed is VLS+10kt but you cannot verify it on your PFD since your speed indication is unreliable. So you need a margin for a probable higher speed than VLS+10kts. And that's why you need a longer landing distance. Perfect! Problem solved. Thanks!