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Why you cannot fly RVSM with both ADR failure?
Professional Pilots - Technical Questions
Written by Capt Lim   
Thursday, 29 July 2010 07:00
Dear Captain,

Could you explain, why you can not fly RVSM (reduced vertical separation minimums) with ADR2 or ADR1 inoperative on the Airbus A320? But ADR3 does not affect RVSM. Are they not similar?

Dear Igor
There are 3 identical ADRs (Air Data Reference is the system that provides the airspeed, angle of attack, temperature and barometric altitude data of an airplane) on the Airbus A320. On a normal flight, only the ADR1 and ADR2 are operating with ADR3 on standby.
When ADR1 or ADR2 fails, ADR3 is selected to replace the failed ADR and the flight continues normally except that the Ground Proximity Warning System is lost in the case of ADR1 failure.  You could fly RVSM.
However when ADR1 and ADR2 fail, the back up is the ADR3.  In such a scenario (Dual ADR lost) you lose, amongst others, the two autopilots. So without autopilots, RVSM flights are prohibited.
So even if the ADRs are similar, you need two of them to be operating at any one time for normal operations and not just one.

ADR1 and ADR2 Fault on an Airbus A320


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Flying in RVSM with ADRs failure
Dear Capt Lim,

Thank you for your reply

Regarding ADRs, Airbus requirements is that ADR1 + ADR2 must be in order in RVSM (2.04.50 special operations RVSM).

According to Airbus MEL, in case of ADR1 or ADR2 failure, RVSM is not approved but nothing is written about RVSM in case of ADR3 failure.

I tried to find the answer but is still unsuccessful. May be this regulation is due to ADR3 being supplied from the standby static line?


Igor , 29 Jul, 2010
Flying in RVSM with ADRs failure
Yes, the MEL says that you require 2 ADRs to fly in RVSM airspace. It did not mention that it must be ADR1 and ADR2. It can be a combination of ADR 1 + 3 or ADR 2 + 3.

So even with ADR 3 failure, you would have the normal ADR 1 + 2 but you no longer have a back up, which means you can still fly RVSM.
Captain Lim , 29 Jul, 2010
Capt Lim is correct and to add to the subject, the FAR,JAA and ICAO regs require 2 operational independent ADRs because of hourly Capt and FO height keeping accuracy on the altimeter.It should not bust 75ft on CRZ and not more than 150ft in RVSM transition airspace.If you refer to the AIB FCOM or JAA TGL, you should find the answers there.if not, just comply. most regs don't make sense anyway. smilies/tongue.gif
Capt BaoLing , 14 Jan, 2011
Flight instrument tolerance
Check 3:04:34 pg 6 you will see that the altimeter tolerance for ADR3 is 355ft at FL300, this exceeds RVSM tolerance, hence only ADR1+2 for RVSM, older A320 only, A321 tolerance all within RVSM tolerances
Capt Kevin , 18 Jun, 2011

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