Why is the NAV ADR 1 ECAM caution a NO GO as per the MEL on the A320?

There isn't much more I can post about the question but in the MEL out of the 3 ADRs if ADR 1 fails on ground the MEL doesn't allow for a dispatch.

• Does Airbus, and/or the airlines that fly them, not provide adequate technical support for these sort of questions? I'm just curious, because this site seems to have a disproportionate number of very specific and detailed A320 questions. If I had a real need to know I would always take it up through proper channels before I would trust a stranger on the internet. Just a random observation FWIW... – Michael Hall Aug 31 at 17:22
• Well, I'm not an A320 pilot or technician, therefore, I'll leave this as a comment and not an official answer. I've done a fair amount of study of the A320 as it was the subject of a research project I did for a technical aeronautics course back in college. I know that ADR1 is specifically tied to the ground proximity warning system and that a fault will result in loss of predictive functions of the GPWS and GPWS TERR FAULT light will come on. Because of this, the GPWS TERR pb-sw should be switched OFF. This can present an undue hazard for departure if the issue is discovered pre-flight. – Aaron Holmes Aug 31 at 18:18
• Closely related: aviation.stackexchange.com/questions/77353/… – Jpe61 Aug 31 at 19:02
• @AaronHolmes I think you are so close I would definitely put that as an answer, as inop GPWS is a no go. – Jpe61 Aug 31 at 19:04
• @Jpe61 Actually the GPWS can be deferred under MEL for 10 days I believe. So, it is not the reason to keep the ADR 1 operative. Yes, most regulators would not allow operations without GPWS but it is not a critical system for a flight. ADR 1 needs to be there simply for the emergency electrical configuration. – Anas Maaz Sep 2 at 12:36

It is to prepare the aircraft for a possible Emergency electrical configuration. I hope you are aware of the fact that in an A320, if the AC BUS 1 and AC BUS 2 were to fail in flight for some reason, the RAT (Ram air turbine) extends which in turn powers an emergency electrical generator. When the aircraft is in emergency electrical configuration, only the very essential systems are powered and the others are shed. Some of the systems that are shed include the ADR 2 and ADR 3, leaving only the ADR 1 functional. The ADR 1 will continue to supply air data to captain's PFD. In this situation, the first officer's displays goes blank and the captain assumes the flying duties, if the first officer was the flying pilot when the failure occurred.

If the aircraft is dispatched with ADR 1 deferred, and if it undergoes an emergency electrical configuration, as soon as the ADR 2 and ADR 3 are shed, all the flight displays will go off, including the captain's side instruments which are supplied by the ADR 1. Then you will have to rely on standby instruments to fly which would not be the most ideal condition to be in an already crippled aircraft. Remember, this configuration will force the aircraft into alternate law. Both auto pilots, autothrust and even the flight directors would not be available.

• I would assume this to be the better answer. Look in the MEL and see if one can dispatch with an inoperative EGPWS. If so, the other answer may not be correct. I am not a A320 driver. – wbeard52 Sep 2 at 3:59
• You can defer the GPWS. But of course you cannot operate aircraft without it in many countries. But it is not the reason for ADR 1. The GPWS is not system critical. – Anas Maaz Sep 2 at 11:55

Well, I'm not an A320 pilot or technician, however, I've done a fair amount of study of the A320 as it was the subject of a research project I did for a technical aeronautics course back in college.

The ADR1 is specifically tied to the ground proximity warning system and a fault will result in loss of predictive functions of the GPWS. This is why the GPWS TERR FAULT light will come on after an ADR1 fault. Because of this, the GPWS TERR pb-sw must be switched OFF. This presents an undue hazard for departure if the issue is discovered pre-flight.

• This is possibly the correct answer, as I asked a friend who flies A320 and his reply was nav adr 1 drives all the captains panels. so if that is lost the pic can't fly. however it also drives the egpws and a few other things – Raffles Aug 31 at 20:05
• @Raffles ADR 3 can drive either the Capt or FO displays in place of ADR 1 (Capt) or ADR 2 (FO). But ADR 1 is the only one connected to the TAWS. Air data is necessary for GPWS Modes 1 (excessive rate of descent) and 3 (alt loss after takeoff). If ADR 1 has a fault and is putting out erroneous but unflagged data, it could result in hazardously misleading information (HMI) from the TAWS. It's interesting to note that it is acceptable in the MEL to dispatch with TAWS inop if "alternate procedures are in place", but the OP is correct in that ADR 1 is always required for dispatch. – Gerry Aug 31 at 21:44
• @Gerry your exactly right my whole confusion arose because with a GPWS FAULT ECAM the MEL permits dispatch which arose my suspicion that something larger is at hand – Jai Sep 1 at 3:44