Airbus Adirs vs Boeing

[Marlboro_2002]

Hey,

I just recently found out that the 737NG has only 2 IRS while the A320/A330 has 3.

Does anyone know the reason behing this decision from Boeing?
(having just one operating IRS on the NG limits the aircraft to day VMC operations only)

Arebthey more reliable or more evolved?

Thanks..

[Denti]

Actually, an inflight fault doesn't limit you except that autopilots cannot be engaged, however normal manual flight IFR/IMC operation is possible on the remaining one.

I guess the original decision (classics or jurassic?) was for cost effectiveness, after all the aircraft was mainly designed for shorthaul traffic.

[Sciolistes]

I presume cost/benefit. They seem to be extremely reliable. Also, looking at the E&E bay, I wouldn't be surprised if space was an issue too.

[Marlboro_2002]

But do you guys know if they are from diferent manufacturers?

I have never experienced an IR failure and do agree that they are in fact very reliable.
But there are other points in consideration:

1) electrical supply failure

2) flight in RVSM/MNPS in case of failure.

3) flight limitations such as DAY flight only

[JABBARA]

I do not want to go details.

Simply Airbus FBW FCC computers use a logic called "Voting". In another word most of the computers compare ADIRS inputs before they transfer command to servo controls. Therefore the number of inputs should be "Odd", not "Even".

For example if two ADIRS output are same and the third one is divergent, these two are considered to be correct as called "Voting" concept. Obviously to be able to do that "3" ADIRS are required; "2" is not enough. If somehow one ADIRS fail, this voting process does not work and, consequently, some systems become inoperative (e.g Cat 3 Fail Operational )

[STBYRUD]

I guess on the bus the ADIRS are even more important than in a conventional aircraft since they are necessary for the flight control system to function normally - On the 737 there are two pilots, two PFDs, two DEUs (or symbol generators on the older ships), two autopilots, two IRSs - they went with simplicity.

[Microburst2002]

I guess airbus FBW are more vulnerable to IR malfunctions so they need more redundancy and also the "voting" capability. The problem is when one IR is correct and the other two are equally wrong.