A-320 different FMA in capt and F/O PFDs?
 

Hi

I just read the issue 11 of Airbus Safety First magazine. In the Radio Altimeter Erroneous Values chapter, I got very surprised.

According to this, if the erroneus RA (capt RA reading -6 ft instead of 1400 ft) occurs during an ILS approach with both APs engaged (according to airbus SOPS) then the captain FMA goes to THR IDLE and FLARE, while the F/Os FMA remains in SPEED , G/S and LOC.

If I was better at computers I would insert the image here...

In the case of only AP 1 engaged, both FMAs would reflect the same (the wrong and dangerous one).

I thought that both FMAs are always "synchronised" and managed by the master FMGC. Apparently this is not the case in case of a AP 1 + 2 ILS. I can't understand why. Does it have to do with CAT III or something?

Affirm. You need two independent autopilots, & 2 radio altimeters & 2 separate (split) electrical sources to do CAT IIIB.

yes, but how can you have an airplane with two different FMAs?

one in FLARE the other in LAND, for instance? In CAT II, III the PNF monitors his PFD, while the captain is monitoring outside. If there is a discrepancy between FMAs...

By the way: why the APU GEN is not capable of achieving the electrical split when it comes in line instead of a failed engine GEN?

Hi MB2002,

By design, it is necessary for each pilot to monitor his respective autopilot during CATIII etc. approaches. So the onside FMA indicates what mode that independent AP is computing. (Like 1 FD 2 compared with 1 FD 1) If one side said "FLARE" before the "One hundred" call from PNF - then you'd know there was something wrong with this picture, compare FMAs then GA.

I don't understand that one because I still can't see why the electrics aren't considered split.

(In the early days on L1011, the APU did not qualify as an adequate electrical source, so with an engine elec geny out, we were downgraded. Sometime later, the APU geny was considered to be satisfactory and approved - then we could do CAT IIIB NO DH even with an engine geny out but with the APU on line.) PFM

I have always believed that this was a certification issue: as it wasn't certified originally, they can't/won't do it subsequently.

The APU generator shares a few relays with the engine generators and can therefore not be considered as fully split.

Thanks Dries.

Voilà!
 

http://nsa20.casimages.com/img/2011/...0643559511.jpg


THEN:

http://nsa20.casimages.com/img/2011/...1144648787.jpg

thanks Drier and shortfuel!

Still it puzzles me that we have "independent" FMAs.

According to FCOM 1.22.30 p69 "the FMGS synchronises the A/THR, approach capability and AP/FD modes to provide identical information on both PFDs."

When we have AP 1 + 2, AP 1 is master. But aren't the FMGCs still synchronised? Is there here another 'transparent to the pilot' thing?

I think it is a flight phase related matter. In this condition both FMGS are in approach phase, as we can see in the picture both are on THR IDLE and both have the same approach capability. The (big) difference is that one system is erroneously in flare while the other one is following the ILS. As the RA1 reads an erroneous height value, it triggers the flare on the FD/AP 1 while the other FMGS, receiving inputs from its onside RA2, has no logic to trigger the FLARE because it actually wouldn't know the reason to do so. When I say it is flight phase related, I mean that there is no flight phase change at that particular moment between the two systems, so they are indeed synchronized...

I see what you mean, but synchronised should mean to me that the active mode (and submode) is the same in both, the master FMGC being the one determining which mode is active.

For instance, when you are flying a non GPS fitted 320, one ND can be do the left of the track, the other to the right, and the roll bars will give opposite commands to intercept the track. The FMGS will follow the master FMGC bar. But the mode is the same: NAV.

I'll get this from the fridge
 

I-2021

Now I notice that A/THR mode is the same in both PFDs!! (in this case the wrong, dangerous one, THR IDLE)

So the corollary is that we can have two different FMAs, with different AP/FD modes, but only one A/THR active mode and only one landing capability. Nice to now!

Hi MB2002, thanks for bringing the subject up again, a nice revision :ok:

Would also note these bug fixes from 10.8A to be fixed in V11.0 (due any day now)

• Dual FMC Restart Due to Unsent ADS Messages
• Intermittent Altitude Intervention Function
• FMC Software Restart Due to Crossover Altitude Computation and the Speed Propagation Feature
• Display of “FMC DISAGREE − VERTICAL” Message during FMC descent in the approach environment.

m2002, just to clarify OEB 26 erroneous RA has been valid for at least 4 years now and it's surely part of the briefing like any other OEBs unless it's been canceled by

for A 320 aircraft is TBD

For A330 aircraft:
FCPC Standard P13M22 2K2, ( MOD/ SB to be determined) for A330-200/-300 aircraft with the electrical rudder, or FCPC Standard P14M23 2K1/2K0 ( MOD/ SB to be determined) for A330-200/-300 with the mechanical rudder.

For A340 aircraft:
FCPC Standard, L23 2K2, ( MOD/ SB to be determined) for A340-300 aircraft with the electrical rudder, or FCPC Standard L24 2K0/2K1 ( MOD/ SB to be determined) for A340-200/-300 aircraft with the mechanical rudder, or FCPC Standard W13 2K2 ( MOD/ SB to be determined) for A340-500/-600 aircraft.