AIRBUS Dual RA fault & Flight control laws
 

Hi - Wondering if any Airbus experts can help me with the following:

When both RAD ALTS fail on the bus, I understand the flight control laws revert to direct law (flare law?) for landing.

My question is; on approach, when the gear is selected down and the AP turned off, and the 330 flight controls revert to FLARE LAW, will there be a "direct law" ECAM triggered ? (and subsequent approach/landing distance procedures).

I ask as I imagine it could pop up at a very inconvenient time during approach and would be much easier delt with prior to comencing an approach.

Any thoughts ?

Cheers.

You make a good point. this is actually only relevant if you're making cat 2/3 approaches. What happens is as you said, the aircraft reverts to Direct Law when landing gear down. There is no inhibition of the RA's warnings during approach until touchdown so what you get is a Autoland degradation from for example CAT3 DUAL to CAT1 SINGLE and the autopilots will disconnect.

GPWS and Automatic callouts will also be inop so hope that the weather is good =)

But there is no LDG DIST PROC Application for this failure since it doesn't affect the aircrafts ability to fly. Just its capability for accurate landing.

You can read more about it in FCOM1 (1.34.40 P2) and FCOM3 (3.02.34 P12)

Kind Regards
Tim

I disagree with Founder there is a landing distance factor and procedure for direct law. The procedure requires you to land Config 3 Vref +10 and a landing factor of 1.35 dry runway. F/CL DIRECT LAW 3.02.27 P9 and 3.02.80 or QRH.
If you have prior warning of the dual RA fault this is my you should refer to the FCOM 3 after completing ECAM. 3.02.34 P12, this then will refer you to the direct law page and associated procedures and factors. I hope this helps?

Dual RA fault
 

Cannot arm the APP mode????? Think about that one.

elac 2

Good point..many miss the direct law checklist:ok:

This is a cut and paste from the Airbus 330/340 FCTM (Flight Crew Training Manual)

"DUAL RADIO ALTIMETER FAILURE

The Radio Altimeters (RAs) provide inputs to a number of systems, including the GPWS and FWC for auto-callouts. They also supply information to the AP and A/THR modes, plus inputs to switch flight control laws at various stages.

Although the ECAM procedure for a RA 1 + 2 FAULT is straightforward, the consequences of the failure on the aircraft operation require consideration.

Instead of using RA information, the flight control system uses inputs from the LGCIU to determine mode switching. Consequently, mode switching is as follows:

·

·

It is not possible to capture the ILS using the APPR pb and the approach must be flown to CAT 1 limits only. However, it is possible to capture the localiser using the LOC pb.
Furthermore, the final stages of the approach should be flown using raw data in order to avoid possible excessive roll rates if LOC is still engaged. Indeed, as the autopilot gains are no longer updated with the radio altitude signal, the AP/FD behaviour may be unsatisfactory when approaching the ground.
There will be no auto-callouts on approach, and no “RETARD” call in the flare
The GPWS/EGPWS will be inoperative; therefore terrain awareness becomes very important. Similarly, the “SPEED, SPEED, SPEED” low energy warning is also inoperative, again requiring increased awareness."

Nice document the Airbus FCTM ;). LGCIU = Landing Gear Control Interface Unit.

Thanks for your comments !

I have reviewed FCOM 3 "NAV RA 1+2 FAULT" (and the FCTM) and whilst they detail how the flight control laws will change to "flare law", there is no reference to "direct law" or the associated direct law checklists/approach procedures.

Should I read into this that since "flare law" is subtley different from "direct law", and there is no mention of direct law, I wont have any nasty ECAM surprise and approach procedure to apply late in the approach ?

No the dual RA is one of those got you type problems. You will get a direct law ECAM when you lower the landing gear. As I mentioned in my previous post always review FCOM 3.02 after you have completed your ECAM. In this case 3.02.34 P12 will then lead you to 3.02.27 direct law. If there are notes in section 3.02 they are worth reading to prevent surprises or highlight why you are doing certain procedures. And of course sometimes there aren't any notes for simple ECAM procedures. I hope this helps?

On the A330, you will not get the FCTL DIRECT LAW ECAM on landing gear extension with a dual RA fault, this may be different to other Airbus. Thus I do not believe you need to refer to the direct law procedure.

It is the same as ELEC EMER CONFIG, again you do not refer to the driect law procedure and do not get the ecam on gear extension (Emer Elec shows ALTN LAW: PROT LOST on the status page)

FCOM dual RA does not say refer to direct law procedure, unlike IR DISAGREE, where it specifically refers you to the direct law procedure.

The direct law ecam will show on the status page DIRECT LAW: PROT LOST. and also FCTL PROT on the inop systems You do not get this on the dual RA ECAM.

Thanks !!!

The 320 paper checklist states "CREW AWARENESS"

This is a nasty problem if you're not ready for the implications. Imagine:-

1. Direct Law for landing - therefore flap 3 and a landing distance increment of 1.35. Not something to discover when you put the gear down.

2. APP pushbutton u/s (LOC will work for a while!)

3. No Autopilot

4. No autocallouts

5. No GPWS
Basically much of your normal backup systems for a anormal approach are gone and it can really throw pilots.

I have seen numerous pilots in the sim totally screw up this problem simply because either they weren't aware of the implications of a seemingly innocuous failure or didn't read FCOM 3. Every time you have a failure get out FCOM 3 after you've finished the ECAM procedure to see what gems may be hidden in the notes etc.

Nice a/c the 320, until you see the problems that such a simple failure can create. In most other types you would merely lose your GPWS and Cat 2/3 capability.

In the 320 it virtually puts you into an emergency situation and you'd better be ready for it. Thankfully, most reputable 320 operators that I know, drill this into their crews in recurrent training.

Words of wisdom.

The point is: Without the RAs there is no way for the computer to know, when to go into the Flare Law. Therefore you cannot engage the Approachmode at all, and it will revert to manual flight at gear down (so it is smart to get it on speed and correct descent rate when lowering the gear, to keep the trim requirements down)

Nic

SumFingWong
Different Airbi have different control laws in this situation - does your question relate to A330/340 or A320?

AIRBUS Dual RA fault & Flight control laws
 

If A319 aircraft got the failure message that "RA1+2 fault" identified by EFCS1&EFCS2 at phase befor 6 inclusive(eg.phase 2)
My question is :
:confused:When approaching and before L/G is down, can the APPR mode be engaged normally ?

Can any one explane why Approach can not be armed in a Duel RA failure.

I guess it's because during the "AP/FD common modes" (FCOM 1.22.30. AP/FD ) Approach modes on the ILS, "LAND FLARE & ROLL OUT" are armed. Since there is no RAD ALT information - the FMGC can't compute LAND & FLARE so "APP" is rejected.

On a previous aircraft (L1011) the AP/FD "Approach" mode was separate to the "A/L" (autoland) mode, so we could do an auto coupled approach to CAT 1 minima with no RAD ALT information. PFM.

Also, from the FCTM.. (A320)
"The final stages of the approach should be flown using raw data in order to avoid possible excessive roll rates if LOC is still engaged. Indeed, as the autopilot gains are no longer updated with the RA signal, the AP/FD behaviour may be unsatisfactory when approaching the ground."
So that's two reasons.

My 2 pennies about this...

1°) Normal law is a load factor control law, which means that the pilot stick or the autopilot are "asking" for a certain Nz demand, that is then computed using a number of information (like mass, CG, flaps configuration, dynamic pressure or mach number...) to give the adequate change of trajectory.
2°) Direct law is a control law that uses no information (or very little) but the gains have been set by defaut to "safe" value. It's a fixed gains mode that is adequate for a range of speeds although precision of flying is degraded and the flight domain may be restrained. Its use is related to failure modes.
3°) Normal Nz law on ground is "catastrophic" while direct law is not.
4°) The flare law is a direct law with additional damping and an additional pitch down term (increasing with time to give the pilot a "natural aircraft" behavior) that is equivalent to a direct law ajusted for low speed (no need to have it valid for 250 kt) that are activated based on RA. No RA => no flare law as such

Based on that if we loose the RA's:
1°) If landing gear is up the aircraft is entitled to think that it is not about to land and therefore there is no reason not to keep the load factor control law, whose gains do not depend on RA information.
2°) A good way to determine that landing is coming is to use landing gear information, then I do not know what the ECAM's or procedures say but the aircraft will be in direct law (or equivalent) since flare law is not available and normal law is not adequate to land.
3°) In normal mode the autopilot gains (pitch and roll) are not valid anymore (too sensitive) due to loss of RA and thus the FD information as well, which explains the loss of the APP functionality.
4°) In direct law the autopilot would still be asking for Nz while the control law is not controlling Nz anymore. Therefore it is not available either, and the "no APP mode" is sensible.

Pierre

Point of order: should be 'Airbii', Latin. Or 'Airbusartum' Or Airbusos (Gr).