A320 IR1+2+3 Fault Ldg Dist Proc

[Mäx Reverse]

Hi there everyone,

with an tripple IR-Fault the A320-QRH calls for a landings distance factor of 2,35 for DRY RWY which is in the magnitude of the factor for EMER ELEC or G+Y HYD Failure.

What's the reason for the huge factor? Except for a Direct Law / Config 3 landing there's no major performance degradation obvious to me. Unfortunately there's no Abn. Proc. for triple IR Fault available to clarify things.

Any inputs appreciated!

Best Regards, MAX

[JABBARA]

Hi

I haven`t checked any tech document yet but the very first reason comes to my mind is "Anti-Skid U/S"

[bflyer]

Hi Max Reverse

Excerpts from FCTM/ABNORMAL OPERATION BRIEFINGS/NAVIGATION/1.03.34/page 1/REV 21/May 98

*Triple IRS or ADR failures are very unlikely
*Do not follow ECAM
*Follow QRH ADR1+2+3 FAULT checklist

The following is from my company's QRH/ABN PROC/page 2.20/REV42/SEQ001/ADR1+2+3 FAULT

( Multiply the landing distance by 1.35 on dry runway,or 1.30 on wet runway, or 1.25 on contaminated runway)

Can you please specify the reference for the 2.35 dry runway landing distance factor...can't find it anywhere
Regards
bf

[Mäx Reverse]

Very interesting so far, thanks!

Since AI revised the Landing Distand Calculation procedure in case of Systen Abnormals (~ 2 yrs ago?), we changed our QRH respectively. These are the Factors for IR 1+2+3 Fault on DRY/WET/CONT Rwys:

A319: 2,55 / 1,95 / 1,55
A320: 2,60 / 2,10 / 1,70
A321: 2,45 / 2,00 / 1,55

An ADR 1+2+3 Fault gives a Factor of 1,35-1,25 on any airframe, so 2,xx seems totally out of place and closer to EMER ELEC or a double HYD Failure where your left with close to nothing in terms of braking.

As we don't us AI FCOMs/FCTMs we also don't have above mentioned procedurer available.

I requested clarification from our training dpt, however any further advice is duly appreciated.

Take care,

MAX

[TyroPicard]

For Anti-skid the BSCU uses ADIRU speed as a "reference speed" for releasing brake pressure to a skidding wheel. No ADIRU = No reference speed, so only the individual wheel speeds are available for comparison with each other. A conservative deceleration target is used, which I believe has been increased for later aircraft. Hence the large factor.
More gen from your training staff if they look at the Airbus October Standards Meeting Report.

[JSF1]

My guess...

Referring to QRH 2.32:

Because we lose both the autobrake & anti-skid, and neither is marked with an asterisk ( * ), we must multiply these factors together.

This factor, accumulated with a 10kt increment to Vref would come pretty close to the IRS triple failure factor.

Just a guess though.

[westinghouse]

no anti-skid.

braking limited to 1000psi.

[TyroPicard]

JSF
QRH 2.32 does not apply to "NO Autobrake", it applies to ECAM A/BRAKE FAULT, when there may be a brake released condition, hence the factor. One can land without using A/BRAKE without making a correction.

All...
See my previous post - the A/SKID still works but because it has no reference speed due to ADIRU failure it releases brake pressure to limit the deceleration to 1.7 m/sec/sec (2.8 m/sec/sec on latest BSCU standard).

[Mäx Reverse]

See my previous post - the A/SKID still works but because it has no reference speed due to ADIRU failure it releases brake pressure to limit the deceleration to 1.7 m/sec/sec (2.8 m/sec/sec on latest BSCU standard).

Still standing by for the answer from our engineers/training gurus, so just a couple of thoughts.

- 1.7 m/sec² is the kind of braking that autobrake low results in - and thats pretty close to no braking at all. That could indeed explain the huge factor.

- You say the BSCU limits deceleration to 1.7 m/sec². When all three ADIRUs are off, where does the BSCU get the limiting decelaration reference from?

- I think at least on a dry runway you can achieve a far better deceleration by using alternate braking w/o antiskid (i.e. A/SKID NWS off) as can be seen by the factor of 1.8 for ANTI SKID Fault (A319, Dry Rwy) compared to 2.55 for the IR1+2+3 Fault.

Even more strange that there is no Abnormal Procedure for this fault.

I'll keep u advised.

Regards, MAX

[TyroPicard]

- You say the BSCU limits deceleration to 1.7 m/sec². When all three ADIRUs are off, where does the BSCU get the limiting decelaration reference from?
Good question! A tachometer on each mainwheel provides speed, and the BSCU uses the highest value as reference speed in the absence if ADIRU information. There's a pic on FCOM 1.32.30 p2 with some maths formulae which explain it all....
Airbus also say that the probability of a triple IR failure is so remote there is no need for a specific ECAM procedure.
Which leads me to thinking of a suitable braking procedure... as the A/SKID is working you can apply max pedal deflection. Anyone see a problem with that?
TP

[Mäx Reverse]

Thanks again for ongoing inputs, tyropicard.

Well regarding the statistical failure probabilities I get a pain the proverbial and to my knowledge there have been at least two tripple IR failures, caused by explosiv nosegear-tire failures. The resulting accelerations when the debris hit the fuselage was outside the scope of what the internal algorithms of the IRs considered valid signals and thus all 3 units where 'faulted' - during rotation.

Although that happened on A310/A300s and there was a mod with respect to altering the mounting positions of the IRs (which formerly were located close to each other) that's enough for me to question the 'highly unlikely' part of the message, especially if it comes from AI.

Commenting further on the topic, regulating A/SKID with reference to tire/wheel speed might work on RWY surfaces with sufficient friction. As soon as it becomes slippery (wet or even worse) and there is seriuos work for the A/Skid there might be a problem. Unfortunately under these conditions a factor of >2 might keep you running out of options rather quickly on a heavy A321 in wintertime, especially if you don't know what kind of braking you're gonna have left.

As I understand our engineers are having a hard time and the phone lines to southern france are pretty busy this time.

Regards, MAX