Question: Stab Trim setting - Airbus
 

Just asking the audience what is the better practice

On dispatch paperwork, Company provides both TO CG%, and Stab Trim (N/U or N/D x.x)

are folks:

1. setting CG on the trim wheel (inner scale, somewhat harder to see)
2. Stab Trim on the trim wheel, or alternatively,
3. setting stab trim using the trim wheel but references the electronic lower screen display when flight controls page is up.

#3 seems better in a dark cockpit and allows both crews to better see it, in my opinion

Thank you

As per the after start checklist verbal reply should be “XX %” set hence using the reading on the scale. However I also prefer to cross-check with the UP/DN value on FCTL page. Don’t think there is any harm in that as an additional check?

After some incidents of setting UP instead of DN and vice versa Airbus changed to setting by percentage. In percentage little more or less is not as critical as setting down instead of up. Besides the MCDU page also shows it in %.

We set the percentage. If it’s off by a bit on the index, you’ll never know the difference.

If I recall, as long as you’re in the green band you’re safe.

For specific values of "safe". ;-)

The mistrimmed takeoff tests for certification generally consider the stab at one end of the band and the aircraft (weight, CG, etc) corresponding to the other end. So if no additional failures or unusual circumstances arise, you are indeed "safe".

If "something else" does happen, though, being fully (or partly) mistrimmed could result in a degraded ability to cope with that event.

Say again? Better yet, put that on my Xmas list, pretty please no sarcasm 100%

Some airlines set the stab after start from fuel predictions page.

​​​​​​​Old discussion on the same subject.

PITCH TRIM
PITCH TRIM handwheel................................................... ...........................................................S ET
Set takeoff CG on pitch trim handwheel.

Same thing here, in my SOP, in the after start flow, the stab trimm should be set following the CG for take off, which should be compared using the TO CG from the loadsheet and the GW CG displayed in the FUEL PRED page. In reality everybody keeps using the DN or UP figures from the load sheet because it is more straight forward, but it has already happened to me taking off using DN instead of UP of viceversa. It had almost NIL effect during rotation because the value of the trim itself was quite close to zero, but I can imagine it being somewhat of an issue with hight values.

Thank you.

I assumed a (non-existent) MOD to display % on the PERF T. O. p.g. in lieu of stab trim units.

Always has been a big fan of ditching the NU/ND +/- altogether, although being forced to the complete opposite corner by present SOP.

Unfortunately what's explained is a complete nono here as the TO pg has TO CG units whereas FUEL has the present calculated CG. The difference being the moment effect of taxi fuel used........... Yes. I know. But 'more busy, more safe', captain.

Enjoy your rules, best wishes.

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An alternative to using the actual +/- physical stab position - which does have a risk of the +/- confusion, or even the "-" sign being dropped from the display, leading to an apparent positive value - is to use a stab "index" rather than values in degrees, with the index chosen such that it is only positive.

In the CRJs, for example, the stab physical travel is +2 to -13 degrees, but the "stab units" value is defined as "2 - stab angle" which results in a value of "0" corresponding to "-2 degrees" and "+15" corresponding to "-13 degrees"; even on the Challenger 600 series, where the range was "0 to -8 degrees" and then "0 to -9 degrees", the "units" concept was used, simply to avoid presenting a negative number with a perceived fear of dropping the minus sign even then. (This may well be a superseded concern, rooted in older display technology)

We already have that in the Airbus, the GW CG... with the added benefit that you'll know straight away (in the A320) that when it is below 27 you need to correct the speeds and flex due to stab trimm drag. I don't know why Airbus chose to set up the stab trimm system using some other made up units, having already the CG already calculated in the load sheet and displayed in the FUEL PRED page...

As I understand you need to enter the NU/ND +/- into the MCDU to ttrigger the F/CTL PITCH TRIM/MCDU/CG DISAGREE caution if the trim is set wrongly.