Originally Posted by
Mad (Flt) Scientist
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 (This may well be a superseded concern, rooted in older display technology)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...