THS calculation on 330/340

[tom2713]

Recently proceeded on an A332 to PMI. I noticed that the captain used some kind of formula to convert the MAC takeoff from the loadsheet into the THS setting before inserting it into the box...

never noticed such thing on the 320. Does anyone of you 330-jockeys know this formula ?

Cheers

[michelda]

For A330:

(35-TO CG)/2


Ciao

[crwjerk]

Not written anywhere as far as i know...... If it's not calculated automatically on the load sheet, just read it from the checklist. I'm sure a formula is hardly an approved method. Please correct me if i am wrong.

[shortfuel]

crwjerk,
it takes less than a minute to figure out that A330 THS setting is a linear function of MACTOW. So if the LTS software uses a formula, why can't you use it yourself when it is that simple? It is an approved method as this is the exact formula not an approximation.

AND...you primarily get your THS setting from the ECAM not from the loadsheet.

What michelda said is 100% correct (for all A330 variants) and I'd just add that your trim would be always UP (or zero).

[blurrrsotong]

So why did AB come up with a separate "THS" linear scale (and its added confusion) instead of just letting the MAC be?

[crwjerk]

100% correct, yes. My point is that If you have nothing to compare with the ECAM, ( if no auto loadsheet ).... I'd be reading it from the bottom of the checklist, no relying on someone's maths, which as we have seen in many cases, can easily be mistaken.

[tom2713]

Michelda,

thats what i was looking for... mille grazie, mate !

[Mad (Flt) Scientist]

Originally Posted by blurrrsotong

So why did AB come up with a separate "THS" linear scale (and its added confusion) instead of just letting the MAC be?

Because although the two items - cg position and takeoff trim setting - are related they are NOT the same thing. If they always presented tail angle as "equivalent MAC" then you'd get all kinds of confusion, because that flap angles other than for takeoff the relationship doesn't hold, so someone landing might be confused that their "stab equivalent cg" wasn't their "real cg".

Stab angle (or THS angle, or units) is what it is, it's what the display shows you and what you need to set. I think pretty much every aircraft with a trimmable stab uses a direct indication of stab position, not some kind of cg equivalent units.