Bendy Airbusses...
 

Excuse me if this turns out to be a stupid question, but it's been bugging me for a while.

I became aware a while ago that Airbus A330/340’s have a built in “sight tubes” for levelling and alignment of the airframe. Presumably, by the very fact that these devices are present, the airframe can become “misaligned”. Under what circumstances can this occur? A landing at MLW in excess of 660fpm, severe turbulence? Does this happen naturally over the lifetime of the airframe, i.e., does the aircraft “sag” over time?

A final question: If misalignment is detected, how on Earth is it corrected? Are there “tensioners” built-in at specific stations along the airframe to “pull in” the stringers, or are adjustments made elsewhere on the airframe, for example, aerodynamic modifications?

Even much smaller aircrafts have some devices to check if the fuselage is still straight after a while or after suspected structural damages.
The ATR, for example, has got 3 triangular markers along the fuselage, and when you watch at them from the nose (or the tail) you must see them aligned.
I think they are used as the first hint only to do much more deep checks, and if there's really a structural damage it's gonna cost you the hell of a lot of money.
Some engineers will enlighten us on that. :oh:

Sorry chaps, I think you'll find that these 'sight tubes' are not for re-rigging 'bent birds', but to allow the aircraft to be checked for level attitude for fuel tank quantity checks.

Hmm, now this is confusing!

A direct quote from the Airbus A330 MFP manual, page 472:

"3. Symmetry and alignment check

Measurement of possible deformation of the aircraft is made by photogrametry."

This is in a section related to the sight tubes.

While on the Airbus subject ...
Can someone kindly explain to me, what on earth are those red fwd facing triangles on the A320 fuselage for ?

I'm not sure which red triangles you refer to, but usually they are markers for airbridge line up, areas for instrumentation, sensors, and "cut here" markers etc.