Nose-down attitudes on the ground
 

Lately I've been wondering why I see so many jet aircraft, from bizjets on up, that seem to have a slight nose-down attitude while on the ground. Obviously the designers didn't forget to make the nose gear long enough, and it seems unlikely that the heaviest part of the aircraft is the nose, so it must be a deliberate design choice, but what's the advantage to it?

I thought of two possibilities, which might or might not make sense. One is that the nose-down attitude helps keep the aircraft on the ground during take-off, until the pilots explicitly rotate it. Another is that the nose-down attitude might be intended to cancel the angle of incidence in the wings, so that the effective angle of attack during forward motion on the ground is zero. I suppose these are variations on the same thing.

Not all jets appear to have this feature, and what's more confusing is that small prop aircraft seem to sit on the ground in a distinctly nose-up attitude.

What's the real reason for the nose-down position?

I may be wrong about this, but I seem to remember reading that the Dash-8 was designed with a slightly nose-down attitude on the ground to keep the door sill within a certain height off the ground. I think it was to avoid the need for emergency exit chutes to be installed.

A330 also appears very nose down...

The A330/A340 family is indeed nose down on the ground, and the reason is indeed that the nose gear is "not long enough". They were unable to fit a longer nose gear!

Couple of possible reasons:

Keeps the deck angle at a more comfortable incline when in straight and level flight (airliners generally fly along with the wing at a positive angle of attack - +4 degrees ish).

Reduces the chances of a tailstrike on certain long types (the Dash 8 which you mention is a prime example).

@ Reimers
 

Check out the bubble on the forward lower fuselage of the A330 Freighter.

No need to lengthen the gear, just mount it lower.

PICTURES: First Airbus A330-200F shows off nose-gear blister fairing

Oddly enough, that looks like a frankentanker.

Nose gear
 

In some cases, it just might mean that the nose gear strut is underinflated. These struts vary in design but many are basically pistons that use air and oil under pressure to remain partially extended under the weight of the aircraft. Depending on temperature, leaks, and perhaps just how much air was added during the last service, they may be slightly low or high.

I should mention that usually nitrogen is used rather than air when servicing the struts because it does not promote corrosion as much as oxygen and the larger molecules don't leake around seals as fast. (Regular air is 80% nitrogen anyway so not really much difference)

Also, depending on the last landing and perhaps bouncing during taxi, sometimes the struts just settle a little high or low.

The Dash-8 comment is correct. De Havilland (now Bombardier) initially designed the 8 with a slightly nose low attitude to remove the need for the slide. They have continued this through the Q400 and into the CRJ.