The Attitude Indicator , also known as 'artificial horizon' appears a little more complex. Unlike the HI, its Gyro spins horizontally on a vertical Gimbal. The vertical Gimbal is attached to a horizontal Gimbal. 'Rigidity in Space' makes the gyro stay in its horizontal position during any 'pitch' (nose high/low) attitudes as well as during any movements around the longitudinal axis (bank).

The blue/brown horizon colored flat metal (face of the instrument) is connected to the vertical Gimbal through an Arm as seen on the picture above.

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Tumbling of the Instrument - How is that possible?

The AI has two limits, after which it will probably tumble (go crazy if you will) if they are exceeded:  
1. Either 60° (degrees) of PITCH UP/DOWN attitude; or
2. 100° (degrees) of BANK (roll)
In either case, the gyro case will come in contact with the outside Gimbals, which produce a strong force, resulting in a tumble.

What brings the gyro back in position?

The AI has so called 'Pendulous Vanes'. They work by the principle of gravity to keep it in balance. What are pendulous vanes? These vanes (tiny holes) swing freely and let air in and out of the gyro's case to slow it down until it's back to equilibrium again. This may take up to several minutes though.

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Things about this Instrument you should be aware of ...
The AI may take up to 5 min to erect itself after engine-start. This is the time it takes the gyro to spin fast enough (10.000-18.000 FPM) for it to rise to its position and reach stability.
Erroneous Indications:

The AI usually shows a slight 'Nose-Up' Attitude or tendency during rapid acceleration and vice versa; 
The AI tends to show a small incorrect 'Bank + Pitch Information after 180 turns'. This error will mostly correct itself after about a min. If you fly a full 360 degree turn instead, this problem does not occur, as it has sort of corrected itself by then.