Wiki has some info on pitot-static, but does not explain. I don't think I've seen this explained elsewhere in a long time, so I'll take a stab at making it simple.
The pitot tube captures impact air, which is sensed by an Air Data Module in the A330, and converted to airspeed. The drain tube allows water to pass without clogging the pitot. To do that, it passes air all the time.
The pitot opening is a specific size, say 5mm dia, and the drain is maybe 2mm. That makes the pitot about 20 sq. mm area, and the drain about 3 sq. mm area, for a ratio of about 7:1. This means the impact pressure at the sensor is about 14% less than actual impact pressure.
When it comes to clogging at high altitude, there are these possibilities:
Pitot clogged, drain clogged at same time: IAS, Indicated AirSpeed, will not change with change of aircraft speed, but IAS will increase with increase of altitude.
Pitot Open, Drain CLogged: IAS will increase 14%. 14% excess IAS will remain regardless of actual airspeed or altitude.
Pitot Clogged, Drain Open: IAS will decrease toward zero as the drain bleeds off the impact pressure to ambient. Increased altitude will not affect IAS.
This last situation: "On 28 October 2009, an Airbus A330-202 (A330) aircraft, registered VH-EBA (EBA), was being operated as Jetstar flight 12... (IAS decreased to about 60 Kn). The airspeed disagreement was due to a temporary obstruction of the captain's and standby pitot probes, probably due to ice crystals. A similar event occurred on the same aircraft on 15 March 2009."
The AeroCaraibe incident showed much the same behavior, so it can be expected AF447 was also the same type failure.
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