Effects of blockages on the pitot system

In addition to the intake, pitot tubes also have a small drain hole through which air can escape (see picture), causing the airspeed indicator to drop to read zero if the pitot tube intake (only) becomes blocked in flight. This aspect isn’t emphasised in the popular training manuals despite this being the more common (and obvious) mode of failure.

The 'airspeed behaves like an altimeter' when both intake and drain in the pitot system becomes blocked is perhaps over-emphasised in the literature because of a major crash in a 727 on climb-out in which this problem featured. Its also a subtle problem which pilots might not notice, hence the emphasis in the texts.

Of course, icing may block both holes, but far more likely is some kind of obstruction in the pitot intake in a non icing situation though – and - the airspeed indicator will read zero in those circumstances. (You did remember to take the pitot cover off during your pre-flight check, didn't you? This would be one of the most common causes!)

See Paul Craig's book (listed below) for an excellent discussion of the problem, the modes of failure and the consequences. It is very well explained here, in much greater depth than in the PPL manuals. See also the online simulator (link below) which helps make this clear.

References:

For an excellent online (and free) simulator, see http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx

Craig, P. (2000) The Killing Zone: How and Why Pilots Die. McGraw-Hill Professional Publishing. (ISBN: 007136269X)

Picture of Pitot

Piper pitot blade assembly. Note drain hole at base.

Atmospheric Picture

FL45, English Channel.