I asked a range of pilots, including experienced instructors, how large they thought the ASI error would be. Invariably, they didn’t really know but guessed at “Ooh, maybe 10 knots, 20 knots?”. The answer is…… the ASI will actually read 140 kt, close to Vne. In other words it over-reads by 75kt! I was in good VMC but you can imagine how things could go badly wrong in IMC, relying solely on the ASI with no visual references.
To convince myself and my sceptical listeners, I ran the maths on a spreadsheet. Using data for the International Standard Atmosphere and carefully converting all units to the SI system, the speed error which I observed proved to be correct. If you want to know more, take a look at http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx which provides an elaborate explanation and a simulator programme to try out the effects.