How I was first alerted to a discrepancy was during an autorotation. The IAS was suggesting a forward speed of about 35 KIAS, but the machine was acting as though it was in a zero airspeed configuration. I could only assume at the time that there was an error with the ASI or if that was correct that the position of the pitot tube was such that Position Error was apparent.
Over the past 18 months when the climatic conditions were favourable (similar temperature and zero winds) I compared climbs at Vy (quoted in the flight manual as 65KIAS at the height band I conducted the tests), and in every instance at a steady IAS of 65Kts the Ground Speed readout on the GPS showed an increase in forward speed. I have done 40 - 50 climbs on different headings and compared GS to IAS and the results were similar.
Inbetween each climb I performed various steady autorotations and again compared GS to IAS and the results confirmed that there was an error here as well.
I fully appreciate that the responsible authorities will want hard evidence that performance data is correct that is to be included in the flight manual.
What I am trying to discover is (a) should I fly at IAS at Vy, (b) during autorotations never fly at less than 70 Kts, (c) notify the manufacturer that I have doubts about their data.
Point of note: 'Where do the authorities get their performance data from to ensure that it is correct'. The answer I'm sure is fron the manufacturers flight test data.