The ETS small emitters tool for calculating carbon dioxide (CO2) emissions is simple at first glance, consisting of no more than an Excel spreadsheet into which ­operators can insert data from flight plans filed with the Eurocontrol central flow management unit. The resulting calculations are based on stored data for fuel burn of listed aircraft types.

But significant discrepancies have been uncovered by early users of the system. Aaron Misko, co-owner of Ohio-based ETS consultants Shockwave Aviation, has crunched numbers for 60 operators covering more than 100 aircraft operating during 2009. He found CO2 emission ­estimates overstated to the tune of 40 to 50 percent in the case of aircraft such as the Bombardier Global Express and Global 5000.

Misko has alerted Eurocontrol and the European Business Aviation ­Association to the inaccuracies. He told AIN that, in reality, the problem is of no direct consequence yet to operators, which will not be accountable for emissions until 2012, and by then the errors could be rectified.

Some ETS experts have acknowledged that the small emitters tool will be inaccurate but have maintained that it is still a financially viable option if it avoids the need to use consultants and verifiers. If European authorities are, after all, going to insist on data being fully verified then this calculation, too, could prove to be ill-founded.