A new European Aviation Safety Agency (EASA) rule covering flight-crew licensing has the potential of grounding hundreds, if not thousands, of general aviation pilots in certain European countries, such as the U.K.

The rule, which took effect on April 8, enacts EASA requirements for pilot training and the issuance of European pilot licenses, ratings and certificates.

One of the more controversial measures in the rule requires European pilots who currently hold third-country licenses to convert their licenses to EASA- or EU-validated licenses. The rule includes a series of requirements that a third-country license holder must meet, including a likely significant jump in classroom hours.

The EASA rule includes a transition measure that permits European nations to allow up to two years for the conversion. But, as of this week, only about half of the countries under EASA oversight have indicated plans to provide the two-year grace period. Others have indicated a much shorter conversion time, including the U.K., which is seeking a July deadline.

The third-country license requirement was developed as a means to ensure proper oversight of pilots in Europe who are flying under the approvals of foreign states, says Jens Hennig, VP-operations for the General Aviation Manufacturers Association (GAMA). While it is not known exactly how many pilots in Europe are operating with third-country licenses, GA groups in Europe estimate the number is close to 10,000, Hennig says. Many of the pilots appear to be based in the U.K., France and Germany, which have the largest GA communities.

A number of pilots may have opted for the third-country licenses–particularly FAA certification–because of the differences in training requirements, Hennig notes. For instance, the U.K. had required all pilots to complete 200 hr. of academic instruction to obtain an instrument rating. FAA requirements, meanwhile, are not centered on specific hours, but on knowledge mastery.

Instrument ratings obtained under Part 61 has no minimum-hour requirements, while those who go through a Part 141-rated school would have to meet the FAA-approved instruction of the individual flight school, which typically means about 30- to 40 hr. The new EASA regulation, which supersedes the former Joint Aviation Authorities requirements, meanwhile, has a 150-hr. instruction requirement for instrument rating.

These requirements mark a major philosophical different between the U.S. and Europe in their respective approaches to training and have served as an incentive for pilots in Europe to seek licensing elsewhere.

GA groups have been pushing U.S. and European authorities to build on the Bilateral Aviation Safety Agreement that took effect last year with annexes that would cover validation and acceptance of licensing, simulators and flight school oversight.

The new European requirement “has caused considerable concern in the general aviation community and threatens to do significant economic harm to an important segment of the European aviation industry unless an appropriate mechanism is established to allow these pilots to comply with new EASA rules from 2014,” GAMA President Pete Bunce has written to FAA and European authorities.

In addition the groups have been working with European regulators on the instrument rating requirements, and EASA is drafting a notice of proposed amendment (NPA) that would create an en route instrument rating for pilots of non-complex aircraft (typically light piston aircraft). The NPA, which could be released later this year or early next year, is expected to lower the classroom instruction time to 100 hr. for an en route rating.

GAMA has been pushing to lower the instruction time even further to 80 hr., saying that there is still instruction included that is more about memorization of superfluous facts, rather than safety-enhancing knowledge.

Implementing new annexes and rules in Europe will help ease the transition for pilots affected by the new rule, Hennig notes. But he adds that getting these measures in place takes time, which is why GA advocates sought a transition period.

Hennig, along with other GA groups, notes that the third-party requirements are not expected to affect U.S. pilots or commercial pilots who already have met a higher training threshold.

EASA is recognizing the International Civil Aviation Organization (ICAO) framework, they say. The National Business Aviation Association notes that under ICAO requirements, a U.S. resident holding an FAA certificate can “freely operate in Europe without needing to obtain a new license or have his/her existing license validated.”

The third-party requirement covers a pilot who is flying an aircraft that is registered in the EU and regulated by EASA, or who is a resident or established in the EU.