From the
WSJ article reporting of the original FAA release:
The Federal Aviation Administration restrictions are interim and many industry officials expect them to be substantially loosened once flight tests are completed. But the decision on extra spacing, communicated to air-traffic controllers last week, raises questions about the extent of operating restrictions the agency may end up imposing on Boeing's 787 Dreamliner and stretched 747-8 jumbo jets.
The interim standards call for all planes, regardless of size, to stay at least 10 miles behind the latest Boeing models during large portions of descents. That's more than twice the distance many planes are now required to fly behind the 747-400, Boeing's largest jet currently in service.
The document announcing interim "separation standards and procedures" calls them "conservative" and indicates "final guidance will become available" after flight-test results have been evaluated. But the document indicates the interim restrictions could remain in effect through October 2011.
The FAA's notice said that studies indicate that wakes generated by 747-8 and 787 models—essentially cones of spinning air spreading out from the tip of each wing—"may be more substantial than those" created by existing wide-body or jumbo jets such as the Boeing 747-400 or Airbus A340.