Raisbeck Engineering has completed flight tests of its "ZR" upgrade kit for the Learjet 35/36 family, but will not begin taking firm orders for the modification until it secures US Federal Aviation Administration approval.

The ZR kit, unveiled at the National Business Aviation Association show in October 2003, includes extensive aerodynamic modifications aimed at reducing drag by up to 10% at the Learjet's nominal high-speed cruise of Mach 0.81.

The kit comprises three main elements including supercritical double-delta inboard wing leading-edge gloves, reflexed extended trailing edge flaps and horizontal winglets or "batwings".

Flight tests are complete, but Raisbeck says the certification process and certain "manufacturing steps" are taking longer than expected. The first kits are now expected to be available early in the second quarter of 2004, rather than March as had previously been expected. The test flight results showed that the expected drag reduction was achieved, says the company, adding: "We have a tremendous amount of interest in the modification, particularly from the larger charter companies."

Source: Flight International