Lufthansa and Boeing have agreed that a 747-8 previously allocated for delivery to the airline will now become a dedicated test aircraft for a set of airframe, system and engine improvements due for delivery in late 2013.

The aircraft, Line Number 1435, first flew in April 2011 but has already been used for some upgrade testing and will now continue in this role rather than be refurbished for Lufthansa. The move, which was triggered by the need to flight test a revised tail fuel system for activation on later aircraft, means that Lufthansa’s firm order book backlog reduces from 20 to 19, though the airline hints that this may only be a temporary reduction.

Lufthansa is still scheduled to take five 747-8s as planned in 2013, and 10 more aircraft by the end of 2015 that will incorporate the lighter structure and improved systems and engines. The airline currently has four 747-8s in service.

Explaining the decision, the German carrier’s 747-8 chief pilot Elmar Boje says the extensive flight test modifications already made to the aircraft would mean that, even after post-test refurbishment, the unit would be a non-standard “white elephant” in the Lufthansa fleet. However, he adds the airline’s long-range fleet group is due to decide on adding additional aircraft by the third quarter of 2013, some of which could include 747-8s.

Although the decision effectively shrinks Boeing’s overall 747-8 backlog to 71, the manufacturer is putting a brave face on the move because a variety of upgrade testing, including the General Electric GEnx-2B performance improvement package (PIP), can now be bundled into one dedicated certification effort. “The plan was to use a 747-8F freighter for the PIP flight test, but when we de-activated the tail fuel we needed an -8 Intercontinental” to flight test the revised system, says 747 vice president and general manager Elizabeth Lund. “That’s when we started our conversation with Lufthansa.”

The aircraft, originally destined to be Lufthansa’s fifth 747-8, will be used for flight testing throughout 2013 and refurbished for onward sale in 2014 as either a standard airliner or for possible business jet modification.

“Lufthansa opted to take a new aircraft instead in 2014,” says Lund. “So as a result, we had to pull this one out and delay delivery to next year. Lufthansa reserves the right to add another at the end. We asked to use it, and it was already instrumented for tail fuel testing. So all in all it’s a good business decision by them,” she adds.

Improvements to be tested next year are concentrated on the upgraded engine configuration and tail tank fuel system, though also include updates to the flight management computer (FMC) to incorporate additional required navigation performance features and a ‘quiet climb’ function.

The FMC 3.0 load is scheduled for the end of 2013.

The 747-8 passenger model’s 3,300 gal. tailplane fuel tank was de-activated before the first aircraft entered service after analysis indicated that, under certain fuel load circumstances, the tail tank could induce flutter.

Re-activation of the tail fuel tank will provide added range and improve the aircraft’s performance says Lufthansa’s Boje. The extra weight of the fuel in the aft of the aircraft can be used to assist in trimming the 747-8 to lower cruise drag. The 747-8’s fuselage extension “tends to be nose heavy so we might gain performance,” he adds.

Lufthansa also plans to ask Boeing to study minor software changes to the fuel transfer system which would extend the length of time the fuel remains in the aft tank, which would increase the trim benefit.