Airbus and Rolls-Royce are believed to be working on a potential thrust margin increase of about 5,000 lb. for the Trent XWB engine that will power the A350-1000, a sufficiently substantial design change that would force the planned entry-into-service target for the larger model to slide from 2015 to at least 2016.

The plans come as key customers, such as Air Lease Corp. (ALC) and Emirates, have publicly voiced doubts that the engine—as currently defined—will be powerful enough.

If the conjecture is confirmed, it is expected to disrupt Rolls’ carefully crafted A350 development plan, which is built around a single engine type focused on the middle aircraft, the A350-900. Originally rated at 83,000 lb. for this version, the Trent XWB power level has already been increased an extra 1,000 lb. following an early Airbus program review. The identical -800 engine, originally de-rated down to 74,000 lb., was also adjusted upward to 79,000 lb.

Currently, for all variants the baseline fan diameter remains unchanged at 118 in. To accommodate the existing thrust needs of the -1000, Rolls planned to run the 22-blade fan faster to increase mass flow. In addition, the fan was expected to be beefed-up with thicker gauge, hollow-titanium blades and a strengthened containment system. Now speculation is mounting over whether this additional increment (about 5.4% thrust growth) is achievable within the existing diameter, or whether more fundamental upgrades are required to the low, intermediate and high pressure systems. Rolls-Royce declines comment on the potential for further XWB thrust growth.

“Based on our detailed analysis, we’ve come to the conclusion that with the R-R powerplant for the 1000-series, it does not have adequate thrust,” ALC president and CEO Steven Udvar-Hazy told Aviation Week on the sidelines of the International Air Traffic Association annual general meeting in Singapore. Airbus had no immediate comment.

Airbus and Rolls-Royce are expected to announce details of the -1000 engine at the Paris air show in mid-June. Hazy says the inadequate thrust means the aircraft’s range performance, runway performance and climb performance “are not adequate to meet customers’ requirements.”

“We have communicated that to Airbus” and the aircraft-maker has responded by “engaging” with Rolls-Royce again, says Hazy. “I believe Rolls-Royce are now addressing this issue and are busy at work to deliver an engine that meets” the aircraft performance specifications. The additional work on the engine development, though, means there could be a “small delay in the delivery time-frame” for the A350-1000, he says.

This is an issue because the A350-1000 is supposed to compete against the extended range Boeing 777s, says Udvar-Hazy. He says Airbus needs to ensure the engine has more thrust because, like with any new aircraft in development, one needs to account for any increase in the aircraft’s weight.

Emirates President Tim Clark is also skeptical about the -1000. If the engine went beyond 100,000 lb. of thrust, it would need a new fan or a new core, he believes. Clark points out that in his view, the A350-1000 is not a replacement for the Boeing 777-300ER, because it is a smaller aircraft with less range. Emirates sees it as a successor for the original 777-300s, seating about 340 passengers for 10-hour sectors.

General Electric, which has consistently opted not to become involved in the A350 for fear of cannibalizing its long-term market with the competing GE90-powered 777 and GEnx-powered 787, remains on the sidelines. The company says development commitments are already at maximum with the Leap, TechX and GEnx family developments. It adds that although “the dialogue with Airbus continues, there is no way to close the business case.”