* Engine nozzle redesigned after cooling problem in test
* Pratt & Whitney says no impact on A320neo schedule, performance
* Airbus confident P&W will meet commitments on revamped jet
* Boeing says completes design for competing 737 MAX 8 aircraft
PARIS/NEW YORK, July 24 (Reuters) - Pratt & Whitney has redesigned a component of a jet engine being developed for Airbus's top-selling A320neo aircraft after tests revealed "distress" in the engine's hot core.
The problem highlights the strict timetable and performance thresholds in one of the aerospace industry's most closely watched developments, but both planemaker and engine designer said they did not expect any delays or drop in fuel savings.
Engine makers routinely push new engines to their physical limits, and well beyond normal operating conditions.
However, Pratt's latest engine is under close scrutiny because it represents the U.S. engine maker's return to the forefront of civil engine manufacturing in a lead role. It is also tied to many billions of dollars in airplane revenues.
In an interview, Pratt & Whitney confirmed that it had noticed "insufficient cooling" on a nozzle inside the high-temperature core of the new engine.
The problem developed when the PW11000G engine was being stress-tested, and the redesign is among dozens carried out at this stage of testing, said Bob Saia, Vice President of Development Programs at Connecticut-based Pratt & Whitney.
The United Technologies subsidiary was responding after an industry source, speaking on condition of anonymity, told Reuters that the engine had suffered "significant hot-section damage" during an incident in testing.
Saia declined to go into details of any damage caused, but noted that engineers had been able to complete their tests.
"We saw very local distress, which we corrected by directing cooling for that area of the part," Saia told Reuters.
The part in question is a nozzle used to direct hot gases into the hottest part of the engine, the high-pressure turbine.
Its modification will not affect the targeted fuel savings which have spurred record sales of the A320neo, nor the delivery schedule of the engine, which is due to begin shipping to Airbus in the third quarter of next year, Saia said.
"We are very confident that Pratt & Whitney will deliver on its commitments," said Airbus spokesman Stefan Schaffrath.
The Pratt & Whitney engine is the first example of a new generation of engines offering significant fuel savings for short-haul aircraft carrying about 150 passengers. Plane sales in that segment alone are forecast at $2 trillion over 20 years.
The prospect of improved engines has prompted both Airbus and Boeing to upgrade their best-selling narrowbody models from the middle of the decade, triggering record-breaking orders from airlines clamouring for relief from high oil prices.
It has also encouraged new competitors such as Canada's Bombardier, with its Pratt & Whitney-powered CSeries, to challenge the dominance of the transatlantic plane giants.
There is no connection between the test result on the Airbus engine and the sister model built for the CSeries, Saia said.
Pratt & Whitney says it has more than 4,500 announced and unannounced firm orders for the same family of engines. More than 2,000 are for the Airbus A320neo.
Both Pratt & Whitney and U.S.-French competitor CFM International have promised double-digit fuel savings in percentage terms by incorporating new technology that must now be proven in development testing, industry analysts say.
The two companies have clashed over the choice of technology, echoing rivalries between leading planemakers.
Pratt & Whitney has built four of eight test engines. The hitherto unreported problem occurred in May and affected a nozzle or "guide vane" on engine number four.
The nozzle is a stationary part that needs to be cooled enough to operate efficiently at what remains extremely high heat and pressure. Engineers were running the engine several hundred degrees above certified limits as part of the test.
"We've made a minor change to the cooling of this vane to correct this problem," Saia said, adding Pratt & Whitney had built in enough development time to prepare for such mishaps.
The PW1100G will be the first engine installed on Airbus's revamped A320neo due to be delivered in the second half of 2015.
It will compete against another new engine model from CFM, co-owned by General Electric and France's Safran .
Airlines can pick which of the two makes of engine to place on the Airbus A320neo family of jets, but CFM is the sole engine supplier for Boeing's upgraded 737 MAX series.
Boeing said earlier it had completed the design for the 737 MAX 8 and repeated plans to deliver it from third-quarter 2017.