by Guy Norris, originally published on May 24th 2016 on Aviation Daily
LOS ANGELES—Pratt & Whitney has confirmed that the first engines incorporating hardware modifications to speed up the start time of the PW1100G geared turbofan have shipped to Airbus for installation on the A320neo.
The arrival of the engines will be welcomed by Airbus, which has stored several A320neos in Toulouse pending availability of the upgraded powerplant. Lufthansa and IndiGo have taken a limited number of initial PW1100G-powered aircraft into service. But other operators such Qatar Airways, the original launch customer for aircraft powered with the engine, have declined delivery of the Pratt & Whitney-powered A320neo. Qatar Airways and other carriers want to wait until the new aircraft start-time requirements are similar to those of A320s powered with in-service CFM International CFM56 or International Aero Engines V2500 engines.
Pratt is tackling the issue, which is related to thermal bowing, or rotor bow, through a combination of software and hardware changes. Rotor bow is caused by asymmetrical cooling after the engine shuts down. Differences in temperature across the shaft section supporting the rotor lead to different thermal deformation of the shaft material. This causes the rotor axis to bend, and compressor tips to potentially make contact with seals, linings and walls.
Although engines can be started normally for the first flight of the day or after several hours of cooling down, the peak deformation occurs around 1–2 hours after shutdown. This is within the usual turnaround time for most short-haul or regional operations. Start-up times, or “motor-to-start” durations, have been prolonged on initial engines to enable cooling to equalize internal temperatures and eliminate any rotor bow.
Slower starting allows more even heating, which eliminates differential thermal deformation. Engine-control software, introduced as an interim measure, interrupts the start sequence at 10% N2 (low-pressure spool speed). The pilots’ central display informs them that the respective engine is “cooling.” Once sensors inside the engine signal that the process has minimized the temperature differences to acceptable levels, normal start-up resumes.
Pratt is delivering revisions that will take the maximum cooling time down to 90 seconds in June and, combined with the latest hardware fixes, will subsequently bring start times in line with the V2500. The upgrade package includes revised tips on the eighth stage of the high-pressure compressor. Introduced into the production process in February, the change adds a toughened coating of cubic boron nitride (CBN) to the tips of the integrally bladed rotor (IBR). The PW1100G’s eight HP compressor stages and three low-pressure (LP) compressor stages are all IBRs.
“The actual start time from ignition to combustion is the same as usual, about 50 seconds,” Danny Di Perna, Pratt & Whitney’s senior vice president of operations, said. “The changes we’ve made with more durability with the CBN coating in Rotor 8 allows [start time] to be more aggressive. Now, you have this tip coating it can rub into the outer air seal. It can take more of a rub, and that is what you want, because it creates this ‘kiss,’” he added, referring to the closer tip clearance this maintains in the compressor.
CBN is the second hardest material after synthetic diamond, and is synthesized from hexagonal boron nitride to create a surface with high hardness; strength; abrasion resistance; thermal and chemical resistance; and an ability to maintain sharp cutting edges.
Other changes introduced from engine No. 160 onward include an improved carbon air-seal assembly around the third bearing. Both the third and fourth bearings have also been modified with dampers to help offset the rotor bow, a change that was introduced around engine No. 141. Another minor change includes a revised Woodward-produced fuel nozzle design. This will address a combustor tone, or “howl,” issue.