The engine manufacturer has also committed to deliver the (fi)rst engines incorporating a (fi)x in April. That likely means GTF-powered A320neo deliveries will be halted for more than two months.
Thank you for the info, zeke
I went to their web site and unfortunately it does not have the same article, but they warned that they are having web site problems.
I got an email from them (that anyone can get after free registration) that has a similar but longer report.
With regard to the current problem, it says:
Pratt says an engineering change was made in mid-2017 to improve durability and introduced into revenue service on customer aircraft in December.
So it arose from an attempt to improve durability rather than a manufacturing process change.
It talks about the final resolution of issues with (a) the bearing seals and (b) the cumbustor, which both were also failed attempts to address durability issues.
For the bearing seals it says:
The chief change to the bearing housing is a switch from the liftoff seal used in the original design to a dry-runningface seal that becomes the standard configuration. The dry-running face seal consists of a rotating mating ring made from a carbide material and a carbon-graphite stationary ring. The faces are flat and held tightly together using magnets or springs to prevent oil leaking through, despite the high revolutions. The liftoff seal, on the other hand, incorporates grooves and wedges to channel a thin film of air between the sliding sealing faces, which creates aerodynamic lift.
“We originally went with a liftoff seal because we thought it would be more durable over time. It turns out the problem we discovered was that the software was misreading the altitude, so the bellows were not putting enough pressure on the carbon seal to create that air bubble. Sometimes it was putting too much pressure, which was causing the carbon flakes to go into the oil side,” a Pratt spokeswoman says. The resulting gap between the carbon air seal and associated seal plate allowed traces of metal particles to enter the oil system and trigger chip-detector warnings.
Pratt paved the way to the final fix by introducing an interim upgrade package in May 2017 that included the addition of a venturi tube to reduce the air pressure directed at the bearing compartment as well as associated modifications to the electronic engine control software to restrict the airflow. The tube was external to the compartment itself but integral to the carbon seal package.
“The improvement package performed better than we had hoped, except for engines that had higher time on them, which had to be reprioritized for overhaul,” Pratt says. The liftoff seal and the improvement package “bought us time to work out whether we needed to develop a permanent fix to the liftoff seal or go to a standard brush seal or dry-face design,” Pratt adds. “We decided to go with a dry-face design, and that’s now part of the standard bill of material.”
For the combustor change it says:
The revised combustor configuration is designed to address the durability problems that led to a rash of premature engine removals by two Indian-based A320neo operators, GoAir and IndiGo. “We have roughly doubled the number of air cooling holes and configured the density of the holes to the lower left of intake/outtake valve. We essentially had the density in the wrong place originally,” the engine maker says.
The revised design is expected to increase durability by a factor of five over the baseline. Although the interim fix fielded in 2017 appears to be adequate for non-harsh environments, the revised combustor design will now be standard to cope with operations in areas such as China, India and the Middle East. All the upgrades have been introduced to production engines and will be woven into engines coming through for overhaul.
So, lots of details on how they addressed the earlier issues, but not much about the current issue, other than the first fix is expected in April after two months of disruption to deliveries.
It sounds like these fixes should be long lasting. It is disappointing that they were not found earlier, but I guess they needed a substantial number of engines in mainline service till the problems were identified.
For those wondering about the earlier shaft bowing problem, https://www.flightglobal.com/news/artic ... lf-424321/
from April 2016 says:
P&W is further addressing the problem by applying a coating to some engine blades and strengthening the engine's third and fourth shaft bearings to prevent the harmonic vibration, Bromberg says.
"Then ultimately we are going to put coating on some of the blades that will improve the sealing function in the compressor," Bromberg says.
The third bearing seal is the one being discussed above.