Evolution of the number 4 Carbon Seal
Effectivity: Challenger 300 and Challenger 350 aircraft
ATA 72
This article describes the background information that led to the final resolution for the number 4 carbon seal failures on Challenger 300 and Challenger 350 aircraft. Honeywell carried out an investigation to determine the root cause of engine events resulting in the failure of the aft sump seal on the S907 engine model. The aft sump is located between the high-pressure and low-pressure turbines and contains the No. 4 and No. 5 carbon seals and bearings.
Seal design configuration
During the investigation, it was discovered that the seal formed a lip which in turn caused friction and heat due to being pinched between the shafts.
Once discovered, it was determined that the best way to control/avoid the formation of the lip, was to introduce a minor design change to the seal. By producing small undercuts on each side of the seal it removed just enough material to prevent the seal from rolling over creating the lip. They were then increased a further .003” (.0015 each side) to reduce potential contact surface.
New seal material
Another concern was the wear characteristics of the existing seal material. Exhaustive testing was carried out focused on three (3) potential materials. Following the testing, the selection was made based on several improved characteristics.
It is important to be aware that while the investigation of the seal issue was in progress, Honeywell at the same time launched a monitoring program to capture potential seal failures. If an operator submitted data downloads, an analysis was carried out to identify the possible seal failure via the Bi-Modality analysis. This was accomplished through CAMP and proved to be very effective in circumventing an Inflight Shutdown, demonstrating the power of the advanced prognostics.
While the corrective action has been released in the way of Service Bulletin AS907-72-9094 in February 2021, the monitoring process will remain in place as added value to the operator.
