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A noncontacting clearance seal consists of a series of centrifugal air pumps and oil/air separators for air turbine starters on the BR710 and the CFM56-5 engines. Conventional contacting seals for rotating shafts are limited in life due to the formation of oxidized oil coke buildup on the sealing surfaces. The clearance seal directly addresses the problem of viscous heat generation in carbon face and lip seals by eliminating contact. Standard contacting seals typically have local temperature increases of 50 to 75 °F higher than the gearbox sump oil. Heat generation by the clearance seal is minimized, resulting in a significant increase in reliability compared to a standard contacting seal. The reliability increase is estimated for various flight-hour-to-start-cycle ratios in air turbine starter gearboxes.

Analytical predictions and laboratory measurements were made for the relative total temperature experienced at the tip diameter of a radial-inflow turbine used in an air turbine starter (ATS). The predictions showed that at freerun conditions the blade tip temperature would be significantly higher than the turbine inlet temperature. Tests to confirm this prediction were performed on an ATS modified to accept an optical pyrometer. The pyrometer was focused on the suction side of the blade at the tip radius. Blade temperature measurements conducted at the maximum attainable speed of the ATS verified the prediction to be within the error of the pyrometer measuring system.