Fault Investigation

APU fuel cluster oil leakage


CU-A1 fuel cluster P/N 441761-7 was returned from customer to our facility in December 2005 for oil leakage at the overboard drain port. Oil leakage from the CU-A1 fuel cluster has been experienced four times in the field, causing an increased removal rate of the fuel cluster.

Background
The CU-A1 fuel cluster was designed and manufactured by Honeywell, and is utilized on 331-500 and 331-600 APU engines. The CU-A1 fuel cluster is made up of the fuel pump, fuel filter, torque motor, pressure regulator, flow divider, etc. The fuel pump has a pump housing, boost pump, impeller, gear set, two bearings, two seals, seal rotor, packing, etc. See Figure 1. The gear shaft is connected to the APU oil sump and lubricated by oil from the oil sump. On the oil side, sealing is achieved by packing (115B) which seals between the fuel pump housing and outer diameter of carbon seal (30), packing (100B) which seals between gear shaft (105) and the inner diameter of seal rotor (45) and packing (120) captured between the fuel pump outer surface and fuel pump bore in fuel cluster. Seal rotor (45) rotates with gear shaft. Carbon seal (30) remains static. Sealing between seal rotor (45) and carbon seal (30) prevents oil leakage.

The fuel cluster component maintenance manual requires that the fuel cluster drive spline is lubricated with lubricating oil MIL-PRF-7870 Type II at pressure of 1-2 psig and leakage of one or more drops in five minutes from overboard drain is not acceptable during final test.

Testing, measurement and overhaul
Tests were performed when fuel cluster CU-A1 was received on test stand at static (0 percent), start (10 percent), full speed (100 percent) and shut-down (0 percent) for five minutes at each point. Oil pressure was maintained at 2.0 psig. Oil leakage from the overboard drain was confirmed. The oil leakage rate increased in proportion with the fuel pump rotation speed.

Fuel cluster CU-A1 was disassembled and inspected and the findings were seal (30) and seal rotor (45) were worn out and were chipped at the rim. See Figure 2. Wear was also detected on the surface of bearings (60) and (65). See Figure 3. In the wave springs (70) and (85) load test, spring load is 9.6 pounds and 10.4 pounds respectively at a spring work height of 0.117 inches. The out of limit value is 12 pounds at 0.117 inches. Therefore the spring failed the load test. Wave springs are shown in Figure 4. Additionally, severe wear was detected on plate (50) during inspection.

Testing was performed on fuel cluster CU-A1 after seal (30) and seal rotor (45) were replaced.

Test results showed that oil leakage was reduced by 95 percent after seal (30) and seal rotor (45) were replaced.

Fuel pump bearings, wave springs, seal, seal rotor, gear set, housing and packings were replaced. All parts were cleaned and reassembled in accordance with customer request and fuel cluster component maintenance manual.

The overhauled fuel pump was reinstalled in the fuel cluster and tested on test stand. The pump passed the break-in test. An endurance test was performed.

The final test was performed on test stand according to test specification in CU-A1 fuel cluster component maintenance manual. The fuel cluster passed final test. No oil leakage was found during final test after overhaul.

Analysis and summary
Three failure modes have been identified that can contribute to the oil leakage problem, one of which is failure of sealing between carbon seal (30) and seal rotor (45). The root cause is excessive wear of carbon seal (30) and seal rotor (45). See Figure 1.

The second failure mode is failure of packing sealing due to side-loading of the gear shaft mechanism at high rotation speed. Two root causes for this failure mode have been identified: excessive wear of bearings (60) and (65); and malfunction of wave spring (70) and (85).

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