# SDR ASAA2019070629972

- Type: Service difficulty report (as reported; unverified)
- Difficulty date: July 6, 2019
- Submitted: July 9, 2019
- Operation: Air carrier
- Aircraft: [N477AS](https://openairdocs.org/aircraft/N477AS), registered now
- Make and model: Boeing 737990ER
- Aircraft serial: 40716
- Aircraft time: 16,405 hours, 5,235 cycles
- System (JASC): [4960 · APU Controls](https://openairdocs.org/jasc/4960)
- Part: Sensor
- Part number: 38762231
- Condition: Inoperative
- Location: Apu
- Stage of operation: TX
- How discovered: O
- Nature of condition: O
- Precautionary procedure: K
- FAA source: FAA Service Difficulty Reporting System (https://sdrs.faa.gov/)
- Page: https://openairdocs.org/sdr/ASAA2019070629972

## Discrepancy

DURING WALK AROUND, HEARD LOUD "BACKFIRE" TYPE NOISE FROM APU EXHAUST AREA, COUPLED WITH A DRAMATIC REDUCTION IN AIRFLOW NOTED. HEARD X4 EVENTS AT 30 SECONDS APART, AND X2 MORE WHEN INTERNAL TO THE FLIGHT DECK. APU ON ELEC & BLEED AIR, EXT POWER AND AIR WERE BOTH DISCONNECTED. GROUNDED - YES. PER FIM 49-60 TASK 817 APU CURRENT FAULTS STILL HAS 49-61262 SPEED SENSOR 2 SHOWS OPEN CIRCUIT. SPEED SENSOR RESISTANCE TEST FAILED. SPEED SENSOR REMOVED AND REPLACED IAW AMM 49-61-21. APU BITE SHOWS NO CURRENT FAULTS. APU RUN CHECK NO ISSUES. NO LEAKS. INSTALLATION TEST GOOD.

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