Embraer ERJ 190-100 IGW N258JB incident near Nassau, BS

Probable cause

failure of the engine high pressure turbine active clearance control valve fuel rod supply tube coupling seal due to a cross-threaded B-nut as installed during original manufacture.

NTSB analysis

A No. 1 engine under cowl fire occurred following landing. An emergency evacuation was performed, but the fire was quickly extinguished. The airplane's fuel system was pressurized and fuel sprayed from under the No. 1 engine high pressure turbine active clearance control (HPTACC) valve rod tube valve fitting B-nut. The coupling was properly safetied, but the B-nut was cocked on the fitting, and 3-4 fitting threads were exposed below the B-nut, consistent with the B-nut being cross-threaded on the fitting during installation. The HPTACC valve fuel rod tube coupling has both primary (metal-to-metal) and secondary (O-ring) seals between the fitting and the tube ferrule. Pre-disassembly X-ray photographs of the coupling provided B-nut stack measurements and showed a gap between the fitting and the ferrule at the primary seal location, indicating that this seal had not been engaged when the B-nut was tightened on the fitting. Disassembly inspection of the coupling found that the O-ring (secondary) seal was radially separated and extruded from its gland, consistent with having failed in service. A high-torque B-nut connection and thread damage to the B-nut and fitting threads were further evidence of an improperly installed (cross-threaded) B-nut. Maintenance and manufacturing records indicated that the coupling had not been disassembled since the engine was assembled at manufacture. An extrusion calculation found that the damage to the O-ring seal was consistent with the engine total time since new. Based on this evidence, it is likely that the coupling was cross-threaded during manufacturing final assembly, and there was no engagement of the metal-to-metal seal. As a result, the O-ring acted as the primary seal, until it failed in service, resulting in the leak.

Text from the NTSB. Open the NTSB docket for ENG09IA017 for the full record.