Incident
During the landing roll at Paris Roissy Charles de Gaulle, the flight crew of a scheduled passenger flight heard a loud detonation and perceived a smell of smoke. The oxygen pressure indication for the crew system was lost on the EICAS display. After clearing the runway, the crew requested fire safety assistance for an external inspection of the aircraft. Upon arrival at the parking position, maintenance technicians found the crew oxygen pressure sensor damaged and bearing burn marks.
Aircraft and Operator
The aircraft involved was a Bombardier CRJ700, registered F-GRZF, operated by Brit Air. The flight was a scheduled public passenger transport service. The flight crew consisted of a captain (pilot flying) and a first officer (pilot not flying).
Sensor Description and History
The pressure sensor (part number CMC 1904) had been installed as a replacement for the original sensor (P/N CMC 1901-1) following the application of Service Bulletin 670BA-35-010. This replacement was prompted by the detection of electronic failures on many sensors installed across the CRJ700 fleet, particularly corruption of the motherboard memory component.
The sensor consisted of a high-pressure fitting, a measurement cell, an electronic compartment, and an electrical connector. Oxygen pressure acted on a metal membrane, which transmitted force to an incompressible fluid, then to a piezo-resistive device that converted pressure to an electrical signal. The motherboard was powered by 28V DC with a consumption of less than 5 mA.
Examination Findings
Examination of the damaged sensor revealed explosion traces and rupture of the sensor body at the input/output electrical connector. Tests applied 870 psi of oxygen (about half the nominal cylinder pressure) to the metal membrane, uncovering a micro-leak of approximately 2 milliliters per hour between the oxygen inlet and the electronic compartment.
The measurement cell was removed and examined. The upper weld connecting the membrane to the cell body, as well as the cell body material, exhibited cracks. Welding operations during production on material with a probable local defect was identified as a contributing factor to the formation of these cracks. The cracks could have appeared either during welding or during service under the combined effect of oxygen pressure.
The motherboard showed extensive damage: the voltage regulator component was missing, and black residues were present, particularly around the missing component. This component was a power component, implying a local temperature increase on the circuit board at that location.
Conclusion and Safety Lessons
The investigation concluded that the explosion of the pressure sensor was very likely the result of auto-ignition of a voltage regulator component on the motherboard in an oxygen-rich environment under pressure. The micro-leak of oxygen between the high-pressure side and the sealed body containing the electronics, over time, generated this oxygen concentration.
Safety lessons noted that the sensor was installed while likely containing undetected production defects, and the sensor design allowed a high concentration of pressurized oxygen in an area not intended for that purpose. Following the incident, the manufacturer decided to verify the tightness of all measurement cells at the end of production.
Recommendation
The BEA recommended that EASA ensure that the design of gaseous oxygen systems does not allow a concentration of pressurized oxygen in areas not intended for that purpose.