What happened
On 13 December 2018, a DHC-8-402 Dash 8, registration G-ECOC, was performing a commercial passenger flight from Aberdeen to Birmingham. While climbing through approximately 18,000 ft, the crew heard a loud noise and observed a maximum differential cabin pressure warning. Although the cabin altitude appeared normal, the aircraft began to depressurise.
As a precaution, the pilots attempted to activate the fixed oxygen system. However, the oxygen masks failed to inflate, and no oxygen was available to the crew. This unexpected failure caused a delay in the crew completing the Quick Reference Handbook (QRH) procedures. The pilots proceeded to level the aircraft at 19,000 ft, reset the pressurisation system—which then functioned normally—and continued the flight using portable oxygen from the cabin. Upon landing at Birmingham, a subsequent check confirmed that while the pressure gauge indicated normal levels, oxygen flow dropped to zero when demanded.
The investigation
The investigation focused on the crew oxygen cylinder assembly. Laboratory analysis of the regulator revealed that the crew oxygen supply port was obstructed by a small, 6 mm piece of debris. This debris was identified as a tapered, left-hand threaded metal fragment, similar in geometry to a screw extractor tool.
Microscopic examination showed that the fragment had caused abrasion damage to the valve pin and had cut into the smooth bore of the supply port. The metal composition of the debris was a low alloy steel, which differed from the corrosion-resistant steel used in the regulator components. Investigators found that the debris had likely been inserted into the port and snapped due to torsional overload. While the blockage was significant enough to prevent oxygen flow during use, it was not complete enough to prevent the pressure gauge from registering a normal reading on the flight deck.
Findings
- A piece of debris, suspected to be the tip of a screw extractor tool, was lodged in the crew oxygen supply port.
- The presence of the debris was sufficient to restrict oxygen flow during demand but allowed the system pressure to appear normal on the cockpit gauge.
- No evidence could be found to explain how the debris entered the regulator or why it was not detected during previous maintenance or certification cycles.
Safety action
- The operator has updated its 'first flight' check procedures. The flight deck fixed oxygen checks now include a momentary flow of oxygen to verify that the system can maintain pressure while supplying flow.