What happened
On 15 November 2018, a DHC-8-402 Dash 8 Q400, registration G-JECR, was operating a commercial passenger flight from Exeter Airport to Charles De Gaulle Airport. During the climb to 19,000 feet, the flight crew observed an intermittent altitude mismatch message on the primary flight displays. As the aircraft reached its assigned altitude, significant discrepancies were noted between the various altimeters: the commander’s instrument indicated 18,860 feet, the co-pilot’s showed 19,000 feet, and the standby altimeter read 18,920 feet. Additionally, a difference in airspeed indications of approximately 3 to 4 knots was observed between the two primary displays. Following standard operating procedures and consultation with the operator, the crew elected to return to Exeter Airport, where the landing was completed without further incident.
The investigation
Post-flight inspections of the aircraft revealed a white, crystalline deposit covering three of the four static pressure holes on the left primary pitot static probe. Chemical analysis of residue recovered from the probe identified the presence of silicone and fluorine, which were inconsistent with the approved lubricant (LF5050) but matched a silicone-based grease.
Investigators examined the air data accessory kit used during maintenance at Exeter prior to the flight. They found that the kit was missing the recommended lubricant and contained other substances, such as 'Snoop'. Interviews with avionics technicians suggested that establishing a proper seal between the test adaptors and the probes can be difficult, leading to the use of various unapproved products to facilitate the connection. The investigation also noted that the aircraft maintenance manual and the kit manufacturer's instructions lacked specific guidance on the installation of adaptors or warnings against using non-approved materials.
Findings
- A silicone-based grease was found blocking three of the four static pressure holes on the left primary pitot static probe.
- The blockage of these holes prevented the proper averaging of static pressure, leading to the altitude mismatch between the primary flight instruments.
- The use of an unapproved lubricant was likely a result of the approved lubricant being unavailable in the maintenance kit at the time of the leak test.
- Inadequate instructions in both the aircraft maintenance manual and the air data accessory kit manual contributed to the use of non-standard maintenance practices.
Safety action
- Two Safety Recommendations were issued: one to the air data accessory kit manufacturer (Nav-Aids Ltd) to improve installation instructions and include warnings against unapproved materials, and one to the aircraft manufacturer (De Havilland Aircraft of Canada Ltd) to update the maintenance manual with similar precautions.
- The maintenance organisation has implemented stricter controls over its tool stores and purchased new air data accessory kits to ensure all required components and approved lubricants are available.