What happened
On 22 November 2021, a SpiceJet DHC-8 Q400, registration VT-SUW, was operating a scheduled flight from Gorakhpur to Delhi. The flight proceeded normally until the descent phase. During the approach, a passenger noticed unusual fluttering from the right-hand engine's upper forward cowl and alerted the cabin crew.
An onboard maintenance engineer inspected the exterior through a window and confirmed that the cowl was fractured and deteriorating. Following this assessment, the flight crew implemented precautionary measures, including limiting the engine torque to 15%. The crew notified Delhi Air Traffic Control of the situation and requested a precautionary landing. The aircraft landed safely at Delhi Airport; however, once parked, two pieces of the engine's outboard upper forward cowl detached and fell onto the apron. There were no injuries among the 85 people on board, though the aircraft sustained minor damage to the engine cowling, propeller blades, and hub.
The investigation
AAIB India examined the maintenance history and mechanical condition of the aircraft. The investigation focused on the structural integrity of the engine's upper forward cowl and the propeller vibration levels. Investigators reviewed maintenance logs, including recent inspections and the status of deferred defects. The inquiry also looked into the functionality of the Propeller Balance Monitor System (PBMS) and the adherence of maintenance personnel to established engineering protocols and OEM requirements.
Findings
- Improper maintenance practices: Maintenance personnel failed to follow engineering circulars regarding the documentation of missing or loose fasteners. Additionally, shims were found to have been used in inappropriate locations, contrary to design requirements.
- Structural failure mechanism: The failure was driven by inappropriately installed and missing fasteners on the upper forward cowl. This led to uneven load distribution and stress concentration, causing the titanium strap rivets to shear and the cowling to fracture and flutter mid-air.
- Unmonitored vibration: There was a lack of effective troubleshooting regarding the propeller's vibration levels. The last recorded vibration reading was from 03 November 2021, and subsequent data was unavailable due to ineffective troubleshooting of the PBMS.
- Maintenance delays: A required functional test following a propeller blade re-torque was not performed until months after the incident.
Safety action
- The operator initiated a one-time fleet inspection to check the condition of engine forward cowl fasteners.
- The investigation noted the need for regulatory audits to ensure compliance with maintenance approvals and deferred defect policies.