Flight Details
Aircraft Type: Boeing 737-800 Operator: WestJet Airlines Registration: C-FYBK Flight Number: WS1433 Route: Los Angeles International Airport (LAX), USA to Edmonton International Airport (YEG), Canada Date of Incident: 10 April 2025 Total Occupants: Not publicly specified; typical 737-800 capacity around 160–180 passengers Weather Conditions: No adverse conditions reported during departure
Introduction
On 10 April 2025, a WestJet Boeing 737-800 operating flight WS1433 from Los Angeles to Edmonton experienced a critical oil system failure on its left-hand CFM56 engine shortly after departure. After indications of low oil pressure and rising oil temperature, the flight crew performed a precautionary shutdown of the affected engine and returned to Los Angeles for a safe single-engine landing.
Subsequent inspections revealed substantial internal engine damage, prompting a full engine replacement. The aircraft was returned to service after corrective maintenance on 20 April 2025.
Sequence of Events
Following a standard departure from Los Angeles, the aircraft was climbing towards cruise altitude when the flight crew noticed:
Low oil pressure on the left engine (CFM56-7B series), initially in the yellow caution range No immediate secondary indications of failure, prompting monitoring during climb
Upon reaching FL340, the crew reassessed the oil parameters and chose to level off instead of continuing to climb. The crew then:
Notified ATC of the technical irregularity Initiated QRH (Quick Reference Handbook) checklist procedures for low oil pressure Requested to return to Los Angeles
Shortly after initiating the return, the left engine oil temperature spiked into the red limit zone, indicating an imminent lubrication or mechanical failure. In response:
The crew conducted an in-flight engine shutdown of the affected powerplant The aircraft was configured for a single-engine approach and safely landed on runway 24R about one hour after departure
Taxiing was completed under the power of the remaining right-hand engine without further incident.
Crew & Communication
The cockpit crew demonstrated strong operational management by:
Closely monitoring engine parameter trends, not relying solely on threshold alarms Choosing to halt climb at cruise altitude for further assessment Executing a timely diversion decision based on multiple degrading parameters Maintaining continuous coordination with Los Angeles ATC for priority handling Completing a controlled and unhurried single-engine approach and landing
Cabin crew were likely briefed accordingly, though no emergency evacuation or alerting of passengers beyond standard precautionary advisories was necessary.
Aircraft Systems & Technical Analysis
The CFM56-7B series engine is highly reliable but remains susceptible to oil system-related failures, particularly involving:
Accessory Gearbox (AGB) seals Transfer Gearbox (TGB) interfaces Oil line and scavenge system failures
Post-landing, maintenance inspections identified:
A pool of oil beneath the Integrated Drive Generator (IDG) drain on the left engine A damaged gearbox seal, leading to oil loss Metal particles found in both the Accessory Gearbox/Transfer Gearbox (AGB/TGB) screen and the magnetic chip detector
These findings are indicative of:
Severe internal component wear or failure due to lubrication breakdown Potential gear tooth or bearing damage within the gearbox assembly Systematic contamination that made engine repair impractical
Corrective action involved:
Replacement of the left engine’s starter (damaged by debris contamination) Complete engine replacement with a serviceable CFM56 unit System flush and inspection of connected hydraulic and electrical systems
The detailed presence of metal debris suggested that the engine was nearing catastrophic failure had it remained operating, validating the in-flight shutdown decision.
Passenger Experience & Cabin Conditions
There were no reports of abnormal vibration, smoke, or audible engine noise disturbances in the cabin. Passengers were informed of a technical return to Los Angeles, with standard inflight service suspended during the precautionary return phase.
Passengers were reaccommodated on alternate WestJet services to Edmonton later that day.
Emergency Response & Aftermath
No emergency services were required at landing, as the approach and rollout were routine under single-engine operations. Ground handlers met the aircraft for normal tow-in operations.
The Canadian Transportation Safety Board (TSB) recorded the incident, given the mechanical failure severity, but has not opened a full public investigation, considering the event contained and without injuries.
Investigation Status
Although no full public investigation was initiated, internally WestJet’s maintenance control and technical quality teams would have conducted:
Full fleet-wide monitoring of CFM56-7B gearbox seals Inspection campaigns for IDG and AGB interfaces on similar airframes Analysis of engine trending data to identify early warning signs missed prior to the failure
CFM International (a GE/Safran joint venture) may also review component batch histories to determine if broader action is needed across the fleet.
Root Cause & Contributing Factors (Preliminary)
Primary Cause:
Seal failure in the left engine’s Accessory Gearbox (AGB), leading to critical oil loss and metal contamination.
Contributing Factors:
Progressive internal wear undetected prior to flight Inability of onboard oil monitoring systems to provide sufficient early warning at the leak’s onset Mechanical damage affecting ancillary components such as the starter motor due to debris spread
Safety Recommendations & Industry Impact
The incident reinforces:
Early and proactive crew action when monitoring oil trends, even without full system failure warnings Importance of scheduled engine health monitoring with focus on accessory drive assemblies Need for rigorous maintenance inspection around IDG drains, gearboxes, and magnetic chip detectors Continued awareness campaigns around oil system integrity and engine health on aging 737-800 fleets
Operators may opt to increase frequency of magnetic chip detector checks during overnight maintenance or trend oil analysis sampling.
Conclusion
WestJet flight WS1433’s safe return to Los Angeles following the detection of low oil pressure and a subsequent engine shutdown reflects the professionalism of the flight crew and the critical importance of prompt decision-making when facing engine lubrication anomalies. The identification of significant gearbox damage underscores the hidden risks of oil system failures and the necessity for rigorous engine monitoring.
The event concludes with the full recovery of the aircraft after major maintenance action and no injuries or significant operational disruptions beyond the initial flight delay.
Disclaimer
This article is based on publicly available information and reports at the time of writing. While every effort has been made to ensure accuracy, we cannot guarantee the completeness of the information provided.
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