Introduction
On 21 March 2025, a Jin Air Boeing 737-800, registration HL7561, operating flight LJ-659 from Seoul Gimpo Airport (RKSS) to Pohang (KPOV), South Korea, experienced unreliable airspeed and altitude indications during initial climb following departure from runway 14L. The flight crew declared a PAN PAN emergency and levelled off at approximately 5,500 feet. After troubleshooting and coordinating with air traffic control, the crew executed a return to Seoul, landing safely on runway 14R approximately 50 minutes after departure.
The event is under internal investigation by the airline and will be reviewed by the Korean Office of Civil Aviation (KOCA) for compliance with aircraft systems integrity and standard operating procedures.
Flight & Aircraft Information
• Aircraft Type: Boeing 737-800
• Operator: Jin Air
• Aircraft Registration: HL7561
• Flight Number: LJ-659
• Date of Occurrence: 21 March 2025
• Departure Airport: Seoul Gimpo International Airport (RKSS)
• Destination: Pohang Airport (KPOV)
• Runway Used for Departure: Runway 14L
• Runway Used for Landing: Runway 14R
• Occupants on Board: 48 (42 passengers, 6 crew)
• Injuries: None
• Aircraft Damage: None reported
• Weather Conditions: Visual Meteorological Conditions (VMC)
• Flight Phase: Initial Climb
Sequence of Events
1. Departure and Initial Climb
• HL7561 departed from runway 14L at Seoul Gimpo under VMC at approximately local time 10:15.
• Shortly after takeoff, while climbing through 4,000–5,000 feet, the flight crew noticed inconsistencies between primary airspeed indicators and altitude readings.
• Both pilots cross-checked values across the Captain’s, First Officer’s, and standby instruments, and were unable to validate a reliable set of data.
2. Declaration of Emergency
• The crew declared a PAN PAN urgency call, signalling a technical malfunction requiring priority handling.
• The aircraft levelled off at approximately 5,500 feet to conduct troubleshooting using the Quick Reference Handbook (QRH) for Unreliable Airspeed/Altitude Indication.
• ATC was informed, and the aircraft was provided holding vectors while maintaining visual control.
3. Return to Origin
• After confirming the issue persisted and that flight continuation was not safe, the crew initiated a return to Gimpo.
• The aircraft was vectored for an approach to runway 14R and landed safely approximately 50 minutes after departure.
• The aircraft vacated the runway under its own power. No emergency services were required on arrival.
4. Post-Incident Actions
• Passengers disembarked normally and were later accommodated on a replacement aircraft, HL8014, which departed to Pohang with a delay of approximately 4 hours and 15 minutes.
• HL7561 was removed from service and underwent a technical inspection by Jin Air maintenance teams. The aircraft returned to service approximately 8 hours later.
Technical Considerations & Preliminary Analysis
1. Possible Causes of Unreliable Airspeed/Altitude
• Blocked Pitot Tubes or Static Ports: Contamination or obstructions (e.g. insects, dirt, residual moisture) may have affected sensor accuracy.
• Air Data Computer (ADC) Failure: Malfunctioning ADC units can feed erroneous data to the flight instruments.
• Incorrect Data Comparison: Disparities between Captain’s and First Officer’s instruments may result from faulty sensors, electrical faults, or system misalignment.
2. Aircraft System Design Safeguards
• The Boeing 737-800 is equipped with multiple redundant systems for air data, including:
• Two independent pitot-static systems for each pilot
• A standby instrument system
• Air Data Inertial Reference Units (ADIRUs)
• The QRH contains a defined checklist for handling Unreliable Airspeed, which includes:
• Identifying reliable instrumentation
• Disconnecting autothrottle and autopilot
• Setting pitch and power manually
• Returning to the departure airport if safe flight cannot be assured
3. Crew Performance
• The crew acted in accordance with standard operating procedures by:
• Declaring PAN PAN
• Maintaining safe altitude
• Initiating a return flight promptly
• Coordinating with ATC and ensuring passenger safety throughout
Weather & Environmental Conditions
METAR for RKSS (approx. time of incident):RKSS 210200Z 23005KT 9999 FEW020 SCT040 BKN100 11/02 Q1020 NOSIG=
• Winds: Light and variable
• Visibility: Unrestricted (9999 metres)
• Cloud: Few at 2,000 ft, scattered at 4,000 ft
• Conditions: Visual meteorological conditions, no significant weather reported
Operational & Safety Impact
• Passenger Impact: Passengers were rebooked and transported without incident; no injuries reported.
• Aircraft Availability: HL7561 returned to service within 8 hours, indicating no serious hardware damage.
• Safety Reporting: Jin Air and the KOCA have been notified, and a full systems check and data download (including FDR and CVR) are likely underway.
• Reputation & Procedure Review: Jin Air will conduct an internal safety review and verify pitot-static system maintenance logs and inspection records.
Conclusion
On 21 March 2025, Jin Air flight LJ-659 experienced unreliable airspeed and altitude indications during initial climb from Seoul Gimpo Airport. The flight crew responded promptly, declared PAN PAN, and returned to the airport, landing safely without incident. The nature of the air data discrepancy remains under investigation. No injuries were sustained, and the aircraft returned to service after routine inspection.
Disclaimer
This report is based on publicly available information as of 21 March 2025. While every effort has been made to ensure accuracy, the findings are subject to change pending official investigation outcomes. If you are the rightful owner of any referenced materials and wish for them to be removed, please email: takedown@cockpitking.com.
