What happened
On August 29, 2020, at 17:30 JST, a Skymlar Airlines Boeing 737-800, registered JA73NM, departed Tokyo International Airport for Fukuoka Airport. While the aircraft was climbing through 8,500 ft at a speed of 240 knots, the flight crew heard a loud impact sound near the left side of the captain's seat. This was immediately followed by a transient, burning smell within the cockpit.
Following the impact, the crew monitored all flight instruments, including engine performance and pressurization, and found no immediate abnormalities. Due to the high workload of navigating the departure, the crew discussed the event after being transferred to the Tokyo Area Control Center. The captain reported the incident to the company via radio and requested a post-flight inspection. The crew elected to continue the flight to Fukuoka, opting to fly at a lower cruising altitude and at a turbulent-air penetration speed to mitigate structural load and ensure a quicker descent to a safe altitude if an emergency arose.
Upon arrival at Fukuoka, a physical inspection by maintenance personnel revealed a blood-like residue on the left outer skin of the aircraft's nose, along with several dents in the outer skin and deformations in the inner frame and stiffeners. No engine damage or bird remains were found during the internal engine inspection.
The investigation
The Japan Transport Safety Board (JTSB) investigated the incident, involving assistance from the United States' NTSB as the state of design and manufacture. Investigators analyzed flight data from the FDR and cockpit voice recordings from the CVR.
Because the biological residue was wiped away during the initial damage assessment, the specific species of the bird could not be identified. However, the JTSB utilized numerical simulations provided by the NTSB to estimate the bird's mass. Based on the deformation of the airframe at the recorded impact speed, the investigation concluded the bird likely weighed between 1.8 kg and 3.6 kg (approximately 4 to 8 lbs). The JTSB noted that the bird may have belonged to the hawk or heron families.
Findings
- The aircraft sustained structural damage to the nose section due to a collision with a bird during the initial climb.
- The abnormal smell in the cockpit was likely caused by bird fragments being ingested into the left engine's compression stage and subsequently heated.
- The flight crew was likely unable to take evasive action because the approach of the bird was not visually detected.
- The decision to fly at a reduced altitude and speed was a prudent measure to reduce structural stress and decrease the time required to descend to a safe altitude in the event of a secondary emergency.