Firefighters conduct a search near the accident site at Muan International Airport in South Jeolla Province on Sunday, the 15th day of the Jeju Air crash investigation. Yonhap

The investigation into the Jeju Air crash has hit a major roadblock after it was revealed that the aircraft's black box did not record the final four minutes leading up to its collision with a localizer at Muan International Airport. Experts warn this will make determining the precise cause of the accident exceedingly difficult.

The investigation borad under the Ministry of Land, Infrastructure and Transport announced Sunday that the U.S. National Transportation Safety Board (NTSB) had extracted and analyzed black box data between Jan. 7 and 11 (local time).

The cockpit voice recorder (CVR) last saved data at 8:58:49 a.m. on Dec. 29, moments before the captain declared a mayday at 8:59 a.m. The flight data recorder (FDR) also ceased recording at a similar time.

The investigation board said it will investigate why the black box stopped recording. Two investigators currently in the U.S. will return on Jan. 13 with the black box for further analysis in South Korea.

An investigation board official said, "We identified on Jan. 4 that the CVR had stopped recording but delayed announcing the findings until the NTSB completed cross-verification."

Police officers examine the Jeju Air crash site at Muan International Airport in South Jeolla Province, Sunday. Yonhap

Missing records hinder crash analysis

Experts express concern over the loss of the most critical data, as the CVR records conversations between the captain and co-pilot, while the FDR captures detailed aircraft performance data, including engine power.

Kim Kwang-il, a professor of aviation operations at Silla University, said, "Without recordings of the decisive moments, investigating the cause of the crash has become extremely challenging."

Key questions remain unanswered, such as when the bird strike occurred, which engines failed and when and whether the landing gear stopped functioning.

While experts agree the crash resulted from a bird strike that caused engine failure and forced an emergency belly landing, this remains speculative in the absence of direct evidence.

Crew decisions remain unclear

The missing data also obscures the pilots' decision-making process. The CVR could have clarified why the crew chose not to land immediately on runway 1 but instead executed a go-around and attempted a belly landing in the opposite direction on runway 19.

Without these records, the reasons behind these critical decisions remain a mystery.

Professor Kwon Bo-heon of Far East University highlighted the limitations of analyzing only landing footage. "For instance, without precise data on altitude, speed, engine status and power supply at the time of the bird strike, it's difficult to refute alternative theories, such as the engines failing due to reasons other than the bird strike," Kwon said.

Kwon suggested that both engines likely failed during or shortly after the bird strike, as the black box relies on engine-generated power.

While auxiliary power units (APUs) could have provided power, it is widely believed the pilots neither had the time nor the need to activate them. Additionally, investigators must determine why the two backup batteries, designed to supply power automatically in case of failure, did not function as intended.

The missing data also complicates assigning responsibility. Whether a bird strike occurred within or outside the airport boundary could influence the level of accountability borne by the Korea Airports Corp. or other authorities.

It remains unclear if the CVR recorded the captain's "mayday" call. While acknowledging the black box data's importance, the Aviation and Railway Accident Investigation Board (ARAIB) emphasized its commitment to investigating other evidence to determine the cause of the crash.

By the end of the month, wreckage from the engine and wings will be transported from Muan International Airport to the ARAIB's test and analysis center at Gimpo International Airport for further examination.

Bereaved families visit the Jeju Air crash site at Muan International Airport, Saturday, two weeks after the accident. Yonhap

Plane lacked backup power to black box

Several media outlets reported that the crashed Jeju Air plane lacked a Recorder Independent Power Supply (RIPS). This backup battery, connected to the CVR, provides over nine minutes of power during a shutdown to ensure critical cockpit audio, including communications from the captain, is recorded in emergencies.

South Korea's Operational Technical Standards for Fixed-Wing Aircraft, updated on Jan. 1, 2018, mandate that all aircraft introduced domestically after that date must have RIPS installed on CVRs. However, the rule is not retroactive, so the accident aircraft, introduced in 2017, was exempt from the requirement.

Unlike the CVR, FDRs — which log electronic signals such as flight paths and speeds — are not required to have backup power. This is because once an aircraft shuts down, all electronic signals cease, leaving no data to record. Thus, a backup power supply for FDRs is deemed unnecessary.

This article from the Hankook Ilbo, the sister publication of The Korea Times, is translated by a generative AI system and edited by The Korea Times.