Water contamination in hydraulic fluid caused ice formation that led to the Jan. 28 crash of an F-35A Lightning II at Eielson Air Force Base, Alaska, resulting in a $196.5 million total loss, according to an Air Force accident investigation board report released this week.
Senior Airman Jose Miguel T. Tamondong, Public domain, via Wikimedia Commons
The mishap pilot from the 355th Fighter Squadron ejected safely with minor injuries after the aircraft became uncontrollable following a second touch-and-go landing attempt. The investigation determined that approximately one-third of the hydraulic fluid in the aircraft’s landing gear struts was water, which froze during extended ground operations in sub-zero temperatures and prevented critical flight control systems from functioning properly.
Colonel Michael B. Lewis, the investigation board president, found by preponderance of evidence that the contaminated hydraulic fluid combined with extreme cold temperatures caused ice to form inside the landing gear struts. When the struts could not fully extend due to ice blockage, Weight-on-Wheels sensors incorrectly indicated the aircraft was on the ground while it was actually airborne, causing the flight control system to switch to “on-ground” mode and rendering the aircraft uncontrollable.
Technical Failure Cascade
The failure began during takeoff when ice formation prevented the nose landing gear strut from fully extending. This caused the gear’s uplock hook to miss its roller, leaving the nose wheel canted approximately 17 degrees to the left and triggering landing gear malfunction warnings.
After consulting with Lockheed Martin engineers for 50 minutes, the pilot attempted two touch-and-go landings to recenter the nose wheel. During these maneuvers, ice continued forming in the main landing gear struts, preventing them from properly extending and causing multiple Weight-on-Wheels sensors to report false “on ground” readings while the aircraft was flying.
The F-35A’s flight control system relies on these sensors to determine which control laws to apply. When sensors indicated the aircraft was on the ground, the system automatically switched to ground-handling mode, which is not designed for flight operations. According to the investigation, the aircraft becomes uncontrollable while airborne if operating under ground control laws.
Air Force Research Laboratory analysis of hydraulic fluid recovered from the wreckage found water contamination levels of approximately 33 percent in both the nose and right main landing gear struts. The contaminated fluid failed to meet minimum cleanliness standards by more than double the allowable particulate limits.
Maintenance and Oversight Failures
The investigation identified significant maintenance procedure violations and oversight failures as contributing factors. Personnel from the 355th Fighter Generation Squadron stored hydraulic fluid barrels outdoors during deployments to Kadena Air Base, Japan, and other locations, exposing them to humid conditions and inclement weather in direct violation of Air Force regulations requiring storage in dry, controlled environments.
The unit’s hazardous materials program lacked adequate supervision, with no primary program manager assigned at the time of the accident. Personnel frequently rotated through the program without proper training, and documentation was incomplete or missing. One hydraulic barrel marked as “empty/consumed” in April 2024 remained in use and contained significant water contamination when tested after the mishap.
Quality control procedures failed to detect the contamination before the hydraulic fluid was used to service the aircraft on Jan. 25, three days before the crash. The investigation could not determine exactly when water entered the system due to inadequate record-keeping.
Decision-Making Under Pressure
During the emergency, the pilot initiated a conference call with five Lockheed Martin engineers through the supervisor of flying to develop recovery options. The engineers recommended touch-and-go attempts to recenter the nose wheel, despite acknowledging the approach had a low probability of success.
The investigation found that while all personnel involved demonstrated competence under difficult circumstances, the decision to attempt multiple touch-and-go landings ultimately led to the aircraft’s loss. Critical health reporting codes indicating main landing gear Weight-on-Wheels sensor faults were reported during the emergency but may not have received adequate consideration in the decision-making process.
A Lockheed Martin maintenance newsletter from April 2024 had specifically warned that Weight-on-Wheels sensor faults during cold weather operations could result in erratic flying qualities making it difficult for pilots to maintain aircraft control. The investigation noted this guidance was potentially overlooked during the emergency response.
Fleet Implications Demonstrated
The failure mechanism was validated nine days later when another F-35A from Eielson experienced a similar malfunction but landed successfully. Post-flight testing of that aircraft confirmed that water contamination in landing gear struts prevented proper extension in freezing temperatures, replicating the exact conditions that caused the Jan. 28 crash.
In that subsequent incident, the pilot was unaware the nose wheel was initially canted 10 degrees left of center and landed without difficulty after the wheel corrected to 5 degrees during touchdown. This suggests the mishap aircraft might have been recoverable had a direct landing been attempted instead of additional touch-and-go cycles that allowed more ice to form.
The investigation noted that both aircraft had flown previous sorties with contaminated hydraulic fluid without incident, likely because shorter ground exposure times and warmer temperatures prevented ice formation.
Systemic Program Concerns
The findings raise broader questions about maintenance quality control across the F-35 program, particularly for units operating in extreme cold weather environments. The investigation highlighted gaps in contamination prevention protocols and the need for enhanced hydraulic fluid quality verification procedures.
Human factors analysis identified ineffective team resource management, inadequate supervisory oversight, and failure to identify hazardous conditions as contributing elements. The investigation applied the Department of Defense Human Factors Analysis and Classification System to evaluate organizational and individual performance factors.
Flight records showed all personnel involved were current and qualified for their duties. The mishap pilot, an experienced evaluator pilot with 554.6 hours in the F-35A and 2,702.5 total flying hours, received commendation for his performance during the emergency.
Recommendations and Response
The investigation resulted in recommendations for enhanced hydraulic fluid quality control procedures, improved hazardous materials program oversight, and updated cold weather operations guidance. The Air Force has not yet publicly detailed specific corrective actions being implemented across the F-35 fleet.
The mishap occurred during a routine training mission where the four-ship F-35A formation was serving as adversary aircraft for air combat maneuvering training. Weather conditions included temperatures of 1.4 degrees Fahrenheit (-17°C) with light winds and scattered clouds.
Emergency responders reached the pilot within one minute of ejection. The aircraft debris remained contained within Eielson Air Force Base boundaries. The investigation took place from Mar. 24 to Apr. 9, with the final report completed in July.
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