Kalitta Air Boeing 747-4HQF accident

On August 17, 2024, a Kalitta Air Boeing 747-400F cargo airplane experienced abnormal airframe vibration while climbing after departure. The flight was operating as a non-scheduled international cargo flight under the provisions of Title 14 Code of Federal Regulations Part 121 from Spangdahlem Air Base (ETAD), Spangdahlem, Germany, to Chek Lap Kok International Airport (VHHH), Hong Kong, Hong Kong.

According to the flight crew, the aircraft departed ETAD and climbed uneventfully to FL250 (25,000 feet). The flight was subsequently cleared to FL310. During the climb at an airspeed of 334 knots, the flight crew detected an unusual vibration that was not consistent with turbulence. They arrested the climb at FL290 and reduced their airspeed to 310 knots, at which point the vibration ceased. The crew then resumed the climb and requested FL310. During the climb, as the airplane accelerated to 334 knots, the vibration reoccurred. The crew again reduced speed to 310 knots, where the vibration again ceased, and the airplane subsequently leveled off at FL310.

During the second climb to FL310, a relief pilot entered the main cargo deck to assess the vibration. The relief pilot reported that the vibration was primarily perceptible in the forward section of the airplane and not near the aft fuselage. While subsequently climbing to FL370, the crew noticed a disagreement between the right and left outboard ailerons on the aircraft status page. The right outboard aileron was deflected about 2 degrees further down than the left outboard aileron. During this phase of flight, the outboard ailerons should be locked out at neutral. A crewmember returned to the main cargo deck and visually confirmed a vibration around the right outboard aileron through a window. After discussing the situation with their company, the crew elected to divert to Istanbul Airport (LTFM), Istanbul, Turkey. The aircraft landed without further incident.

The right outboard aileron is normally attached to the wing at 6 locations numbered 1 to 6 from outboard to inboard. The numbers 1, 2, 3, 5, and 6 locations are similar and consist of an aileron hinge-rib installation that attaches to the wing trailing edge and a fitting and retainer assembly that attaches to the aileron leading edge. The number 4 location is different and is where the aileron actuator attaches to the aileron.

Post-flight inspection of the airplane found three of the five right outboard aileron hinge ribs common to the fixed trailing edge of the wing fractured. The fractured hinge ribs at 3 locations would adversely affect the structural strength of the aileron connection to the wing. Replacement of the damaged components was required, and the extent of the damage meets the criteria for substantial damage as defined by NTSB regulations. The operator removed and replaced the right outboard aileron, fractured hinge-rib installations, and the fractured trailing edge structure. During the rigging of the ailerons after the replacement, the right outboard aileron was found to be drooping about 3 degrees down from its normal position due to low cable tension. The right outboard aileron was rerigged in accordance with the maintenance manual.

The right outboard aileron and damaged components were removed and sent to the Boeing Engineering Test and Technology (ETT) facility in Huntington Beach, California for examination and analysis under NTSB supervision.

Metallurgical examination of the hinge components determined that the three fractured hinge fittings exhibited features consistent with fatigue cracking. Additionally, most of the fastener holes in all the components were found to be worn beyond limits. The mis-rigged condition of the right outboard aileron likely increased loads on the hinge fittings during cruise flight, contributing to the initiation and propagation of fatigue cracks.

Based on their event, Kalitta developed additional maintenance inspection procedures for the ailerons, hinge assemblies, and fitting assemblies for their fleet of 747-400F airplanes to be accomplished at A-checks and heavy maintenance visits. The first set of inspections was a detailed visual inspection of the left and right aileron hinge and attach fittings to be accomplished at each A-check (1200 hours) until the heavy maintenance check inspections. The next set of inspections was to be accomplished at the next scheduled C-check (every 28 months) and included measurements of the various fastener holes in the hinge and fitting assemblies, measurement of the aileron cable tensions, and examination of the rigging status of the ailerons.

As of this report, all the Kalitta airplanes were inspected at least once per the A-check inspection. These inspections revealed that numerous airplanes exhibited low cable tensions in the aileron control system, and nearly all airplanes displayed hinge wear on both the wing and aileron sides that exceeded component maintenance manual (CMM) limits.

In response to the NTSB’s inquiry, two other U.S. operators with Boeing 747-400F fleets reported no history of in-flight vibration events associated with the outboard ailerons. However, both operators identified instances of wear and damage to aileron hinge fittings and related components. One operator noted a single replacement due to a worn fastener hole and subsequently enhanced their inspection protocols to focus on hinge integrity. A review of ten years of fleet data revealed multiple flight control cable-related issues, including significant out-of-limit cable tensions, prompting the implementation of routine tension checks across their fleet. The second operator reported several hinge assembly replacements since 2008, including four due to cracking, and initiated a detailed inspection program for aileron components and cable systems with defined intervals. These findings underscore the importance of proactive maintenance and inspection practices to mitigate potential structural and control system degradation over time.

Verbatim NTSB analysis and probable cause from the NTSB dataset