Body Surface Landing

Belly landing is a controlled landing in which an aircraft touches down on the runway directly on its underside without deploying its landing gear. This situation typically occurs due to landing gear malfunction, hydraulic system failure, or the pilot forgetting to extend the landing gear. A belly landing is a low-speed impact event performed at a controlled approach speed and usually at a three-degree descent angle.

^{Belly landing of a training aircraft without deployed landing gear (}AA⁾

To mitigate the structural effects of belly landings, modern light transport aircraft (LTA) are equipped with an aluminum alloy underbody structure known as a “belly fairing.” This structure houses components such as the fuel system, landing gear bays, and electrical systems, while also aiming to protect passengers inside the cabin during accidents. Energy absorption during a belly landing is achieved through the deformability of the underbody cladding and frames.

The impact velocity of an aircraft during a belly landing is typically around 3 m/s, classifying the event as a “low-speed impact.” The impact occurs with the rear portion of the aircraft slightly inclined downward, and the dissipation of forces generated during impact is related to the flexibility of the fuselage surface. Analyses generally consider the dynamic nature of the collision, examining time-varying force and velocity parameters.

Finite Element (FE) methods are preferred for modeling belly landing events. In this approach, the fuselage structure is modeled with a denser mesh, particularly in the impact region. Thin-walled aluminum structures are represented using two-dimensional QUAD elements. The impact surface is treated as a rigid ground surface (concrete or compacted soil). Nonlinear solvers such as ABAQUS Explicit are used in simulations.

The Johnson–Cook material model is preferred for realistically modeling structural deformations. This model defines the material’s stress–strain relationship based on plastic deformation, strain rate, and temperature effects, thereby determining the material’s energy absorption capacity during impact. Analysis results are typically evaluated using stress, velocity, kinetic energy, and acceleration graphs.

Dynamic analyses show that during a belly landing, the impact force concentrates initially at the point of contact and then distributes along the fuselage frames. During this process, local crushing and buckling occur in the lower fuselage region. However, within expected impact severity ranges, deformations remain limited and the cabin integrity is preserved. Acceleration values decrease over time and remain within acceptable limits for passenger safety.

Energy graphs indicate that a significant portion of the impact energy is dissipated through plastic deformation. This finding demonstrates that the underbody structure can absorb impact energy, meaning that only localized repairs are typically required after such an event.

Belly landing is a critical accident scenario to consider in aircraft design. Finite element analyses are crucial for predicting fuselage behavior during such incidents and for providing the necessary strength evidence during certification processes. Simulation results demonstrate that when the underbody design is appropriately engineered, it can effectively distribute impact energy, preserve passenger safety, and maintain substantial structural integrity. ^【1】 Such analyses serve as a fundamental engineering tool for verifying compliance of both civil and military aircraft with crashworthiness standards.