Bell Helicopter Crash History
A Bell Model 205A-1 helicopter, owned and operated by the LAFD crashed in Griffith Park during an airlift rescue operation in Los Angeles on March 23, 1998. The helicopter was airlifting an injured child from a car accident when the helicopter’s tail rotor yoke failed and caused the aircraft to crash, destroying the helicopter and killing the child, two LAFD paramedics and an LAFD helicopter apparatus operator.
The following animations were prepared for Baum Hedlund to be shown as evidence during the 2006 product liability trial at Los Angeles Superior Court against Bell Helicopter Textron, Inc., and Bell Technical Services, Inc. The trial was conducted by lead trial counsel Ronald L. M. Goldman and J. Clark Aristei of Baum Hedlund Aristei & Goldman.
Fatal Helicopter Crash Flight Path in Griffith Park
This animation shows the path the Los Angeles Fire Department Bell 205A-1 helicopter took through Griffith Park seconds before the crash.
Helicopter Crash Yoke Breakdown – Tail Rotor of Helicopter Involved in Deadly Los Angeles Crash
This animation shows the LAFD Bell 205A-1 helicopter and zooms into the tail rotor yoke, disassembling all parts on the way. It includes a depiction of the removal of the tail rotor blades, the crosshead, the trunnion set with deformable flapping stops, the indicator, balance arm, control tube, and all bolts, bushings, washers, and nuts.
Helicopter Shot Peening Process – Griffith Park Helicopter Crash Trial Animation
The Bell 205A-1 tail rotor yoke is heat-treated, forged 15-5 PH (precipitation hardened) stainless steel with an ultimate tensile strength of 170-200 KSI. The yoke has cross-sections of reduced thickness to allow for flexure and cadmium plating for corrosion resistance.
After the yoke, in this case, was machined, it underwent a process called “shot peening.” This animation depicts the shot peening process – tiny beads are blasted onto the yoke. This process induces residual compressive stresses that impart greater fatigue strength to the yoke. One of the issues in the case was whether this Bell 205A-1 tail rotor yoke had been subjected to shot peening that did not conform to Bell Helicopter Process Specification (BPS) 4409. Inadequate shot peening reduces the residual compressive stress and fatigue strength of the yoke.
Bell 205A-1 Helicopter Part Fails – Bell Helicopter’s Cracked Yoke Leads to Crash
This animation depicts the forces being exerted on the tail rotor blades, which weaken the tail rotor yoke and resulted in a crack in the tail rotor yoke. If large enough, these forces may result in “static overload” damage, which creates a loss of compressive residual stress and a resulting fatigue fracture. Static overloads occur when the tail rotor is stationary, not when a helicopter is in flight. Static overload can be caused by improper ground handling (such as using the tail rotor blade as a handhold to move the helicopter), collision with a vehicle, improper bearing removal while the yoke is off the helicopter, and wind gust or jet blast. The resulting crack eventually propagates throughout the yoke, causing it to separate and fail in flight.
Griffith Park Helicopter Accident Experienced Flexure Loss
This animation shows the cross-sections of the tail rotor yoke. The cross-sections are of reduced thickness to allow for flexure (bending). The yoke is designed to bend within fixed parameters without losing its strength. If a force strikes the tail rotor large enough to bend it beyond its design tolerance, it may result in “static overload” damage, which creates a loss of residual compressive stress and a resulting fatigue fracture. Static overloads occur when the tail rotor is stationary, not when a helicopter is in flight. Static overload can be caused by improper ground handling (such as using the tail rotor blade as a handhold to move the helicopter), collision with a vehicle, improper bearing removal while the yoke is off the helicopter, and wind gust or jet blast. The animation shows the flexure of the yoke 1) within its design tolerance; and 2) outside of its design tolerance.
