Crashworthy Design of Military Rotorcraft Presented by: Dr. Akif Bolukbasi Senior Technical Fellow Phone: (480) 891-5111 E-mail: akif.o.bolukbasi@boeing.com The designs of military rotorcraft are significantly influenced by the crash safety requirements. This presentation will give an overview of the crash impact scenarios, design requirements, and guidelines to achieve the desired level of crashworthiness in a cost and weight effective manner.

Systems Approach to Crashworthy Design Key Subsystems Landing Gears Airframe Structure Seats & Restraint Systems Fuel Systems Rotorcraft crash safety can be defined as the capability of the aircraft to reduce the injuries and prevent fatalities in survivable accidents. An accident is considered survivable if the forces transmitted to the occupants through the seats and restraint systems do not exceed the limits of human tolerance and the aircraft structure remains substantially intact to provide a livable volume to the occupants. The most important rotorcraft subsystems in helping to prevent injuries are the seats, airframe structure, landing gears, and crash resistant fuel systems. Crashworthy design of rotorcraft requires a systems approach with the landing gears, the fuselage structure, and the seats absorbing the rotorcraft kinetic energy and reducing the loads experienced by the occupants to tolerable levels.

Military Rotorcraft Crash Environment The crashworthiness design criteria for military rotorcraft are based on the definition of the crash environment. The crash environment is in general defined in terms of typical crash scenarios including impact velocity, aircraft attitude, and impacted terrain. Other significant parameters including acceleration pulse peak, onset rate, and duration are also utilized to develop design criteria for subsystems such as seats, landing gears, and fuel systems. The frequency of occurrence of crash impacts in terms of impact velocities and terrain are shown in the Figures. Vertical Velocity Longitudinal Velocity Impact Terrain

Military Rotorcraft Crashworthiness Design Criteria (MIL-STD-1290) The current crashworthiness technology and design practices are based on 40 years of military and civil rotorcraft research and development (R&D) activities. Starting in 1960s, the U.S. Army Transportation Research Command has initiated a long-term program to study all aspects of rotorcraft safety and survivability. Under contract to the U.S. Army, The Aviation Safety Engineering and Research (AvSER) Division of the Flight Safety Foundation, Inc. has investigated the problems associated with occupant survivability in rotorcraft crashes. The investigations included accident analyses, full scale aircraft drop tests, and development of design criteria and prototype crashworthy systems. In 1965, the U.S. Army has consolidated the series of reports generated during this study into the first edition of the Crash Survival Design Guide, TR 67-22. Since then the Crash Survival Design Guide has been revised and expanded four times to incorporate the results of the continuing research in rotorcraft crashworthiness technology. The latest edition was published in 1989. Much of the information in the Crash Survival Design Guide was also incorporated into the MIL-STD-1290. MIL-STD-1290 crash impact design conditions for U.S. Army rotorcraft are summarized in the Table.

Military Rotorcraft Crashworthiness Design Criteria (Performance Requirements) MIL-STD-1290 crash impact design conditions for U.S. Army rotorcraft include combined high angle impact conditions also required to be met using an aircraft roll and pitch envelope of up to 10 degrees roll, 15 degrees nose up and 5 degrees nose down aircraft pitch attitudes. Crash performance requirements under these impact conditions are shown in the Table.

Military Rotorcraft Crashworthiness Design Criteria (Major Mass Item Retention Strength) MIL-STD-1290 provides for the ultimate load factors (g) for retention engines and transmissions. Other areas addressed in the MIL-STD-1290 include landing gears, seat systems, occupant and fuel tanks. Specific specifications cited for crashworthy design of these subsystems include MIL-T-27422B - Fuel tanks and MIL-S-58095 Seats . The landing gears are designed to absorb all aircraft kinetic energy during hard landings and crash impacts of up to 20 ft/sec. The fuel tanks are designed using flexible bladders with high tear and puncture resistance. In addition the fuel tank are drop tested at 65 ft/sec impact speed. The fuel lines are also required to incorporate self sealing valves and break-away fittings to minimize fuel spillage during crash impacts.

Vertical Impact Design Requirements Crash Sink rates 26 fps (civil), 42 fps (military) Damage to the fuselage and mission equipment becomes unavoidable. The emphasis is on protecting the occupants from injury. The fuselage may undergo large deformations but should maintain a livable volume for occupants. Post crash fires are minimized using crashworthy fuel systems. The vertical impact energy is absorbed by the landing gears, airframe structure and crashworthy seats. The energy absorption provided individually by each of these subsystems needs to be apportioned achieve the desired level of crash protection with minimum system weight and cost. In addition, a protective shell around the occupants should be maintained, aircraft interior including instrument panels and controls should be delethalized, and post-crash fires should be minimized. Crash-resistant fuel systems also significantly reduce fatalities due to post-crash fires.

Landing Gears Normal Landing Sink Rates 8 - 10 fps No yielding of the landing gear (military) Hard Landing Sink rates 10 fps (civil), 20 fps (military) Emphasis on preventing fuselage and mission equipment damage Plastic deformation or damage to the landing gear is acceptable The landing gears are designed to absorb all aircraft kinetic energy during hard landings and crash impacts of up to 20 ft/sec. Their primary function during hard landings is to protect the airframe and minimize damage.

Effect of Roll and Pitch Requirements The crash survivability of the aircraft is affected by the roll and pitch attitude of the aircraft during crash impacts. Typical design conditions are 10 degrees roll, 15 degrees nose-up pitch and 5 degrees nose-down pitch.

Landing Gear Testing Landing gear qualification includes drop tests of individual gears. Although not required for qualification, tests of multiple gears using and iron bird test fixture is highly desirable.

Skid Landing Gears Slid landing gears are typically used in commercial helicopters. The energy absorption capability is provided by elastic-plastic bending of the landing gear cross tubes and by dampers (if available).

Military Rotorcraft Crashworthiness Design Criteria (Seats) There are two seat dynamic test requirements specified. Test Condition 1 provides downward, forward, and lateral loads. The triangular pulse parameters as follows: Peak Acceleration = 48g Velocity Change = 50 ft/sec Time Duration = 0.065 sec. Test Condition 2 provides forward and lateral loads. The triangular pulse parameters are: Peak Acceleration = 30g (Cockpit) 24g (Cabin) Time Duration = 0.103 sec (Cockpit) 0.130 sec (Cabin) The peak acceleration can be reached between 0,4t and 0.8t, where t is the duration of the triangular pulse.

Crashworthy Seat Testing A drop tower or a sled may be used to conduct seat dynamic qualification tests. Drop tower is preferred for Seat Qualification Test #1 since the primary acceleration is in the vertical direction. A sled is preferred for Seat Qualification Test #2 since the primary acceleration is in the longitudinal direction. Dynamic Test #1 (Fwd & Down) Dynamic Test #2 (Fwd & Side)

Cost-Benefit Trade Study for Crashworthy Rotorcraft Design The initial cost and weight associated with incorporating crashworthy deign features can be offset by cost savings associated with reduced occupant injuries and aircraft damage over the life of the aircraft. The optimum level of crash protection depends on the aircraft configuration and requires a trade study as shown in the Figure. The desired level of crashworthiness can be achieved in the most cost and weight effective manner during the preliminary design stage of a new rotorcraft. Crash safety of existing rotorcraft can also be improved by retrofitting crashworthy subsystems if it is practical to do so. However, special care should be taken to ensure compatibility between the retrofitted subsystems and other parts of the rotorcraft to ensure that the retrofit will be effective to improve the crash safety of the rotorcraft.

Conclusions Rotorcraft should be designed to prevent fatalities and minimize injuries during survivable crash impacts. The probability of occupant survival will be increased if proper attention is given to these design guidelines. In developing crashworthy systems through a Systems Approach, design reliance on any one of the subsystems should be avoided. The landing gears for example may not be effective during crash impacts on soft soil and water. The effectiveness of the fuselage energy absorbing structure may also be significantly reduced due to aircraft impact attitude and impact terrain. Introduction of composite materials into modern rotorcraft airframe structures also present challenges to provide energy absorption capability comparable to conventional aluminum structures.