The CH-53K King Stallion is the U.S. Marine Corps (USMC) next-generation, state-of-the-art, heavy-lift helicopter. Designed to be a survivable platform when operating in a combat environment, the aircraft was designated as a covered program under Live Fire Test and Evaluation (LFT&E). It was granted a waiver from testing a production aircraft due to the excessive associated expense. Under U.S. Title 10 code statute (e.g., USC 2366 or 4172), however, a covered program may not proceed beyond low-rate initial production (LRIP) until realistic survivability testing and supporting vulnerability assessment of the system are complete. On 21 December 2022, the CH-53K entered full-rate production (FRP) and its deployment phase, following a decision review by Mr. Frederick (Jay) Stefany, Acting Assistant Secretary of the Navy for Research, Development, and Acquisition. The approving decision occurred after a review assessing the results of Initial Operational Test and Evaluation (IOT&E), LFT&E, production readiness reviews, and risk and affordability analyses.
For the CH-53K LFT&E program, “realistic survivability testing” included evaluation of the vulnerability of the system against threats expected to be encountered in combat. Live fire testing results supporting the FRP decision covered both component- and system-level evaluations against ballistic threats targeting flight-critical components and systems using CH-53K production-representative parts and ground test vehicle (GTV) aircraft. Of the live fire testing completed, the CH-53K full-up system-level (FUSL) LFT&E testing, referred to as the LF12 series, stood out in achievement and raised the benchmark for system-level testing for next-generation combat aircraft systems.
THE H-53 CAPABILITY AND MISSION
The current USMC rotary heavy-lift aircraft, the CH-53E, was designed in the 1960s and introduced in 1980 as an Engineering Change Proposal (ECP) to the CH-53D. Subsequently, the CH-53K was developed to address and satisfy the future needs and requirements of the USMC, with improvements in operational capability, interoperability, survivability (vulnerability), reliability, and maintainability while also reducing total ownership costs.
The CH-53K heavy-lift helicopter is a major systems acquisition program managed by the Naval Air Systems Command (NAVAIR) H-53 Heavy Lift Helicopters Program Office (PMA-261). The aircraft, developed by Sikorsky Aircraft Corporation, is a ground-up redesign that incorporates the latest in helicopter technology, including new General Electric GE38-1B engines, fly-by-wire digital flight controls, and composite airframe structures. The advanced capabilities of the drive and rotor systems enable the aircraft’s lift performance to achieve (in certain conditions) nearly three times the lift capability of its predecessor, the CH-53E.
The CH-53K is designed to operate day and night in adverse weather and low-visibility conditions and is capable of rapidly embarking aboard, and operating from, amphibious assault aviation ships, air-capable ships, and aircraft carriers. It provides the capability to move and sustain forces and operate from a sea base and is equipped with various assault vehicles. In addition, the heavy-lift capability provides sustainment to forces ashore by being able to move large amounts of fuel, water, food, ammunition, and equipment.
THE CH-53K LFT&E PROGRAM
Detailed planning of LFT&E for the CH-53K began by taking the critical design review (CDR) vulnerability assessment, risk reduction live fire testing, and analysis trade study results and identifying agreed-on focus areas for LFT&E. The CDR baseline vulnerability analysis assessment on the CH-53K platform identified flight-essential (critical) components and systems contributing to the vulnerability of the overall system. As a follow-up, a comprehensive strategy or plan was developed, laying out the approach, tasking, resources, and methods to be taken for LFT&E and its execution. The ballistic testing data gathered, combined with incorporation of the results back into the analyses, would provide insight and understanding of the CH-53K capabilities and impacts caused by threats expected to be encountered in combat.
Testing was conducted on critical systems (i.e., fuel propulsion, drive, hydraulic, rotor, flight control, and structure) and comprised evaluation against both specific threats defined in the CH-53K Air Vehicle Specification (AVS) and other threats, including small- and large-caliber projectiles. To accomplish the overall objective, numerous live fire tests were designed to produce sufficient data to accomplish the following subobjectives:
- Evaluate ballistic damage effects, including synergistic effects, due to ballistic threat impact on components and systems to support assessment of operational impact.
- Determine if the component/system meets specification for maintaining operation after a hit.
- Provide insight into the CH-53K battle damage assessment and repair processes and procedures for the expected threat-induced damage.
- Gather data in support of verifying AVS vulnerability specification design requirements and updating component vulnerability probability of component dysfunction given a hit [Pcd|h] values.
- Provide insight into potential design changes that would lead to reduction in the CH-53K vulnerability.
CH-53K FUSL LFT&E TESTING (LF12 SERIES)
The objective of CH-53K FUSL live fire testing (LF12 series) was to provide information to assess the overall system-level vulnerability (attrition and forced-landing kill levels) of CH-53K flight-critical systems to threats expected to be encountered in combat. System-level objectives in testing focused on specific vulnerability issues identified in the program for fuel, propulsion, drive, hydraulic, flight control, rotor, and structure systems.
GTV Article
The CH-53K GTV (shown in Figure 1) was leveraged to support LFT&E program efforts. The GTV was a near fully functional, instrumented preproduction engineering development aircraft, and production-representative in the areas evaluated under LFT&E. Modifications incorporated into the GTV included additional instrumentation (thermocouples, pressure transducers, accelerometers), added hardware for remote flight operation, added T-taps on aircraft digital buses monitoring and recording of data (e.g., ARINC-429, RS-422, MIL-STD-1553, ethernet and PCM bus identifications), and onboard firefighting and fuel tank inerting to combat any ballistically induced fires and fuel vapor reactions created during testing.
Testing Approach
The Naval Air Warfare Center Weapons Division (NAWCWD), Aircraft Vulnerability Division, and Weapons Survivability Laboratory (WSL) at China Lake, CA, were responsible for oversight, planning, and execution of CH-53K FUSL live fire testing conducted from April 2020 through December 2021. The testing was designed to address vulnerability data gaps for CH-53K systems (i.e., fuel, hydraulic, structure, flight control, drive, tail rotors, engine bay fire suppression effectiveness, and cascading effects). Testing on the GTV included ballistic shots taken under static nonoperating and dynamic in-hover conditions, as well as simulated ballistic damage evaluations for other nonoperational and operational conditions. Testing was executed in three phases (Phases I, II, and III) with the test event order executed from the least to the greatest risk of causing catastrophic damage to the GTV aircraft.
LF12 Phase I Testing
LF12 Phase I testing focused on understanding the sponson fuel tanks bladder self-sealing performance and fuel leakage, the likelihood of fire ignition/sustainment, and the airframe/sponson structure response to ballistic impacts. Testing included ballistic impacts to the sponson fuel tanks under replicated airflow of cruise and hover flight conditions. WSL’s four-engine High-Velocity Airflow System (HIVAS) served as the source for airflow coverage, providing a realistic environment for fire ignition and sustainment. To achieve airflow coverage and speed requirements, a custom nozzle was developed, having HIVAS adaptability and interchangeability to support the aircraft’s right- and left-side testing. The setup (shown in Figure 2) included the GTV configured for static nonoperating condition with the fuselage resting on three jack stand supports that interfaced with existing aircraft jack points. The aircraft was held in place with a series of steel cables that attached to aircraft tiedown rings (fuselage, landing gear) and test pad rail locations. A facility fuel conditioning and storage unit provided the conditioning of fuel at the test specified temperature.
LF12 FUSL Phase II/III Testing
LF12 FUSL Phases II/III testing focused on understanding the vulnerability of CH-53K systems (i.e., fuel, hydraulic, structure, flight control, drive, and rotors) evaluated at the system-level (operational) but also capturing synergistic (cascading) effects and situational awareness available to pilot/copilot and crew during and after a hit. The setup for Phase II/III testing accommodated dynamic operation of the GTV aircraft when placed in a 1-G hover-in-ground-effect (HIGE) condition. As shown in Figure 3, the aircraft was mounted atop a hover stand, comprising three tower supports that interfaced with the aircraft’s landing gear. Each tower has provision to allow for aircraft motion but within a limit to assure safe operation during a 1-G hover flight. In addition, the aircraft was elevated with the bottom of the fuselage positioned 115 in above the pad surface. A facility fuel conditioning and storage unit was also present on the pad for preheating onboard fuel to the test-specified temperature.
GTV Hover Stand
A custom hover stand was developed to allow the CH-53K GTV to dynamically operate in hover during testing. The stand allowed the aircraft (up to 74,000 lbs) to safely achieve a test hover condition (HIGE) while ensuring precise gun aiming. The hover stand comprised three tower sections, which interfaced with the aircraft’s tires and landing gear strut assemblies (see Figure 4). The three towers were of similar design, and each consisted of a floating post to which the aircraft was attached and a fixed restricted frame that anchored to the facility test pad. Each post allowed limited movement in all three planes. Twenty-four rubber pneumatic actuators (airbags) provided dampening of resonating helicopter vibrations to mitigate the potential for entering ground resonance conditions. Additionally, the airbags provided aircraft stabilization to ensure precise gun aiming in hover. Each tower also included a quad set of upper and lower airbags. During operations, all the airbags were remotely filled and/or vented, with airbag pressures monitored by an operator from the control room.
During testing, the airbags were first filled to position so the floating posts and aircraft aligned with the center position of the movement guides. With minimal collective set, the aircraft’s engines were then started and engine throttle controls were increased to bring main rotor speed to FLY operation speed (103% Nr). Collective control was then slowly added while bottom airbag pressures were lowered to maintain the posts in the center position of the movement guides. Videos of all three posts and the rear of the aircraft were viewed and monitored, and rudder control was applied to counter any visual aircraft yaw as collective was added. The aircraft achieved hover when the pressures in the bottom airbags were equal to or lower than the upper airbags. The airbags were used to pinch the floating posts, providing just enough stabilization to preclude the helicopter from wandering in hover. Shutdown of the aircraft was similar but in reverse order.
GTV Remoting and Controls
FUSL live fire testing of the CH-53K GTV required the aircraft to be controlled and operated remotely from a test control room located a safe distance away from the aircraft (1,000 ft). To accomplish the remote operation capability, the aircraft’s flight controls (collective, yaw, and cyclic [pitch and roll]) and several other cockpit functions performed by pilots were remoted through a controller box located in the GTV’s cabin (see Figure 5).
Movement of flight controls (collective, yaw, and cyclic) during operation were accomplished through four remotely controlled electric linear actuators connected to pilot cockpit controls interfaces (shown in Figure 6). Collective and yaw were controlled each by a single actuator, and the cyclic inceptor was controlled by two actuators in such a way to achieve fore-aft and side-to-side movements. Four cockpit panels and their functions were remoted and replicated for aircraft operations controlled from the facility control room. Remote control capability for testing was achieved by computer program, touchscreen displays, throttle controls, CompactRIOs, relays, voltage dividers, and power supplies. The remote control system developed for testing fully replicated cockpit panel functions and interfaced in the same way as the production panel would with the aircraft’s systems. Two additional control function overrides were also needed to remotely operate the aircraft. The first provided provision to override the Weight-on-Wheels (WOW) indication to the fight controls (even though the landing gear was down, resting on the airbag-cushioned hover test stand mounts). The second provided independent battery backup override provision for the three engines (Number 1, 2, and 3) and auxiliary power unit (APU) fuel feed selector valves to move to stop-fuel-feed in the event of an emergency (e.g., a facility power failure or loss of GTV control).
Data Acquisition, Monitoring, and Recording
A custom Data Acquisition System (DAS) was also developed for the CH-53K FUSL testing (see Figure 7). The DAS was based on the National Instruments (NI) LabVIEW development environment and was tailored for the CH-53K GTV. The data messages recorded and monitored live during testing originated from 58 ARINC-429 buses residing on the GTV. All data messages collected were time-stamped with the Inter-Range Instrumentation Group time code B (IRIG-B) standard for synchronization with other data (standard video, high-speed video, analog data, and firing trigger signals) gathered during testing.
For every dynamic hover operation performed, more than 21,000 digital channels of data were recorded at a rate of 40 Hz for up to 2 hr. Of these channels, a subset of 161 critical data parameters was displayed live in the control room for situational awareness monitoring by operators and test engineers (see Figure 8).
In addition to aircraft buses data, facility instrumentation (81 analog channels) was added throughout the aircraft to provide additional situational awareness during testing. Numerous thermocouples, pressure transducers, accelerometers, and other sensors were installed at targeted areas of the aircraft; were recorded; and were monitored live in the control room. Further, several radial and triaxial accelerometers were installed around the aircraft (at the cockpit, hanger bearings, and tail boom), and vibrations were monitored and recorded live during dynamic tests to ensure they were within threshold and to avoid potential damage to the aircraft. When vibrations were a primary data source, raw data were analyzed post-test using Fast-Fourier Transform (FFT) NI DIAdem software.
CH-53K FUSL LFT&E TESTING SUMMARY
Over the three phases of execution, the CH-53K FUSL LFT&E testing (LF12 series) program completed a total of 105 test events in a 19-month period (Figure 9). Systems evaluated in testing included fuel, hydraulic, drive propulsion, rotors, and structure. Actual results and findings from testing could not be included because of their sensitive nature.
The realistic in-operation FUSL testing successfully showed how the systems responded to ballistic damage and provided additional information on situational awareness of pilots and crew on the post-hit state of the aircraft and onboard conditions that could not be duplicated by other modeling or simulation methods.
Overall, the testing program completed 801 ballistic and nonballistic test events and successfully demonstrated the robust design of the CH-53K helicopter. In terms of survivability, the CH-53K met program requirements. Moreover, the testing and analysis performed for this program answered all the LFT&E critical issues and verified compliance to both the Survivability Key Performance Parameters (KPP) and Force Protection KPP identified in the Capability Production Document (CPD) for the CH-53K program.
ABOUT THE AUTHOR
Mr. Marty Krammer leads the Aircraft Vulnerability Division at NAWCWD, where he supports multiple Navy, Air Force, and Army aircraft programs. He has specialized expertise in aircraft fire, fuel tank self-sealing, explosion protection, and aircraft LFT&E and has supported many aircraft vulnerability reduction and testing programs, including the AV-8B, F-15, F-14, F/A-18, JSF, AH-1, UH-1, H-60, V-22, and CH-53 programs. Mr. Krammer holds a bachelor’s and master’s degree in mechanical engineering from California State University, Chico, and California State University, Northridge, respectively.
