By: Marty Krammer

Figure 1. CH-53K Heavy Lift Helicopter.

As part of the CH-53K Heavy Lift Helicopter (shown in Figure 1) Live Fire Test & Evaluation (LFT&E) Program, the LFT&E team recently completed a series of tests to assess the tail drive system (TDS) vulnerability to ballistic damage. The TDS comprises flight-critical components, including multiple tail drive shafts, couplings, support bearings, support mounts, and gearboxes, that provide a means for driving the tail rotor during flight. The live fire tests were designed to support vulnerability analysis by helping to understand the CH-53K system vulnerability and impact to mission expected threat encounters.

The live fire testing of the TDS – conducted at the Weapon Survivability Laboratory (WSL), Naval Air Warfare Center, Weapons Division, located at China Lake, CA – included the following:

  • CH-53K Horizontal TDS Components Test (LF05)
  • CH-53K Pylon TDS Components Test (LF06)


As illustrated in Figure 2, the CH-53K TDS transmits torque while accommodating angular and axial deflections of the tail boom from the main transmission’s tail take-off (TTO) output flange and directs it toward the tail rotor. The horizontal segment of the TDS configuration spans nearly 40 ft and consists of six drive shafts, four hanger bearing assemblies, six multi-disc diaphragm couplings, and one splined disconnect coupling. The tail pylon segment of the TDS spans nearly 20 ft and consists of the intermediate gearbox (IGB), pylon drive shaft, two diaphragm couplings, and the tail gearbox (TGB). The IGB and TGB are each fitted with updated technology, including emergency lubrication and gearbox health monitoring (vibration) capabilities. Each gearbox is designed to handle 30 min of secondary (emergency) gearbox lubrication in the event the primary oil system is compromised and pressure loss occurs. Compared to the CH-53E, the CH-53K TDS is designed to reduce the number of parts, reduce servicing needs, simplify installation and removal, and provide improved ballistic tolerance capability against threats encountered in combat.

Figure 2. CH-53K TDS Description.


The live fire testing performed verified CH-53K Air Vehicle Specification (AVS) vulnerability design requirements and determined TDS flight-critical components vulnerability and capabilities after being hit by ballistic threats. Production CH-53K TDS components were evaluated after being hit while under representative operational conditions (loads and speeds), including pilot actions over a 30-min fly-home scenario. The testing performed helped determine the structural integrity, dynamic stability, and load-carrying capability of TDS components after damage.

The testing addressed the following questions:

  • Can any modifications be made to reduce vulnerability on the CH-53K?
  • What is the CH-53K system-level vulnerability (attrition, forced landing kill levels) to flight-critical systems given a hit by expected threats?
    • Does the component or system continue to function after taking a hit?
    • Can a 30-min post-impact flight operation be maintained?


The testing of the TDS required additional consideration of mission scenarios when defining the test conditions. To fully represent and best assess the TDS performance after a hit, a CH-53K’s 30-min fly-home spectrum (FHS) was formulated by selecting a subset of flight maneuvers or regimes from the CH-53K’s usage spectrum. The 30-min period identified for LFT&E is considered a suitable time for the CH-53K to leave a hostile region and return and land in a safe area of operation. The FHS represents the helicopter’s time spent at each maneuver, torque fluctuations within each maneuver, potential torque fluctuations when transitioning from one maneuver to another, and the rotating speeds of the TDS components. To accommodate the WSL facility’s test equipment and capabilities, steps were taken that simplified the TDS loading process for test operation. An equivalent 30-min CH-53K fly-home spectrum was generated, reducing the number of flight regimes from 23 down to 8 equivalent steps. Regimes or maneuvers having similar power magnitudes were combined and averaged, adjusting for the number of cycles and times operated. As shown in Figure 3, the mission scenario used for LFT&E represented the CH-53K being hit while in a Hover-Out-of-Ground-Effects (HOGE) condition, then transitioning and climbing, cruising, and flying for 30 min prior to landing.

Figure 3. CH-53K TDS 30-min FHS.

Two dynamic fixtures (shown in Figures 4 and 5), were specifically developed for the testing. They included a prime mover T-64 turboshaft helicopter engine, industrial reduction gearbox, and a water brake dynamometer to generate and apply torsional loads and speeds necessary to replicate dynamic operation of the CH-53K TDS FHS specified in Figure 3.

Figure 4. Horizontal TDS Test Setup.

Figure 5. Pylon TDS Test Setup.

Each test began by operating the TDS in the HOGE flight condition and then shooting the targeted TDS component. Once hit, attempts were made to continue operations of the damaged TDS component for an additional 30-min FHS of operation and determine the TDS ballistic tolerance and performance capability. Aircraft TDS sensors were monitored “live” and recorded during testing to determine the TDS health status during operation. The type of sensors included in testing were hanger bearing temperatures and vibrations, as well as gearbox (IGB, TGB) oil temperature, oil pressure, vibrations, and chip detector.


As mentioned previously, testing was conducted at the WSL main site, addressing system-level vulnerability and ballistic tolerance of the TDS against ballistic threats (Figure 6). A total of 17 ballistic tests were performed on TDS components, including shots taken on the tail drive shafts, couplings, bearings, and mounts. Shot selections on TDS components focused on maximizing each threat’s ability to remove material during penetration. Testing explored both shaft and coupling ballistic tolerance to single-aperture wounds and the percent of circumference removed under dynamic conditions. Tests performed fully evaluated the TDS’s performance over mission-representative flight spectrum conditions. The testing showed favorable results for the TDS’s ability to withstand damage and maintain operation for 30 min.

Figure 6. LF05 Horizontal TDS LFT&E – Coupling Evaluation.


Testing was conducted at the WSL main site, addressing system-level vulnerability and ballistic tolerance of the TDS against ballistic threats (Figure 7). A total of 23 ballistic tests were performed on TDS components, including shots taken on the IGB, TGB, pylon shaft, and coupling components. Shot selections on components focused on maximizing threat penetration and material removal on coupling, pylon shaft, and gearbox internal gear teeth, shafts, and bearings. Testing demonstrated each gearbox’s ability to maintain operation for 30 minutes after a hit for times when the loss of lubrication occurred with the primary system and activation of the emergency lubrication system was initiated. Test results were favorable, demonstrating each component’s high resilience to damage and ability to maintain operation for 30 min.

Figure 7. LF06 Pylon TDS LFT&E – TGB Evaluation.


Overall, the results of the CH-53K TDS live fire testing described herein were favorable, and in many cases the TDS performed better than predicted. The testing conducted is considered the most realistic combat helicopter system-level testing performed. The dynamic test capabilities developed and the approach taken provided a complete mission-level live fire evaluation and understanding of TDS vulnerability and capabilities to ballistic threats.


Mr. Marty Krammer is an aircraft vulnerability engineer at the Naval Air Warfare Center Weapons Division in China Lake, CA, currently leading live fire test and evaluations activities on the CH-53K and CMV-22 programs. With more than 27 years of experience, he has supported numerous aircraft vulnerability reduction and live fire test programs, including AV-8B, F-15, F-14, F/A-18, JSF, AH-1, UH-1, H-60, V-22, and CH-53, and has provided subsequent recommendations to reduce the vulnerability of these aircraft. Specializing in aircraft fire, fuel tank self-sealing, and explosion protection, Mr. Krammer also serves as the Navy co-chairman of JASPO’s Vulnerability Reduction and Analysis Subgroup, as well as the Navy Deputy Test Director for the Joint Live Fire Aircraft program, investigating vulnerability issues associated with fielded Navy aircraft. He holds bachelor’s and master’s degrees in mechanical engineering from California State University, Chico and California State University, Northridge, respectively.