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CH-47D Chinook

The CH-47D aircraft has a design service life of 10,000 flight hours. The MH-47 engineering staff estimated that the service life of the MH-47G is 3,000 hours. The MH47G aircraft are remanufactured from CH-47D, MH-47D and MH-47E aircraft, which already had an average of 6,400 hours on their airframes for an airframe total service life of 9,400 hours. At the end of the service life, a decision is made to recapitalize or replace the aircraft. A recapitalization results in a type designation change, capability upgrades, and additional service life.

The CH-47D was the result of June 1976 contract for a modernized Chinook. The Army recognized that that the Chinook fleet was rapidly reaching the end of its useful life and signed a contract with Boeing to significantly improve and update the CH-47. Three airframes, CH-47A, CH-47B, and a CH-47C, were stripped down to their basic airframes and then rebuilt with improved systems to provide three CH-47D prototypes. The first CH-47D was rolled out in March of 1979 and the aircraft became operational with the 101st Airborne Division in 1984. A total of 472 CH-47A, B, and C model Chinooks were converted to CH-47D's.

The Army announced August 9, 1999 that it had grounded the Chinook helicopter fleet following the discovery of cracked transmission gears during an aircraft overhaul. The U.S. Army Program Executive Office, Aviation at Redstone Arsenal, Ala. and the Boeing Company in Philadelphia were working with aviation units in a records search to locate all gears of the suspect type and design. Army leaders took this action as a prudent measure to ensure the safety of soldiers. No accidents or mishaps had been attributed to these gears. The Program Executive Office for Aviation, with Boeing and the Army Aviation and Missile Command were also investigating the condition of all similarly designed transmission gears. The most plausible cause of the problem was poor grinding of the gear's spherical bearing raceway during manufacture. Improper grinding is a critical flaw because it could produce surface cracks and potential gear fractures. Such cracks and fractures could cause the rotor blades to come out of phase, mesh with each other, lock the transmissions, and potentially lead to a crash.

The CH/MH-47 programs experienced increased usage due to combat and contingency operations. For example, the CH-47 program was flying at up to three times peacetime programmed rates due to ongoing GWOT operations. Usage data at the asset level for CH-47 aircraft is available, along with usage data for other Army aircraft and ground systems. Usage data at the type designation (mission design series) level for the MH-47 program is available via a manual data call from PEO Rotary Wing. Through this study, it has been confirmed that usage data is generally available for aircraft programs. However, a requirement should be established that requires usage data to be collected and reported in enterprise-level systems on a routine basis. This will provide enterprise-wide data visibility and availability for use within the OPTEMPO methodology for budget justification and capital planning purposes.

In addition to increased usage, the CH/MH-47 programs have experienced structural fatigue due to operating at increased weights or at the maximum weight capacity for extended periods of time. Harsh environmental conditions (extreme temperatures, salt water, sand, mountainous climate and terrain), higher altitudes (which stresses the airframe), increased takeoffs and landings, and more aggressive maneuvering (turns and banks) are also leading to increased stress. These conditions led to increased strain and stress on the airframes, including cracking in the airframe, as the increased weight coupled with combat maneuvers and altitude continued to stress the airframe and shorten life expectancies, despite solid maintenance procedures and effective service life extension programs. The sand environment also had a significant impact on the rotary wings.

Structural fatigue is evaluated by manual inspection and testing for the CH/MH-47 aircraft. The program does not have a method for quantifying the overall impact of fatigue on the airframe due to the factors and conditions noted above. CH/MH-47 engineers indicated that the primary cause of structural fatigue is the operating weights at which the aircraft are flying to support combat and contingency operations and that this should be addressed in the methodology by factoring the percentage increase in weight carried to support combat operations above the aircrafts typical peacetime operating weight.

Similar to other aircraft programs, such as the C-17, the CH-47 program has developed a fatigue-tracking capability. The CH-47 is developing the Health and Usage Monitoring System (HUMS), which is a capability that will measure and track structural fatigue impacts on the program when future F model aircraft are fielded. When HUMS data is available, this data would replace the existing fatigue estimate in the OPTEMPO methodology.

The OPTEMPO methodology factors total losses, utilization expressed in flight hours, and a fatigue multiplier at a rate of 42.86%, which is based on the percentage increase in weight carried, to support combat operations, above the aircrafts typical peacetime weight. For the Army CH-47D, the typical peacetime operating weight is approximately 35,000 pounds, while the weight typically carried to support combat operations is 50,000 pounds, the maximum weight capacity. This equates to a 42.86% increase in weight above that normally sustained during non-combat operations.

Data provided for the D fleet, current as of February 14, 2008, reported that the D model fleet had 336 active assets. From program inception in 1982 for the D model, battle losses totaled nine and non-combat losses were seven. The total number of hours flown on the active assets for the D fleet is 2,155,354. Of these hours, the wartime hours flown is 325,835.60, which supported GWOT operations from 2001 to p2008 in Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF). This results in a peacetime usage of 1,829,518.40 hours. The fatigue multiplier, which is applied to the wartime hours to account for asset fatigue that is occurring during contingency/GWOT operations, is calculated by adding one to the 42.86% weight increase carried for GWOT operations. The peacetime hours and the wartime hours, after being weighted by the fatigue multiplier, are summed and divided by the total hours available (10,000 hours per aircraft times 336 active assets). The resulting number is multiplied by the active asset quantity. Battle losses and non-combat losses are then added to this number to determine the number of equivalent lives consumed. The result is 245.50 equivalent lives consumed, or 69.89%, or 70%, [246/352] of the program service life had been consumed.



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