Under a modification program, 161 Air Force Reserve and Air National Guard tankers were re-engined with the TF-33-PW-102 engines. The re-engined tanker, designated the KC-135E, is 14 percent more fuel efficient than the KC-135A and can offload 20 percent more fuel. The present KC-135E model is the result of a 1982 modification program to the KC-135A fleet with the goal of reducing noise, increase take off thrust, and reduce fuel consumption. The modifications entailed changing the J-57 engine for theTF-33-PW-102 engines, improved breaks, avionics up grades and new horizontal stabilizers.
This fleet has been slowly modernized over fifteen years, largely as a result of funds added by Congress for the KC-135E to R engine conversion program. The Air Force initially replaced the TF33 engines on 20 Guard and Reserve KC-135E aircraft with CFM-56 engines at a cost of about $436 million. These aircraft represent the last of the KC-135s for which re-engining funds have been approved. The last four aircraft were completed in fiscal year 1998. By the completion of the program, it had installed CFM-56 engines on 410 KC-135s.
KC-135E Service Life
The Air Force projected that E and R models have lifetime flying hours limits of 36,000 and 39,000 hours, respectively. According to the Air Force, only a few KC-135s would reach these limits before 2040, but at that time some of the aircraft would be about 80 years old. The Air Force estimates that their current fleet of KC-135s have between 12,000 to 14,000 flying hours on them-only 33 percent of the lifetime flying hour limit and no KC-135E's will meet the limit until 2040. Flying hours for the KC-135s averaged about 300 hours per year between 1995 and September 2001. Since then, utilization is averaging about 435 hours per year.
Only six KC-135s would need to be retired by 2040 because they would exceed their airframe life. According to 1996 letter from the defense secretary's office, the planes still had 35 years left in them.
According to the Air Force, the Mission Capable Rate for KC-135 tankers is 81 percent-the highest in the Air Force inventory. For comparison purposes, the KC-10 fleet is entirely in the active component, and the 59 KC-10s had an average mission capable rate during the same period of 81.2 percent. The B-2 Mission Capable Rate by comparison was 39 percent.
The March 2004 Defense Science Board Task Force Report on Aerial Refueling Requirements found that "Usage, which induces material fatigue, is not the driving problem. Total flying hours are relatively low for the KC-135s: the current airframe average is about 17,000 hours. Fatigue life is estimated to be 36,000 hours for the E, 39,000 hours for the R. Cycles are commensurately low on average (3800 for the R and 4500 for the E). Thus, the airframes should be capable to the year 2040 based on current usage rates."
A 2005 Air Force study [KC-135 Assessment Report. Air Force Fleet Viability Board. Wright-Patterson AFB. September 2005] estimated, with numerous caveats, that KC-135E aircraft upgraded to the "R" configuration would remain viable until 2030. The "Air Force Fleet Viability Board, KC-135 Assessment Report" cautioned that, before retiring KC-135s, the Air Force needs to conduct destructive testing so it can proceed on an informed basis. However, the Air Force has not complied with that recommendation.
The E-model economic service life is markedly different because of the difference in age and technology of some of its major components, most notably the engines. The basic airframe should, in theory, last as long as the R-model, but the age of the engines points to the likelihood that upkeep could become expensive (in terms of parts and maintenance man-hours). The TF-33 (E-model) engines were previously used but refurbished to an expected 6,000 hour service life. At current use rates, the TF-33 began to need another major overhaul around the turn of the century. Additionally, since the TF-33 does not meet FAA Stage III noise requirements for the year 2000, more time and money must be expended to ensure compliance.
KC-135E Engine Struts
The KC-135E engine struts were obtained from retired 707 and 720 airframes. Because of their exposure to engine heat, severe heat-induced corrosion and fatigue have occurred. The Oklahoma City Air Logistics Center at Tinker Air Force Base depot developed an "interim" strut repair for about $100 thousand per strut, awaiting an FY06 program initiation of a fully reworked strut repair with a cost of about $1 million per strut.
The March 2004 Defense Science Board Task Force Report on Aerial Refueling Requirements found that "The struts that attach the engine to the wings of the KC-135E models are a prime example of the problems of aging and environment. The struts are near the end of their service life due to exposure to high temperatures and corrosive environments and, assuming the KC-135Es are not retired, a major structural repair to the KC-135E struts is planned for initiation in FY06."
Secretary of the Air Force Dr. James G. Roche tasked the board in June 2004 to perform an independent, in-depth analysis of a portion of the KC-135E fleet affected by a problem with the engine pylon support struts. The board, a team of experienced aircraft engineers, evaluated 30 aircraft throughout a two-month period, assessing the engine strut problems and overall health and viability of these aircraft. General Handy was briefed on their draft recommendations Sept. 13 and directed that 29 aircraft be removed from the daily flying schedule while the board's findings are thoroughly reviewed and evaluated. A decision was to be made on further disposition of the affected 29 aircraft sometime after senior Air Force leaders were to be briefed.
On Sept. 16, 2004 AFPN reported that Gen. John W. Handy, commander of Air Mobility Command, had directed 29 KC-135E Stratotanker aircraft with identified engine strut problems be removed from the flying schedule while Air Force leaders evaluated a report from the Fleet Viability Board and recommendations of the Oklahoma City Air Logistics Center's KC-135 system program office. That decision was based on flight safety considerations associated with this model of the KC-135.
The Senate amendment to the National Defense Authorization Act For Fiscal Year 2008 contained a provision (sec. 142) that would prohibit the Secretary of the Air Force from retiring any KC-135 aerial refueling aircraft during fiscal year 2008 unless the Air Force provides the congressional defense committees with a request to retire KC-135E aircraft during fiscal year 2008 in accordance with established procedures similar to those used for prior approval reprogramming requests. The House bill contained no similar provision. The House receded with an amendment that would allow the Secretary of the Air Force to retire up to 48 KC-135E aircraft in fiscal year 2008.
Sec. 135. "Limitation on retirement of KC-135E aerial refueling aircraft" in the National Defense Authorization Act For Fiscal Year 2008 [December 6, 2007] provided that the Secretary of the Air Force may not retire more than 48 KC-135E aerial refueling aircraft of the Air Force during fiscal year 2008, except as provided in subsection (b). Subsection (b) provided Contingent Authority to Retire 37 Additional Aircraft -- Effective as of the date specified in subsection (c), the number of such aircraft retired by the Secretary of the Air Force during fiscal year 2008 may not exceed 85. Subsection (c) Specified Date -- The date specified in this subsection is the date that is 15 days after the date on which the Secretary of the Air Force submits to the congressional defense committees the Secretary's certification that -- (1) the system design and development contract for the KC-X program has been awarded; and (2) if a protest is submitted pursuant to subchapter 5 of title 31, United States Code -- (A) the protest has been resolved in favor of the Federal agency; or (B) the Secretary has authorized performance of the contract (notwithstanding the protest).
KC-135E CFM-56 Re-engine
Air Force Guard and Reserve tankers were fitted with the TF33 engine as a low-cost interim measure to improve performance and operating capability until the state-of-the-art CFM-56 engine could be installed. Boeing has completed work on a program to re-engine all KC-135As in the Air Force Reserve and Air National Guard fleet -- a total of 161 airplanes [of which 157 remained in service as of late 2001]. In that modification program, which began in 1981, KC-135As were modified with refurbished JT3D engines taken from used, commercial 707 airliners. After modification, the airplanes are designated KC-135Es.
This upgrade, like the KC-135R program, boosts performance while decreasing noise and smoke-position levels. The modified KC-135E provides 30 percent more powerful engines with a noise reduction of 85 percent. The program included acquisition of used 707s, procurement of purchased parts and equipment, basic engineering, some parts manufacturing and refurbishment and installation of the engines, struts and cowling. The kit also included improved brakes, cockpit controls and instruments.
The KC-135R's operational range is 60% greater than the KC-135E for comparable fuel offloads, providing a wider range of basing options. The increased capability of the KC-135R eliminate the operational constraints placed on the KC-135E due to performance limitations and significantly enhances hot weather operations ensuring interoperability of reserve component crews with their active duty counterparts.
The KC-135E constituted 53% of the Air National Guard's tanker forces, and an interim fix is required until a modern tanker replacement aircraft is procured. The KC-135E fleet faces decreasing reliability and maintainability due in large part to continued reliance on the aging TF33-102 engine. Many options were available, including Low-time TF33-P-7 engines from the retiring C-141 fleet, the CFM-56, the JTD 8-219, and/or other appropriate buy or lease options. The re-engining of KC-135Es with improved engines provides the following benefits: increased reliability and maintainability, increased air refueling capability, Stage III Noise Compliance, increased takeoff performance, better climb performance, improved cruise range, higher mission altitude, and projected life cycle cost savings.
The Air Force could spend up to $600 million to overhaul or up to $3.3 billion to replace the TF33 engines on 139 Air Reserve Component (ARC) KC-135E tankers to keep them operational and reduce support costs. The TF33 engines are used, refurbished commercial engines which, at current usage rates, needed a major overhaul around the turn of the century. This will include related repairs to cracked and corroded struts and engine shrouds. Even if overhauled, the TF33 engines will not meet federal noise and pollution standards. The Federal Aviation Administration imposed even more stringent noise standards in 2000. Since many ARC aircraft operate from commercial airports, they could be subjected to operating hour restrictions or other penalties that could affect tanker operations and flexibility.
An option is to replace the TF33s with the CFM-56 engine, which according to Boeing meets noise and pollution standards, requires less maintenance, has more thrust, and is more fuel efficient. The increased thrust enables CFM-56-equipped KC-135s to take off from shorter runways (increasing basing options) and, when combined with better fuel efficiency, increases the amount of fuel available for offloading by about 25 percent. The Air Force conducted a life-cycle cost analysis to determine the economic payback point for replacing the TF33 engines with CFM-56 engines. It used this analysis to determine whether to modify the remaining ARC aircraft. The average replacement cost per aircraft is about $24 million.
The Reliability, Maintainability, and Availability (RM&A) indicators for the KC-135E with the old TF-33 engines highlight the urgent need for re-engining. E-model engines are removed for repair/overhaul at a rate 17 times greater than R-model engines. Additionally, the overhaul costs for the E-model engine increased to $1.039M in FY01 versus $400K for the R-model engine. The KC-135 fleet re-engining is already a validated conversion program.
CFM [a joint company of Snecma, France and General Electric Company, USA] worked with both the USAF and Air National Guard to reengine the remaining KC-135E fleet, which would encompass more than 100 aircraft. On 29 May 1998 Boeing Defense and Space Group, Wichita, Kan., was awarded a $14,327,000 face value increase to a firm-fixed-price-contract to provide for two Group A re-engining kits applicable to the KC-135E aircraft. Expected contract completion date was October 2000. In 1999 the House Defense Appropriations Subcommittee initially provided $208 million in funding for eight KC-135 conversions. Following conference negotiations on the defense bill with the Senate, funding was reduced to $104 million for four conversions. In July 2000 the US Air Force released funding to purchase 36 additional CFM56-2 engines to re-engine four KC-135 tanker and five RC-135 recognizance aircraft for delivery in the 2001/2002 time frame.
The National Guard Association of the United States in September 1999 requested that the National Guard Bureau, the United States Air Force, the Department of Defense and the Congress of the United States provide funding to support the procurement of a modern air refueling aircraft replacement for the KC-135E fleet as appropriate to meet United States and allied requirements.
By the year 2000 engines for 139 E-models require $59M/year for maintenance, while engines for 408 R-models only require $4M/year. Converting to reliable R-models will reduce engine maintenance costs by 95%, increase fuel capacity by 7%, reduce fuel consumption by 15% and will provide significant Life Cycle Cost (LCC) savings which are estimated to be $3 to $7 billion dollars over the expected lifetime of the KC-135.
The Boeing 767 leasing proposal was intended to allow the rapid retirement and replacement of the KC-135Es. Audits have shown it would cost about 15 times more to lease a modified 767 than to re-engine a KC-135E.
GAO estimated the cost to lease 100 Boeing 767 tankers for 6 years to be $20 to $30 billion. GAO estimated that the cost to modernize and upgrade 127 KC-135 Es to "R" Models was $3.6 billion; a $22.4 billion savings to leasing 100 tankers. GAO estimated the cost for building new infrastructure for 100 Boeing 767 tankers to be $1.7 billion, the same cost to modernize 59 older KC-135 tankers.
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