Paul Lewis/PHILADELPHIA
When it comes to aeronautical longevity little equals the B-52 bomber other than perhaps the CH-47 Chinook helicopter from the same Boeing stable. With no successor in sight, 300 of the US Army's tandem-rotor helicopters are about to pass through Boeing's gates for the third time.
The Improved Cargo Helicopter (ICH) calls for the hand-over of the first remanufactured CH-47F in 2003, more than 40 years after the inaugural -47A was first delivered and over 20 years since completion of the first rebuilt -47D. By time the initial projected Joint Transport Rotorcraft (JTR) is deployed in 2020, many of the machines will be almost 60 years old.
"Structurally, the airframe can fly virtually forever if you keep fixing it," says Jerome McMullan, CH-47 business development senior manager. "By remanufacturing the aircraft, you avoid having to overhaul it, save on costs and reduce downtime, while allowing for improvements to be made to the helicopter.'' .
The ICH programme entails the most comprehensive modernisation and relifing of the heavylift helicopter. The work will encompass freshly manufactured subassemblies, structural strengthening, a replacement avionics suite, uprated engines and new electrical wiring and hydraulic tubing.
CH-47Ds on arrival at Boeing's Philadelphia plant will be disassembled and inspected for structural cracks and corrosion. New corrosion prevention treatment will be applied to the helicopters underfloor bilge area. The ICH programme is also intended to finally address the Chinook's high level of vibration.
"We couldn't really solve the natural frequency issues until we got into the 1980s and '90s and had more powerful computer technology," says McMullan. The Boeing-funded Bearcat 3 trials in 1996 at Fort Rucker demonstrated a 50% reduction in vibration throughout the helicopter. These changes will be incorporated into the ICH programme.
Boeing has achieved a 3/rev and 6/rev vibration reduction with thicker gauge forward side and shoulder panel skins and strengthened forward butt line 18 beams. This has allowed for fewer shock absorbers and a corresponding weight saving. The change in the airframe's natural harmonics will reduce component failure and aircrew fatigue .
New cockpit more viable
The reinforced forward cabin will be mated with a new section 41 cockpit. "With all the rewiring and tubing, new cockpit instrumentation and structural stiffening to lower vibration, we did a study and found it more viable to build a new cockpit rather than modify the old one,'' says McMullan.
Housed in the new structure is a Rockwell Collins integrated avionics suite drawing on elements from the MH-47E special forces version of the helicopter and Raytheon C-12 communications turboprop. The US Army's primary interest is enhanced long range communications and more precise navigation capability for the helicopter, as its mission evolves from that of a simple heavylifter to being a large battlefield area tactical transport.
The US Army's chosen configuration differs from the Honeywell integrated suite offered on the improved international CH-47SD or Super Chinook in having two large liquid crystal (LCD) multifunction displays rather than four. "The army's focus was on the mission and affordability is a big issue, so the they did not want a full glass cockpit," says McMullan.
To the right of each of the large format LCDS to be used for digital map and tactical displays are two smaller vertically stacked electronic flight information system displays (EFIS). The central consoles retains theCH-47D's conventional analogue flights instruments as an EFIS back-up. Other additions to the MIL STD1553 standard suite include two CDU-900 control display units, DR-200 data loader and new voice and data communications systems.
The CH-47D's twin AlliedSignal T55-L-712 turboshafts are being upgraded under a separate army programme. The improved 3,655kW (4,900shp)-rated -714A with full authority digital engine control delivers 22% more hot-and-high power at 4,000ft (1,200m) and 35¹C, 5% better specific fuel consumption and a 131% drop in unscheduled depot-level visits.
There is no increase in either the helicopter's 24,495kg (54,000lb) maximum take-off weight (MTOW) or 12,700kg payload, but the improved engine enables the CH-47F to better perform its existing mission. For the US Army's CH-47F, which operates at a lower 22,680kg maximum take-off weight, this translates into being able to move a 7,265kg external load more than 90km (50nm), previously a requirement it could not meet.
The US Army has for now decided against the new 7,830litre (2,070USgal) crashworthy single-point pressure refuelling tanks being offered by Boeing. The Royal Air Force has opted to fit its final batch of eight Chinook HC Mk3s with the long-range fuel system, which is also employed by the US Army special forces MH-47E version. The ICH will retain the 3,915litre tanks.
Another item not part of the ICH programme, but which has potential application for the CH-47F and SD is the low maintenance rotor (LMR). The newly designed main rotor head would eliminate the use of oil lubricants and rolling element bearings. The LMR instead uses elastomeric or dry bearings, reducing maintenance time and the component part count by 70%.
A 1980s upgrade programme extended the original hubs lifespan from 33,000h to 50,000h, but it needs to be replaced. "We got a lot of mileage out of this hub and it now needs to be addressed," says McMullan. LMR is the subject of a joint four-year $24 million UK/US development contract. The plan had been to retrofit the LMR during scheduled overhauls, but the US Army hopes, funding permitting, to incorporate it as part of the ICH.
US engineering and manufacturing (EMD) development expenditure on the ICH so far totals $76 million, enough to modify two test helicopters. These were delivered to Philadelphia in January for tear down. The 36-month long EMD phase calls for the first remanufactured CH-47F to fly in July 2001. It will be redelivered to the US Army following a six-month flight test programme.
Multi-year buys planned
Boeing hopes to secure a long lead item contract by the 2000 and a two-year low rate initial production order for the first 30 machines 12 months after that. Rather than award an annual series of annual production contracts similar to the CH-47D, the US Army is planning a series of multi-year buys, covering 26 aircraft a year through to 2013.
The US Army plans to modify 302 of its 431 surviving CH-47Ds, including the two test aircraft at a cost of around $3 billion. The number is based on the assumption the JTR will be in service sometime between 2015 and 2020. If the JTR is delayed, the number of CH-47F rebuilds could increase to at least 400.
While ICH is in principle a remanufacturing programme, Boeing is considering building new CH-47Fs for domestic and international use. The US Army is 14 helicopters short of its required inventory number, but except for one new -47D attrition replacement for US Army Reserve being built, all future new production will revolve around the CH-47F, says Boeing.
International production is moving to the CH-47SD, with which the CH-47F shares 80% commonality. The final four of six cancelled CH-47D orders will go to Egypt later this year and the remaining two to Australia in 2000. "The market wanted technical improvements, and to be competitive, we had to update the aircraft ahead of the CH-47F," says Samual Mayer, Boeing regional sales manager.
The first -47SD model is scheduled to roll out in October and is one of six ordered by Singapore. The helicopters, which will be fitted with the long-range fuel system, will supplement six earlier CH-47Ds that Singapore may also upgrade to a similar standard.
The Royal Netherlands Air Force has received six new hybrid versions, with seven upgraded ex-Canadian machines, incorporating monochrome glass cockpit, -714A engines and modified structures. The RAF has taken delivery of six Chinook HC Mk2s incorporating the -47SD modified structure. Its next eight Mk3s due for delivery in 2000 will essentially be -47SDs fitted with the MH-47E's enlarged nose for a weather radar and refuelling probe.
International sales prospects include Greece, which has signed a letter of intent for seven helicopters with improved Rockwell Collins avionics. Taiwan has been offered nine Chinooks, while Turkey has an outstanding need for eight heavylift machines. The CH-47SD is also in the contest for a multirole pan-Scandinavian requirement for 40-50 helicopters.
"Our goal is to sell 24 helicopters a year to maintain an economical flow-we have orders booked through to early 2001," says Mayer. The CH47F and -47SD promise to provide Boeing with a healthy heavylift output well into the first decade of the next century, which by any reckoning is a remarkable measure of the CH-47's enduring success.
Source: Flight International
