Graham Warwick/WICHITAILLUSTRATION BY Giuseppe Picarella
BY EARLY NEXT century, US Air Force and Navy pilots will undergo primary training on the same aircraft type - the Raytheon Beech Pilatus PC-9 MkII. This unprecedented co-operation will be made possible by the $7 billion Joint Primary Aircraft Training System (JPATS) programme.
Raytheon Aircraft was awarded a $4 billion contract in February 1996 to develop, produce and support the Beech MkII turboprop-trainer. As JPATS prime contractor, Raytheon is also orchestrating the competition for a $3 billion contract to design, build and support the associated ground-based training system, including aircrew training devices.
USAF students will come to Beech MkIIs fresh from Slingsby T-3 Firefly flight-screening aircraft and move on to upgraded Northrop T-38 Talon supersonic advanced-trainers. USN students will begin their flying training on Beech MkIIs and progress to McDonnell Douglas T-45 Goshawk carrier-capable advanced jet-trainers.
TRAIN AND SAVE
Despite their different origins and destinations, students from both US services will undergo primary training on identical aircraft - even down to the paint scheme. Beech MkIIs will replace the Air Force's side-by-side, twin-turbojet Cessna T-37s and the Navy's tandem-seat, turboprop Beech T-34s. Operating the identical training systems is expected to save the services billions of dollars over the life of the JPATS programme.
David Reimer, vice-president of Raytheon Aircraft's Trainer Systems division, gives the services credit for making the programme possible. "The Air Force and Navy did an exceptional job early on to get joint requirements. They had their differences, but they were always able to agree in the end," he says. Perhaps their greatest achievement is to agree that a tandem-seat turboprop is the best for the job.
The USAF initially attempted to replace the T-37 with the Fairchild T-46. This was a twin-turbofan aircraft with side-by-side seating, but the programme was cancelled after it fell significantly behind schedule and went seriously over budget. Raytheon began studying primary trainers, Reimer says, "-when we realised that the T-46 was not going to make it".
Reimer believes that the T-46 programme failed because Fairchild and the Air Force were reluctant to reconsider the early decisions which shaped that aircraft. "The T-46 concept was wrong, fundamentally wrong," he says, but "-our first decision - to pick the PC-9 - was absolutely right", he adds.
Raytheon's own preliminary-design studies resulted in primary trainers which looked like the PC-9, he says, "-so why start from scratch?" The company evaluated all of the then-existing aircraft. Only the Agusta S.211 jet-trainer was not available, having been selected by Grumman for its JPATS bid.
"The only aircraft we were convinced would be a winner was a derivative of the PC-9. It had the performance and handling for a primary trainer. We knew budgets would get tighter and life-cycle cost would be an issue, so a turboprop was cheaper. The other aircraft were too big, too heavy, too costly to build. The PC-9 was an aircraft with which we could come up with a winning strategy," he says. Raytheon's goal from the outset was the "-lowest acquisition-cost, lowest life-cycle-cost aircraft which meets the requirements at the lowest risk". Affordability is critical in primary training, Reimer believes. "You can't simulate fear in a simulator. You have to get students psychologically and physically ready, so there is a certain amount of flying you have to do. If that gets expensive, then you have a problem," he says.
SWISS ROLE
Raytheon teamed with Pilatus in 1990, and began tailoring the PC-9 to JPATS requirements. "Use of the PC-9 avoided a preliminary-design phase and allowed us to move rapidly to missionisation," says Reimer. "We knew the aircraft would need missionisation, and we decided to do it all before contract award. All the changes were in the prototype provided for the [JPATS] flight evaluation," he says.
The PC-9 has "excellent" spin characteristics which Raytheon did not want to lose in making the changes needed to meet the JPATS requirement, Reimer says. The flight characteristics were improved, but most of the changes made were to the powerplant and systems.
A more-powerful Pratt & Whitney PT6A-68 turboprop was installed to provide the sustained-manoeuvre performance required. The engine is themodynamically rated at 1,270kW (1,700shp), but is derated to 820kW to increase the time between overhauls and so reduce life-cycle cost. Pressurisation was added, a zero-airspeed/zero-altitude escape system installed and the canopy and wing redesigned to withstand a 1.8kg birdstrike at 270kt (500km/h). Other changes included single-point pressure refuelling and a missionised JPATS cockpit. Raytheon received two PC-9s from Pilatus. One was used as an engineering prototype and was modified incrementally with the changes planned for the Beech MkII, to make sure that the basically good handling qualities of the PC-9 were not lost in the missionisation process.
Flight-control "tuning" included elevator changes to provide linear and stable pitch control; rudder redesign to reduce yaw-control forces and provide good rudder-centring; a ventral fin to increase directional stability; wing leading-edge stall strips to provide roll control throughout the stall; and rotary pressurisation-seals to minimise friction.
The biggest handling improvement is the introduction of a trim-aid device (TAD) to balance rudder, aileron and elevator trim requirements and reduce the propeller-torque effects. "The PC-9 rudder trim requirement is four times that in roll and pitch. The TAD balances and harmonises trim, so that pitch, roll and yaw are roughly equal," Reimer explains. "You can do a touch-and-go feet on the floor," he claims.
The TAD computer uses power, altitude, airspeed and pitch-rate inputs and drives the same actuator as that of the pilots' controls, to reduce rudder-trim requirements. It is an "open-loop" control system, Reimer explains, "-so it does not fight you like a [closed-loop] yaw damper would and it doesn't automatically trim the aircraft".
JET MIMICKED
Another major improvement over the PC-9 - and other turboprop trainers - is the introduction of full-authority digital engine-control. The Beech MkII's power-management system (PMS) controls engine and propeller, providing single-lever power control with "student-proof" features such as torque and temperature limiting. Most importantly, the PMS modifies the PT6A's power response to mimic that of P&WC's JT15D turbofan.
In a conventional turboprop, there is a significant delay in engine response to power-lever movement as the gas generator spools up. Propeller pitch kicks in at the last moment, forcing the student to "-get on the rudder quick", says Reimer. In the Beech MkII, the PMS brings in propeller pitch as the gas-generator spools up, and so replicates the more-linear power-response curve of a turbofan.
The PMS provides a more-immediate, jet-like power response and avoids the propeller-pitch kick-in. Together with the TAD, the system eliminates most of the undesirable handling-characteristics of a turboprop trainer, Reimer believes. The combination made it easier for Raytheon to pursue its goal of achieving lowest life-cycle cost by using a turboprop.
Raytheon's biggest design effort concerned the cockpit, with changes required to incorporate pressurisation, zero-zero ejection seats, birdstrike-resistant canopy and missionised avionics. All these design changes were incorporated in two Beech MkII production prototypes built by Raytheon, with only the wings supplied by Pilatus. To reduce the risks associated with its JPATS bid, Raytheon conducted 11 sled tests of the Martin-Baker MkUS16A seats, using a Beech MkII forebody which was later refurbished for use in the competition as a cockpit mock-up. The aircraft has a through-canopy "no-dead-seat" ejection system. "A canopy-fracturing system ensures the occupant never has to wait for the canopy to jettison before the seat moves," explains Reimer.
The canopy is substantially different to that of the PC-9, with a separate windshield and reprofiled forward section to meet the JPATS birdstrike requirements. The windshield is of 20mm-thick stretched acrylic, raked at 34.5° to the horizontal, while the 21°-raked forward section is 9mm thick. In seven 1.8kg/270kt birdstrike tests conducted in late 1994, there was no penetration into either cockpit, Reimer says. The single-piece, side-opening canopy has generously curved frame corners to avoid pressure-seal leaks, and is designed to be manually operated. Raising the canopy provides maintenance access to behind the front-cockpit instrument panel. Other maintainability changes include a lower cowling-break to provide better engine access and an avionics bay behind the rear cockpit, easily reached by a mechanic standing on the ground.
DESIGN CHALLENGE
The biggest design challenge, Reimer says, was meeting crew-accommodation requirements. The JPATS programme is intended to increase the population from which the USAF and USN can draw students, and particularly to improve chances for women to enter pilot training.
This resulted in a requirement that the aircraft accommodate pilots ranging from a 1.5m (4ft 9in)-tall, 50kg person to one of 1.93m-tall and 113kg. The challenge was in designing a cockpit which would allow students at both extremes to reach all of the controls. "As the seat rides higher, the occupant moves away from the controls, and vice versa," explains Reimer.
Raytheon started with anthropometric computer-models, moved on to tests of a wooden cockpit-mock-up with employees matching the JPATS design-size cases, and refined the design with the production prototypes. "We spent the most money on cockpit anthropometrics," says Reimer. Considerable time was also spent in designing the instrument panel. The Beech MkII uses commercial avionics, including AlliedSignal electronic flight-instrument and global-positioning systems and Smith Industries liquid-crystal air-data and engine displays. The latter, five in all, are identical, "plug-in" interchangeable displays.
The panel layout was evolved in consultation with the customer, Reimer says, beginning with a Velcro board and instrument pictures, followed up by Dassault CATIA computer-aided-design drawings and completed with construction of the functioning cockpit mock-up which was required for the JPATS competition.
DESIGN GOAL
Simplicity was a design goal, and systems were not included on the aircraft unless there was a perceived training benefit, Reimer says. Examples include the omission of a flight director, which was considered to be too costly, and the inclusion of an onboard oxygen-generation system, which eliminates the need to replenish oxygen between missions. "Acquisition cost is higher, but safety is better and it's cheaper in the long run," he says.
Systems which have been installed in the aircraft include selectable nosewheel steering; a speedbrake, for operational-effects training; a closed-cycle cockpit-heating/cooling system; automated fuel management; centralised communication/navigation tuning; and a collision-avoidance system.
The JPATS requirements document set "required" and "desired" levels of performance. Raytheon went for the "desired" level, with few exceptions, says Reimer. One was pressurisation: Raytheon had opted for 0.24bar (3.5lb/in2) before the services came out with a desired requirement for 0.34bar. The company decided not to redesign the aircraft and so saved weight, reduced fuel consumption and increased performance, he says.
In other areas, the Beech MkII exceeds the basic requirements, Raytheon says, with a +7/-3G capability (compared with the required +6/-3G); 270kt sustained speed (versus 250kt); 1,200m runway capability (versus 1,500m); and a 25kt crosswind limit (versus 15.5kt).
Reimer says that many T-37 missions are lost to crosswind-landing limits: "This will not be a bottleneck with JPATS," he says.
The JPATS specification called for a set of required training manoeuvres to be accomplished in 20min: these and desired additional manoeuvres were demonstrated with the Beech MkII in 18min, Reimer says. This is a result of the aircraft's high power-to-weight ratio, which allows height lost during manoeuvres to be recovered quickly. While lost altitude can be regained more quickly in a jet-powered trainer, manoeuvres can be accomplished in a smaller volume of airspace with a turboprop, saving time, he argues.
Evaluation of the JPATS bids gave the highest ranking to operational utility, a combination of operational capability and crew accommodation. Reimer says that the Beech MkII's operational capability includes the ability to perform consistent, repeatable, stalls and spins. "One of its greatest features is that you brief the student and get no surprises," he says.
Flying qualities were so critical to the JPATS requirement that all of the contenders were evaluated by the Air Force and Navy before proposals were submitted. Bidders were required to provide an aircraft for the flight evaluation which was aerodynamically representative of the production design. Raytheon furnished its second production prototype, which included all of the planned aerodynamic, powerplant, system and cockpit changes.
"Handling was the key to winning. The MkII was the only competitor with no operational deficiencies," says Reimer. He also cites the attention to detail in the production prototypes. "The evaluation aircraft was exactly like the production aircraft. That was a strength of our bid," he says.
Raytheon was selected in May 1995, but award of the JPATS contract was delayed after protests from losers Cessna and Rockwell until February 1996. The protests were over-ruled and Raytheon's bid confirmed as offering the "best value". The company was then awarded a manufacturing-development (MD) contract to build one production aircraft. Contract options cover a further 141 of a planned total of 711 production aircraft - 372 Air Force and 339 Navy - over 20 years.
The company's first task is to certificate the Beech MkII to US Federal Aviation Administration Part 23 standards. This is under way, using the second production-prototype, and will be concluded using the MD aircraft, which is scheduled to be flown in the third quarter of 1998. This aircraft will also be used for instructor training.
The first Beech MkII delivery to the Air Force is scheduled for early 1999, with the first Navy delivery planned for early 2002.
TARGETING COSTS
Reimer says that the MD aircraft will incorporate manufacturing improvements planned for the production aircraft. These are required to meet Raytheon's target average unit cost of "under $2 million", in 1992 dollars. Meeting this cost goal has been a "considerable challenge," he admits, and required simplification of the PC-9 structural design.
Raytheon amended its agreement with Pilatus in 1993 to give it total programme responsibility, including manufacture of the entire airframe. The Swiss company, which was to have manufactured the wing, will now receive a royalty instead. According to Reimer, the Beech MkII wing is structurally significantly different to that of the PC-9, with fewer parts and making greater use of forgings, castings, high-speed machining and automatic riveting (which cannot be used on the PC-9 wing, he says).
The entire aircraft was redrawn using CATIA. "There are no Pilatus part numbers on the aircraft," which is Raytheon's first all-CATIA product, Reimer says. A special division formed to handle the JPATS bid, and to focus early attention on the transition to production, is now building an $11.5 million assembly plant at Wichita.
"The same people who produced the prototypes and wrote the proposal will execute the contract," Reimer emphasises.
Source: Flight International
