A marriage of new and old technology
By Greg Napert
Because of its size, the Rolls-Royce Tay engine is considered somewhat of a transitional engine between corporate aircraft and transport category aircraft. But it's more than its size that makes it a transitional engine. Its technology is such that it combines older hydromechanical controls with sophisticated aircraft electronics to bring proven and reliable technology up to the 21st century. There are over 1,500 Rolls-Royce Tay engines operating worldwide on Gulfstream, Fokker-100, and some Boeing 727 aircraft. There are basically four derivatives of the Tay — the 611, 620, 650, and 651 with thrust ratings from 13,850 pounds to 15,400 pounds.
According to John Masella, senior instructor for Rolls-Royce Product Support Canada Inc., "If you are familiar with the Spey, you see that the Tay has many of the same components, so one might assume that it works the same way. That's essentially true at the component level, but the way in which the Tay engine is controlled is quite different." To understand the control system on the Tay, it helps to have an understanding of the older Spey engine. Masella explains, "The older Spey's control system is designed from the outset to be a full throttle, fully-automated control system. Speys use engine pressure ratio (EPR) just for takeoff. During the climb and cruise phase, the pilot sets the thrust using temperature and shaft speed.
"When the Tay came along, some method had to be created to adapt a 30-year-old hydromechanical control system to a 1980s electronic aircraft. The key to doing that was to introduce digital EPR. Unlike the Spey, the Tay has an EPR system that's used throughout the flight envelope. Because it's digital, it can talk to the autothrottle system and the flight management system. The important difference on the Tay is that the control system isn't intended to give the pilot takeoff thrust at full throttle — instead the pilot (or autothrottle system) moves the throttle lever until the required EPR for the day is achieved. A digital EPR that can talk to an autothrottle system is the key," he says.
"In other words," Masella says, "the Tay's digital EPR signal can be used by a modern aircraft's autoflight system for thrust management while allowing the engine to retain the N2 governing hydromechanical fuel control system, which was originally designed for use on the Spey engine.
"It's the best of a hydromechanical design that's inherently quite reliable and simple, and vastly cheaper than fitting a brand new electronic control system."
Troubleshooting and maintenance
Besides the required overhauls, the Tay is relatively maintenance free. Even the fuel nozzles and igniters experience very few problems and need to be addressed only on rare occasions.
David J. Hewitt, director of Customer Support and Marketing for Rolls-Royce Canada, agrees, "The Tay is generally a very reliable engine. It doesn't give too many on-wing problems and we're very fortunate."
As with any piece of equipment of such a complex nature, there are always problems which need to be addressed as part of the troubleshooting and maintenance regime.
Hewitt says, "Of the few problems we experience with the Tay, one of them that can be troublesome is vibration problems related to the low pressure fan. Generally speaking, this is rectified by a trim balance of the fan, where we actually do a balance by putting weights on the fan disc and balancing the fan out itself with the fan installed on the aircraft. The flight crew will notice the vibration through a combination of instrument indications and a buzz in the cockpit. It then is verified through a spectrum analysis conducted with vibration monitoring equipment. The noticeable vibration is sometimes interpreted by the flight crew as a synchronization problem similar to that which is encountered with propeller-driven aircraft. However, this is not the case with turbine fans — it is strictly related to vibration/balance characteristics. As little as 3.0 to 4.0 grams of weight removed from or added to a blade can have a large impact on the vibration characteristics of the engine," he says.
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