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.
"We have always had success eliminating these vibrations in a minimal amount of time. The Tay is easy to balance because the imbalance is almost always a condition of uneven weight distribution. We don't really have problems on the Tay with blade hang-up or what we call 'shingling' as is the case on the older Spey. The Tay fan is a 'wide chord' fan — there is no mid span supports onto which the blades can hang up."
A common question from the field is how the fan becomes out of balance in the first place. Doesn't the fan get balanced during the manufacturing process, and aren't these problems worked out in the test cell prior to installation on the aircraft?
Hewitt explains, "It's really impossible to totally eliminate this problem in the overhaul shop. It's simply a characteristic of the engine we have to deal with. For instance, we can have an engine that is perfectly balanced in the shop and it passes the test on the test bed and has gone through all the vibration surveys, and we install it on the wing of the airplane and may have problems with balance. Although vibrations were not registered in the test cell, the fact that an airplane is a relatively good amplifier results in minute vibrations being picked up and transmitted throughout the aircraft to the rudder pedals, power levers, and in other areas of the aircraft as a buzzing sound.
"Some of the other vibration indications that we see in the cockpit often turn out to be false. We occasionally see stray signals that are the result of dirty cannon plugs or connectors for the transducers, or a bit of oil gets on the transducer which transmits a false signal. Under these circumstances, you will not hear the vibration or feel it. Instead, you will see it on the vibration monitor and it is typically a fluctuating signal. This is often indicative of a contaminated plug or loose cannon plug. Air in the engine fuel system could have a similar effect. This slug of air can be induced via the fuel drains mechanism," he says.
Real balance problems," Hewitt explains, "typically only occur after a major event such as an overhaul or a fan change, or if you have had some damage that required blade dressing. Once you've performed the trim balance, it's very rare to have any repeat problems. I don't recall having to go back after several years and having to rebalance the fan. The fan is a very robust piece and once you trim it properly, there shouldn't be any further problems.
"In the event of a blade that is damaged, for one reason or another, it is quite easy to find a replacement blade that is the exact same weight as the damaged one. If we can't do that, we replace the damaged blade and the blade directly opposite it with two evenly weighted blades. The manufacturing processes and engineering involved in manufacturing the blades is so good today that the blades are very closely matched in terms of weight and form. As a result, it is rare that we have to change opposite blades. If we have a bird strike, for instance, we can usually get away with changing the damaged blades and then performing a trim balance," Hewitt says.
On rare occasions, a trim balance may not work. Hewitt says, "This may be due to the blade and hub combination and in this case, we resort to actually pulling the fan blades from the engine and re-indexing the blades to the hub. This simply consists of removing the blades from the hub and shifting their position relative to the hub. A final step, if all else is unsuccessful, would be to 're-clock' or 're-datum' the hub. This entails removing the blades from the hub, removing the fan hub, and reclocking the same hub up to 180 degrees from its original coupling location. Blades are then replaced in their original slots. The trim balance operation will confirm success. This procedure can be accomplished with the engine installed."
The typical Tay installation today, such as on the GIV, includes a continuous vibration monitoring system which monitors the LP and the HP system. You can actually select each one of these frequencies to monitor it during flight or at any time.
A marriage of new and old technology
By Greg Napert
P3 air gasket leaks
Another problem that can challenge technicians and create troublesome operational characteristics on the Tay is related to P3 air gasket leaks. Hewitt explains, "The fuel flow regulating system on the engine is such that you set the power lever at an HP speed setting and the engine will automatically maintain that HP setting with minimal throttle lever movement. The fuel flow regulator controls the HP setting and senses various delivery air pressures throughout the engine such as P2.6 and P3 — that's HP1 inlet pressure and HP12 delivery pressure. If you get a leak at either of those air signals to the fuel flow regulator, it will give you a false scheduling for your particular climb performance, cruise performance, or takeoff.
"Any of these could cause the fuel flow regulator to malschedule. Some external seals on the fuel flow regulator have generated leaks, which usually show up as slow acceleration or "hunting throttles" with autothrottle selected. Hunting throttles occur when throttles are set at cruise and one throttle hunts back and forth and all of the parameters follow suit. What's really happening at the fuel control is that there may be an uncontrolled air scheduling leak and the unit is trying to correct for that leakage.
"These seals or gaskets, once made from asbestos, held up well in the heat being generated by air coming off the HP compressor and into the fuel flow regulator. Following the ban on asbestos, other materials have been used, but they haven't worked as well.
"Lucas, which manufactures the fuel control, has been working on modifications which should eliminate the occasional air leak problem, but it still exists as we speak.
Hewitt continues, "When any of these symptoms appear, we change these seals in the field right away. We were not allowed to change the seals previously because of the procedures necessary to seat the seals properly. We now have clearance to change the majority of these external seals with the engine installed on the aircraft. Specific procedures for installing these seals involve removing the fuel flow regulator from the engine, installing the new seals, heating the unit in an oven, and heat soaking it for about five hours with the new seals, which are re-torqued throughout the heat soaking process. An alternative is to remove the unit and send it to Lucas and they will perform the procedure and have it back to you in a short time.
"To determine whether or not you have a problem, use a soapy solution and spray it in the general area of the seals with the engine running. If you see small bubbles, you know you have a problem, because even a small amount of leakage can cause a scheduling problem. Some internal seals in the fuel regulator can cause problems which are more difficult to diagnose. Essentially, if you have performance problems or throttle hunting problems, and you can't find a leak externally, you can pretty much deduce that it's an internal malfunction," he says.
Masella adds, "There were some fundamental changes made to the fuel flow regulator which have reduced the number of seals for both the fuel and air, resulting in less leakage. The problem principally is at the inlet connector, so using soapy water at the inlet connector can tell you if it is leaking. A note of caution: next to the inlet connector, there is a small pinhole moisture vent that will produce bubbles during a leak check. Don't mistake this for a leak — it's perfectly normal, but there shouldn't be any bubbles anywhere else. Masella continues, "To repair the seals, you need to remove the P3 block which is removed with four bolts. Keep in mind that if you do that, the P3 block must be returned to the same fuel flow regulator as they are calibrated together as a unit.
Hewitt says, "Quite often if we are unsure of exactly what the problem is, we will change the fuel flow regulator as a matter of course to verify that the problem is with the regulator."
Hewitt continues, "I like to think of the fuel flow regulator as the carburetor of your engine — it houses all the control functions of the engine.
"A problem with hunting throttles can also be attributable to programming of the autothrottle electronics. There have been some software updates and this can also solve the problem. I have seen cases where correcting a hunting autothrottle can require a combination of fixing P3 air leaks and software upgrades."
Increase in LP at the top of climb
Hewitt says, "All Rolls-Royce engines have what we refer to as a Ôbelt and braces' controlling mechanism. There are four of these control mechanisms or Ôlimiters': the LP governor, HP speed, overspeed high turbine gas temperature (TGT) control mechanism, and P3. If the engine goes into an overspeed condition or the temperature rises to the maximum limit, the engine will be controlled automatically.
"The Tay is known as a pilot's engine because if an emergency requires the use of full throttle, the limiters within the control system will prevent engine rotor speeds, temperatures, and pressures from reaching unsafe values. One of these limiters is the LP speed governor which limits low pressure fan rotational speed. We have had events related to the LP governor, associated with a sudden increase in LP speed and a resulting exceedence at the top of climb when the aircraft levels off at cruise. We have found that this is generally caused by the LP governor setting drifting on the high side. This results in a minor exceedence of the LP assembly that is dispatched as an alert on the flight panel. If this happens, the pilot can simply pull back on the throttles to correct it temporarily and report it to the maintenance department. Typically, all that is required is a simple adjustment to the LP governor."
By-pass duct air leak
Hewitt adds, "Another area that can cause the crew to report a discrepancy between LP engine speeds is if you have any kind of an air leak at the outer bypass duct panels. This can be as simple as one of the bypass panels not being shrouded properly. A small leak of bypass air in the outer air duct could offload the LP compressor and the speed will rise marginally. Any of the access panels or a combination of panels on the outer bypass duct can cause this. It's like opening a bleed air valve and releasing bleed air — the rpm will go up. A dark stain on the trailing edge of a panel can be a tip-off to this leakage.
"We had an instance where the crew had been complaining for over a year that the LP speed on one engine was consistently higher. The maintenance department checked but couldn't find anything. It wasn't until later, when the time expired engine was removed, that we exposed a certain bushing on the inboard side of the engine that was causing an air leak. It was where the fuel burner nozzle or arm goes through the bypass panel. The sleeve had caulked, and there was air being released from this sleeve," Hewitt says.
Hewitt explains, "A power run-down is a tendency for the engine to run down to sub-idle condition. We saw only one case on the GIV but more on the F-100. This turned out to be a calibration drift of the fuel flow regulator or incorrect acceleration settings. During service, you can check the acceleration time, and if the acceleration time is slow, you could have an under-fueling condition. That under-fueling condition has an effect on the operation of the engine throughout its operating range. If you've lost power, or the load on the engine changes with increased air bleeds or electrical loads, the engine doesn't have the degree of over-fueling that's required to maintain the selected power. That degree of over-fueling is set by the acceleration control on the fuel flow regulator. If your acceleration times and deceleration times are set incorrectly, it can have an effect on the engine throughout its operating range — including the engine start sequence. Results can be hot or hung starts, etc. Setting the acceleration and deceleration settings correctly will usually rectify run-down and starting problems. Occasionally, however, you do have a calibration drift of the fuel flow regulator and you will need to pull it and have it recalibrated on the bench."
Too much oil
Hewitt says, "Remember to wait 15 to 30 minutes after shutdown to service the oil on the Tay engine. By over-servicing the Tay you'll end up with oil covering the engine. If you leave it longer than 3/4 hour, then you should fire up the engine and run it at idle for a couple of minutes and then shut it down to check the oil."
Masella adds, "Despite the use of correct servicing procedures, the engine can still show evidence of air/oil mist, for example, at the engine's cooling air outlet duct. A current Mod Action from Gulfstream improves the sealing between this vent outlet and the cowl to reduce the amount of oil staining on the engine."
Ready to go
Masella says that Rolls-Royce is continuously working on making changes to improve the operating characteristics of the Tay engine. In addition to working with Lucas to eliminate P3 air leaks, upcoming changes to the Tay include elimination of the temperature limiting actuator to simplify engine controls; the addition of an air canister which serves as an "air capacitor" for the pneumatic chamber on the fuel flow regulator (to dampen the rate of engine response at altitude to prevent surges); and a modification to the fuel drain collector tank to reduce overboard spills.
Hewitt adds, "The name of the game in corporate aviation is dispatch reliability. When the chairman of the company calls and says he needs the aircraft at such and such a time, it better be ready. Because of this, the corporate aircraft can be a bit more expensive to maintain than the average airline aircraft. The corporate operator wants 100 percent dispatch reliability and this can cost money. Nonetheless, it's the maintenance personnel that must take the wrath of the chairman if the aircraft isn't ready — that's just the nature of corporate aviation."