"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.