PT6T Removal and Installation
By Greg Napert
For the average helicopter technician, an engine change is not a very common event. In the case of the Pratt & Whitney PT6T Twin-Pac® installation, if the engines are operated within acceptable parameters and the number of engine operating hours is average, the technician may go five or more years before having to pull an engine.
There are some unscheduled events, such as performance deterioration, that may result in having to pull modules of the engine individually. An individual engine change, for example, can be done easier just by removing that engine and reinstalling it in the airframe.
However, removal of the gearbox, for instance, is much easier to accomplish after removing the entire Twin-Pac assembly. Additionally, a dual hot section inspection or gearbox repair, combined with engine inspection, often dictates that you pull the engines from the airframe as an assembly.
Dave Mills, instructor at Bell Helicopter Textron's Commercial Customer Training in Fort Worth, TX, says, "Because it's inevitable that you're going to eventually have to pull the engines, yet infrequent, it's really important that you tap into help from good sources of information."
First and foremost, he explains, is the helicopter manual.
"The aircraft manual, available from Bell really is a great source of information. Keep it with you at all times during the installation and removal. Additionally, we have a full time technical support staff. Never be afraid to call and ask. I'm not too proud, even as an instructor and technician for 21 years — I learn something new during every class. If I can make it easier for someone else, I really enjoy doing so."
Bell's five-day course consists of a day of theory using a computer-based program that shows you how the engine works, along with fluid flows, and operation. Next, the student is led through an actual engine removal and installation. They then return to the classroom to review the removal and installation, and review any problems or challenges they encountered. Finally, the students return to the engine to perform the full rigging and adjustments of the fuel transducers, ITT, etc. The rigging is performed twice just to make sure they know what they are doing.
Probably the most difficult and time consuming part of removing the Twin-Pac assembly from the helicopter is removal of the firewalls and cowling. The firewall installation is quite extensive on the Bell 412 and 212. Essentially, the firewall surrounds each engine and the gearbox; and the hot section of each engine is separated from the cold section.
"There is a lot of firewall, mounting brackets, and a lot of fasteners. Removal of these items can require some finesse as much of the hardware is difficult to gain access to. Additionally, organizing and tracking the parts removed can be a chore," says Mills.
The amount of time it takes to perform a quick engine change on average can vary quite considerably. An experienced technician can perform the change in about four hours, but the uninitiated can take two days or more.
"It's something that requires some practice and can be done rather expeditiously when you have done it before. The first time is very slow, however," says Mills. He recommends that if possible, all hardware used on the firewall assembly be replaced. "This will make it much easier for you the next time you remove the hardware. If this is not possible, at least try to replace any hardware that shows even the slightest sign of rounding over or stripping," he says.
The following are procedures which are listed in Bell's PT6 Quick Engine Change Notebook which outline the procedures for removal and installation of the PT6 Twin-Pack. Although these procedures are very good, they are for training purposes only. Remember that the procedures vary from installation to installation and helicopter model to helicopter model.
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