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.
Typical Twin-Pac removal from Bell Model 412 Helicopter
1. Cap all open lines and fittings. Cover both section air inlet and exhaust ducts to prevent foreign objects from entering.
2. Use standard precautions for use of the maintenance hoist; do not stand under powerplant during removal procedure.
3. Preservation should be accomplished, if applicable before powerplant is removed.
4. Turn off all electrical power and disconnect battery.
1. Open and hinge forward the forward pylon fairing, remove the forward transmission fairing.
2. Disconnect anti-collision light electrical connector at top panel. Remove the upper pylon cowling.
3. Release and remove left and right engine inlet fairings.
4. Open and remove left and right engine lower cowls.
5. Open engine upper cowls, disconnect starter-generator cooling ducts, fire detection element lead wires, and FM antenna and remove left and right engine top and side cowl assemblies.
6. Remove left and right reduction gearbox inspection doors and side panels.
7. Remove left hand combining gearbox top cowl with side cowl attached.
8. Disconnect oil filler hose and oil vent hose from oil filler adapter.
9. Remove right hand combining gearbox top cowl with the center panel and side cowl attached.
10. Remove top portion of cooler support cowl.
11. Disconnect ejector drain tubes, vent hoses and remove vent tube and bolts securing ejector to brackets.
12. Remove screws and hardware securing firewall, upper aft, and remove ejector and firewall section.
13. Remove main drive shaft and, using T101588 or 412-20-001-101 holding wrench, remove engine drive shaft adapter.
14. Remove screws to detach center section of aft firewall, and center firewall through, disconnect lines to differential switch and remove centerline firewall. Aft firewall center section will be removed with powerplant.
15. Remove upper section of induction baffle, disconnect air management valve electrical connector, disconnect engine wash hose at spray ring, disconnect vent line and remove transition duct as an assembly.
16. Remove screws, detach and flex left and right inboard ears of lower middle firewall and induction baffle forward to allow clearance and secure with lockwire.
17. Remove screws to allow removal of panels from lower middle firewall. remove access doors from aft cabin bulkhead at each side of pylon, in cabin and disconnect bonding strip from each tach generator.
18. Drain engine oil and disconnect engine control tubes, electrical connections, fuel, oil, bleed air and drain lines as required.
19. Loosen clamps and slide oil cooler blower boots aft.
20. Install Engine sling, SWE13833, attach a hoist using the fifth hole from the rear, and take up the slack.
21. Disconnect right forward engine mount vertical tube at lower end and left forward vertical tube upper end. Remove bolts and disconnect powerplant from bipod and tripod mounts.
22. Lift engine clear of helicopter after checking all lines, electrical wiring and linkages are disconnected.
23. Remove right forward engine mount vertical tube from accessory gearbox.
24. Position powerplant to engine stand and secure engine mount pads with locking pins.
25. Ensure stand position locks are applied and remove sling.
Typical Twin-Pac installation
1. Transfer from removed powerplant any parts not provided on replacement assembly. Such parts include exhaust ducts, aft center firewall section, manual fuel control levers and oil filler adapter, and vent hoses.
2. Install engine sling, SWE 13833, attach a hoist, using the fifth hole from the rear, and take up slack.
3. Remove pins and lift powerplant from stand.
4. Install left and right forward engine mount vertical tube upper end to fitting. Bolt heads should be positioned forward for the front engine deck fittings and bolt heads inboard for upper fittings on the accessory gearbox.
1. Lower powerplant carefully, monitoring all electrical wiring, control tubes, fuel and oil lines, and firewall. Use caution to prevent damage to any of these components.
2. The forward and center fire seals are properly positioned when they are located aft of the lower induction baffle and lower middle firewall. The aft firewall should be forward of the lower aft firewall.
3. Align powerplant to engine mounts as powerplant is lowered. Install aft mount bolts, then forward mount bolts. Use two washers and a nut for each bolt. Tighten to standard torque and install cotter pin.
4. Remove sling and hoist.
5. Connect engine control tubes, electrical connections, fuel, oil, bleed air, and drain lines. Slide oil cooler blower boot forward, tighten clamps.
6. Connect bonding strip on each accessory gearbox tachometer generator. Install access doors on aft cabin bulkhead each side of pylon support.
7. Install left and right inboard ears of lower section of induction baffle to lower center line firewall.
8. Attach panels to lower section of middle firewall.
9. Install upper sections of induction baffle and middle firewall with attached inlet duct, air valve and transition duct, as an assembly, to each side. Connect vent lines as necessary.
10. Connect air management valve electrical connector in front of middle firewall above inlet duct.
11. Connect each engine wash hose to spray ring connection and install left and right side panes of induction baffle.
12. Install centerline firewall upper web (trough) to lower web and to middle firewall and induction baffle. Connect fuel lines to each differential switch.
13. Install engine driveshaft adapter using T101588 or 412-240-001-101 holding wrench and install main driveshaft.
14. Install aft firewall center section to aft firewall lower section and to flange of centerline firewall. This section is installed with powerplant.
15. Install both ejectors with upper section to aft firewall, connect ejector links to brackets and install stiffener sections.
16. Connect ejector drain tubes, vent tubes and vent hoses.
17. Install top portion of oil cooler support cowl.
18. Install right hand combining gearbox top cowl with anti-collision light and side cowl attached.
19. Connect oil filler hose and vent hose to oil filler adapter.
20. Install left hand combining gearbox cowl with side cowl attached.
21. Install left and right combining gearbox side panels with inspection doors.
22. Install left and right engine top side cowl assemblies. Connect starter-generator cooling ducts, fire detection element and lead wires and FM antenna lead.
23. Install left and right engine lower cowls.
24. Install left and right engine air inlet fairings.
25. Install the upper pylon cowling and connect the anti-collision light.
26. Install forward transmission fairing and hinge forward pylon fairing to closed positions and latch.
27. Rig engine controls to nominal adjustments.
Final rigging will be accomplished in conjunction with ground run and flight tests. Finally, the engine lubrication system needs to be primed, the gearbox properly lubricated, the fuel system primed, a wet motor run performed, and rigging adjustments made per the Bell maintenance manual.
Review of the Engine Control System for Bell 212February 1999
continued from Main Article...
The following is taken from the Pratt & Whitney PT6T Engine/Airframe Interface Rigging and Troubleshooting Training Manual. It is for review purposes only. For applicable data, refer to the Helicopter manufacturer's maintenance manual.
To understand rigging, it is helpful to understand the operation of the Droop Compensator and the Power Turbine Governor.
Droop Compensator -- Most output shaft Nf governors are of the droop type. A droop governor is used to ensure stability of operation at all power settings. This means that for each power setting, there is a separate and unique output shaft speed at any fixed setting of the Nf governor. The droop type governors are normally designated in terms of percent of droop (usually 6 to 16 percent). For example, a 10 percent Nf governor droop means that if the rotor speed were set at 100 percent under a no load (flat pitch) condition and full load was demanded, the output shaft speed would droop to 90 percent. In order to achieve 100 percent at full load, the Nf governor would have to be reset. Now, if power demands were reduced, the rotor would overspeed. In order to avoid this problem, a droop compensator is normally used to control the output shaft speed at constant value throughout the power range, regardless of the load applied to the rotor blades.
In other words, using a droop compensator, the engine power required will be a function of collective pitch setting, ignoring the effects of tail rotor and cyclic pitch power demands which are combined; these are usually not only less than 10 percent of total power available but also of a transient nature. The droop compensator mechanism input signal is provided by the collective pitch control system.
This input signal, which is a function of the collective control position, is being fed to a compensating cam positioned in the Nf governor control system. The droop compensating cam in the Nf governor control arm will be constantly changed to reset the Nf governor to give the same output shaft speed regardless of the collective pitch setting. This will maintain constant rotor rpm.
Power Turbine Governor -- The power turbine governor is mounted on the RGB and is driven at a speed proportional to N2 speed. In order to control the N2 speed, the AFCU supplies a pneumatic signal (Pg) to the power turbine governor. This again, changes the gas generator speed upon sensing an off-speed condition of the power turbine to keep constant N2 speed regardless of loading (collective pitch). During normal helicopter operation, the throttle lever twist grip is positioned against the maximum speed stop, and N1 is controlled by the Pg pneumatic signal supplied by the AFCU.
Power Turbine Governor Paralleling -- Before attempting to adjust the governors, perform the following:
• Collective pitch control system rigging
• Droop compensator controls rigging.
• Torque indicating system check/calibration.
In order to prevent interference of the TCU with the adjustment of the P.T. governor paralleling, it is recommended to disconnect the torque control unit pneumatic lines. Cap off all connections and lines.
Before starting, note the maximum rotor speed limitation given in the Bell 212 Flight Manual. Now you're ready to start the actual rigging procedure:
1. Actuate rpm switch to full Decrease.
2. With collective in down position (flat pitch, turn ENGINE 2 throttle grip to full INCREASE. Record rotor rpm. This should give the full DECREASE rotor rpm which should be 95 percent.
3. Slowly actuate rpm switch to full INCR (do not exceed 100 percent). Record rotor RPM. This should give the full INCREASE rotor rpm which should be 99 percent. If range and/or spread is incorrect, adjust as follows.
a. If range is incorrect, adjust actuator rod, lengthen or shorten so range is 95 to 99 percent.
b. If spread is incorrect, adjust stroke of actuator to give four percent spread (actuator to be in mid travel position when adjustment is made.)
4. Repeat steps 2 and 3 above for recording rotor rpm by using ENGINE 1. The range again should be 95 to 99 percent. If adjustment is required, adjust control tube (12, Fig. C-1). If this does not provide the desired range, move rod end to a new hole in jackshaft arm.
Although the initial adjustment is necessary for single power section rotor rpm, the following adjustment is used to finalize by achieving proper twin power section rpm range, and to avoid torque needle split.
5. With rotor at flat pitch, actuate rpm switch to full INCR. Turn both power section throttles to full INCREASE. Record rotor rpm.
6. Actuate rpm switch to full DECR. Record rpm.
7. Adjust length of actuator rod end to obtain 97 to 101.5 percent. Shorten rod to raise rpm or lengthen to lower rpm. When performing checks of the next step, the helicopter will become airborne.
8. Reconnect pneumatic lines to TCU.
9. With rotor at flat pitch, turn ENGINE 1 and 2 throttle grips to full increase. Beep to obtain 100 percent rotor rpm. Increase collective pitch slowly in a series of equal steps from flat pitch to full power (either engine temperature/rpm limit or transmission torque limit). Rotor rpm should remain at 100, ±1 percent throughout the power sweep. In the meantime engine torque should be matched within 4 percent during steady state operation. Ensure that the torque pressure transmitters are properly indexed to the numbers stamped on the RGB dataplate.
10. If torque does not match within four percent, perform the torque matching procedure given in the next paragraph.
Where torque matching requirement conflicts with the beep range, or the relationship of N1, ITT, or fuel flow between the two engines, torque matching shall be favored.
11. If rotor rpm droops (decays below tolerance) or overspeeds above 100 percent during power sweep, the compensation cam rate should be changed.
After each cam adjustment, an adjustment of control tube (6) may be required to ensure that cam slot does not bottom out during full down collective.
12. Check for any interference and for security of parts.
13. With collective full up and rpm beep switch at INCR, adjust max. power stop screw on governor to within 0.010 inch of stop arm.
Finally, perform Torque Matching -- Before attempting to obtain correct torque matching, ensure that the torque pressure transmitters are properly indexed to the numbers stamped on the RGB data plate; also, check the integrity of the airframe indicating system before attempting to alter the rigging of any components.
The engine controls rigging shall allow steady state single engine flight at 97 percent Nf or higher with either engine.
If torque split exceeds 4 percent during steady state operation, adjust control tube (12) to match both power sections. Lengthen control tube if ENGINE 1 torque is too high and shorten if it is too low. Don't forget to check the governors to ensure some clearance exists between stop arm and minimum stop screw.