TPE-331 Prop Governing Systems
Theory and Troubleshooting Tips
by Dan Ankarlo
Fundamental differences between Garrett (AlliedSignal) and Pratt & Whitney turbine engines dictate differences in propeller governor function. Whereas the Pratt & Whitney PT6A is a free turbine design; the Garrett engine is direct-drive from the engine to the propeller. This would seem to make the job easier for a governor installed on a Garrett engine than it would a Pratt & Whitney design; however, some special issues must be dealt with. An examination of the basic governor functions will reveal critical differences between the two models.
AlliedSignal and Woodward Governor both manufacture governors installed on Garrett TPE-331 engines. It is interesting to note that the AlliedSignal model of governor contains parts manufactured by Woodward (the base and body sections), and each manufacturer assigns its own part number to the assembly. This can lead to confusion regarding part numbers in the field.
For example, an AlliedSignal governor can be assigned three different part numbers, which are installed on two different dataplates. One of the part numbers is a base/body assembly number as assigned by Woodward. The second part number is the Woodward number for the complete governor assembly. The third part number is the corresponding AlliedSignal number for the assembly. In addition, a governor installed on a TPE-331 engine can also apparently have two different serial numbers. One serial will be for the base/body assembly, and the other serial will be for the completed assembly. Regardless of how many different numbers are stamped into the dataplate(s), both manufacturers' units operate in fundamentally the same way.
Garrett engine speed is controlled directly by prop blade angle. High blade angles (high load) equate to lower engine rpm, while lower blade angles (low load) equate to higher engine rpm. The governor is supplied with engine oil (typically at 95 psi). The governor contains an internal pump and pressure relief system that boosts this pressure to approximately 485 psi and supplies it at capacities of up to 10 quarts per minute. A set of spinning flyweights inside the governor sense engine rpm and direct the pilot valve to move in reaction to changes in rpm other than the desired setting.
In both Garrett and Pratt applications, servo oil (oil supplied to the prop piston) moves the blades toward the low blade angle, or high rpm position. This is designed so that a loss of engine oil pressure will automatically allow the propeller to move toward the feathered position (through the use of prop blade counterweights and feathering spring). The pilot valve position therefore determines whether oil will be ported to the propeller (increasing rpm) or to drain from the propeller (decreasing rpm). A speeder spring directly connected to the flyweights and pilot valve determine the position of the valve and thus a corresponding rpm. Speeder spring pressure is altered through the governor control shaft and associated linkage to the cockpit.
During "on speed" condition, the forces of the spinning flyweights equal the force of the speeder spring, and, as a result, no oil is ported to or from the propeller. A constant rpm results until an upsetting force changes the balanced state. It is important to realize that in reality, the governor is continually porting oil to the propeller to compensate for internal leakage between the gearcase and propshaft.
An underspeed condition will result if the aircraft is pitched nose up or if the pilot selects a greater prop rpm. In this case, the spinning flyweight forces are reduced and the pilot valve moves, resulting in an opening of the servo oil ports that supply high-pressure oil to the propeller servo. The addition of servo oil moves the blades to a lower blade angle and results in greater rpm or a restoration of original prop rpm.
Overspeed conditions are the result of unloading the propeller blades as in the case of a descent, or the pilot selecting a lower prop rpm. When an overspeed condition is sensed, the increased flyweight force overcomes speeder spring force and the pilot valve is moved to a position whereby oil is allowed to drain from the propeller. As a result, blade angle increases (due to propeller counterweight force) and rpm is thus reduced.
Propeller beta mode is defined as any blade angle between idle and full reverse. In beta mode, the pilot can manually adjust prop blade angle to control reverse thrust power during aircraft deceleration. A design problem must be overcome to allow the pilot to manually select blade angle. The governor must be held in an underspeed condition to eliminate interference between pilot command and speeder spring command.
To solve this problem, governors that are installed on Garrett engines have a piston assembly attached to the speeder spring that can alter speeder spring pressure. The governor control shaft runs through the middle of this piston. On one side of the piston, there are spring tension washers and on the other side of the piston, high-pressure oil (200 psi) is supplied to act against the force of the spring washers. In normal operation, the presence of this high-pressure oil will compress the spring tension washers. Beta mode is entered as the propeller pitch control uncovers ports allowing the oil to drain away. As a result, the spring tension washers move the piston into a position that causes an increased speeder spring force. This is a simulated underspeed condition, which will not allow the governor to interact with the propeller until approximately 107 percent of rated rpm. In this way, the prop is allowed to travel through the zero thrust range and into reverse. Actual control of the propeller at this point is accomplished by the pitch control sleeve that surrounds the beta tube in the engine. It is a means of mechanically controlling propeller blade angle and rpm without interaction from the governor. As propeller blade angle approaches the desired setting, pressure ports in the beta tube are uncovered, which will maintain the blade angle. When further travel into reverse is desired, the power lever resets the sleeve in the pitch control, allowing oil to move the propeller deeper into reverse.
Synchronizing On multi-engine applications, synchronizing and synchrophasing between two engines is accomplished through the use of a speed bias coil inside the governor. The coil is located between the pilot valve and the spinning flyweights. When energized, the coil exerts a magnetic force on the flyweights causing the pilot valve to move in small amounts. The synchronizing computer receives speed signals from both engines and varies the coil voltage of each governor to attain synchronization between both engines.
Look Mom, no linkage
Perhaps the most interesting Garrett engine governor is found on the TPE-331-8 (or -10) as installed on the Cessna Conquest II. This particular governor (P/N 897160-X) has no mechanical linkage for actuation. The fuel control computer and an actuator in the governor accomplish control of engine speed. The actuator in the governor is actually a torque motor that can alter speeder spring pressure and, thus, rpm. The governor has an integral speed pickup that sends rpm indications to the fuel control computer. The fuel control computer compares the actual engine rpm with the rpm selected by the pilot. It then sends power to the torque motor to change speeder spring pressure and rpm as required. In a similar fashion to other Garrett engine governors, beta control is achieved by simulating an underspeed condition within the governor. When ground idle is chosen by the pilot, a signal is sent to the governor that resets the piston/ speeder spring to 105 percent of rated rpm. This allows the pilot to control blade angle without interference from the governor.
As is the case with any multi-engine aircraft, isolating suspected governor problems can be easily accomplished by swapping the governors side-to-side on the airframe.
Service difficulties with Garrett engine governors usually fall within two categories: synchronizing problems and beta problems. To isolate synchronizing problems, speed pickup output voltages need to be checked and reset as required. In some cases, this is accomplished much easier on a test bench than on the aircraft. Voltages on Garrett governors are measured peak-to-peak using an oscilloscope (not using RMS figures with a volt/ohm meter). At International Governor, we have seen many cases where magnetic speed pickups and flyweight assemblies have been damaged due to incorrect procedures being used when checking speed pickup voltages. Typically, the technician performing the work will not have the correct voltage indications (because it's being measured in RMS), and will turn the speed pickup into the point where it contacts the flyweight assembly. This can be a costly error. After speed pickups have been verified, the coil assemblies should be checked for continuity. Airframe power to the coil should be checked at this time as well.
Difficulties achieving beta blade angles are usually the result of weak spring tension washers inside the governor or a blocked pressure line to the reset piston.
Again, isolating this type of problem is best accomplished by swapping the governors side-to-side on the aircraft. Don't be afraid to use this technique as it can save the aircraft owner money by not ordering unnecessary exchange units.
Hydraulic Systems Tubing
Lifelines to power and motion control
By Terry Karl and Mark Morrow
Generic tube repair
Step by step description of a basic tube repair:
Tube cutting - Cut the tube using a chipless tube cutter, high-grade hacksaw, or other production method that ensures a square-cut end, with a minimum amount of burrs. The cutter should be moved slowly and uniformly to ensure that tube deformation does not occur.
Tube deburring - After cutting the tube, carefully remove any burrs from both the outside diameter and inside diameter of the tube. Use of a deburring tool helps to prevent the inclusion of metallic chips inside the tube which would contaminate the hydraulic system. Cut and deburred tube ends should be protected from further damage or the collection of dust or dirt if they are to be left unattended for any period of time.
Tube bending - In most cases it will be necessary to form the replacement section of tubing to fit the aircraft. Obtaining a smooth bend with an absolute minimum of tube flattening is needed to ensure high integrity and life of the replacement section as well as avoiding any unwanted fluid flow restriction. Tubes may be formed either by hand or by powered tube benders, but care must be taken, depending upon the nature of the tube material, as well as the wall thickness. Excessive flattening, kinking, wrinkling, or other deformation of the tube must be avoided. Table I on pg. 52, shows the acceptable limits of tube flattening.
The maximum OD and the minimum OD are the largest and smallest cross-sectional diameters measured within the area of the bend. The ovality in the bend area should not exceed the values in Table I. Recommended minimum bend radii are as shown in Table II, pg. 52.
Depending on the capability of the apparatus used to bend the tube, it may be necessary to use larger bend radii.
Installing tube fittings - After the replacement section has been formed and is ready for installation, attachment of the permanent tube fittings is required. While each of the permanent tube fitting styles has some unique requirements, there are generic steps which are similar:
1. Mark the parent tube and the replacement section to indicate where the repair fitting is to be positioned
2. Position the tube fitting relative to the tubing
3. Position the installation tooling
4. Install the tube fitting
5. Inspect the installed joint
Figures 5(a) through 5(e) depict this sequence of events for the axially swaged, Rynglok® Tube Fitting System.
Inspection of the repaired tube assembly - If possible, the repaired tube assembly should be proof tested using appropriate equipment, in accordance with the airframe manufacturer's maintenance manual instructions, prior to being installed on the aircraft. Alternatively, the repaired tube assembly may be installed on the aircraft and tested as the hydraulic system is tested before deeming the aircraft flight-worthy. Care should be taken to perform all normal hydraulic system tests.
Permanent tube fittings: A bewildering array - Over the years, quite a few permanent tube fitting styles have been developed, both for production of the aircraft's hydraulic tube system, as well as for repair of the aircraft once in service. Development of this wide variety of choices is due in large part to the complex variety of installations on the aircraft. Many mechanics have often encountered situations where, among other things, they have wondered exactly how the installation was designed and installed on the aircraft and whether any thought was given to the poor mechanic who one day might have to maintain these tubes. As a result, each of the types of permanent tube fitting styles offers attractive attributes, depending upon a variety of circumstances.
Correspondingly, each tube fitting system also entails some less than desirable attributes. Each mechanic and system maintenance engineer must examine the primary criteria of their aircraft needs, fleet needs, logistics, mechanic skill and training levels, and other pertinent factors, when deciding what tube fitting styles best meet their requirements. Having said that, it is worth mentioning the most prominent permanent tube fitting styles available today:
Weld-style fittings: Widely used to produce aircraft hydraulic tube assemblies, this method of attachment may also be used for repair, but requires the proper weld equipment and inspection facilities and is more difficult to accomplish on board the aircraft. Mechanic skill levels are relatively high. Welded tube connections, accomplished correctly, create joints of equal or greater strength than the parent tube, and are light in weight.
Bite-style fittings: This fitting style relies on sleeves that literally bite into the parent tube to effect the connection. They are relatively simple to accomplish, require a lesser skill level on the part of the mechanic, but also require a larger envelope in which to turn wrenches that install the fitting.
External swage-style fittings: Although used industry-wide, external swage-type fittings require a considerable amount of equipment to accomplish repairs and a relatively skilled workforce to install them. The prevalence of the system provides for logistic advantages. In some cases, the production tubes were manufactured with this system, minimizing the envelope restrictions for access to accomplish repairs. This fitting style also uses an elastomeric seal on the interior of the fitting as a secondary seal, if required.
Shaped memory fittings: Advancements in metallurgical science allowed for the development of a special fitting style that relies on the "memory" of the metal. These fittings are stored in cryogenic dewars of liquid nitrogen and removed when needed to be installed on the hydraulic tubes. Special equipment pre-chills the tube ends to properly accept these lightweight fittings; however, since these fittings accomplish their method of attachment to the tubing by warming up in the ambient environment, they are somewhat time sensitive in their installation.
Axially swaged-type fittings: This fitting style axially swages a permanently attached ring around a fitting body, which permanently deforms both the fitting and the tube to effect a metal-to-metal seal without the use of elastomers. The fitting material is compatible with all types of tubing and wall thicknesses. Mechanic training and skill level are relatively low. Installation equipment investment required is more than bite-type fittings and shaped memory fittings, but less than external swage and weld-style fittings.
The road ahead
Care and maintenance of hydraulic system tubing are as important as any other aspect of system maintenance. With the ever increasing complexity of today's aircraft, proper attention to hydraulic tubes on both a preventative as well as a repair and replacement scheme can avert potential problems before they impact the aircraft's flight operations. Older and newer technologies and approaches are available to meet each individual aircraft's needs. Newer technologies attempt to address all elements of cost associated with the repair and maintenance of hydraulic tubing. Each mechanic is encouraged to familiarize themselves with alternative tube repair technologies in an effort to ensure optimized repair capability for the aircraft on which they work.