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- EPI Service Bulletin #1 -


Document Number 260001
A08/22/01Initial Release.
B06/28/02Raised upper oil temperature limit from 225°F to 235°F.


It has come to the attention of EPI, Inc. that in certain cases, the EPI Mark-9 PSRU is being installed on aircraft in violation of the requirements specifically defined in EPI Document Number 240002, entitled EPI Mark-9 Gearbox Installation Manual. Such noncompliance with the installation manual, in the considered opinion of EPI, Inc., the designer and manufacturer of the subject PSRU, renders such noncompliant aircraft unairworthy.


The Installation Manual (EPI 240002) is explicit in its requirement for the installation of the gearbox Chip Detector and an appropriately connected annunciator on the instrument panel, as well as accurate gearbox front gallery oil pressure and exit oil temperature gauges on the instrument panel.

EPI has discovered that several aircraft with EPI PSRU’s installed have been flown in violation of those requirements.

The requirements for that instrumentation are well-defined in the Installation Manual (EPI 240002), and the relevant portions of that document are reprinted in Section 4.0 below. The complete installation manual is available from EPI, Inc.


EPI, Inc., the designer and manufacturer of the subject PSRU, declares any aircraft not in compliance with the mandatory requirements of the Installation Manual, as defined in this Service Bulletin, to be UNAIRWORTHY, and further requires that such noncompliant aircraft not be flown until such compliance has been achieved and documented in the appropriate aircraft logs.

Note that the acquisition of this document by the insurance carriers covering these aircraft would likely render any such insurance void.





There are three pieces of gearbox instrumentation which EPI considers to be FLIGHT CRITICAL: (a) the Chip Detector, (b) the Front Gallery Oil Pressure Gauge, and (c) the Exit Oil Temperature gauge.

EPI considers it to be mandatory that each aircraft which uses a powerplant with the EPI Mark-9 PSRU must contain, on the instrument panel, in a location at least as visible as the other engine instrumentation, (a) an annunciator light to alert the pilot that the Chip Detector has activated, (b) a pressure gauge, with a range from 0 to 50-75 psig, which accurately displays the oil pressure at the Front Gallery tap on the gearbox, and (c) a temperature gauge, with a range from at least 100°F to 250°F, which accurately displays the temperature of the oil which is exiting the gearbox.

This instrumentation is critical to the determination of potential gearbox problems which could cause complete and sudden stoppage of the propeller. These instruments are no less important than the engine oil pressure and temperature gauges required by FAR 91.205{b}. (Note that Part 91 applies to experimental as well as certificated aircraft.)

In no case should flight be attempted without all three of these pieces of instrumentation installed and operating correctly.

In addition to the required instrumentation, EPI recommends that the aircraft instrumenta­tion system include two additional annunciators to draw the pilot’s attention to the oil pressure and temperature gauges anytime pressure is below 25 psi or temperature is above 235 °F.

1.1 Chip Detector

The EPI Mark-9 PSRU is built and tested with the approved Chip Detector installed. The Chip Detector is located in a position so as to enable it to detected the presence of ferro­magnetic particles in the gearbox exit oil. Any presence of such particles is sufficient indication of abnormal wear or deterioration of the gearbox internals so as to warrant termina­tion of flight as soon as the pilot reasonably can do so safely, followed by an inspection of the Chip Detector and of any other systems as required to determine the cause of the warning.

A chip detector is REQUIRED on rotorcraft drive system gearboxes (FAR 27.1337-e), and EPI considers the potential loss of propulsion as a result of gearbox damage to be no less dangerous than a similar loss on a rotorcraft.

The wiring of the Chip Detector can be accomplished as follows: Power from the main bus is supplied to one side of the annunciator light. The other side of the annunciator light is connected to the Chip Detector terminal. When particles are detected, the Detector completes a circuit to ground and lights the annunciator. It is recommended that a momentary contact switch be wired in parallel with the detector. Operation of that switch should be part of the preflight checklist to test the operation of the annunciator.

If, after the gearbox is put into operation, the chip detector triggers an alarm, remove it and examine the particles adhering to the magnet, and contact EPI at once to discuss the findings.

If the chip detector triggers an alarm in flight, reduce power if safe to do so, and land as soon as safely possible. Determine the cause of the alarm and a resolution to the problem before considering any further flight.

1.2 Gearbox Oil Pressure Gauge

Most of the heavily-loaded bearings in the gearbox are journal bearings, just the same as the bearings in the engine. Unlike the bearings in a Lycoming or Continental, even a momentary interruption of the pressurized oil supply to the gearbox (or engine) bearings imposes a significant risk of damage to and failure of these bearings.

The gearbox front gallery oil pressure is a FLIGHT-CRITICAL parameter, and it is essential that there is an accurate oil pressure gauge installed in the aircraft which displays gearbox oil pressure to the pilot.

The PSRU is supplied with a pressure tap at the front of the idler shaft bore (described further in the PLUMBING section) for measuring the oil pressure in the front gallery of the gearbox. It is this gallery which supplies lubrication oil to the front bearings on the prop­shaft, drive gear and propeller thrust bearings. These bearings require a constant supply of oil, pressurized to not less than 25 psi.

Please note that relying on the engine oil pressure instrument or on a gauge displaying gearbox oil supply pressure IS NOT SUFFICIENT. It has been demonstrated in testing at EPI that the pressure in the front gallery can decrease to dangerously low levels and NEVER SHOW A CHANGE ON THE ENGINE OIL PRESSURE OR THE SUPPLY PRESSURE GAUGES.

The gearbox should never be operated at any power setting above ground idle if the front gallery oil pressure is below 25 psig. If this limit is reached in flight, reduce power as soon as conditions will allow a power reduction safely, and land as soon as safely possible to determine the cause of the problem.

1.3 Gearbox Oil Temperature Gauge

The gearbox depends on the circulation of engine oil not only for lubrication, but also for cooling. The meshing of the gears generates a significant amount of heat, and the flowrate of oil through the gearbox has been designed to carry off that heat if the gearbox is oper­ating properly.

The gearbox exit oil temperature is a FLIGHT-CRITICAL parameter, and it is essential that there is an accurate oil temperature gauge installed in the aircraft which displays gearbox exit oil temperature to the pilot. Similar instrumentation is considered mandatory on many rotorcraft.

If something in the gearbox starts to fail (a bearing, for example) the gearbox exit oil temperature will begin to rise very quickly, giving the pilot a certain amount of warning about the impending problem. Even though the oil exiting the gearbox is returned to the engine sump, you cannot rely on the engine oil temperature gauge for this function. Testing at EPI has demonstrated that the gearbox exit oil temperature can be rising significantly above its maximum limit, and yet produce no significant change in the temperature of the oil in the engine sump.

During initial gearbox break-in and testing, gearbox exit oil temperature is monitored very carefully. For approximately the first hour of operation (at power levels less than 300 HP) the exit oil temperature is quite high while the gear teeth (very hard and smooth as manufactured) are mating in. The completion of this process is indicated by a rather quick decline in exit oil temperature to normal values.

The sensor for the oil temperature gauge should be installed in the 1/8-NPT threaded boss on the lower port side of the gearbox case.

Before attempting flight, verify the correct operation of this temperature gauge by placing the sensor in a cup of boiling water and noting the reading on the gauge (212°F ±5 at sea level).

At no time should the gearbox oil temperature be allowed to exceed 235°F. If this limit is reached in flight, reduce power as soon as conditions will allow a power reduction safely, and land as soon as safely possible to determine the cause of the overtemperature.

If the temperature has been allowed to exceed 235°F for more than one minute (cumulative), there is a significant risk that the strength of the gears may have been compromised, and the gearbox must be returned to EPI for a complete inspection.

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