Appendix 1
(1) An engine may not achieve maximum power available or emergency rating because insufficient fuel is supplied. This condition has a number of possible causes:
(i) Low acceleration schedule (Il) Low maximum fuel stop (III) Low fuel pump output
(iv) Restrictions between the fuel control and the combustor
(2) The proposed emergency ratings (OEI) may preclude the use of a topping check to uncover the above problems; therefore the following procedures are advanced which can be used either separately or in combination with other approved methods to assure that the required fuel flow is available.
(3) During engine acceleration the fuel flow rate is considerably higher when compared with the normal steady state condition. This fact can be used to verify the availability of OEl fuel flow. The verification can be done by a direct measurement of fuel flow during an acceleration or derived indirectly from the engine acceleration rate.
It is envisaged that the determination of fuel flow by these procedures should be done by some automatic means.
(4) Figure 1 is a bypass technique in which some of the fuel controls output is routed away from the engine and back to tank. This forces the fuel control onto the acceleration schedule in order to maintain gas generator speed. The design of the system should ensure that with the bypass flowing the fuel control outlet pressure and flow at the OE ratings are simulated. The bypass system can be either permanently installed and operated in flight, (Failure Malfunction Effects Analysis must be provided), or as an item of ground test equipment. The quantity of fuel bypassed should be equivalent to the worst case difference between fuel flow at the 30-second rating and typical power assurance power levels. However, trend monitoring and service history may provide the basis of an alternative to periodic measurement.
b. Limiters. A means must be provided to assure that a lower than required (for OEI power) limiter setting does not exist. Limiters that could prohibit reaching OEI power are as follows:
(1) Ng - Maximum Compressor Speed Limiter or Governor
(2) Measured gas temperature limiter
(3) Output shaft torque limiter Page A1-4
Appendix 1
(4) Np limiter or power turbine governor-power turbine governors can be verified at lower than OEI power conditions.
(5) Fuel flow limiter or maximum fuel flow stop-Fuel flow limiting has been addressed in previous paragraphs.
c. Eailure Modes and Effects Analysis.
Failure modes and effects analysis, along with limited demonstration and suitable engine health monitoring procedures, may provide the basis of an acceptable solution to possible unexpected power limiting due to engine condition. It should be shown in the analysis that there is no probable event or combination of events which can cause a latent problem leading to inadequate fuel flow at high powers. The analysis should include all components of the fuel system such as: pump(s), control system (mechanical, hydromechanical, electronic, etc.) pipework, filters, fuel nozzle(s), and electrical interfaces. It should also address the probable effects of accumulated running time, dirty fuel, and hostile environment.
d. High Corrected Gas Producer Speed.
(1) The proposed OEl ratings will cause the engine to run at high corrected gas producer speeds (Ng/). At high Ng, performance characteristics of components, especially in the compressor, can change significantly and to an extent which would change the extrapolation of low speed run line data.
(2) In operation, the effects of the accretion of dirt, FOD, component deterioration, and erosion of blading may also cause changes in the high-speed performance of an engine.
(3) The above effects must be considered when developing power assurance procedures and data.
e. Special Devices.
(1) The satisfactory operation of devices or systems whose functioning is required in order to achieve the OEI powers should be verified. Devices or systems, which in normal operations are not exercised through the range of travel needed to achieve the OE powers, may require special checks to assure adequate capability.
(2) Special devices that are required only in order to achieve the OEl powers (for example, solenoids to provide additional cooling flow to hot-section components or a water/anti-freeze mixture into the compressor), should be subjected to periodic checks and have a demonstrated high reliability.
Page A1-5
Appendix 1
6. AIRFRAME CONSIDERATIONS.
a. Instrumentation Accuracy.
(1) The accuracy of any power assurance check is strongly dependent on the air data and engine parameters. SAE ARP 1217 (May 1979) provides guidance on the desired measurement accuracy for parameters used for engine health and diagnostic monitoring. The parameters to be considered with their respective functions include:
Pressure Altitude Air data basis for Flight Speed establishing power Free Air Temperature plant inlet pressure (stagnation) and temperature Torque Direct measurement of Power Turbine Speed power output.
Gas Generator Speed(s) Measured Gas Temperature Fuel Flow Primary thermodynamic and limiting parameters Secondary trend monitoring and potential limiting parameter
(2) The overall power check accuracy can be assessed on a suitable statistical basis using equations that link the measured parameters and inserting system accuracy distributions for each value. This approach will provide an overall assessment of power check accuracy and will highlight major contributors to error. The accuracy assessment at each parameter should include the following elements:
Sensor error Indicator error System error Reading error
(3) This assessment might show that while conventional instrument displays of air data are acceptable, servo driven digital displays are desired for engine parameters.
Further, displays that provide a "snapshot" of engine readings at a given moment may be useful in avoiding variation in power level during the finite period needed to manually read and log the set of parameters.
b. Installation Loss Definition.
(1) Installation loss definition is an extremely important aspect of any form of rotorcraft engine performance. Engines are certificated and sold with uninstalled performance guarantees and estimates as to the power output capabilities. Installation of the engine in the rotorcraft imposes power output penalties that must be accounted Page A1-6
Appendix 1 for in any sort of power assurance check procedure. Normal practice dictates that the engine manufacturer provides a computer program that accurately predicts the engine power output capability throughout the approved flight envelope. This computer program has the capability to correct the power output for the losses incurred by the rotorcraft installation.
(2) Losses that can reduce engine power available are as follows:
(1) Air intake total pressure loss (II) Air intake total temperature rise
(iii) Exhaust back pressure
(iv) Accessory power extraction
(v) Compressor bleed air extraction
(vi) Off-optimum power turbine output speed effects
(3) The above items and methods of dealing with them are clearly defined in SAE Aerospace Recommended Practice (ARP) 1702. Typically, these losses will not be a fixed percentage but will vary with engine operating conditions and environment.
(4) Any calculations involving power assurance data should use the approved engine performance program, and the rotorcraft losses should be input on a discrete basis so that the interaction between losses and their independent variability is properly considered. This approach is clearly defined in ARP 1702. Accurate consideration of the losses should produce a Power Assurance Check that will preclude premature removal of acceptable engines or continued operation of inadequate power plants.
7. ROTORCRAFT FLIGHT MANUAL RFM).
a. The Power Assurance Check data for the installed engine (engine data adjusted for inlet losses, exhaust losses, bleed extraction, power extraction, and off-optimum output shaft speed operation) should be presented in the RFM in an easily useable format. The data format may consist of charts of engine torque (at constant power turbine shaft speed) versus allowable values of gas generator speed and gas path temperature covering the range of ambient conditions for takeoff operations.
Associated limitations for the rotorcraft transmission and the engine should be noted.
b. The RFM should also:
(1) Include succinct statements of the reason for the Power Assurance Check and what must be done if the Power Assurance Check results are not acceptable.
Page A1-7
Appendix 1
(2) Clearly state that Power Assurance Check either is a pre-takeoff or in-flight procedure, as required by operations, specifications and/or other approval authority documents.
(3) Be kept simple, easy to use, and identify equipment operation limitations and requirements.
Page A1-8
7/30/97/ FUEL
FUEL FLOW
CONTROL
TRANSMITTER
FUEL PUMP FIGURE АРХ1-1 BY-PASS
TEE NEEDLE VALVE
FUEL PRESSURE
TRANSMITTER * U.S. GOVERNMENT PRINTING OFFICE: 1997 - 430 - 037 / 80092 FUEL FUEL NOZZLE
PRESSURE GAGE
COCKPIT FUEL AIRCRAFT
FLOW GAGE FUEL TANK Appendix 1 AC 27-1A Page A1-9 INSTALLED ENGINE FUEL FLOW CAPACITY CHECK