tfe 731 chap 75 (1)

Garret TFE 731 Turbofan Engine (CAT C)

CHAPTER 75

of 18 FOR TRAINING PURPOSES ONLY © TFE 731 - ISSUE 2, 2010


CHAPTER 75


INTRODUCTION

0 TABLE OF CONTENTS

1 Bleed Air Extraction 3 2 Compressor Stall (Surge) 4 3 Primary Sources of Back Pressure 5 4 Low Pressure Compressor Surge Valve 6 5 Surge Control Valve System 7 6 Surge Bleed Valve 8 7 Surge Valve Open 9 8 Thrust Setting – Normal Mode 10 9 Surge Valve Closed 11 10 Surge Valve

1/3 Open 12

10.1 Troubleshooting 12 11 Spinner Anti-ice System 13 11.1 Anti-Ice Shutoff Valve 14 11.2 Solenoid Operated Control Valve 14 11.3 Anti-Ice Pressure Switch 14


CHAPTER 75


ENGINE AIR

1 BLEED AIR EXTRACTION Compressor bleed airflow is available for aircraft use at all engine operating levels and during all flight conditions, except during starting. Since bleed-air extraction is not permitted during engine starting, all aircraft systems that require engine bleed air, such as ECS and anti-icing, must be turned off during engine starting. Compressor bleed air is also used for engine anti-icing if the engine is equipped with the anti-iced spinner, and to preclude compressor instability (stall) when required. The anti-icing system is controlled manually, while the engine electronic control system automatically adjusts bleed to preclude compressor stall. Compressor bleed airflow for airframe use may be extracted, either separately or in combination, from the low-pressure or the high-pressure bleed ports. The low-pressure bleed air is extracted from the annular plenum located between the low-pressure and high-pressure compressors. High-pressure bleed air is extracted from the plenum surrounding the combustor. The engine normally is equipped with two low-pressure bleed ports and one high-pressure bleed port. To accommodate installations requiring additional high-pressure bleed air, a three-bleed-port plenum configuration is available for all models of the TFE731-3/-4/-5 engines. The engine bleed air ports and other bleed system components are shown here. Airframe connections to the bleed port flanges are depicted in the aircraft maintenance manual.


CHAPTER 75


2 COMPRESSOR STALL (SURGE) A characteristic of gas turbine engines is their tendency, under certain operating conditions, to stall or surge. First, we should understand that compressor stall (surge) is not a phenomenon peculiar to any specific type of engine. It may occur on any turbine engine if the conditions are right. The constant demand for more thrust and lower specific fuel consumption is met by increasing the mass airflow through the engine, increasing the pressure ratio, increasing the turbine inlet temperature, or improving the efficiency of the compressor and turbine sections of the engine. Compressor stall (surge), which occurs in many different forms and under various conditions, is a mixture of many complex phenomena which are neither easy to describe nor to understand. The intent here is to explain surge in non-technical terms. The simplifications and analogies used must not be interpreted as the final explanation, but as insights to understanding what is used to control surge. When an excessive amount of fuel is metered into the combustor, there will be enough air to burn it; but as more air is used for combustion, there will be less cooling air. Thus, burner pressures increase, resulting in an increased volume of gasses to exhaust through the turbine. If the resultant volume of gasses exceeds that which can flow through the turbine, the turbine will choke. When the turbine chokes, burner pressures increase rapidly to a value that is equal to or greater than the compressor discharge pressure. Airflow from the compressor stalls if the burner pressure is equal to the compressor discharge pressure (PCD). If the burner pressure is greater than PCD, the compressor not only stalls, but the gasses will flow from the combustor into the compressor. Either of these will result in the same thing, no airflow into the combustor. As the available oxygen is depleted, the fire now begins to die out for lack of oxygen. This result in a rapid drop in temperature, reduced expansion, and greatly

reduced volume of gasses. Now the turbine no longer chokes and burner pressures drop significantly. The compressor airflow "surges" into the combustor. This rapid movement of air into the combustor moves the flame downstream through the turbine. If the velocity of gasses does not exceed the burning rate of fuel, the flame propagates back through the turbine into the burner. If there is still too much fuel flow, the above cycle repeats itself many times per second. Compressor stalls vary in severity, depending upon whether the stall involves only a portion of a stage, a stage or several stages in a multistage compressor, or an entire compressor. At the beginning of a stall, it may produce roughness with or without audible sounds or rumble or drone. More pronounced stalls might produce noises varying in intensity from pistol shots to small explosions. Extremely severe stalls may produce pulsations that cause flame, vapour, or smoke to exit the exhaust or even the air inlet. An LP compressor stall may also occur when HP compressor speed slows in


CHAPTER 75


relation to LP compressor speed. The LP compressor is developing more airflow than the HP compressor can accept. This flow restriction occurs mostly during deceleration when the HP spool slows faster than the heavier LP spool.

3 PRIMARY SOURCES OF BACK PRESSURE For each compressor RPM there is a particular relationship between its pressure increase and the amount of its airflow. In order to maximise compressor efficiency, it must operate at a point that produces the highest pressure ratio. Since the TFE731 engine is a free turbine, it must be recognised that each spool will turn at the speed that the high pressure gas generator drives it. For this reason, a mismatch of N1 and N2 speeds can occur, resulting in a surge condition. It should be noted that the compressor is merely an air pump and that the air moving through the engine meets many obstacles to its flow, most notably, the pressures within the combustor that are generated by the burning of fuel. If over fuelling occurs, the pressure in the combustor may exceed the compressor discharge pressure, therefore, compressor discharge air cannot flow and the compressor tends to stall or surge. The HP turbine nozzle areas must be sized to produce the required pressure drop to provide desired turbine speeds and burner pressures.


CHAPTER 75


4 LOW PRESSURE COMPRESSOR SURGE

VALVE A natural question to ask at this point would be "since the TFE731 engine is controlled by an electronic engine control, how does it control surge?" During start, fuel is precisely metered by the fuel control receiving commands from the EEC. The EEC is monitoring spool speed and turbine temperature during this time. The LP compressor is dumping some PCD overboard through the surge bleed valve. The valve is open during start. During acceleration, precise fuel scheduling based on the relationship of N1 and N2 speeds controls surge. When decelerating, the potential for surge is minimised by the constant monitoring of spool speeds by the electronic control in addition to a reduction of fuel flow as a function of spool speed and opening of the surge bleed valve to slow the LP compressor. The EEC will select the one-third or full open position as required maintaining the proper spool speeds. In the normal mode of operation, the TFE731 is essentially surge free.


CHAPTER 75


5 SURGE CONTROL VALVE SYSTEM

5.1 Description

The system consists of two solenoid operated control valves (A and B) and a pneumatically operated surge bleed valve. The solenoids are mounted externally on the fan duct at approximately the three o'clock position and are electrically actuated by the electronic engine control (EEC). These solenoid operated valves are spring loaded open (de-energised) and when electrically energised will close, shutting off the P3 airflow to the surge bleed valve. The surge bleed valve is mounted to the compressor housing at three o'clock just aft of the right hand low pressure bleed duct. It is a three-position valve: open, one-third open, and closed. An internal spring holds the valve in the open position while the engine is not running.


CHAPTER 75


6 SURGE BLEED VALVE Looking at this schematic, in a static condition with the EEC off, note that both solenoids A and B are de-energised, and the valves are open. The surge bleed valve has two ports and two air chambers. Notice that the spring in chamber A has positioned the poppet in the open position. The poppet stem is drilled with an orifice. Notice the machined area midway on the poppet stem. This will become significant later in our discussion of the surge bleed valve operation. The following diagrams will depict the positions of the surge bleed valve during a start and run sequence.


CHAPTER 75


7 SURGE VALVE OPEN During the engine start sequence, solenoid B is energised by the EEC, closing valve B. As P3 air becomes available, pressure is applied through valve A to port A. The spring force in chamber A maintains the valve in the open position. With the power lever in the idle detent, the surge bleed valve will remain in the full open position.


CHAPTER 75


8 THRUST SETTING – NORMAL MODE While the power lever is in the thrust range, the surge bleed valve position is controlled by the electronic engine control (EEC) and will open and close as surge conditions warrant. This automatic control of the surge valve is available only after the EEC recognises a power lever position above idle. For rigging purposes, 20° of fuel control input shaft rotation equates to the power lever idle detent position. During normal engine operation, the EEC receives the same electrical signal (idle) until the fuel control shaft rotates to 26 or 40°, depending upon which model fuel control the aircraft utilises. This area above the quadrant idle detent (20°) is known as the idle dead band. When the power lever is within this range, the surge valve remains open and engine speed remains at idle.


CHAPTER 75


9 SURGE VALVE CLOSED With the power lever positioned in the thrust range, Solenoid "A" is energised and Solenoid "B" is de-energised by the EEC. P3 is blocked at Solenoid "A" and any trapped air is vented. P3 passes through Solenoid "B" into Chamber "B" of the surge valve. Pneumatic pressure easily overcomes the spring force and the poppet strokes closed. The surge valve will remain in this position until the EEC recognises a possible surge condition or when the power lever is retarded to idle. When retarding the power lever the electronic control will drive the surge valve to a full open position at 60, 42, or 30 degrees of PLA depending on service bulletin compliance.


CHAPTER 75


10 SURGE VALVE 1/3 OPEN

The surge valve will be positioned one-third open any time both control solenoids are de-energised. While both solenoids are de-energised, P3 is allowed to pass through each into Ports A and B. Pressure flows into Chamber A through the machined area on the poppet stem. The orifice in the poppet acts as a metering port, producing a balancing of forces in both chambers. The valve is held in the one-third open position by these balanced forces. Remember that the position of the surge bleed valve is controlled by the electronic engine control (EEC) based on power lever position and the spool speed relationship between N2 and N1. When the valve is opened, the surge margin is increased. When the surge schedule is far enough above the output fuel schedule the valve closes, first to the one-third position and then fully closed. When the power lever is in the idle dead band range, the bleed valve will remain in the full open position. This position lowers the minimum idle thrust and acts to improve the acceleration rate from idle to maximum thrust. If the EEC transfers to manual mode, both solenoids would be de-energised, causing the surge bleed valve to assume a one-third open position.

10.1 Troubleshooting

Mechanical malfunctions of the surge bleed valve system will usually result in an unwanted "valve open" condition. The mechanical tendency of the surge valve is to remain open so most system failures will result in a leak of compressor air. This condition will appear as a drop in N1 and an ITT increase. Remember that the EEC is controlling N2 and, within limits, will supply as much fuel as needed to obtain the desired N2 speed. With any air leak, the loss of pneumatic energy must be compensated with more fuel, hence higher ITT. A surge valve that leaks or remains open will look like any bleed air leak, slower

acceleration, higher than normal ITT and lower than normal N1. To spot this condition, the operator must be aware of the normal parameters of the engine. Monitoring and recording normal operation is a valuable tool for spotting abnormal conditions. Isolating a bleed air leak from the surge valve is not difficult. The light maintenance manual lists, in detail, the procedure for checking this unit.


CHAPTER 75


Electrical problems concerning the solenoids usually result in the EEC reverting to manual mode. This situation then becomes a search for the circuit problem and making the electrical repair. Leaks may develop in the plumbing that delivers P3 air through the solenoid to the proper chamber of the surge valve. This is a rather straightforward problem and easy to identify using a source of clean air and looking for leaks. If a surge valve leak is suspected, another option is to perform an engine performance evaluation run. This procedure, although time consuming, will spot problems that may only occur during high engine speeds. The procedures for conducting the engine performance evaluation run are found in the engine light maintenance manual.

11 SPINNER ANTI-ICE SYSTEM Bleed air for anti-icing of the engine spinner is extracted from the plenum surrounding the combustor and is ducted internally to the inner surface of the spinner, finally exhausting into the fan air stream just forward of the fan blades. The spinner anti-ice system is utilised on TFE731-2 and -3 engines equipped with elliptical spinners and some TFE731-3A engines. Ice formation on these spinners is possible but preventable using engine high pressure compressed air as an anti-icing agent. Engine anti-icing is controlled by an airframe-furnished cockpit switch that actuates the engine anti-ice control solenoid valve and energises the inlet pressure and temperature sensor (PT2 TT2) electrical heater.

The control solenoid actuates a pneumatic valve located in the chamber between the low pressure compressor and the inner fan duct. A pressure switch, downstream of the anti-ice valve, closes when 6 ±1 psi is available to the engine anti-ice passages. This provides cockpit indication of engine anti-ice operation. Engines equipped with the conical spinner, with the exception of the -3A, do not require spinner anti-icing; therefore, the engine anti-ice valve, pressure switch and associated control systems are deleted. However, the engine inlet pressure and temperature sensor must be anti-iced.


CHAPTER 75


11.1 Anti-Ice Shutoff Valve

The anti-ice shutoff valve is located at approximately the twelve o'clock position, forward, and underneath the upper air/oil cooler. The valve is normally held closed by P3 air pressure exerting a force on the piston in Chamber "A". When this air pressure is interrupted and Chamber "A" is vented to atmosphere, compressed air in Chamber "B" overcomes the opposing spring tension. The shutoff valve poppet then opens and permits airflow to the spinner.

11.2 Solenoid Operated Control Valve

The air supply to the shutoff valve is through the normally open solenoid (C) mounted with the surge bleed solenoids on the fan bypass duct. When the cockpit anti-ice switch is energised, solenoid "C" will close, stopping the flow of air and opening Chamber "A" of the shutoff valve to ambient.

11.3 Anti-Ice Pressure Switch

When the anti-ice valve opens, air pressure is routed to the pressure switch. Operation of the switch completes a ground circuit to illuminate the cockpit anti-ice warning light. The pressure switch is normally located on the outside of the fan bypass duct. Exact location of the switch varies with engine configuration.


CHAPTER 75



CHAPTER 75


PAGE INTENTIONALLY LEFT BLANK

tfe 731 chap 75 (1)

Documents

engine air

surge bleed valve

engine bleed air ports

lowpressure bleed air

bleedair extraction

compressor stall surge

engine antiicing

highpressure bleed ports