Revised 24-Mar-2001
Back To Index

The Boeing 727 is powered by three aft mounted Pratt and Whitney JT8D series dual compressor turbofan engines.  Each engine produces Between 14,000 and 16,900 pounds of thrust at sea level depending on model installed (new fan mod excluded).   The design of the engines includes an integral fan bypass which routes fan air the entire length of the engine. cooling the engine jacket.  The fan air mixes with the turbine exhaust just forward of the reverser section where it increases thrust and reduces engine noise.  The engine has two axial flow compressors.  The low pressure compressor includes the fan stages. and is driven by the low pressure turbine stages.  The high pressure compressor is driven by the high pressure turbine stage.  The dual compressor feature allows for more closely matching compressor blade speed to the increasing pressure and temperature of the air as it passes through the compressor stages.  This results in increased compressor efficiency, a higher compressor ratio. and increased thrust. Engine thrust is controlled by metering fuel to the combustion chambers. The fuel control unit accomplishes this function by sensing, in addition to inlet temperature and pressure and combustion chamber pressure. the RPM of the high pressure turbine-compressor rotor.  The fuel control does not sense low pressure rotor RPM, its speed depends on air flow through the high pressure rotor.

N1 & N2
The accessory section of the engine is driven by the high pressure rotor. The engineering symbol for the RPM of the low pressure rotor is NI, and for the high pressure rotor is N2. Common usage has made the terms NI and N2 refer to the rotors themselves.  NI is measured off the low pressure rotor directly, and N2 is measured at the accessory section. if the shaft to the accessory section from the N2 rotor fails, N2 will read zero even though the rotor may still be turning.  The tachometers in the cockpit for NI and N2 are driven by self powered tach generators.  
Exhaust Gas Temperature
Exhaust gas temperature is measured at the low pressure turbine outlet.  Standby AC power is required to display EGT in the cockpit.
Engine Pressure Ratio
Engine pressure ratio or EPR is the relationship between engine inlet pressure.  PT2, and turbine exhaust pressure.  PT7.  It is a measure of the thrust being produced by the engine.  EPR is displayed on the face of each EPR gauge with a needle and a digital counter. Any blockage of the inlet pressure probe. which is located in the engine nose dome. will result in erroneous EPR indications which can cause serious errors in power settings.  NI RPM is also an excellent measure of power and can be used as a cross check in icing conditions when the EPR probe may be blocked.  Electrical power is required to operate the EPR gauges. The EPR gauges receive inputs from the PDCS when the set knob an the instrument is pushed in.  These inputs drive the internal reference marks or "bug", and a digital counter on the face of the instrument.
Fuel Flow
Fuel flow is measured between the fuel control and the burner nozzles.  Both AC and DC power are needed for the fuel flow gauges.
The engine accessory section is driven by the high pressure, or N2, rotor.  It has pads for the engine's own fuel and oil systems, an AC generator, a hydraulic pump, and a starter.  Only engines one and two have hydraulic pumps installed.  Bleed air is extracted from the sixth, eighth. and thirteenth compressor stages for the pneumatic and anti-ice systems.  When any bleed air valve is opened. that engine's EPR will vary slightly.  
A pneumatically driven starter is attached to the accessory section.  An electrically operated starter valve allows compressed air from the pneumatic system to drive the starter.  The GROUND position of the engine start switch controls the starter valve.  

There is a thrust reverser unit on the aft end of each engine that deflects the exhaust gases forward to shorten  the landing roll. The reversers may be of a cascade type, which consist of internal clam shell doors and external  cascade vanes.  Alternatively there is bucket door system with deflector doors only. The clamshells are pneumatically operated by bleed air from their respective engines. But not by compressed air delivered from the APU or an external air source.  The reverser mechanisms are operated by levers on the throttles. With the throttles out of the idle position. an interlock prevents operation of the reverse levers.  If a malfunction occurs which causes an engine to go into reverse thrust with its throttle in forward thrust, that throttle will move forcibly to the idle  position. When an engine is in reverse thrust, the interlock prevents forward movement of the  throttles.  As with the throttles. if the engine goes into forward thrust with the reverse lever in  reverse thrust. the reverse lever will move forcibly into the forward thrust position.  REVERSER NOT STOWED lights, located above the engine instruments, indicate that the clamshells of the  reverser are not fully stowed in the forward thrust position.  The reverse interlock on each engine will prevent motion of the throttle into the forward thrust range if the engine is actually in reverse thrust. and the reverse interlock will also prevent the reverse lever from applying reverse thrust until the clamshells are in the full reverse position.  
Oil System
Each engine has an independent oil storage and distribution system which provides cooling and lubrication of gears and bearings.  The tank has a useable capacity of four gallons. An engine driven pump pressurizes the oil from the oil tank.  The pressurized oil passes through a filter, is cooled. and is piped to the bearings and accessory section gears.  The oil is then returned to the tank by scavenge pumps. After it leaves the cooler. the pressure and temperature of the oil are measured and the values are transmitted to gauges on the flight engineer's panel.  In addition. a separate pressure switch will turn on a light on the pilots' center instrument panel if oil pressure is too low. If the oil filter is unable to process the output of the oil pump because the filter is clogged. a bypass will open to allow oil to reach the engine.  If this occurs. the difference in oil pressure across the filter will cause the same low oil pressure light on the center instrument panel to illuminate.  The label on the light signifies this dual purpose:  
If the low oil pressure or filter bypass light comes on, the cause can be determined by reference to the corresponding oil pressure gauge.  If the oil pressure reading is normal. the light indicates that the oil filter is being bypassed. The oil is cooled in a heat exchanger through which fuel from the engine fuel system is circulated.  As fuel temperature and flow rate vary. oil cooling will vary, and oil temperature will change. Oil quantity, temperature, and pressure are monitored by gauges on the flight engineers panel.  A test button in the lower right corner of his panel is used to test the operation of the oil quantity gauges.  Standard instrument markings are used on the temperature and pressure gauges: green indicating the normal range. yellow indicating caution. and red showing the operating limit.  

Oil temperature and pressure limits are indicated by colored arcs on the gauges.  The temperature and pressure limits for continuous operation are shown by green arcs.  There is a 15 minute time limit an engine operation with the oil temperature indicating in the caution ranges shown by a yellow arc.  Operation with the oil pressure in the caution range. a yellow arc, is allowed for a short period at reduced thrust.  Red radials show the maximum oil temperature and maximum and minimum oil pressures.  An amber light on the forward instrument panel will illuminate if the oil pressure in the associated engine is too low.  The minimum oil quantity for dispatch is one gallon (US) and a quart (US) for each hour of planned flight.
Fuel is normally transferred under boost pump pressure to the engine from the fuel tanks.  At the engine the fuel is pressurized by a low pressure engine driven pump, passes through a heater and is filtered. The high pressure engine driven pump increases the pressure of the fuel before it reaches the fuel control.  The fuel control modulates the flow of fuel to the engine to maintain its power at the level selected by the position of the throttles in the cockpit. Between the fuel control and the burner nozzle in each engine is a fuel flow transmitter.  It measures the rate at which fuel is delivered to the burner nozzles in that engine and sends this information to the fuel flow gauge on the center instrument panel. To eliminate the need for a drag producing oil cooler in the slipstream, the fuel is used to as the cooling medium to cool the engine oil.  From the cooler the fuel is directed to the burner nozzles by the pressurizing and dump valve.  At low power settings, only the primary fuel manifold is pressurized.  As power is increased. the secondary manifold is also used. At low temperatures the fuel filter is susceptible to clogging by ice particles in the fuel.  If this occurs. a bypass around the filter opens allowing fuel to reach the engine.  The pressure drop across the filter is sensed. causing an amber icing light on the flight engineer's panel to illuminate. Upstream of the fuel filter. a fuel heater is installed.  When the fuel heat switch on the flight engineer's panel is moved to ON, a valve opens allowing high stage engine bleed air to pass through the heater. heating the fuel rapidly.  The heated fuel flows to the fuel filter. melting the ice in the filter.  The heated fuel continues an through the fuel control and on to the oil cooler.  Since the fuel is now warmer. it cannot cool the engine oil as efficiently, and oil temperature on that engine rises. A blue IN TRANSIT light on the flight engineer's panel will illuminate as long as the hot air valve does not match the position of the fuel heat switch.  When the hot air valve opens, a drop in slight drop in EPR can be seen.  A rise in engine oil temperature verifies that the fuel is being heated. The low pressure engine driven pump normally provides for suction feed. if necessary from a fuel tank to force fuel through the fuel heater and filter to the high pressure engine driven pump.  If the low pressure pump fails, a bypass allows fuel to flow directly to the high pressure pump without passing through the heater or filter.  In this situation the fuel cannot be heated and use of the fuel heat switch will not cause a rise in engine oil temperature. if fuel temperature fails to, rise while the fuel heat is in use, the low pressure engine driven pump may have failed.  

Fuel Temp
Fuel temperature in the number one fuel tank is displayed on the flight engineer's panel to determine if the fuel is approaching a temperature limit. or if any fuel filter blockage could be caused by ice.  Number one tank was chosen since its fuel is the coldest.  
Ignition & Starting
Ignition and start valve operation are controlled by the ignition switches and the start levers.  The ignition switches are located on the overhead panel, and the start levers are on the center console throttle quadrant.  Each engine's ignition system provides two levels of ignition energy.  High energy ignition, used for engine starting. receives its power from the DC circuits.  Low energy ignition. which can be used continuously without decreasing the life of the engine igniters. receives its power from the AC circuits. Each ignition switch arms its ignition circuits. however. the ignition will not be activated unless the associated start  lever an the throttle quadrant is moved to start or idle.  With the ignition switch in either flight or ground, and the start lever in starts high energy ignition is activated.  With the ignition switch in either flight or ground and the start lever in idle, low energy. continuous ignition is activated. The ground position of the start switch has the additional function of opening the associated engine start valve to initiate engine rotation for start.  Once the engine reaches the proper RPM, the start lever is moved to start, which causes the high energy ignition to activate as well as the fuel to be introduced to the engine. With the start lever in its normal inflight position idle. moving the start switch to flight provides low energy ignition.  The ground position of the start switch would also activate the low energy ignition but this would open the start valve, subjecting the starter to potential damage.  Low energy ignition should be used during takeoff. landing. in icing conditions. turbulence. and when using fuel heat. Fuel to the engine as well as ignition is controlled by the start lever.  With the start lever in start or idle. the fuel valve in the fuel control is open providing fuel to the engine.  In addition. the fuel shutoff valve at the fuel tank is open.  With the start lever in cutoff. the fuel is shut off at the fuel control and at the wing tank shutoff valve.  
S Duct
The "S" duct., which supplies air to number two engines has an access door for the number two engine inlet.  It is located directly in front of the number two engine inlet in the duct.  A microswitch senses if the access panel is secured.  If not, an amber light labeled ENGINE ACCESS DOOR next to the engine start switches will illuminate.

You can have a closer look at the engine from the contents menu

Back To Index