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The following extracts are from the CFM56-3 Specific Operating Instructions manual, courtesy of Capt Selva Kumar.


  1. Intro
  2. Engine description
  3. Operating Requirements
  4. Ground Operating Procedure
  5. Flight Operating Procedure
  6. Operating Procedure For Abnormal Condition
  7. Emergency Operating Procedure

CFM56 Cutaway


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Engine Description (Section 2)


  1. The CFM56-3 is a high bypass, dual rotor, axial flow turbofan engine. Basic engine specification are provided in Fig 1 and 2.
  2. The integrated fan and booster (low pressure turbine-LPC) is driven by a 4 stage low pressure turbine (LPT). A single stage high pressure turbine (HPT) drives the 9 stage high pressure compressor (HPC)
  3. The two rotors are mechanically independent of each other. Air entering the engine are divided into a primary (inner) airstream and a secondary (outer) airstream (Fig3). After the primary airstream has been compressed by the LPC and HPC, combustion of the fuel in the annular combustion chamber increases the HPC discharge air velocity to drive the high and low pressure turbines. An accessory drive system off the N2 rotor drives engine and airplane accessory components.

Engine Characteristics

1. Trust Class for the CFM56-3B engine………... 22,100 LBS

2. Type of engine……………………………..…...Axial Flow, Gas Turbine Turbofan

3. Number and Type of Combustion Chamber…...One / Annular

4. Type of Compressor ………………………..….Two spool,13 stage compressor consisting of a 4 stage low pressure compressor (includes 1 stage fan) and a 9 stage high press compressor.

5. Direction of Rotation (Both Rotors)…………..Clockwise (as viewed from the rear looking fwd)

6. Type of Turbine………………………………....5 stage, split, consisting of a 1 stage high pressure turbine and a 4 stage low press. Turbine.

7. Engine Weight (Bare weight)………………….4290 pounds dry (approximate)
(Eng. With QEC)……………...5390 pounds (approximate)

8. Engine length (with flame arrestor)……………114.5 inches (approximate)
Installed Engine Length (with inlet cowl)….. 192.6 inches (approximate)

9. Engine diameter
Largest diameter (without inlet cowl……… 87.6 inches (approximate) Incl. Accessory gearbox)
Largest diameter (with inlet cowl)………… 88.4 inches (approximate)

10. Ignition system
Ignition exciter and plug……………………… Two per engine.

11. Lubrication system……………………………..Oil type (Class B [ type 2 ])
Min.Eng.Oil Stating Temp -40C

12. Fuel system (fuel specification)……………...D1655- JET-A, -A1, -B, MIL-T-5624G JP-1,JP-4,JP-5, MIL-T-83133 JP-8


Operating Requirements (Section 3)

Engine deterioration is directly related to engine hot section time/temperature (EGT) exposure. Rapid temperature transients are also detrimental to hot sections part life. Therefore, to obtain maximum engine service life, time at takeoff / high thrust should be held to the practical minimum, and rapid throttle movements avoided during normal flight operations.

Ratings, Restrictions and Power Management

Takeoff Rating-The Takeoff Rating is the certified rating or ratings defined in the regulatory agency Approved Airplane Flight Manual and is limited to 5 minutes. The 5 minute limit applies to all operation above maximum continuous thrust.

NOTE: The normal 5 minute takeoff time limit may be extended to 10 minutes for engine-out contingency if authorized by the regulatory agency Approved Airplane Flight Manual in the country of registration of the specific airplane (airplane registration) involved. If the 10 minute contingency time is utilized, the total operating time at takeoff thrust must be recorded in the flight log.


CFM56-3B-2 Performance and Operating Limit

100% N1= 5175 RPM

100% N2 = 14460 RPM

Thrust Rating (lbs):

  • Takeoff, flat rated, ISA+15C: 22,100
  • Max cont, flat rated, ISA+10C: 20,500
  • Max climb, flat rated, ISA+10C: 20,500
  • Max cruise, flat rated, ISA+10C: 19,193


Ground Operating Procedure (Section 4)

Starting Notes

  1. Starter air pressure lower than recommended may result in slow N2 acceleration and subsequent rpm hang-up.
  2. Rapid EGT rise accompanied by slow N2 acceleration, hang-up or deceleration requires immediate start abort. These symptoms may indicate incorrect fuel scheduling, faulty instrumentation, engine damage, low starter pressure or excessive deterioration. The cause must be investigated as per the Aircraft Maintenance Manual, and corrected before further attempts to start.
  3. Starts with slow N2 acceleration from light-off to idle, accompanied by low EGT (lean fuel schedule), may be continued provided starter limits are not exceeded.
  4. Starts in excess of 725 deg.C must be reported for corrective actions.

Engine Shutdown

Engine Shutdown After Landing

Following high power operation, such as maximum reverse thrust during landing or maximum power assurance check, it is recommended that the engine be operated at, or near idle for 3 minutes prior to shutdown to thermally stabilize the engine hot section. If operational requirements dictate, the engine may be shut down with a one minute cooling period.

After shutdown, monitor EGT and engine rpm, to be sure that the temperature and rpm decrease, indicating fuel shutoff. Monitor EGT for indication of post shutdown fire.
Note: After engine shutdown is complete and all rotation has ceased, EGT will normally increase due to temperature soak-back.


Flight Operating Procedure (Section 5)

Setting of Takeoff Thrust

The desired thrust is obtained by setting the throttle to obtain the target N1 as determined from the Airplane Flight Manual performance data (or equivalent) for the applicable total air temperature, pressure altitude, engine bleed configuration and PMC status (on or off).

During takeoff, engine instruments must be monitored to make sure that the engine limitations are not exceeded, e.g.
- Exhaust Gas Temperature (EGT)…930 DEG C
- N1 and N2.not to exceed allowable limits.

- Oil pressure…min 13psi
At 85 % N2 normal range is between 18psig-65psig
At 90 % N2 normal range is between 21psig-72psig
At 95 % N2 normal range is between 24psig-80psig

Oil pressure is oil supply pressure measured relative to sump/vent pressure. Pressure surges may occur during cold soak , sub zero temp starts and takeoff. Under these conditions, high oil pressure is acceptable.

- If any overspeed or over-temperature has occurred, the duration and the highest RPM or EGT attained should be recorded as an engine discrepancy in the Aircraft Flight Report.

Voluntary Shutdown In Flight

Retard throttle slowly and smoothly to idle.
NOTE: Gradual power reduction to idle during voluntary shutdown promotes thermal stabilization before fuel shutoff. Operate engine at idle for 3 mins if feasible.

Airstart attempts may be made following a voluntary shutdown at any altitude and airspeed; however if the start cannot be achieved, establish flight conditions within the airstart envelope.
Warning: Be prepared to abort the start if a rapid EGT rise occurs, approaching starting limits

-Light off normally occurs within 2 to 3 seconds after placing the fuel lever in the ON position. Observe same EGT, Fuel flow and RPM limits as for ground start .
-If the light-off does not occur within 30 seconds, abort the start by returning the fuel lever to the OFF position. Let the engine windmill for 30 seconds to purge the combustion section of residual fuel before attempting another start.
-Starter off at 50 percent N2
-When the engine stabilized at idle N2
(a) Oil pressure…………Check
(b) Ignition off…………..Pilots discretion
-It is recommended that the engine be stabilized at flight idle for 3 minutes prior to high power application following an airstart.

Windmilling Restart Procedure

Caution: Starter assist should be used if N2 is below 15 percent. If a windmill start must be attempted at less than 15 percent N2 because of starter unavailability, a Hung Start or Hot Start may result. Increasing kias will increase windmill speed. Airstarts in most of the starting envelope require starter assist due to the low windmilling characteristics of the engine.


RPM Hang-up During airstart

During airstarts at high altitudes and low airspeeds, rpm hang-up may be experienced after obtaining initial light-off and acceleration. The hang-up is characterized as a stabilized engine speed lower than idle, with no response to throttle advancement and may be the result of either too rich or too lean fuel scheduling, low airspeed or excessive altitude.


Operating Procedure For Abnormal Condition (Section 6)

No Start

- If it is noted during a start procedure that a no light-off is indicated within 10 seconds after fuel is applied, the start procedure should be discontinued.
-Before attempting a second start, dry motor the engine for 60 seconds for ground starts or windmill for 30 seconds for airstarts. Use alternate ignition system for second start attempt. (Observe starter limitations)

Note: CFM International does not supply the starter or thrust reverser for CFM56-3B-2 Engines on the 737-400 aircraft. The BOEING company will determine and supply the starter duty cycle limits, starter re-engagement limits, and starter air pressure requirements ,etc. which are compatible with the engine, for the 737-400 Aircraft Flight Manuals.

-If the second start attempt fails, no further start attempt should be made before the cause of the start failure has been determined and corrected (ground only).
-If the failure to start is attributed to either ignition system, it should be recorded in the Aircraft Log, investigated and remedied at the earliest opportunity.

Unsatisfactory Starts

If a unsatisfactory start should occur, it will most likely be accompanied by one of the following conditions :


A. Hot Starts

A potential hot start is indicated by an abnormally rapid EGT rise after light-off. By monitoring fuel flow and EGT, a hot start can be anticipated before the 725 C limit is exceeded.

Hot Starts may be caused by :
(a) Inadequate starter air pressure, resulting in N2 too low to provide sufficient compressor air flow.
(b) Faulty starter valve action, preventing proper operation of starter, with same result as item (a)
(c) Premature starter deactivation
(d) Incomplete purging of fuel in the combustion chamber from the previous start attempt.
(e) Foreign object damage (FOD) preventing sufficient engine acceleration and airflow.
(f) Faulty pressurizing valve (hung open) resulting in fuel, under low pressure, puddling in the combustion chamber before light-off.
(g) Faulty main engine control (MEC) resulting in incorrect start fuel scheduling.
(h) Incorrect scheduling of variable stator vanes (VSV)

B. Hung Starts

A hung start is identified by light-off followed by abnormally slow acceleration and rpm stabilization below idle. A hung start may be result of fuel scheduling being either too lean or too rich. A lean hung start is associated with low fuel flow and proportionally low EGT. A rich condition can be recognized by a high fuel flow and an EGT rise which may tend to develop into an over-temperature condition and possible compressor stall.

Hung Starts may be caused by :
(a) Starter air pressure too low to accelerate engine to self-sustaining speed
(b) Premature starter deactivation
(c) FOD to compressor
(d) Faulty pressurizing valves not opening at scheduled fuel pressure setting.
(e) Incorrect scheduling of HP compressor IGV and variable stators.
(f) Turbine section damage.


Oil system malfunction

Exercise caution when operating an engine with oil pressure outside the normal pressure range.
Oil pressure fluctuations, or pressure shifts exceeding +/- 5psid (69kpa diff.) is cause for investigation.


N2 (Core Engine) and N1 (Fan) Transitory Overspeed

The throttle must be retarded to reduce core engine speed below limits.
Caution: All N2/N1 overspeed conditions must be reported for maintenance action.
The maximum speed attained and the duration of the overspeed condition should be recorded in the Aircraft Log.
The following operational procedure are provided for guidance and are recommended in the event certified N2/N1 limits are exceeded.

Speed range % --- Procedure
N2 : 105.1-106.0 --- Normal operation to next landing
N2 : Over 106.0 --- Precautionary shut down
N1 : 106.1-109.0 --- Normal operation to next landing
N1 : Over 109.0 --- Precautionary shut down

EGT Transitory Over-temperature

EGT above the takeoff and maximum continuous limits in figure 1 is considered over-temperature. Inspection and/or troubleshooting in accordance with instructions in the Aircraft Maintenance Manual must be performed, if EGT exceeds the normal EGT limit (930deg C). The maximum EGT attained and the duration of the condition should be recorded in the Aircraft Log.
Caution : All EGT transitory over-temperature that exceed time and temperature limits
must be reported for maintenance review and/or action.

The following operational procedures are provided for guidance and are recommended in the event EGT limits are exceeded. The throttle must be retarded to comply with EGT limits.

EGT Range C --- Procedure
931-950 --- Normal operation to next landing after returning EGT to limit (930 deg C)
Over 950 --- Precautionary shut down


Engine Stalls

Al stalls are an indication of an engine malfunction, or an indication that the engine was operated in excess of the authorized operating envelope, i.e. excessive wind component, high N1 speed vs KIAS in reverse, etc.

A normal engine will operate stall-free throughout the entire authorized operating envelope. Engine stalls are caused by a disruption of the normally smooth airflow through the compressor due to FOD, distorted airfoils, VSV's off schedule, etc. A stall may be indicated by varying degrees of abnormal engine noises, accompanied by flame from the engine exhaust and possibly from engine inlet in severe cases, fluctuating performance parameters, sluggish or no throttle response, high EGT, and/or a rapid EGT rise when throttle is advanced.

If an engine stalls, the following procedure are recommended :
Caution : The decision to continue operation of an engine that has encountered a stall (or stalls) must be with the consideration that possible additional stalls and increased engine damage may occur. Continued operation must be with caution. If high EGT is evident, or if a rapid EGT increase occurs during slow throttle advance, or if excessive vibration is detected (sensed or indicated), the engine should be returned to idle. If the malfunction continues per item 4 below, shut the engine down.

1) Retard throttle to idle to clear the stall. Verify that EGT and N2 decrease to normal idle and engine vibration levels appear normal.

2) Turn on all available bleeds for the affected engine.

3) Advance throttle slowly. Observe that N1,EGT and N2 follow throttle movement and vibration levels appear normal. If stall does not recur ,engine operation may continue. If stall recurs, reduce thrust and operate below the stall threshold or shut the engine down at the pilots discretion.

4) If an engine malfunction is evident in idle, as evidenced by high EGT/ low N2/detectable vibration (sensed or indicated), shut the engine down.

Engine Malfunction

A. The engine should be shutdown as soon as possible after discovery of a serious malfunction. Severe damage to the engine, and possibly to the aircraft, can result if engine operation is continued with a critical deficiency. The longer the delay between detection of a malfunction and engine shutdown, the more severe will be the resulting damage. Caution is advised when dealing with an engine malfunction or failure. Instrumentation problems resulting in abnormal indications should not be misconstrued as an engine malfunction or failure. Consequently, proper analysis and good judgement is as important as prompt action when dealing with an engine malfunction or failure.

B. The following indications should be recognized as symptoms of a serious engine malfunction and/or impending failure:

  1. An increase in engine vibration accompanied by higher than normal EGT or fuel flow.
  2. Repeated or uncontrollable engine stalls.
  3. Loss of thrust.
  4. A shift in engine to engine parameters, or in the relationship of one parameter to another during steady state operation.
  5. An oil pressure increase or decrease of +/- 5psi or more from the normal steady state operating pressure, and/or an increase in oil temperature, or indications of oil filter bypass.
  6. Any combination of the foregoing symptoms.

Thrust Reverser Operation

A. In a emergency, maximum reverse thrust may be used on the ground at any aircraft speed commensurate with the emergency. Under these conditions, maximum allowable reverse thrust may be used to zero knots.

B If thrust reverser system fails to stow in flight or on the ground, a normal engine shutdown may be made with reverser deployed.


Emergency Operating Procedure (Section 7)

Engine fire on ground.

A. Internal Engine Fire
(1) An internal engine fire may be evidenced visually (tailpipe fire) or by failure of EGT to decrease after turning the fuel off (post shutdown burning)
(2) For either case, the engine should be isolated from the aircraft fuel supply.
(3) When N2 on the affected engine is below maximum starter re-engagement speed (20%N2) ,the starter should be engaged and the engine motored until burning stops.
(4) If the fire cannot be extinguished by motoring the engine or if motoring is not possible ,close the aircraft (emergency) fuel shutoff valve and extinguish the fire with ground equipment.
(5) Use of fire extinguishing agents must be reported for maintenance action.

B. External Engine Fire
(1) An external engine (nacelle) fire will be indicated by the aircraft fire warning system. The use of fire extinguishing agents must reported for maintenance action.

Engine Failure/Malfunction

A malfunctioning engine is evidenced by abnormal engine parameters, noise or vibration. Continued operation with a known engine malfunction may lead to engine failure. In the event of a actual or impending failure ,the engine should be shutdown by turning the fuel and ignition off. If the engine operates normally at idle, it should be allowed to idle for 3 minutes prior to shutdown if practical. Restart attempts are not advisable, as further damage may result.



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