A detailed technical comparison of the Boeing 737NG vs 737 MAX. Explore key differences in CFM LEAP-1B engines, cockpit avionics, MCAS, and fly-by-wire spoiler systems.
| System / Area | 737 NG (800) | 737 MAX (8) |
| Flight Deck | ||
| Display Units | 6 x CRT/LCD (Common Display System) | 4 x 15.1″ Landscape LCDs (MAX Display System) |
| Aux Display | Not available | Located on the Outboard Display Unit. It displays following information: – Chrono Dial and read out displyed when a CLOCK switch is selected on glareshield. – Flight Number, XPNDR Code, Sel Cal, Tail Number. – The clock and flight information is located in the upper part of the AUX display. – Elapsed time- Starts at weight off wheels, stops 30 seconds after weight on wheels and resets on power up or new origin airport entered. – UTC Time. |
| MAINT Light | PSEU Light: Fault is detected in PSEU. | When designated faults are detected, a MAINT light on the aft overhead panel illuminates, and the OVERHEAD system annunciator light and MASTER CAUTION lights illuminate. The MAINT light is inhibited: in flight, after engine start switch is placed to GRD during the first engine start, for 30 seconds after landing. |
| Landing Gear Lever | 3-Position (UP-OFF-DOWN) Hydraulic pressure from gear upline is removed when gear lever is moved to OFF. | 2-Position (UP-DOWN) Hydraulic pressure is automatically removed 10 secs after gear retraction (UP and Locked). |
| L/G LOCK OVRD | Override Trigger allows LANDING GEAR lever to be raised, bypassing the L/G lever lock. | LOCK OVRD switch releases landing gear lever lock. |
| Display Alerts | “CDS MAINT” / “CDS FAULT” | “DSPLY SOURCE” (indicates specific Display Computer failure) |
| Autobrake Switch | Located above engine display | Relocated to the lower center panel |
| Tire Pressure & Brake Temp indication | Manual check | Tire Pressure & Brake Temp indicated on MFD SYS Page |
| Engines | ||
| Engine Model | CFM56-7B (26k/27k Thrust) | CFM LEAP-1B (28k Thrust) |
| Fan Diameter | 61 inches | 69.4 inches |
| Bypass Ratio | 5.1:1 | 9.0:1 |
| Nacelle | Round intake | Chevrons on trailing edge (Noise reduction) |
| Start Duty Cycle | 2 minutes | 3 minutes (Normal) |
| Start Logic | Standard start | Bowed Rotor Motoring (BRM): Auto-motors 6-90s. Only active for ground starts. |
| Protection | Mechanical Governors | TCMA (Thrust Control Malfunction) & EOS (Electronic Overspeed System) |
| Engine Indications | Standard N1/EGT | FUEL FLOW (Amber) & THRUST (Amber) alerts for deviation (additional) |
| THRUST Alert | Not applicable | Steady: – The thrust is more than the commanded thrust; or – The thrust is less than the commanded thrust, – Displayed in conjunction with amber N1 command sector for affected engine. Blinking: – With a condition of more than or less than commanded thrust. The alert and entire block will blink for 10secs and then the alert only will remain steady until the condition no longer exists. |
| FUEL FLOW Alert | Not applicable | Steady: Engine fuel flow is abnormally high when compared to FMC expected fuel flow. Blinking: with fuel flow abnormally high, the alert and entire block will blink for 10 secs and then the alert only will remain steady until the condition no longer exists. |
| Reverser Lights | REVERSER (Amber) one or more of following has occurred: – isolation valve or thrust reverser control valve is not in commanded position. – one or more thrust reverser sleeves are not in commanded state. – auto–restow circuit has been activated. – a failure has been detected in synchronization shaft lock circuitry. | REVERSER COMMAND light: the reverse thrust lever is not in the down position in flight. REVERSER AIR/GRD Light: the air/ground thrust reverser logic is failed. REVERSER LIMITED Lights: A fault in the thrust reverser system limits reverse thrust. Thrust reverser will not deploy or reverse thrust will be limited to idle if commanded. |
| Reverser System | A pause in movement of the reverse thrust levers past detent No. 1 toward the stow position may cause MASTER CAUTION and ENG system annunciator lights to illuminate. A pause of approximately 16 seconds engages the electromechanical lock and prevents the thrust reverser sleeves from further movement. Cycling the thrust reversers may clear the fault and restore normal operation. | A pause in movement of the reverse thrust levers past detent No. 1 toward the stow position may cause MASTER CAUTION and ENG system annunciator lights to illuminate. A pause of approximately 8 seconds engages the electro mechanical lock and prevents the thrust reverser sleeves from further movement. Cycling the thrust reversers may clear the fault and restore normal operation. The MAINT light illuminates 30 seconds after landing. |
| Engine Core Anti-ice | Not applicable | Core anti-ice operation is automatically controlled by the Electronic Engine Control (EEC) with no crew input or indication of normal operation. The EEC controls the core anti-ice system by directing bleed air to the engine core as needed based on engine parameters and atmospheric conditions. |
| ENG ANTI-ICE light | Not applicable | Illuminates when the cowl thermal anti-ice system has been inhibited due to a system failure or when an engine core anti-ice valve fails closed. |
| COWL VALVE light | COWL VALVE OPEN (blue) illuminates. | Will illuminate if: Momentary – related cowl anti-ice valve is in transit. Steady- cowl anti-ice valve position disagrees with related ENGINE ANTIICE switch position. |
| L VALVE light R VALVE light | L/R VALVE OPEN (Blue lights) | Illuminated (amber) – momentary – related wing anti–ice valve is in transit. steady – wing anti–ice valve position disagrees with related WING ANTI–ICE switch position. Extinguished – related wing anti–ice valve is closed (switch OFF), or related wing anti–ice valve is open (switch ON). |
| Flight Controls | ||
| Flap Indicator | Physical Gauge | Digital indication on PFD. Flap position indicator & LE FLAPS EXT, LE FLAPS light are displayed on inboard display along with engine indications. Flap Position indicator only indicates position of right TE flaps when Battery is the only source of electrical power. FLAP LOAD light illuminated when flaps retract due to air load because of airspeed. |
| Stab Trim Switches | The STAB TRIM MAIN ELECT cutout switch and the STAB TRIM AUTOPILOT cutout switch, located on the control stand, are provided to allow the autopilot or main electric trim inputs to be disconnected from the stabilizer trim motor. | PRI- Primary B/U- Backup Main Electric Stabilizer Trim Switches (spring-loaded to neutral) Push (both): – electrically commands stabilizer trim in desired direction, – autopilot disengages if engaged, – overrides Speed Trim System (Speed Trim function and MCAS function) if active. |
| Spoiler System | Mechanical Mixer & Cables | Fly-by-Wire (Spoiler Control Electronics – SCE) |
| SPOILERS Light | Not applicable | One or more spoiler pairs are inoperative. Illuminated (amber): – activated by signal from spoiler control electronics unit, – indicates spoiler system fault. |
| Load Alleviation | Not available | Maneuver Load Alleviation (MLA): Retracts speedbrakes under high G |
| Landing Assist | Not available | Landing Attitude Modifier (LAM): Uses spoilers to adjust nose angle |
| Speedbrake protection | A lever stop feature is incorporated into the SPEED BRAKE lever mechanism. The lever stop prevents the SPEED BRAKE lever from being moved beyond the FLIGHT DETENT when the airplane is in flight with the flaps up. In the event of the loss of electrical power the lever stop is removed and full speed brake lever movement is available. | Speedbrake commands are electronically limited at the FLIGHT DETENT. In the event electrical power to the SCE (Spoiler Control Electronics) is lost, all spoilers are inoperative and will retract if extended. |
| Speedbrake Limit | Standard usage | In flight, do not extend the speedbrake lever beyond the ARMED detent with flaps 40 selected. In flight, do not extend the speed brake lever beyond the FLIGHT detent. Do not use speed brakes below 1000 feet above the surface. |
| Emergency Descent | Manual speedbrake deployment | EDS (Auto-extends speedbrakes >30k ft if cabin alt warns) |
| Elevator Jam Landing Assist Switch | Not Available | If a jam occurs in the aft elevator control mechanism, both control columns have a limited range of motion. During approach and landing, the Elevator Jam Landing Assist System uses the flight spoilers for small changes to the flight path. To activate the system, the Elevator Jam Landing Assist switch must be selected ON, the actual flap position must be 1 or greater, and the autopilot must be disengaged. With the system activated, the flight spoilers deploy to a preset position. The control wheel steering force sensors detect forces applied to the control column. A push on the control column causes the spoilers to extend farther, increasing the descent rate. |
| Air Systems | ||
| Bleed Air Source | 5th & 9th Stage | 4th & 10th Stage |
| Bleed Light | BLEED TRIP OFF Light will illuminate if: Bleed air over temperature or over pressure – Related engine bleed air valve closes automatically. – Requires reset. | BLEED Light will illuminate if: 1. Excessive engine bleed air temperature, over-pressure or under-pressure. – related engine bleed air valve closes automatically. – requires reset. 2. A failure within the bleed air system. – Bleed system failure may not be resettable. 3.An incorrect bleed air configuration after takeoff or go-around. – Turning either BLEED switch ON will extinguish both BLEED lights if lights illuminated because of a configuration problem. |
| Engine Bleed | Do not operate more than one pack from one engine. | In flight one engine bleed is capable of supplying bleed air for two packs. On the ground, do not operate more than one pack from one engine. |
| Integrated Air Systems Controller (IASC) | Not applicable | The flow of bleed air from the main bleed air duct through each air conditioning pack is controlled by the respective pack valve. It is pneumatically actuated and electrically controlled by the Integrated Air Systems Controller (IASC). Dual IASC Failure: Packs auto-shutdown on overheat. |
| Fan Air Modulating Valve | Not applicable | The Fan Air Modulating Valve (FAMV) is electrically controlled and pneumatically activated. It is spring loaded to the open position. |
| Precooler | Not applicable | The precooler is used to cool the hot bleed air from the engine. Cooling airflow is extracted from the engine fan-air duct and controlled by the FAMV. |
| PACK light | PACK Light will illuminate amber if: Pack control failure or overheat. | PACK Light will illuminate amber if: – indicates pack trip off or failure of both primary and standby pack controls, or – indicates failure of the Flow Control Valve to open when commanded on, or – an incorrect pack switch configuration after takeoff. – turning either PACK switch ON will extinguish both PACK lights if lights illuminated because of a configuration problem. |
| EQUIP SMOKE Light | No Light | EQUIP SMOKE Light (Auto-response: 5 mins fan isolation) |
| Fuel System | ||
| Fuel Tank | No. 1 (3915 KG), No. 2 (3915 KG), CTR (13066 KG). Total = 20896 KG | No. 1 (3869 KG), No. 2 (3869 KG), CTR (12990 KG). Total = 20728 KG |
| Filter Bypass | Independent lights | Latched Logic: If both bypass, lights stay on until ground shutdown |
| FUEL DISAGREE amber alert | Not applicable | The totalizer fuel quantity and the FMC calculated fuel quantity disagree. |
| FUEL FLOW alert | Not applicable | Illuminated (amber) – the difference between actual engine fuel flow rate and the FMCS expected fuel flow rate exceeds the MEDB threshold value for five continuous minutes. The message is displayed independently for each engine. |
| USING RSV FUEL INSUFFICIENT FUEL | Shown only on FMC scratch pad message. | Shown below the fuel quantity indications along with FMC scratch pad message |
| APU | ||
| APU Inlet Door | Vortex Generator/Fixed | Has 3 positions: Ground Open, Flight Open, Closed. Single Door (Closed / 45° Ground / 17° Flight) |
| APU Cooling Air | Air for APU cooling enters through a cooling air inlet above the APU exhaust outlet. | APU cooling air routes to the APU through the air inlet door. |
| APU Door Light | MAINT Light (Blue) | DOOR Light (Amber) |
| APU EGT Gauge | Analog on overhead | Removed (Digital pop-up on MFD only) |
| Gear/Exterior | ||
| Nose Strut | Standard | Extended 8 inches (20 cm) – to provide clearance for the larger LEAP-1B fans. |
| Tail Cone | Standard rounded cone | Extended, pointed cone (No vortex generators) |
| Tail Strobe light | 1 tail strobe light | 2 tail strobe light |
| Winglets | Blended or Split Scimitar | Advanced Technology (AT) Split-Tip Winglets |
| Winglet Aft Marker Lights | Not applicable | Winglet aft marker lights illuminate the lower portion of the winglet. Controlled by POSITION Light switch. |
Thrust Control Malfunction Accommodation (TCMA)
This is a ground safety feature. If the EEC detects an engine producing uncommanded high thrust (e.g., during taxi), it automatically shuts off fuel to that engine to prevent runway excursions.
Thrust Control Malfunction Accommodation is an EEC function that provides protection against idle thrust asymmetry conditions while on the ground (RTO or landing roll). The TCMA function does not compare thrust between the engines, each engine is monitored independently. The EEC commands shutdown of the affected engine when the:
- airplane is on the ground, and
- thrust lever is at idle, and
- engine is above idle speed and not decelerating normally
After TCMA activation, the amber ENG FAIL alert illuminates when N2 falls below the ground N2 idle speed.
EOS
EOS is an EEC function that provides protection against the exceedance of engine structural design limits. If an uncontrollable N2 overspeed condition is detected in flight, the EEC automatically shuts off fuel to the affected engine. The amber ENG FAIL alert illuminates when N2 falls below the schedule N2 idle speed determined by flight condition. THRUST alert may or may not appear depending on flight condition, power setting, and ENG FAIL timing.
Fly-by-Wire Spoiler System
The MAX uses Spoiler Control Electronics (SCE) instead of a mechanical mixer, enabling:
- Maneuver Load Alleviation (MLA): Automatically retracts speedbrakes during high-G maneuvers to protect the wing structure. The MLA system reduces wing and aft fuselage bending loads during certain maneuvers and normal load factors. The MLA system retracts extended speedbrakes during maneuvers that result in higher load factors at certain gross weights. The SPD BRK lever does not move during MLA activation. When the conditions for MLA are no longer present, the speedbrakes will return to the selected position.
- Landing Attitude Modifier (LAM): On approach, this system deploys spoilers slightly to dump lift, forcing a higher nose-up attitude to keep the longer nose gear safe from early touchdown. The first LAM function applies when the flaps are in the 30 or 40 position. To maintain acceptable nose landing gear contact margin, LAM symmetrically deploys flight spoilers on approach to reduce lift and force the airplane to use a higher angle of attack. The second LAM function applies when flaps are positions 15 through 30 and the thrust levers are near idle. This function also symmetrically deploysflight spoilers, in order to generate additional drag.
- Emergency Descent Speedbrakes (EDS): If a cabin altitude warning occurs above 30,000 ft, speedbrakes automatically extend beyond their normal flight limit to maximize drag. In order to minimize exposure time of passengers and crew to high cabin altitude, an EDS function is included. EDS is armed when the airplane is above 30,000 feet and Cabin Altitude Warning is active. Moving the speedbrake lever activates the function. When activated, the EDS raises the speedbrakes to a higher than normal position when the speedbrake lever is in the flight detent.
EQUIP SMOKE
Two smoke detectors are installed downstream of the fans. The EQUIP SMOKE, MASTER CAUTION, and OVERHEAD System Annunciator lights will illuminate if smoke is detected. When smoke is detected the system will automatically set the packs to high flow, turn the E/E Cooling Supply Fan(s) OFF for approximately 5 minutes, overboard exhaust valve opens to the smoke position and the recirculation fans turn off. The E/E Cooling Supply Fans return to normal operations 5 minutes after the first detection of smoke. If smoke is re-detected, the Supply Fans turn OFF for another 5 minutes. The OFF light for the supply fan does not illuminate during this automatic shut off.
Note: Light extinguishes 30 seconds after smoke is no longer detected.
Engine Start System
737-8 (MAX)
In the GRD position, the engine start switch uses battery power to close the engine bleed air valve and open the start valve to allow pressure to rotate the starter. When the start valve opens, an amber START VALVE OPEN alert is provided on the MDS Engine Display. The starter rotates the N2 compressor through the accessory drive gear system. The EEC performs Bowed Rotor Motoring (BRM) after the Engine Start Switch has been moved to the GND position and before the Engine Start Lever is moved to IDLE. BRM straightens the N1 and N2 rotor shafts which will bow due to thermal buildup after the engine is shutdown. BRM is active for 6-90 seconds on the ground only. During BRM, MOTORING is displayed on the N2 gauge between 18% and 24% N2. When the engine accelerates to the recommended value (25% N2 or max motoring), moving the engine start lever to the IDLE position, the EEC performs a test of the TCMA and EOS functions. During this test, fuel flow will indicate zero and the engine fuel shutoff valve will open and close repeatedly. The ENG VALVE CLOSED light will illuminate and remain bright blue during this logic test, and will extinguish when the test is complete. Once the test is complete, the engine start sequence continues, the fuel valves on the wing spar and engine opens, and causes the EEC to supply fuel and ignition to the combustor where the fuel ignites. At starter cutout speed (approximately 63% N2), power is removed from the start switch holding solenoid. The engine start switch returns to OFF, the engine bleed air valve returns to the selected position, and the start valve closes.
737-800 (NG)
In the GRD position, the engine start switch uses battery power to close the enginebleed air valve and open the start valve to allow pressure to rotate the starter. When the start valve opens, an amber START VALVE OPEN alert is provided on the upper display unit. The starter rotates the N2 compressor through the accessory drive gear system. When the engine accelerates to the recommended value (25% N2 or max motoring), moving the engine start lever to the IDLE position opens the fuel valves on the wing spar and engine, and causes the EEC to supply fuel and ignition to the combustor where the fuel ignites. Initial fuel flow indications lag actual fuel flow by approximately two seconds, therefore, during engine start, an EGT rise may occur before fuel flow indication. At starter cutout speed (approximately 56% N2), power is removed from the start switch holding solenoid. The engine start switch returns to OFF, the engine bleed air valve returns to the selected position, and the start valve closes.
Bowed Rotor Motoring (BRM): The LEAP-1B shaft can thermally “bow” after shutdown. When starting the engine, the EEC will initially maintain an N2 between 18% and 24% depending on certain conditions. The time spent motoring is called BRM. BRM is active for approx. 6-90 secs and is only active for ground starts.
- Thermal bow is a temporary condition that may occur in certain applications of metal parts. The cause of thermal bow is differing temperatures on either side of those parts.
- For example, when steel doors experience a temperature difference between the door faces, the top and bottom of the lock edge of the door begin to distort either toward or away from the frame stop. The larger the temperature gradient, the more extreme the bowing.
- In turbine engines, Rotor Bow, or thermal bowing, is normally due to asymmetrical cooling after shutdown on the previous flight. Differences in temperature across the shaft section supporting the rotor lead to different thermal deformation of the shaft material, causing the rotor axis to bend. This results in an offset between the center of gravity of the bowed rotor and the bearing axis, causing a slight imbalance and potentially reducing the tight clearance between the rotor blade tips and the compressor wall.
- Maintaining this clearance as closely as possible is critical to engine efficiency. Slower starting allows more even heating, eliminating differential thermal deformation.
- Operating an engine in the bowed-rotor condition causes unnecessary wear-and-tear, reduces critical seal clearances, and may even be catastrophic due to high levels of vibration.
Speed Trim System
The Speed Trim System (STS) provides speed and pitch stability augmentation. Speed stability augmentation is provided by the Speed Trim function in the Speed Trim System. Pitch stability augmentation is provided by the MCAS function in the Speed Trim System. The Speed Trim System only operates while the autopilot is not engaged. If the SPEED TRIM FAIL light is illuminated, Speed Trim System (Speed Trim function and MCAS function) operation is inhibited. Main electric stabilizer trim switches can be used to override Speed Trim System inputs. If the left and right AOAs disagree by 5.5 degrees or more with the flaps retracted, the SPEED TRIM FAIL light illuminates and the Speed Trim System (Speed Trim function and MCAS function) is inhibited for the remainder of the flight.
Note: The Speed Trim System (Speed Trim function and MCAS function) is inhibited with either the STAB TRIM PRI cutout switch or the STAB TRIM B/U cutout switch in the CUTOUT position.
Speed Trim Function
Speed trim is a speed stability augmentation function designed to improve flight characteristics during operations with low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. The purpose is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The Speed Trim function monitors inputs of stabilizer position, thrust lever position, flap position, airspeed and vertical speed to determine the amount of stabilizer trim and rate at which to control the stabilizer trim motor. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction needed to return the airplane to the trimmed speed. This increases control column forces to encourage return to the trimmed speed. As the airplane returns to the trimmed speed, the commanded stabilizer movement is removed.
The Speed Trim function contains a High AOA mode that is used to provide increased stabilizer commands at elevated AOA. As AOA increases toward stick shaker, the Speed Trim function transitions to the High AOA mode to command additional nose down stabilizer to increase control column forces. AOA is used only to transition into and out of the High AOA mode and does not determine the amount of stabilizer command. The Speed Trim function controls the stabilizer trim motor at a high rate with flaps extended and low rate with flaps retracted. The Speed Trim function operates most frequently during takeoff, climb and go-around. Conditions for operation are listed below:
- Up to Mach 0.68
- 10 seconds after takeoff
- 5 seconds following release of main electric stabilizer trim switches
- Autopilot not engaged
- Sensing of trim requirement
Maneuvering Characteristics Augmentation System (MCAS) Function
MCAS is a pitch stability augmentation function designed to operate at elevated angles of attack (AOAs). The purpose is to increase control column forces by commanding the stabilizer in the nose down direction at elevated AOAs. The MCAS function becomes active when the airplane exceeds a threshold AOA. Stabilizer trim inputs in the nose down direction are added incrementally if AOA continues to increase. Stabilizer trim commands are based on AOA and Mach number. As AOA is reduced below threshold AOA, the commanded stabilizer trim is removed. If AOA is again increased above threshold AOA, MCAS becomes active and again commands stabilizer trim inputs in the nose down direction. The MCAS function controls the stabilizer trim motor at the flaps extended Speed Trim function rate. The FCC uses a corrected AOA value to activate MCAS. This logic uses both AOA vanes and filters out small differences between the vanes and provides a single corrected value to the MCAS function.
The MCAS function contains logic that limits the amount of nose down stabilizer trim movement to preserve elevator authority for pitch control. When MCAS activates, the command limit logic computes a nose down stabilizer trim limit. If stabilizer trim reaches the computed limit, the SPEED TRIM FAIL light illuminates and the Speed Trim System (Speed Trim function and MCAS function) is inhibited for the remainder of the flight. Manual stabilizer trim, main electric stabilizer trim and autopilot trim are not affected by the command limit logic. The command limit logic inhibits the Speed Trim System when the computed nose down stabilizer trim limit is reached:
- within five minutes following MCAS activation, or
- before the autopilot has been engaged for one continuous minute following MCAS activation.
The command limit logic is reset after five minutes or after the autopilot has been engaged for one minute, whichever occurs first. Conditions for operation are listed below:
- AOA above threshold
- Up to Mach 0.84
- 10 seconds after takeoff
- Flaps up
- Autopilot not engaged
