B737 B737MAX Differences

Pneumatic Systems

While pilot operation of the system remains unchanged, the system has a few differences:

• The system is electronically control and pneumatically activated
• Bleed air comes from the 4th and 10th stages as opposed to the 5th and 9th stages on the NG.
• A single bleed source can operate both packs in flight.
• There is automatic fault detection and isolation
• BLEED TRIP OFF light is replaced with BLEED and the ram door open lights are gone. The lights illuminate when:
  • Engine bleed air over temperature or overpressure
  • System fault or failure
  • Incorrect configuration after takeoff or go-around
• There is an EQUIP SMOKE light to alert for smoke detected in the equipment cooling system

Anti-ice

The MAX LEAP engine includes a system called “core anti-ice”, which sends bleed air to the engine core as needed without flight crew input. This operation is controlled by the EEC.

There is also an ENG ANTI-ICE alert, that illuminates if:

• Core anti-ice inop
• Cowl anti-ice inop

Valve position disagreements are shown by an amber light, instead of bright blue. Additionally, the lights are inhibited on the ground.

APU

The MAX APU control panel does not contain an EGT or maintenance indication.

The MAX APU inlet door has three positions; ground open, flight open, closed. It comes along with an amber DOOR light to indicate door is not in commanded position. Note that the light also illuminates on the ground with the APU shutdown.

Engines

The 737MAX is powered by CFM LEAP-1B engines.

When starting the engines, the EEC will maintain 18-24% N2, depending on the time required for Bowed Rotor Motoring, which is used to straighten the rotor shaft that naturally “bows” due to thermal buildup after an engine is shutdown.

When active, the N2 display will indicate MOTORING. After the light disappears, the engine start procedure continues the same as the NG.

Normal start attempts are limited to 3 minutes on the Max, while starter usage is limited to 5 minutes (for first two attempts). On the third and subsequent attempts the starter usage is 5 minutes with 10 minutes off.

During a normal start, the starter cutout will occur at 63%.

The EEC on the MAX incorporates two shutdown parameters as well:

• Electronic Over-speed
• Thrust Control Malfunction Accommodation

Both shutdown modes will announce an ENG FAIL.

The ENG VALVE cLOSED light illuminates bright while the engines conduct a self test of the overspeed and TCMA functions. During this time, fuel flow indicates zero and the engine fuel SOV opens and closes. Once the test is complete, the start continues normally.

Prior to take-off, warm up times for the engines is incrased to 3 minutes rather than two minutes, and the oil temp must be at or above 31 degrees.

When engine anti-ice is required and the OAT is <3 degrees:

• Increase thrust on each engine to >=50% N1
• Decrease thrust to idle
• Repeat (do not exceed 60 min between intervals)

Engine Ground Wind Operations:

• Crosswinds > 43 kts → limit thrust to taxi setting
• Winds > 58kts → limit thrust to idle

The EEC is has an additional icing idle mode, which is not available above 30,400 ft. It increases while on descent to 22,000 ft. It is activated when:

• Engine anti-ice on
• Flaps up
• Gear up
• At or below 30,400 ft.

There are also two additional engine alerts:

• THRUST: Illuminates when thrust is less or more than commanded thrust
• FUEL FLOW: illuminates when fuel flow is abnormally high compared to FMC expected value

Autopilot

Changes:

• Auto disengage of autopilot after stick shaker
• Temp removal of flight director after stick shaker
• Inhibit of autopilot nose up trim when airspeed within min maneuver speed (amber bar)

MCAS

On the 737MAX, pitch stability augmentation is provided by MCAS, which is part of the STS system.

The MCAS system is designed to operate at high AoA when the autopilot is not engaged and the flaps are up. It functions by increasing control column forces by commanding the stabilizer into the down direction.

Trim inputs are placed in the down direction incrementally if the AoA exceeds a threshold. As the AoA is reduced, the trim commands are reversed.

The commands are based on AoA and Mach number. Conditions for operation include:

• Autopiot off
• Flaps up
• AoA above threshold
• < Mach 0.84

• 10 seconds after takeoff

The FCC features three MCAS logic:

• AoA comparison
• Maximum command limit
• Single MCAS Activation

It uses a comparison between both AoA vanes. Should there be a disagreement ofn >5.5 degree the SPEEDd TRIM FAIL light will illuminate and the STS system will be inop for remainder of flight.

The nose down limit is established by reference to the trim position prior to MCAS activation. To inhibit the system, the limit needs to be reached (whichever comes first):

• within 5 minutes following MCAS activation
• before auto pilot is engaged for one continuous minute following MCAS

Flight Controls

Fly-by-wire spoilers, controlled by spoiler control electronics unit. Translaties signals from speedbrake lever, FO’s control wheel, and air data, and sends inputs to appropriate PCU.

Maneuver Load Alleviation: with speed brakes extended at high loads, the speed brakes automatically adjust to avoid excessive load.

Landing Attitude Modifier: with flaps in 30 or 40 position and speed 10 above Vref, LAM deploys flight spoilers to match pitch attitude to match NG gear contact margins. They also deploy between flaps 15 to 30 with thrust near idle to generate additional drag and match flight characteristics of the NG.

Emergency Descent Speedbrakes (EDS) armed whenb >FL300. The speed brakes are increased to higher than normal position to help with an emergency descent.

Elevator Jam Landing Assist: if pitch controls are jammed, EJLA can be used to manage flight spoilers to provide small sink rate control. Pushing the control column deploys the spoilers, while a pull retracts them.

Warnings

The ROLL/YAW ASYMMETRY alert:

• Replaces the active autopilot status annunciation when the autopilot has reached 75% of its total roll authority.

• Is replaced with ROLL AUTHORITY when 100% of the auto pilot roll authority is required.

• Is replaced by the active autopilot status annunciation when less than 50% of the autopilot roll authority is required.

Software

Updated FCC software P12.1.2 incorporates the following AOA Signal Processing algorithms:

• The AOA MVS outputs to MCAS the middle value between ADIRU L, ADIRU R, and previous MVS output which mitigates unintended MCAS activation that could be caused by a single erroneous AOA, including small differences between AOA vanes.  The output of the MVS algorithm is used by the FCC to control MCAS.  The AOA MVS reduces opportunities for MCAS activation if there are small differences between the AOA vanes, up to the 5.5-degree difference.