Aircraft Specific Questions: B737

The 737NG anti-ice system protects several critical components using both pneumatic and electrical means:

Pneumatically Heated (Bleed Air):

• Engine cowl lips – protected by the Engine Anti-Ice system
• Three inboard leading edge slats on each wing – protected by the Wing Anti-Ice system

  Note: The wing anti-ice system does not include the outboard slats or the leading edge flaps. It is effective with the slats in any position.

Electrically Heated:

• Pitot probes (3 on the fuselage)
• Elevator pitot probes (2 on the tail)
• AOA (alpha) vanes (2)
• TAT (total air temperature) probe
• Flight deck windows (via the window heat system)
• Windshield wipers

Yes, the wing anti-ice system is available on the ground, but only under specific conditions:

• Thrust on both engines must be below the setting that triggers the takeoff configuration warning
• The temperature inside the wing distribution ducts must be below the thermal switch activation threshold

When these conditions are met, positioning the WING ANTI-ICE switch to ON opens both control valves.

Additional Notes:

• If either engine’s thrust exceeds the takeoff warning threshold or if there is a duct overtemperature, both valves close automatically.
• The switch remains physically in the ON position on the ground even if the valves close.
• At lift-off, the system automatically trips OFF when the air/ground sensor switches to air mode.

Automatic load shedding is a protection feature designed to prevent electrical overload when operating on a single generator or under limited power conditions.

It occurs in the following situations:

• Engine generator operation (single source):
• Galleys and main buses on transfer bus 2 are shed first
• If overload persists, galleys and main buses on transfer bus 1 are shed
• If still overloaded, IFE buses are shed
• When two generators are available, all buses are automatically restored

• Manual restoration of galley and main bus power can be attempted by toggling the CAB/UTIL Power Switch OFF then ON

• APU operation (in flight, APU as sole source):

• All galley and main buses are shed automatically
• If overload continues, IFE buses are also shed
• On the ground, the APU attempts to carry the full load and sheds buses only if overload is detected
• Manual restoration is also possible via the CAB/UTIL Power Switch

Note: Load shedding helps prioritize essential systems by temporarily disabling non-essential loads.

The Mach trim system provides speed stability at higher Mach numbers. Above Mach 0.615, it automatically adjusts the elevators relative to the stabilizer to compensate for Mach tuck.

The system uses Mach data from the ADIRU to determine the required adjustment. A Mach trim actuator repositions the elevator feel and centering unit, effectively modifying the neutral position of the control column as speed increases.

Hydraulic system B powers the following components:
- Ailerons
- Rudder
- Elevators and elevator feel
- Two flight spoilers per wing
- Leading edge flaps and slats
- Trailing edge flaps
- Normal brakes
- No. 2 thrust reverser
- Autopilot B
- Alternate nose wheel steering
- Landing gear transfer unit
- Autoslats
- Yaw damper

Autobrake disarms automatically when:
- Manual brakes are applied
- Speedbrake lever is moved to the down detent
- Advancing the forward thrust lever(s), except during the 3 first seconds after touchdown
- System fault occurs