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Boeing 737-800 NG · Study guide

How it works

Plain-language chapters explaining each system: what it is, how it works, how it fails and what that means for the crew and the mechanic.

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Chapter 1 · ATA 27

Flight Controls

The 737 is a conventional, hydraulically powered aircraft with manual reversion for the primary controls.

Primary controls

Ailerons, elevators and rudder are driven by hydraulic power control units (PCUs) from systems A and B. The column and wheel link mechanically to the surfaces through cables, so what the pilot feels at the controls is a real, direct connection. Flight spoilers assist roll, and ground spoilers deploy on landing to dump lift.

If both hydraulic systems fail, the ailerons and elevator revert to manual control via the cables and tabs, with greatly increased control forces. A standby hydraulic system powers the rudder, thrust reversers and leading-edge devices as backup.

Stabiliser trim

The horizontal stabiliser is trimmed by an electric motor, commanded by trim switches or the autopilot. Stabiliser trim cutout switches allow the crew to isolate the motor in a runaway trim situation, in which case manual trim wheels are used.

High-lift devices

Trailing-edge flaps and leading-edge slats and Krueger flaps increase lift at low speed. They are hydraulically driven, with alternate electrical and manual-extension options in some failures.

Technical detail — hydraulic power and manual reversion

Conventional cable-driven controls with hydraulic power control units (PCUs). Ailerons and elevators are powered by Systems A and B; if both fail, the pilot moves them manually (manual reversion) through the cable and tab system — heavy but controllable. The rudder has a main PCU (A and B) and a standby PCU (standby system).

The stabiliser is moved by an electric trim motor or manually with the trim wheels. STAB TRIM CUTOUT switches stop electric trim. On the MAX, MCAS adds nose-down trim at high angle of attack with flaps up; after the accidents it was redesigned to use both AOA sensors and activate only once.

Technical detail — high lift

Trailing-edge flaps (triple-slotted on Classic, double-slotted on NG/MAX) driven by System B hydraulic motor; alternate electric drive is slow. Leading-edge slats and Krueger flaps extend with the flaps. Flap positions 1, 2, 5, 10, 15, 25, 30, 40.

Technical data

Hydraulic powerSystems A, B, standby
Flap positions1, 2, 5, 10, 15, 25, 30, 40
BackupManual reversion (ailerons/elevators)

Key points

  • Cable-linked controls with hydraulic PCUs.
  • Manual reversion is possible with high forces.
  • Stab trim cutout switches for runaway trim.

Reference: 737 FCOM Vol 2 Chapter 9 (Flight Controls); QRH

EDUCATIONAL USE ONLY. Content is summarised from manufacturer documentation for learning. It is not approved flight or maintenance data. Always use the current, aircraft-specific POH/AFM, FCOM, QRH, AMM and operator procedures.