ATR 42/72-600 Systems Guide — Chapter O: Landing Gear

2026-06-23

Chapter O. LANDING GEAR (FCOM DSC 32)¶

1. LANDING GEAR STRUCTURAL MECHANICAL ARCHITECTURE (Page 160)¶

The landing gear framework uses a retractable tricycle design configuration optimized to withstand heavy secondary field landing tracking loads.

Main Landing Gear (MLG) Mechanics¶

Retraction Path: The twin trailing-arm MLG leg assemblies fold directly inward into fairing bays constructed beneath the lower fuselage floor section.
Mechanical Uplocks: Once fully enclosed inside the fairing bays, heavy mechanical hooks latch over the strut alignment pins. This locks the gear assembly securely inside the belly fairing without relying on continuous hydraulic line pressure.

Nose Landing Gear (NLG) Mechanics¶

Retraction Path: The NLG leg assembly retracts forward up into a sealed nose bay box located directly beneath the forward flight deck avionics bay.
Bay Enclosure Doors: Mechanical linkages link the nose gear doors directly to the main shock strut frame. As the gear retracts or extends, the gear doors actuate automatically, sealing the nose bay closed to optimize aerodynamic drag metrics.

Chapter O. LANDING GEAR (CONTINUED)¶

2. LANDING GEAR COCKPIT PANEL & CONTROL LEVER LOGIC (Page 161)¶

The primary landing gear control panel is located on the center instrument panel glaresheld support layer. It coordinates mechanical selection commands with electronic safety interlocks.

  LANDING GEAR CONTROL PANEL LAYOUT
  ┌────────────────────────────────────────────────────────┐
  │                     LDG GEAR                           │
  │                                                        │
  │                        ┌───┐                           │
  │                        │ █ │ ◄── UNLK lights (Red)     │
  │                        └───┘                           │
  │                        ┌───┐                           │
  │                        │ █ │ ◄── GEAR DOWN (Green)     │
  │                        └───┘                           │
  │                                                        │
  │                     [  LEVER  ]                        │
  │                      ( UP / DN )                       │
  │                                                        │
  └────────────────────────────────────────────────────────┘

2.1. Mechanical Lever Lock Solenoid¶

Ground Protection Logic: To prevent accidental gear retraction while the aircraft is parked on the ramp, an internal electrical lock solenoid engages mechanically inside the lever slot whenever the main landing gear shock-strut proximity switches register a weight-on-wheels (WOW) state.
In-Flight Release: At liftoff, as the shock struts expand into a weight-off-wheels (WLO) configuration, the solenoid retracts to allow the pilot to select the lever to UP.

2.2. Annunciator Status Profiles¶

UNLK Annunciators (Red): Illuminate continuously if a landing gear leg or its associated gear bay door is out of agreement with the selected cockpit lever position (gear in transit phase).
GEAR DOWN Annunciators (Green): Symmetrical trio of light boxes. Illuminate once the forward nose gear and both main gear legs reach full extension points and lock securely into their over-center drag structural hooks.
Extinguished State: Indicates the landing gear assembly is fully retracted, up-locks are closed, and hydraulic line pressure has been automatically isolated from the retraction loop components.

3. PROXIMITY SENSING CONTROL UNIT (PSCU) & MFD SYNOPTICS (Page 162)¶

The positions of all landing gear legs, lock links, and enclosure doors are tracked and verified electronically by a dual-channel Proximity Sensing Control Unit (PSCU).

3.1. Proximity Sensor Operations¶

Solid-State Non-Contact Targets: The airframe eliminates mechanical microswitches in favor of inductive proximity sensors. These sensors detect the presence of target metal plates on moving gear components, calculating lock status without physical component friction or line wear.
MFD WHEELS Page Presentation: Selecting the SYS / WHEELS synoptic display page translates data into a clear graphic matrix. Green square icons represent a confirmed down-and-locked gear profile, amber triangular hashes map an active transit sequence, and hollow outlines confirm the gear is nested inside the up-locks.

4. WHEEL BRAKE TEMPERATURE MONITORING SYSTEM (BTMS) (Page 163)¶

The multi-disc carbon brake assemblies convert aircraft kinetic energy into thermal loads during landing deceleration. These thermal limits are tracked dynamically via individual thermocouple loops.

4.1. Thermal Alert Parameters¶

Live MFD Value Readouts: Continuous numerical readouts are displayed adjacent to each main wheel icon on the MFD display.
Brake Over-temperature Alert: If any brake core temperature scales past a calibrated structural safety boundary ($> 250^\circ\text{C}$):
The FWS triggers a single chime Master Caution alert line.
Displays the amber flashing BRAKE OVHT message on the central EWD.
The respective numerical temperature digits on the wheel synoptic page change color to a high-visibility Amber.
Operational Turnaround Rules: Following a high-energy reject takeoff or extreme braking landing run, crews must verify the BTMS readings drop back below safety limitations before initiating a subsequent departure. This ensures adequate heat absorption margins remain available in the carbon stacks for an emergency stop profile.