12.4 Position & Warning Systems: Landing Gear Horns, Stall & Warning

12.4 Position & Warning Systems: Landing Gear Horns, Stall & Warning

FAA Airframe Subject Matter Focus: Aviation maintenance technicians must master the electrical, mechanical, and pneumatic operation of aircraft position and warning systems. This includes landing gear uplock/downlock limit switches, throttle and flap warning horn logic circuits, ground safety squat switches (weight-on-wheels), stall warning suction reeds and electric lift vanes, stick shakers and pushers, takeoff configuration warnings, and centralized Master Warning / Caution annunciator systems.

1. Landing Gear Position & Indication Circuits

Retractable landing gear systems require continuous, unambiguous position feedback to the flight deck. The status of each gear (nose, left main, right main) is sensed by heavy-duty, environment-proof microswitches (limit switches) positioned along the mechanical gear linkage.

               LANDING GEAR POSITION INDICATION ARCHITECTURE

        Gear Fully Retracted               Gear In-Transit                Gear Down & Locked
     ┌────────────────────────┐      ┌────────────────────────┐      ┌────────────────────────┐
     │ Uplock Switch CLOSED   │      │ Uplock Switch OPEN     │      │ Downlock Switch CLOSED │
     │ All Lights EXTINQUISHED│      │ RED / AMBER UNSAFE ON  │      │ THREE GREEN LIGHTS ON  │
     └────────────────────────┘      └────────────────────────┘      └────────────────────────┘

Sensed Positions & Cockpit Displays

1. Down-and-Locked (Three Green): When each gear strut extends fully and its mechanical drag brace locks over-center, a dedicated downlock microswitch closes. This completes the electrical ground circuit to illuminate three individual green annunciator lamps (NOSE, LEFT, RIGHT). All three green lights confirm the gear is mechanically locked down and capable of supporting ground landing loads.
2. Gear Unsafe / In-Transit (Red or Amber): A red or amber warning light (GEAR UNSAFE or IN TRANSIT) illuminates whenever the landing gear is in an intermediate, unlocked position—specifically when the gear selector handle position disagrees with the actual gear position, or when any gear is neither fully locked up nor locked down.
3. Gear Fully Retracted (All Lights Out): When all three gear assemblies reach their full-up position, uplock microswitches open to extinguish the red unsafe lights. In standard transport configurations, no lights remain illuminated in cruise flight.
4. Lamp Test & Dimming Circuits: Annunciator panels incorporate a spring-loaded PRESS-TO-TEST feature to verify filament integrity across all bulbs simultaneously. A day/night rheostat or instrument light sensor inserts dropping resistors into the circuit to dim annunciators during nighttime flight to prevent pilot glare.

2. Landing Gear Warning Horn Logic & Ground Safety Squat Switches

To prevent inadvertent gear-up landings, FAA airframe airworthiness regulations (14 CFR §23.1315 and §25.729) mandate automatic aural warning systems that alert the flight crew whenever the aircraft is configured for landing without the gear locked down.

                   LANDING GEAR WARNING HORN CIRCUIT LOGIC

      +28 VDC Bus
           │
           ├─── [ Throttle Microswitch ] ───┐ (Closed when Throttle < Cruise / Idle)
           │                                │
           │                                ├─── [ Any Gear NOT Locked Down ] ──┐
           │                                │    (Downlock Switches Open)       │
           ├─── [ Flap Microswitch ] ───────┘                                   ▼
           │    (Closed when Flaps > 25°)                             ┌───────────────────┐
           │                                                          │ GEAR WARNING HORN │
           │                                                          │ (Continuous Tone) │
           │                                                          └─────────┬─────────┘
           │                                                                    │
           └────────────────────────────────────────────────────────────────────┴── Ground

Warning Horn Activation Logic

• Throttle Retardation Trigger: A microswitch mounted on the throttle quadrant (or manifold pressure switch) closes whenever the engine throttle lever is retarded below a predetermined cruise power setting (typically $<12\text{ to }14\text{ inHg}$ manifold pressure or flight idle). If any of the three landing gear is not down and locked, the circuit completes, sounding the landing gear warning horn.
  • Horn Silence Button: Pilots can temporarily mute the horn during normal descent at low power using a cockpit horn silence push-button. However, advancing the throttle resets the silence relay.
• Flap Extension Override (Non-Cancelable): A flap position microswitch connects in parallel with the throttle switch. If the wing flaps are extended beyond a normal approach setting (typically $>25^\circ$ or full landing flaps) while any landing gear is not down and locked, the warning horn sounds and CANNOT be silenced or muted by the pilot.

Ground Safety Switch (Squat Switch / Weight-on-Wheels)

A mechanical squat switch (Weight-on-Wheels / WOW switch) is mounted on the torque links or scissors of the main landing gear oleo strut.

• On the Ground (Strut Compressed): The aircraft weight compresses the oleo strut, holding the squat switch contacts open (or closed, depending on circuit design). This de-energizes the landing gear selector valve solenoid or mechanically locks the cockpit gear handle in the DOWN position, completely preventing accidental gear retraction on the runway.
• In Flight (Strut Extended): Upon takeoff rotation, the oleo strut extends fully, allowing the squat switch contacts to transition state. This unlocks the landing gear selector handle and energizes flight subsystems (e.g., enabling flight thrust reverser logic, flight recorder mode, and stall warning systems).

3. Stall Warning & Prevention Systems

An aerodynamic stall occurs when the wing's angle of attack (AOA) exceeds the critical angle of attack ($\alpha_{\text{crit}}$), causing the airflow to separate from the upper camber of the wing, resulting in a sudden loss of lift. Stall warning systems must provide clear, distinct warnings to the flight crew at a margin of 5 to 10 knots above stall speed ($V_s$) under all flight configurations.

                     STALL WARNING TRANSDUCER OPERATION

     1. NORMAL FLIGHT (LOW AOA)            2. APPROACHING STALL (HIGH AOA)

          Airflow Over Wing                     Airflow Over Wing
         ═════════════════════>                ═════════════════════>
       (                       )             (                       )
        (  [Vane Pushed Down] )               (  [Vane Deflected UP] ) ──> Closes Switch
         ( ───\              )                 ( ───/               )     Sounds Horn!
         ══════\══════════════                 ══════/═══════════════
                Stagnation Point                      Stagnation Point
                is ABOVE Vane                         moves BELOW Vane

Stall Detection Mechanisms

Active Stall Protection: Stick Shakers & Stick Pushers

On swept-wing and T-tail transport category aircraft susceptible to hazardous pitch-up or "deep stall" phenomena, passive acoustic horns are supplemented with active control column devices:

• Stick Shaker: An unbalanced eccentric electric motor mounted on the control column yoke. At approximately $1.05 \times V_s$, the motor spins violently, shaking the control column and creating a loud tactile and auditory rumble that cannot be overlooked by the pilot.
• Stick Pusher: If the pilot ignores the stick shaker and AOA continues to increase toward $\alpha_{\text{crit}}$, a fast-acting pneumatic or hydraulic actuator forcibly drives the control column forward with 30 to 80 pounds of force, reducing the angle of attack automatically to prevent wing stall entry.

4. Takeoff Configuration, Annunciator & Central Aural Warning Systems

                MASTER WARNING & ANNUNCIATION HIERARCHY

  ┌─────────────────┬────────────────────┬──────────────────┬────────────────────────┐
  │ Level / Alert   │ Visual Annunciation│ Audio Sound      │ Required Pilot Action  │
  ├─────────────────┼────────────────────┼──────────────────┼────────────────────────┤
  │ MASTER WARNING  │ RED (Flashing)     │ Fire Bell / Siren│ IMMEDIATE corrective   │
  │                 │                    │ / Klaxon / Voice │ emergency action.      │
  ├─────────────────┼────────────────────┼──────────────────┼────────────────────────┤
  │ MASTER CAUTION  │ AMBER / YEL        │ Single Chime /   │ Immediate awareness;   │
  │                 │ (Steady)           │ Caution Tone     │ subsequent action req. │
  ├─────────────────┼────────────────────┼──────────────────┼────────────────────────┤
  │ ADVISORY / STAT │ CYAN / WHITE       │ None / Subtle    │ Operational monitoring │
  │                 │ / GREEN            │ Beep             │ and system status.     │
  └─────────────────┴────────────────────┴──────────────────┴────────────────────────┘

Takeoff Configuration Warning Systems

To prevent aircraft from attempting takeoff with improper control configurations, a Takeoff Configuration Warning System monitors essential airframe systems:

• Trigger Conditions: Activated when the engine throttles / thrust levers are advanced into the takeoff power range while the aircraft is on the ground (sensed by the WOW squat switch), if any of the following unsafe conditions exist:
  1. Wing Flaps / Slats: Not extended to an approved takeoff detent.
  2. Horizontal Stabilizer / Pitch Trim: Set outside the approved green takeoff band.
  3. Speed Brakes / Ground Spoilers: Not fully retracted and stowed.
  4. Parking Brake: Applied or partially engaged.
  5. Control Locks: Flight control gust locks engaged.
• Aural Alert: Sounds a distinctive intermittent warning horn, siren, or repetitive synthetic voice warning ("CONFIG FLAPS", "CONFIG TRIM", "CONFIG SPOILERS").

Central Aural Warning Units (CAWU) & EICAS / ECAM

Modern glass cockpit aircraft consolidate all airframe position, warning, and engine alerts into centralized electronic display architectures:

• EICAS (Engine Indicating and Crew Alerting System) / ECAM (Electronic Centralized Aircraft Monitor): Replaces hundreds of dedicated individual panel lamps with high-resolution LCD displays.
• Priority Logic: Critical emergency alerts (e.g., Engine Fire, Cabin Depressurization, Overspeed, Terrain Pull-Up) automatically override lower-priority caution messages and mute non-essential alarms, preventing pilot sensory overload during high-workload emergencies.