เซ็นเซอร์น้ำหนักบนล้อ

ภาพรวม

The weight-on-wheels (WOW) sensor — also called the squat switch or air/ground sensor — is a proximity switch or micro-switch assembly mounted on the main gear shock absorber strut that detects whether the aircraft is airborne or in contact with the ground. Despite its apparent simplicity, the WOW signal is one of the most consequential logic inputs in the aircraft systems architecture: it gates or arms a remarkable number of critical functions that must behave differently in the air versus on the ground, and an incorrect WOW state can simultaneously affect autobraking, spoiler deployment, thrust reverser arming, ground spoilers, nose wheel steering, pressurisation, and numerous other systems.

The WOW signal transitions from AIR to GROUND as the oleo-pneumatic strut compresses under aircraft weight, bringing a target (flag) on the inner cylinder into proximity with the inductive sensor on the outer cylinder. This transition typically occurs when the strut compresses a few centimetres below its fully extended position, and the transition is virtually simultaneous with touchdown. The reverse transition — GROUND to AIR — occurs as the strut extends after liftoff, typically within one to two seconds of wheel departure from the runway.

หลักการทำงาน

Modern WOW sensors use inductive proximity switches rather than mechanical contact switches to provide long service life and resistance to contamination. The sensor consists of a coil energised by a high-frequency oscillating signal; when the metal target on the inner cylinder enters the sensor's electromagnetic field at close range, it induces eddy currents that change the sensor's inductance and trigger a logic output. This contact-free detection eliminates mechanical wear and provides reliable switching despite contamination by water, ice, and debris.

Most aircraft use redundant WOW sensors — typically two per main gear, connected to different system channels — with the AIR or GROUND state determined by a voting or priority logic. On the Boeing 737, both main gear WOW sensors must indicate AIR before the aircraft logic considers itself airborne; either WOW sensor indicating GROUND causes the logic to revert to ground state. This safety-conservative approach prevents inadvertent in-flight arming of ground-only systems in the event of a single sensor failure.

The WOW signal gates or arms the following functions (among others): thrust reverser deployment permission; autobrake activation; ground spoiler automatic deployment; nose wheel steering enable; differential braking during takeoff and landing roll; pressurisation mode (ground versus flight); terrain avoidance warning system (TAWS) mode; and numerous maintenance mode functions accessible only on the ground.

ส่วนประกอบหลัก

• Inductive Proximity Switch: The sensing element, typically embedded in the outer cylinder of the oleo strut. Rated for millions of cycles and qualified for the temperature, vibration, and fluid-contamination environment of the wheel well.
• Target Flag: Metal plate or collar on the inner cylinder that enters the sensor's detection range on strut compression. Surface condition and clearance to the sensor are critical to reliable switching.
• Proximity Switch Electronics Unit (PSEU): Central module receiving raw WOW signals from all gear proximity switches (uplock, downlock, WOW) and distributing processed logic states to consuming systems via ARINC 429 or discrete wiring.
• Wiring Harness: Shielded, fluid-resistant cabling routing WOW signals from the rotating/telescoping strut environment into the aircraft structure, typically via strain-relieved connections at the trunnion pivot.

การใช้งานในอากาศยาน

• Boeing 737-800 — dual proximity switches per main gear; PSEU processes all gear signals; WOW gates thrust reverser deployment and autobrake arming
• Airbus A320-200 — LGCIU (Landing Gear Control and Interface Unit) processes WOW; dual LGCIU1/2 with cross-monitoring for fault isolation
• Boeing 777-300ER — main gear and body gear WOW inputs; BSCU and PSEU integrate all signals; required for brake-by-wire ground mode
• Boeing 787-9 — WOW integrated with BSCU; critical for electric brake mode switching between air and ground brake control laws

ข้อดีและข้อจำกัด

The proximity switch WOW design is extremely reliable — typical MTBF (Mean Time Between Failures) exceeds 100,000 flight hours — and requires minimal maintenance beyond periodic functional checks and target-to-sensor gap verification. The contact-free operation eliminates the most common failure mode of earlier mechanical squat switches (contact welding or contamination-induced failure to switch).

The most significant limitation is the consequence of incorrect WOW state during critical flight phases. A GROUND state during flight could inhibit go-around thrust, deploy spoilers, or trigger autobraking while airborne — all potentially catastrophic. Conversely, an AIR state on the ground could prevent thrust reverser deployment, autobrake activation, and ground spoiler extension after touchdown, significantly increasing stopping distance. Aviation history includes several incidents where WOW sensor failures or incorrect rigging contributed to accidents. These consequences drive redundant sensor architectures, careful maintenance procedures for sensor gap adjustment, and extensive testing in ground and flight conditions.