What Triggers Airbags?
What triggers the airbag?
The airbag is triggered by the airbag control unit when crash sensors measure a deceleration of at least 3 to 5 g and the calculated change in velocity exceeds a threshold of approximately 25–30 km/h. What matters is not the driving speed, but the severity of the impact.
The control unit makes the decision to deploy or not deploy the airbag (Fire/Non-Fire) within milliseconds. It evaluates data from multiple independent sensors and performs a plausibility check.
Which sensors trigger the airbag?
Acceleration sensors in the control unit and front sensors in the front area measure impact deceleration. Pressure sensors in the doors detect side collisions for the side airbag. At least two independent sensors must agree before the airbag deploys.
An overview of the different sensor types:
| Sensor | Location | Function |
|---|---|---|
| Acceleration sensor | In the control unit (center console) | Measures deceleration during frontal and rear impact |
| Front sensor | Front area / engine compartment | Detects frontal crash early |
| Pressure sensor | In the doors | Detects side collision through pressure increase |
| Safing sensor | In the control unit | Prevents accidental deployment |
Safing sensor: The safety level
The safing sensor is an additional safety instance. The airbag is only deployed if both a crash sensor and the safing sensor detect a collision. This prevents a single faulty sensor from causing a false deployment.
How does the control unit decide whether to deploy?
The airbag control unit measures vehicle deceleration every 1 millisecond. When 2 g is exceeded, a calculation cycle begins that computes the collision-related change in velocity. This integral value is the immediate criterion for the firing decision.
The process in detail:
- Measurement: Sensors continuously detect deceleration
- Threshold: When over 2 g is exceeded, crash calculation begins
- Integration: Change in velocity is calculated from sensor data
- Plausibility check: Multiple sensors must agree
- Decision: Fire (deployment) or Non-Fire (no deployment)
- Ignition: Electrical signal to the gas generator
Additionally, the control unit evaluates other vehicle data: engine load, ESP signals, steering angle, and brake status. This allows it to better classify the type of accident.
At what speed does the airbag deploy?
Front airbags deploy at a collision-related change in velocity of approximately 25–30 km/h – equivalent to an impact at this speed against a rigid wall. According to the German Federal Highway Research Institute (BASt), deployment occurs in 50% of cases starting at 20 km/h.
Important: The differential velocity at impact matters, not the driving speed. A car traveling at 100 km/h that grazes a guardrail may not deploy an airbag – but an impact at 30 km/h against a tree will.
Why doesn't the airbag deploy sometimes?
Soft obstacles like guardrails or parked cars slow the vehicle more gradually. The deceleration peaks are lower, and the threshold is not reached. In these cases, the seat belt alone provides sufficient protection.
What severity levels are there for deployment?
Modern airbag control units categorize accidents into four severity levels and adjust deployment accordingly. In minor accidents, airbags remain inactive; in severe crashes, both stages are deployed – for maximum occupant protection.
| Severity level | Type of accident | Response |
|---|---|---|
| Level 0 | Minor accident | No deployment – seat belt is sufficient |
| Level 1 | Moderate accident | Possible deployment of stage 1 |
| Level 2 | Severe accident | Deployment of stage 1 |
| Level 3 | Very severe accident | Deployment of both stages |
Two-stage airbags inflate more slowly at the first severity level – this is gentler for the occupant. In a very severe impact, the airbag inflates faster and to full volume.
How quickly does the airbag respond?
The driver airbag is deployed within 30 milliseconds after impact and is fully inflated after 55 milliseconds. The passenger airbag follows 10 milliseconds later, as the greater distance to the dashboard allows more time.
For comparison: A blink lasts approximately 300 milliseconds. The airbag is thus ready for deployment in a fraction of an eye blink. This extreme speed is only possible through the gas generator, which fills the airbag in milliseconds.
Differences between front, side, and head airbags
The deployment conditions differ fundamentally depending on the airbag type. Front airbags respond to deceleration sensors, side airbags to pressure sensors in the doors, and head airbags protect in side and rollover accidents.
- Front airbag: Acceleration sensors, deployment during frontal impact at approximately 25 km/h differential velocity
- Side airbag: Pressure sensors in the doors, faster deployment required (less crumple zone)
- Head airbag (curtain): Combination of pressure and acceleration sensors, also protects during rollover
- Knee airbag: Same sensor technology as front airbag, protects legs from the dashboard
Frequently asked questions about airbag deployment
Here you'll find answers to the most important questions about airbag deployment.
Can the airbag deploy from a pothole?
No. The safing sensor system and plausibility check prevent false deployments. A pothole does not produce sufficient deceleration in the direction of travel and is not recognized as a crash by the algorithm.
Does the airbag deploy in every accident?
No. In minor accidents (severity level 0), the airbag remains inactive because the seat belt provides sufficient protection. Deployment only becomes necessary above a certain impact severity.
What happens to the control unit after deployment?
The airbag control unit stores the crash data and locks itself. It must then be repaired or replaced. Specialized shops can delete the crash data and restore the control unit to working order.
Can an airbag deploy without an accident?
Extremely rare, but possible – for example, due to a faulty sensor or a short circuit. This is why it's important to take the airbag warning light seriously and have it checked immediately.