Why didn't my airbag deploy?

Last Updated Feb 12, 2017

You crash your car. It's violent, brutal and chaotic. Maybe there’s a lot of damage. Maybe you sustain some injury - bit of whiplash, bruises, a few cuts. You look at the wreck and you discover the damn airbags didn’t even deploy. And you presume your car is defective. You’re understandably angry with the manufacturer. Maybe you can smell a lawsuit. But before you retain Denny Crane and his team of associates, let’s look at the line between life and death in car crashes, and the mad science of survival.


I get questions about this airbag non-deployment issue a lot. People say: "My airbags didn’t deploy in a crash: Should I sue the manufacturer?"

Or they say:

"I’m never buying another [INSERT BRAND NAME HERE]. Never."

The latest such inquiry came from Sumit, after a drunk driver T-boned his wife in their Hyundai Elantra. She suffered whiplash and shock, but was otherwise OK, and the car’s a write-off. Sumit says:

Another related question this week from Javier who was concerned about the safety of side airbags for non-human cargo. Javier says:

Both of these questions - and the dozens more I get every year on this tell me there’s a fundamental misunderstanding about the dynamics of violence in car crashes. The physics. The engineering. So let’s clear that right up.

In emergency medicine, they call them ‘high mechanism’ injuries. The injuries where you acquire so much energy that evolutionary biology just has not prepared you for survival. If you break your leg playing football, it snaps into two pieces. Hurts like a bastard. They set the bone. You get better. Nobody dies of this in the first world.

If you break your leg in a high mechanism car crash, it breaks into maybe thirty-two pieces. Or something. Just shatters. If you don’t die of hypovolemic shock at the roadside, you endure hours of surgery. Months of external fixation. More months of physiotherapy. You’re maybe left with a leg that’s intact, but perennially cold, nerve-damaged and essentially painful and useless. One day the doctor might recommend amputation as a therapeutic option - to improve the quality of the rest of your life.

Learn more about the safest cars on the ANCAP website >>


We’re talking about those kinds of high-mechanism injuries - and worse. To really understand what’s going on here, you need to slow a car crash right down, like in a crash test video. Watch it unfold like a story in three acts, which, essentially, it is, rather than a brutal real-time event that’s over in the blink of an eye, and of which you cannot have any detailed memory.

When you do that, slow it down, you actually see three crashes - and only one of them kills you.


First thing that happens, crash one: The car hit something else (a tree, another car, a power pole - something significant). If we do a bit of physics in the beer garden here, you see metal start to bend. It’s hard to bend that metal. It absorbs energy. Therefore, the car decelerates. If the thing you hit is big and hard, the deceleration is massive. That’s crash one. Between the car and the thing it hits.


Crash two happens just a few milliseconds later, when you hit the inside of your car. Just before the crash, the car’s doing 60, therefore you’re doing 60. Then the car slows down. You’re still doing 60. If the impact is in front, you feel yourself getting thrown forward, hard.

You’re actually just trying to keep going at 60 (it’s called ‘inertia’). It’s the car that’s slowing down violently, but it feels like you getting thrown forward. You hit whatever slows you down inside the car. That’s crash two. Your chest and hips hit the seatbelt, your knees might hit the dash, you headbutt the airbag - whatever. Crash two is between you and the car, and you’re still quite okay at this point.


Crash three is the one that kills you. It’s occurring entirely inside you, a few milliseconds later. Your internal organs move forward, and they hit your skeleton. All of that gruesome glop that keeps us all ticking over - the component parts of the machine we call homo sapiens - your heart, lungs, liver, spleen, major bood vessels … they all jam up hard against the inside of you rib cage. Your brain runs into the inside of the skull. None of this is good.

All of those parts have structural limitations. Load limits. This many Gs for that brief time duration, they burst, they rupture. There’s no ‘afterwards’ when that happens.

If you decelerate at 80 G for 100 milliseconds - literally the blink of an eye - it’s like lying on the floor and having five Toyota Corollas stacked on your chest, really briefly.

Good luck with that.


If these loads rupture your aorta - the body’s biggest blood vessel, you bleed out on the spot. Broken ribs can puncture your lungs, or lacerate your liver. Blunt chest trauma can rupture your heart. Your heart’s in a leathery bag called the pericardium, and if your heart starts leaking into that space, it compresses your heart, which eventually fails.

That’s called a cardiac tamponade - a really serious problem with the plumbing. I once stood in the operating theatre and watched a team of emergency cardio-thoracic surgeons fix exactly this problem in a young man who’d been T-boned in a car crash. Incredible to be invited over, and peer into someone’s chest, and see the exposed human heart leaking like that.

You lacerate your liver. That’s bad. It’s a sponge full of blood. Very difficult to repair. But easier now thanks to the efforts of Professor Russell Strong in Queensland in the 1970s and 1980s, when road trauma was really out of control - he’s the globally recognised pioneer of advanced liver surgery.

If your brain hits your skull, a blood vessel might rupture. Your brain lives in a rigid box made of bone. Very good for protection if another primate swings out of a tree and punches you in the head, or if you trip over and fall down. Not so good in a car crash. If you start bleeding into your skull, it compresses your brain. You feel OK - then you’re dead.

Treatment options for brain injury? Medieval - they sedate you and intubate you. Pump you full of oxygen. Hope for the best. Oxygen’s a vasoconstrictor - clamps your circulation down. Throttles back the blood flow. Next time you hear the words ‘induced coma’ in a news report about a car crash, it’s a euphemism for some poor bastard suffering a profound traumatic brain injury.

I’ve gone into some detail here because this is exactly what you’re looking down the barrel of, in the Twilight Zone between crash two and crash three. Think about it like this: You’re on a train. It’s the fastest train you’ve ever been on. You’re at crash two. Next stop: crash three. Evolution cannot protect you from the physics of driving - we’ll have to give it another 100,000 years to do that.

Need to know more about first aid? The Red Cross can help >>


Between now and then, all we’ve got is advanced technology. Airbags - together with load-limiting seatbelt pre-tensioners and crumple zones - they’re all brilliantly designed to stretch out the time duration of crash number two. What this does is it makes you hitting the car softer, and consequentially it decreases the critical loads experienced by your internal organs in crash number three. This is the underlying physics of saving your life.


In every crash, you are completely safe over most of the event. It’s not hurting you. It cannot hurt you. But there is a critical differential element of time, a wafer of a just few milliseconds - a time element none of us can feel or physically appreciate because it is just too brief - but this is when the window opens, and the grim reaper steps into the room.

The brilliance of the technology is that it rockets into action, and identifies this window of time. Not only that - it gets all the hardware into place at exactly the right time, and it slams the window shut just before that death-dealing motherf#@ker can stride through and chalk you up on the pavement.

It’s unbelievably clever. But there is a catch...


The reaction of the system is brutal. Old Testament brutal. Airbags, the most obvious protective feature in play, do not just “pop” as Sumit said in his e-mail. They explode violently towards you at something like 300km/h. They must act that fast to get to you before crash #2 does.

You think about it: the car has a crash sensor that spends its entire life listening for just the right (or wrong) crash. And then it says is ‘go’. You think about it: you’re steering, braking, programming the GPS, making calls, perving at some young hottie waiting for the bus. All the crash sensor does is listen. You have to be well inside crash number one for it to do its thing. And if crash number one steps over a pre-determined severity threshhold, it say go. Flicks the switch. Bang - it’s over.


Deploying all those explosive devices is not a risk-free undertaking. Airbags and pretensioners can injure you seriously (especially if you are out of position when they go off). We’ll get to that. Airbag deployment is an engineering last resort, like fixing bayonets in the trenches, running low on ammo. Like, you’re likely to die - this is the best we can do.

The fact that Sumit’s wife did not suffer severe injuries is all the evidence I need to conclude that the Elantra’s restraint systems functioned brilliantly during that crash. Mrs Sumit would absolutely have been more at risk if they had deployed. I’m sure the event was violent, brutal and more than a little upsetting, but the fact that she was uninjured tells me everything functioned as it was intended to. The crash sensor said, nah - not serious enough.

But I do truly hope the cops and the courts nail that drunk bastard who crashed into Sumit’s wife.


And I guess that brings us inevitably to Javier’s problem about the safety of the pooch in transit. Will the curtain airbag crush the dog’s neck in a crash? It’s really not a fair question in terms of how the curtain airbags operate.

I mean, yeah - if you’re driving along, dog has its head out the window and ‘bang’ spontaneous, uncommanded curtain airbag deployment - yeah - that’ll probably kill the dog. But it doesn’t happen like that. The curtain airbag deploys only in situations that are so severe they would otherwise kill you. In a crash like that, the dog is dead either way.

Airbags don’t deploy in non-life-threatening situations.


Which brings us to: personal responsibility. You’re sitting in an environment with six or eight explosive airbags, and four or more explosive seatbelt pre-tensioners. If you are out of position when they go off, they will injure you. So if you are leaning into the footwell to pick up the phone you just dropped, or reaching around to pick up the baby’s dropped bottle, those devices will hurt you.

A seatbelt pre-tensioner is more than strong enough to break your collarbone. That can puncture your lung - great way to suffer a tension pneumothorax at the roadside. And if you’re one of those dickheads who drives with the outboard hand gripping the edge of the roof, out the window (always vigilant; making sure the roof stays on), consider this:

The curtain airbag deploys down at 300 kilometres an hour. Do you really want it to hit your arm? A former colleague of mine was an intensive care paramedic in the Victorian Ambulance Service. He turned up on a T-bone car crash one day, and the driver (otherwise intact) has a broken humerus - the big bone in your upper arm - and the ball in his shoulder has been ripped out of the socket. That’s gotta hurt. It’s what happens when you drive hanging onto the roof and the curtain airbag deploys. Up to you.

A couple of other classic irresponsible moron manoeuvers: driving one-handed is especially retarded - merely from a car control perspective. But if you crash, and you’re driving with your right hand at 10 o’clock on the wheel, or your left hand at two … because, I dunno, Hollywood … then the airbag is going to deploy into your arm, which will blow your arm into your face a lot harder than you can punch yourself … and your arm will spoil the deployment and ruin the airbag’s capacity to save your life … so that’s bad.

Finally, and not to be sexist, but if you’re one of those chicks who, after a long, salty day at the beach, rides home in the passenger’s seat with her feet up on the dash, think about this. No matter how pleasant it may be to waft a pleasantly cool stream of climate control into the underlying ady parts - and, when I think about these things, it is fairly pleasant - but if you crash, you’re fucked. And not by Chris Hemsworth. By angry, outlaw bikers seeking revenge.

Resting your feet - or anything else - on or across the airbag module on the dashboard is a very bad idea. It’ll potentially smash your feet, ankles and legs into the windscreen, spoil the deployment, blow your knees into your head and maybe ‘submarine’ you into the footwell - none of which sounds particularly fun.


I guess the three critical points to remember are:

  1. If you crash and the airbags don’t deploy - and you walk away relatively unscathed - then the crash wasn’t severe enough to threaten your life. Instead of suing the car company, send them a note of sincere appreciation. Unless you are severely injured you simply have no grounds for complaint.
  2. Any time you drive, remember you are surrounded by explosive devices. This should not especially trouble you - it’s not like they were put there by ISIS. But there are responsibilities if you want to maximise your safety.
  3. If you one day have a real serious crash and all the hi-tech shit comes out to protect you, you won’t remember it anyway. Because our memories don’t function on that brief timeline.

In any other culture they’d call walking away from that kind of event magic, or at least miraculous - which is exactly what advanced engineering is, if you don’t understand it. 


Cars experience significant controlled deformation at the front. This absorbs energy before it gets to you. Literally, they sacrifice the front end so that you may live

Cars experience significant controlled deformation at the front. This absorbs energy before it gets to you. Literally, they sacrifice the front end so that you may live


Hi John,

Cars that are literally ripped  apart in an accident. Has any one ever tested any of these cars for metal fatigue? Some of the boom boxes in these cars you can hear them half way up the street and when they pull up beside you your car and ear drums vibrate. Surely this must be causing metal fatigue.

Also: Poor seating posture, especially with our young drivers. I see so many of them, my son son included, drive with there seat laid back like a lounge seat. Surely they can't see or handle a car properly in the case of an emergency.

I have written to NRMA twice in recent years but they wrote back saying they don't think there is any interest in the story at this time. What do you think?



Hi Craig,

Good questions - thanks.

Boom boxes & metal fatigue: 
Boom boxes are annoying but they don't cause metal fatigue. Metals have two deformation ranges - elastic deformation and plastic deformation. In the elastic range (which varies from metal to metal based on composition and heat treatment) the metal can be deformed and released an unlimited number of times without fatigue.

Example: a spring in the suspension - never wears out; experiences millions of deformation events in its life. Never fatigues. The so-called plastic range is where some of the deformation is permanent. The metal gets bent permanently. Metals that are deformed repeatedly in the plastic range get fatigue: crystalization occurs at the point of fatigue, causing brittleness. Example: bend a coathanger back and forward. It fatigues, crystalizes and breaks through fatigue.

It's inconceivable that a speaker, no matter how 'boomy' in a car, could cause metal fatigue.

Crash deformation:
The fact is, new cars are designed - for a very good reason - to deform tremendously in crashes. The reason they do this is to absorb impact energy before it gets to the driver. If the car were very rigid (think HQ Holden) the deceleration of the occupants in a collision would be very sudden. If the car had carefully engineered crumple zones (think: any car with a five-star ANCAP safety rating) the deceleration of the occupants is more gentle because it occurs over a greater period of time.

Example: Thomas Kelly. Killed not directly by a sucker punch thrown by a coward, but rather by the impact of his head on the pavement as a result of that punch. (I'm not making excuses for his killer, who is demonstrably a scumbag.) The impact with the rigid, unyielding pavement was so brief that the victim was unable to survive the deceleration of his brain inside his skull. Its physical limits were exceeded, blood vessels were damaged, an intra-cranial bleed was suffered and the rigid box of bone (the skull) left nowhere for the pressure to go. Death resulted.

If Mr Kelly's head had hit a few empty egg cartons on the pavement, he would undoubtedly be alive today. The egg cartons would be damaged, but the brain would have survived because the cartons absorbed the impact in a  controlled way, absorbing energy and extending the time to reduce the magnitude of the deceleration.

If you compare the two impacts, when it's head-versus-pavement, the head loses and the pavement wins. The pavement looks (and is) undamaged. When it's head-versus-egg-cartons-on-pavement the egg cartons lose and the head survives. 

The head V pavement scenario is crashing in a 1980s car. The head V egg carton scenario is crashing in a modern car. The structure is a shock absorber. This is one of the principle reasons why the road toll is way down. It's not policing, that's for sure.

There's a huge body of technical research in support of this fact. Unfortunately, you need a degree in applied physics or engineering to understand it. If your head decelerates at 80G for more than a few milliseconds, you're dead. That's what the engineers aim to prevent - by using the car like the egg cartons as a shock absorber. (Similar limits apply for thoracic deceleration, and loads on lower limbs.)

Personally, I'd rather have the car be completely rooted, and me be okay after a crash - because the alternative is the other way around. (And you can't have both - something has got to give in this game.) Older cars weren't better built; they were death traps.

Seating position:
We agree on this. As upright as possible is a great idea. It boosts both vision and control as you say. It also maximises the capacity of the restraint system - seatbelts, pre-tensioners and airbags - to protect you in a crash. In a severe crash, if you're sitting too far reclined, you'll slide through the seatbelt, into the footwell. Ask paramedics and the fire brigade. Something should be done about this, but I'm not sure what. Probably a public education campaign.

Thanks for the questions. I understand how you think that about modern car construction. I've been asked that several times. A high level of technical training is required to understand the reason for all that deformation.

Thankfully though, they don't make 'em like they used to. Trust me on this; that's a good thing.