Ruapehu Bus Crash- Inquest

keytag, Nov 12, 8:26am
I’m curious about the following statement by a mechanic in relation to the inquest into the Ruapehu bus crash.

The mechanic is saying the driver ‘may’ have bumped a switch which ‘may’ have caused air to bleed from the bus braking system.

“ a mechanic brought by the driver's lawyer said it was highly possible the driver accidentally bumped a switch - which was not installed by the manufacturer - which subsequently allowed all the compressed air in the brake tanks to be released.
The mechanic, Mike Brown, said the driver was experienced, with approximately 500 trips up and down Mt Ruapehu under his belt.
An inadvertent knock of the auxiliary switch, which would have been easy to do, would have left the bus without brake power within 30 seconds, he said. “

I thought you needed air to hold the brakes OFF and you put the brakes ON by releasing air from the calipers, such that if a truck or bus has no air in the tank or an air leak then the default is that the brakes are applied, not released ?

keytag, Nov 12, 8:27am

harm_less, Nov 12, 9:52am
I was involved in an accident involving lack of brakes in a heavy vehicle. The driver of a waste water tanker we were following repeatedly used his brakes on a downhill run and soon after missed a gearchange on the following uphill run. The truck started rolling backwards towards us with no brakes and shunted us off the road into a fence before coming to rest at the base of the hill. Definitely driver error in a similar vein to what is being suggested in the Stuff article. Long story short: no air pressure = no brakes.

keytag, Nov 12, 3:49pm
The waste water truck lack of braking would likely have been because of excessive brake use, wrong gear, no exhaust brake, causing the brakes to overheat.

Pushing the pedal releases air from the brake cylinders and low air pressure applies the brakes, the more it is pushed the more air is released, the less air in the tank the harder the brakes come on.

If my truck air tank is empty the brakes are locked on, they won't come off and I can't move until the engine has been started and the air pressure is above 8 bar (120psi).

I have always known one of the fail safes for trucks is that no air or low air = brakes are locked on.

keytag, Nov 12, 3:55pm
So in simple terms, when at rest truck brakes are jammed on by super strong springs.
Start the engine and push the brake pedal, high air pressure pushes against the springs and takes the brakes off.
Pressing the brake pedal, or blowing an air hose or anything that causes low air pressure causes the springs to default and pull the brakes on.

bwg11, Nov 12, 4:59pm
Only speculating, air pressure holds the brakes off, but if the brakes are cooked from misuse, springs can't stop the vehicle with grossly faded or overheated brakes.

tweake, Nov 12, 5:26pm
those small buses probably run air over hydraulics. i'm not up on the system but i think if they run out of air you can have no brakes.

big buses run full air brakes which is basically spring powered brakes which uses air to control the brakes rather than power the brakes. release the air the brakes come on.
watching a truck loose air while driving is very smoky as the wheels lock up.

the waste water tanker mentioned above would simply have had worn out brakes due to lack of maintenance.

keytag, Nov 12, 6:02pm
I'm likewise not up on air over hydraulic, but I have a 4.6 tonne, dual axle air over hydraulic trailer.

If there is no air then the brakes are on, you need air pressure to release the brakes, so the only way to get the brakes off is to air couple to the truck.

Once had the truck/trailer air coupling come off and it auto locked up the trailer air over hydraulic brakes.

There is a backup air tank on the trailer with a yard valve, so to move the trailer without hooking up to the truck air you have to charge the brakes via the yard valve and if the trailer has been sitting for awhile and the air tank is empty then you can't get the brakes off without coupling to the truck air

So methinks even air over hydraulic are fail safe, no air, brakes are on.

keytag, Nov 12, 6:07pm
I found this:
* As a fail safe, air-over-hydraulic braking systems required by regulation to use tandem system design so that one system can compensate if the other fails.
*All vehicles are required to have park brake system that can act as emergency brake should there be a failure of the service brakes.

keytag, Nov 12, 6:10pm
And I found this about fail-safe on air brakes;
A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes. Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi).

tweake, Nov 12, 6:14pm
not to mention they also have a low air pressure warning alarm.

redhead18, Nov 12, 6:20pm
Sad reality is no matter how many times a driver has been up and down a mountain.
Where the same driver claims to have passed a course and test on driving a FULLY LADEN bus-and has lied.
The Police have admitted had they known details that came out at the inquest they WOULD have charged the driver.

https://www.stuff.co.nz/national/123290284/mt-ruapehu-bus-crash-inquest-driver-wrongly-said-hed-completed-downhill-assessment https://www.stuff.co.nz/national/123277748/ruapehu-bus-crash-inquest-driver-says-he-wasnt-trained-to-deal-with-emergencies
So insert WEATHER and the day the bus goes up laden and due to WEATHER comes down soon after and the driver does NOT use the exhaust brake and overheats the brakes misses gears and free wheels down closes his eyes and hits a bank=one fatal mess,,,,

keytag, Nov 12, 6:25pm
Interesting that the 2 x police mechanics "found no issues with the brakes"
From experience single (or short term) overheating of the brakes with decent shoes doesn't leave too much of a tell tale sign, other than the smell.
But long term sustained over-heating often 'purples' the brake disc, and super shines the pads. a half way decent mechanic can tell if the brakes have had repeated over-heating.

So in the case of this bus incident if it was short term (single event) over heating there may be no obvious signs.

HOWEVER if the bush mechanic is correct that the driver knocked some sort of air switch then the police mechanics should have noted the position of the switch when they checked the brakes, assuming part of their tests were to start the bus, charge the tank etc. They did say they tested them over VTNZ rollers, so it is reasonable to assume this was with the engine going.

Of course the driver 'may' have knocked the switch on, crashed and then 'may have knocked it back off.
BUT if knocking the switch drained the air then Google says the brakes should have come on. the only other possibility is if the bus was going too fast downhill, in the wrong gear, with the exhaust brake off, then even in fail safe the brakes may have struggled to pull the bus to stop.

keytag, Nov 12, 6:30pm
I'd like to think that based on the new evidence the Police have the option to now lay proper charges.

tweake, Nov 12, 6:59pm
thats far more likely.

john1623, Nov 12, 7:28pm
Sounds like it was an older bus with straight air over hydraulics not spring or maxi brakes.

john1623, Nov 12, 7:43pm
Have driven a lot of trucks over the years and unless they have full engine brakes such as Jacobs or Dynatard, the flap type exhaust brakes are about as useful as dragging your foot along the ground.

alowishes, Oct 25, 5:41pm
Some driving related offenses have a six month limit and after that it’s too late to charge the driver.