Five firefighters were injured when a hose exploded as they tackled a major blaze.
Some are understood to have been thrown several feet into the air when the hose burst and flew at them.
A source said: "This was a shocker. They were wiped out.
"The only saving grace is that the hose caught them on the legs - if it had been at face height, it could have taken their heads off."
The incident happened as several crews tackled a huge fire at the former Keil School in Dumbarton early on Thursday. Local firefighters were called to the scene after flames were spotted shooting from the roof of the derelict building.
A £500,000 high-reach aerial rescue pump (ARP) was drafted in from Polmadie in Glasgow but had problems taking on water from a hydrant.
The firefighter in the cage attached to the vehicle's extending arm was told to come down while the blockage was investigated - but at that point, the hose burst.
Of the five firefighters who were knocked off their feet, two had to be taken to hospital.
One had serious leg injuries while the other sustained a back injury. Both were later released.
Three "walking wounded" were treated at the scene.
Crews battled the flames well into the afternoon but the building was left gutted.
Keil, one of Scotland's top feepaying schools, closed in 2000 and is now the site of a major housing development.
A police spokeswoman said yesterday: "The fire is being treated as wilful. A joint investigation is being carried out by police and fire services and inquiries are ongoing."
s r e ob " ARP trucks are supposed to do the job of a normal engine and a "height appliance" combined - but firefighters have complained they are not fit for purpose.
fire-Safety fears were raised after one toppled over at a miniroundabout near Hamilton. And a firefighter was injured in 2011 when he was trapped on a faulty ARP ladder above a burning building in Tradeston, Glasgow.
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