The ATSB has concluded that a Bambi Bucket cable became hooked over the skid of a Bell 204 during uplift operations, creating a centre-of-gravity change that caused the helicopter to plunge into a Queensland dam.
According to the ATSB report released this week, VH-EQW was on firefighting operations near Tarome in September last year when it tracked to a dam to uplift water in the bucket. After filling the bucket, the helicopter pilot attempted to depart, but lost control of the aircraft, which plunged into the dam and rolled over.
The pilot was able to escape with minor injuries, but the helicopter was destroyed.
Investigators determined that the cable suspending the 1230-l Bambi Max bucket below the helicopter became snagged over the left rear skid, which shifted the aircraft's CoG aft, resulting in asymmetric lift loads, loss of control and collision with the water.
Calculations showed that a load of 300 kg applied to the left rear skid would take the helicopter outside its CoG envelope.
“The cables attaching the Bambi Bucket to the helicopter had been caught over the left rear skid by the time the helicopter was on approach to the dam and when it was uplifting water into the bucket,” said ATSB Director Transport Safety Kerri Hughes.
“As a result, due to the tethered weight over the left rear skid, when the pilot began lifting the bucket from the dam, the helicopter’s centre of gravity moved aft and left.
"This created asymmetric lift loads, resulting in the helicopter’s loss of control.”
As Australia enters the 2024-25 fire season, the ATSB has warned operators of the risk of conducting external load operations over water, which is thought to increase risk of an accident involving a firefighting helicopter two-fold compared to private helicopter operations.
“There can be a lack of visual references, visual illusions over water, limited visibility and vertical reference of the hook and external load through mirrors and bubble windows, which all add complexity when operating helicopters with external loads over water,” Hughes said.
“Fouling of external load suspension cables on the airframe can lead to rapid changes in weight distribution, asymmetric lift and loss of control.
The pilot was able to escape the submerged helicopter by opening the left rear door, which the ATSB credits to the pilot having completed Helicopter Underwater Escape Training (HUET) previously.
“The pilot had undertaken HUET about 2.5 years prior to the accident, which increased their chances of survival when the helicopter submerged in the dam,” Hughes said.
“Helicopters tend to be ‘top heavy’, thanks to their engine/s, transmission and main rotor system being above the cabin, and so can flip over quickly once hitting water.
"As such, this accident reinforces just how important conducting HUET is to increasing helicopter occupants’ chances of survival in the event of impact with water.”
The full report is on the ATSB website, along with a video on the value of HUET for helicopter pilots operating over water.