Searching for the phantom bendy bus brake bug

Sydney bus drivers have refused to drive all of the city's articulated or "bendy" buses, because of alleged problems with their electronic braking system.

Sydney bus drivers have refused to drive all of the city's articulated or "bendy" buses, because of alleged problems with their electronic braking system.

According to Raul Boanza, secretary of the bus division for the Rail Tram and Bus Union, there have been over 100 cases reported when drivers step on the brake, nothing happens, and then the system locks up, sending passengers flying.

Each of the 80 bendy buses has a group of around seven electronic control units (ECU) which manage systems on the bus such as engine, transmission, braking and suspension. Braking software controls how the three types of bus brake — engine brake, transmission brake, and service brakes — work together, altering the mix of the three to get the best braking performance, a process called brake blending.

The brake disc from a bendy bus
(Credit: Volvo)

Although the State Transit Authority is aware drivers have been lodging reports of "brake blending events" on the buses since early 2007, CEO Peter Rowley says the buses are safe: "At no time have these Volvo bendy buses been at risk of brake failure," he said in a statement.

One of the buses which had been constantly reported as experiencing problems was monitored under a 53-day trial by the authority, with no problems.

Volvo, which makes the buses, has also checked out the reports, but could not replicate the problems experienced by the bus drivers.

"It's a difficult one because we're being asked to fix something that's not broken," said David Mead, general manager Volvo Buses Australia. "The oldest bus went into service on 30 September 2005. Reports only emerged in recent months."

Boanza, however, said that there have been incidents for as long as the buses have been on the roads.

When the system encounters a fault, it should log fault codes in the electronic control unit — no faults have been recorded, according to Volvo.

Boanza believes this is due to the system's programming: "The computer can only record whatever fault you program it to record," although Mead disputes this: "The ECUs are programmed to find any fault in the system," he said.

Although Volvo has been unable to replicate the fault, the State Transit Authority has asked the company to develop customised software to solve the problem. Volvo has developed the software which changes the way the brake blending is handled, making the buses brake more like the older vehicles in the fleet.

"All of the hardware remains the same, all of the braking systems remain the same — just a change to the software itself," Mead added.

Last Friday, the new Volvo software was fitted in a single bus for trial. It solves the problem, according to Boanza, and drivers want to keep the bendy buses off the road until all vehicles are equipped with the software.

The Industrial Relations Commission is looking into the issue.


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