Machine Design

Mower Design Flaws Spark Deadly Fire

A man died from serious burns after the garden tractor he was riding caught fire. He had been mowing a flat area for less than 5 min when gas in the fuel tank exploded and knocked him off the seat.

Fire investigators concluded a spark from the engine or another part of the mower lit a trickle of leaking gasoline. The flame climbed the leak path back to the gas tank where it ignited the remaining gasoline in the nearly full tank.

The design of the 30-year-old mower created a tunnel that funneled the flames through the instrument panel and toward the driver. The driver received second and third degree burns over 75% of his body that caused his death a few days later.

A more-detailed examination of the mower after the incident revealed the mower’s generator and the wire leading from the generator to the ignition coil were directly below the gas tank. Over time, a clip meant to hold the wire in place had broken and the wire’s insulation had worn away, creating ideal conditions for a spark.

The operator could have occasionally inspected the generator wire for worn insulation and proper attachment. If such inspection was required, the owners manual should have recommended it and detailed the dangers of exposed wiring near the fuel tank. Ideally, designers should have relocated the wire or the fuel tank to eliminate this danger.

Another danger the owners manual neglected was that of leaking or spilled gasoline, and there were no warnings on the mower. The gas cap screwed on with a single thread, a design that made cross-threading and leakage more likely. Finally, even if the operator was aware of the dangers of leaked gas, he could not have seen them when filling the mower or during operation.

The design of the mower cover that guided flames toward the operator compounded these design and documentation lapses. Other designs, including vents on the sides of the cover or a repositioned filling neck would have let flames escape away from the driver and turned a fatal fire into a scary close call.

The mower designers should have taken into account reasonably foreseeable wear and tear and operator error into account when designing the machine. Ideally, they should use materials and construction methods that last the lifetime of the part and errorproof parts of the machine that users interact with. But, as a minimum, the owners manual should warn users of potentially dangerous situations and there should be warnings at the point of use.

This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].

© 2010 Penton Media, Inc.

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