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CDL · Outage and Loading

Why is a tank that is 50% full more dangerous for highway transport than a completely full tank of the same total weight?

Correct answer

A half-full tank has room for liquid to surge — the free surface effect means the liquid can shift dramatically during braking and turning, creating amplified force on the tank walls and on the vehicle's stability; a full tank moves as a solid mass with no surge room

  1. A Half-full tanks weigh more
  2. B A half-full tank has room for liquid to surge — the free surface effect means the liquid can shift dramatically during braking and turning, creating amplified force on the tank walls and on the vehicle's stability; a full tank moves as a solid mass with no surge room
  3. C A half-full tank is actually safer
  4. D Full tanks are more dangerous because of weight

Why this is the answer

FREE SURFACE EFFECT: When liquid has room to move in a container, it can surge and shift — dramatically amplifying forces during vehicle maneuvers. A completely full tank: the liquid has no room to move; it behaves like a solid load; no surge effect. A 50% full tank: the liquid has maximum surge room; waves can build up during extended braking and slosh forward; lateral sloshing during turns shifts center of gravity toward the outside of the turn; the combined effect of already-high center of gravity PLUS sudden lateral weight shift is the most dangerous combination for rollover. The most dangerous fill levels are approximately 20-80% — enough to allow significant movement. Empty or completely full are the most stable.
Source: FMCSA CDL Manual, Chapter 10, Tank Vehicles, Partial Loads and Free Surface Effect

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