What is 'outage' in tanker loading and why must it be provided?

OUTAGE (also called 'ullage'): The empty space deliberately left at the top of a tank to allow the liquid room to EXPAND.

Why: Liquids expand when their temperature rises (e.g., loaded cool in the morning, warming during the day, or moving from a cold to a warm climate); without outage, the expanding liquid has nowhere to go — it could rupture the tank, force product out of the vents/relief valves, or create dangerous pressure.

Amount: Different liquids expand by different amounts, so the required outage varies by product; the loader/shipper specifies the proper outage; FOOD vs CHEMICALS vs FUEL: Each has expansion characteristics; OVERFILLING is dangerous — leads to spills, pressure buildup, and potential tank damage; this is part of why tankers aren't simply 'filled to the top'; LOADING considerations combine: weight limits (heavy liquids), outage (expansion room), and surge (partial loads have more surge); the driver and loader must ensure proper outage; outage is a tanker-specific concept tested on the endorsement — it's about leaving room for thermal expansion to prevent tank damage and spills.