Outage and Loading
Why tanks are loaded with empty space and how outage protects you.
Endorsement: Tanker (N) · Source: FMCSA CDL Manual (public domain)
Outage is the empty space deliberately left in a cargo tank above the liquid surface. Liquids expand as temperature rises, and a tank loaded completely full with cold liquid would burst as the liquid warmed and expanded. Federal regulations require specific outage percentages for different products: typical fuel oils require about 1 percent outage; gasoline often requires 2 percent or more; and liquefied petroleum gases require still more, with the outage calculated based on the highest temperature the tank could reach in service.
Loading correctly is also a weight-distribution problem. A bulkheaded tank with multiple compartments must be loaded in a sequence that keeps each axle within its rated weight. Loading the front compartments first overloads the steer axle; loading the rear compartments first overloads the drive axles. The standard procedure is to load the front and rear compartments in alternation, then top off the middle compartments. Many tankers have placards near the loading inlets specifying the maximum load per compartment in pounds and gallons.
Loading and unloading procedures also involve specific safety steps. The vehicle must be parked on level ground, the engine must be off (for flammable liquids), the parking brake must be set, the wheels must be chocked, and the truck must be electrically grounded if loading or unloading flammable liquids by pumping. The driver must be in attendance throughout. Spilled product must be contained immediately; even a small surface spill at a fuel terminal can result in heavy fines. The exam tests both the conceptual reason for outage (thermal expansion) and the specific values for common products, plus the loading-procedure safety steps.
Key terms to memorize
- outage
- surge
- baffle
- smooth-bore tank
- liquid-tight integrity
- rollover threshold
Other Tanker (N) topics
- Liquid Surge — Why partial loads slosh, why that is dangerous, and how to drive for surge.
- Baffled vs. Smooth-Bore Tanks — Three tank designs, three different handling profiles.
- High Center of Gravity — Why tankers roll over so easily and what the safe-cornering speeds really are.
- Emergency Procedures for Tankers — Leaks, rollovers, and what to do in the first minutes.
Test what you learned
Now that you have the Outage and Loading material in your head, drill the Tanker (N) practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Tanker (N) test for your jurisdiction.