CDL Greenlight

Study guide

CDL tanker endorsement: surge, baffles and outage

Learn why a tank of liquid handles like no other load, and the rules that keep it upright and stopping where you mean it to. Then take the free CDL practice test.

Who takes it
A Class A or B driver hauling a liquid or gas in large tanks. Class C too, with a hazardous load.
To pass
80 percent.
Questions
20 in Pennsylvania. Check your state's manual.
On your license
Endorsement N. With hazmat, X.
On this page
  1. The figures that make it a tank vehicle
  2. Quick answers: surge, baffles and outage
  3. Inspecting a tank vehicle
  4. Worked example: three loads, three dangers
  5. Five common mistakes
  6. Three practice questions
  7. Sources

The figures that make it a tank vehicle

119 gallons
A tank counts when its own rated capacity is more than this, attached to the vehicle or the chassis for good or for the trip. Manual 8
1,000 gallons
And the tanks' rated capacity is this much or more together. Then the vehicle is a tank vehicle and the driver needs the endorsement. An empty storage tank not built for transport, carried on a flatbed trailer, does not count. Manual 8
2 times
Wet roads double the normal stopping distance. An empty tank vehicle may take longer to stop than a full one. Manual 8.3

Quick answers: surge, baffles and outage

What is outage?

The room you leave in a tank for the liquid to expand. Liquids expand as they warm, so a cargo tank is never loaded totally full. Different liquids expand by different amounts and need different outage: know the requirement for what you haul. Manual 8.2

How are bulkheads different from baffles?

Bulkheads divide a tank into several smaller tanks, so when you load and unload them you watch the weight on the front and the rear. Baffles are bulkheads with holes: the liquid flows through, and they slow the forward-and-back surge. Manual 8.2

Do baffles stop side-to-side surge?

No. They help control the forward-and-back surge. Side-to-side surge can still occur, and it can cause a rollover. Manual 8.2

How is a smooth bore tank different to drive?

Nothing inside slows the liquid, so forward-and-back surge is very strong. These un-baffled tanks usually carry food such as milk, because sanitation rules forbid baffles. Drive slowly and carefully, above all when starting and stopping. Manual 8.2

How do you control surge when you stop?

Keep a steady pressure on the brakes and do not release too soon. Brake far in advance of the stop and increase your following distance. In a quick stop to avoid a crash, use controlled or stab braking, and do not steer quickly while braking: the vehicle may roll over. Manual 8.3

What is the most important thing to check on a tank vehicle?

Leaks. Check under and around the vehicle. Carrying liquids or gases in a leaking tank is a crime: you will be cited and prevented from driving further, and you may be liable for the clean-up. Manual 8.1

Why might you fill a tank only partly?

A full tank of a dense liquid, such as some acids, may exceed legal weight limits. How much you load depends on how much the liquid will expand in transit, its weight and the legal weight limits. Manual 8.2

Is the posted speed for a curve safe in a tanker?

Not always. Tests have shown that tankers can turn over at the speed limits posted for curves. Slow down before the curve, then accelerate slightly through it. Manual 8.2, 8.3

Inspecting a tank vehicle

Tank vehicles come in many types and sizes, so check the operator's manual for how to inspect yours. And never drive with open valves or manhole covers. Manual 8.1

  1. Leaks

    The most important item. Look under and around the vehicle, and check the tank's body or shell for dents or leaks.

  2. Valves

    Check the intake, discharge and cut-off valves. Each must be in the correct position before you load, unload or move the vehicle.

  3. Pipes, connections and hoses

    Check them for leaks, especially around the joints.

  4. Manhole covers and vents

    The covers need gaskets and must close correctly. Keep the vents clear so they work.

  5. Special purpose equipment

    If the vehicle has it, make sure it works: vapor recovery kits, grounding and bonding cables, emergency shut-off systems, a built-in fire extinguisher.

  6. Emergency equipment

    Find out what your vehicle must carry, and make sure you have it and it works.

Worked example: three loads, three dangers

Set each load against what the liquid does inside the tank.

  • A milk tanker, half full, stopping at an icy intersection.

    The surge can push you into the intersection. A smooth bore tank has nothing inside to slow the liquid. When the wave hits the end of the tank it pushes the truck the way it is moving, and on ice it can shove a stopped truck out. Brake early, keep a steady pressure, and do not release too soon.

  • A baffled tank, partly full, on a tight off ramp at the posted speed.

    Too fast. Baffles slow only the forward-and-back surge. Side-to-side surge can still roll the vehicle, and tankers can turn over at the posted speeds for curves. Slow down before the curve.

  • A tank of acid, filled to the top to make the trip worth it.

    Wrong twice. A cargo tank is never loaded totally full: the liquid needs room to expand. And a full tank of a dense liquid may exceed legal weight limits.

Five common mistakes

  • Common answerBaffles keep the liquid from moving.

    The manual 8.2They help control the forward-and-back surge only. Side-to-side surge can still occur and can cause a rollover.

  • Common answerA full tank is the steady one, so fill it to the top.

    The manual 8.2Never load a cargo tank totally full. Liquids expand as they warm, and a full tank of a dense liquid may exceed legal weight limits.

  • Common answerThe posted speed for a curve is safe.

    The manual 8.2, 8.3It may be too fast for a tank vehicle. Tankers can turn over at the speed limits posted for curves.

  • Common answerAn empty tanker stops quicker than a full one.

    The manual 8.3It may take longer to stop than a full one.

  • Common answerEase off the brake as the truck slows down.

    The manual 8.3Keep a steady pressure on the brakes and do not release too soon when coming to a stop.

Three practice questions

Pick an answer. The reason and the manual's own sentence follow.

Why must you never load a cargo tank totally full?

Show the answer

The answer is A. The liquid expands as it warms, so you leave room for it. That room is called outage, and different liquids need different amounts.

Commercial driver manual, section 8.2Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid.

Your tank has baffles. Which surge can still roll the vehicle over?

Show the answer

The answer is A. Baffles help control the forward-and-back surge. Side-to-side surge can still occur, and it can cause a rollover.

Commercial driver manual, section 8.2Side- to-side surge can still occur. This can cause a roll over.

How do you brake to a stop in a partly loaded tanker?

Show the answer

The answer is A. When you stop, the liquid surges back and forth, and the wave pushes the truck the way it moves. A steady pressure, held until the truck has stopped, keeps the push under control.

Commercial driver manual, section 8.3Keep a steady pressure on the brakes. Do not release too soon when coming to a stop.

Now answer 5, free.

5 questions from the course, no signup. Miss one and see exactly why.

Try 5 questions free The whole course, $99 once Every CDL knowledge test, lesson by lesson: 97 lessons, more than 2,000 practice questions and mock exams at test length.

Sources

Reviewed by CDL Greenlight on October 23, 2026. CDL Greenlight writes and checks this page. Every figure is the one the commercial driver manual prints, with its section named beside it. Where your state has a rule of its own, your state's manual decides.

Found a mistake? Write to support@cdlgreenlight.com. Corrections we have published