An unlined cargo tank hauling a corrosive product is measured during a periodic thickness test. The result comes in below the minimum prescribed thickness plus one-fifth of the tank's original corrosion allowance. Under 49 CFR 180.407(i)(1), what happens to that tank's testing schedule going forward?

49 CFR 180.407(i)(1)

This exception is a safety escalation rule: the thinner a tank's walls get, the more often it has to prove it's still safe. Knowing that thinning tanks move to annual testing (rather than staying on the standard 2-year cycle) is exactly the kind of federal retest-requirement detail tested on the tanker endorsement knowledge exam.