California Code, Revenue and Taxation Code - RTC § 5367

If any person who is requested to file a statement pursuant to Section 5365 fails to file such statement by the time specified by the assessor, a penalty of 10 percent of the market value of the unreported aircraft shall be added to the value of the aircraft of such person which is placed on the current roll.

If the assessee establishes to the satisfaction of the county board of equalization or the assessment appeals board that the failure to file the statement was due to reasonable cause and not due to willful neglect, it may order the penalty abated;  provided, that the assessee has filed with the county board written application for abatement of the penalty within the time prescribed by law for the filing of applications for assessment reductions.

If the penalty is abated, it shall be canceled or refunded in the same manner as an amount of tax erroneously charged or collected.