California Code, Revenue and Taxation Code - RTC § 1152

(a) The proportionate amount of time, both in the air and on the ground, that certificated aircraft have spent within the state during the 12-month period from January 1 through December 31 of the previous year immediately preceding the lien date as compared to the total time in the 12-month period from January 1 through December 31 of the previous year immediately preceding the lien date.

(b) Time in the air consists of flight time and taxi time within California's borders. Time in the air shall be based on the State Board of Equalization's “California Standard Flight Times” table in the most recently published Letter to Assessors that addresses intrastate and interstate standard flight times. These standard times shall be multiplied by the number of departures to and from the airports listed in the Letter to Assessors.

(c) Ground time is all time in the state that is not flight or taxi time. Ground time at each airport shall be reported on a summary basis by fleet type pursuant to subdivision (m) of Section 441. All ground time allocated to heavy maintenance that requires a certificated aircraft or scheduled air taxi to be removed from revenue service shall be excluded. An air carrier claiming an exclusion for heavy maintenance time shall identify such maintenance and supply sufficient documentation that will enable the assessor to confirm the amount of time the aircraft was not in revenue service. Routine line maintenance that does not require removal from revenue service shall not be excluded from time allocable to the airport.

(d) Time allocable to each airport is the amount of time a certificated aircraft or scheduled air taxi is on the ground at the airport computed pursuant to subdivision (c), plus the portion of incoming and outgoing flight time computed pursuant to subdivision (b).

(e) All time, both in the air and on the ground, that certificated aircraft have spent within the state prior to the aircraft's first entry into the revenue service of the air carrier in control of the aircraft on the current lien date shall be excluded from the time-in-state factor.

(f) This section shall become operative on January 1, 2020.