The defense claimed that California Civil code section 3482 states "Nothing which is done or maintained under the express authority of a statute can be deemed a nuisance," therefore plaintiffs cannot claim that the runway noise is a nuisance. Source: Informal Hearing Brief of Defendant, City and County of San Francisco, May 21, 2019

The law says otherwise.

Section 3482 was rejected in the Court of Appeal in a 1979 lawsuit.  In that case, the defense's two claims were that

1) the federal government has totally preempted the regulation of jet aircraft noise, and

2) the use of jet aircraft....is an activity done ....under the express authority of various statutes, etc., or their equivalent, and therefore the noise resulting from such use cannot, under Civil Code section 3482, be deemed a nuisance.

The Court of Appeal, Second District, Division 3, California in Greater Westchester Homeowners Association, Civ. 51698, decided February 28, 1979, rejected both of the defense claims.

As one source said, the California Supreme Court concluded that the statutory sanction plea was unavailable to the municipal airport proprietor concerning acts, which, by their very nature, constitute a nuisance. The Court said these acts must be expressly authorized by the statute to ensure that the legislature intended to sanction a nuisance. In other words, while the statute designates the use of the runways for aircraft, it does not expressly authorize the excessive noise levels of the aircraft on the runways.