The noun 'aircraft' is an uncountable noun. The singular and the plural possessive form is aircraft's.
EXAMPLES
One aircraft's owner has not paid the airport fee. (singular)
All of these aircraft's owners are delinquent in their fees. (plural)
Both the singular and the plural is aircraft.The plural possessive form is aircraft's.Example: All of the aircraft's inspections have been completed.
Singular possessive: secretary's Plural: secretaries Plural possessive: secretaries'
The plural is lads. The plural possessive is lads'.
The plural is relatives. The plural possessive is relatives'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural form is stepsisters. The plural possessive is stepsisters'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is inches. The plural possessive is inches'.
Studies' is the plural possessive of study.
The plural possessive is pals'.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.