If you had to do this as an enumeration, I'd use the following:

<xsd:schema elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <xsd:element name="airportType">
        <xsd:simpleType>
            <xsd:list>
                <xsd:simpleType>
                    <xsd:restriction base="xsd:string">
                        <xsd:enumeration value="civil"/>
                        <xsd:enumeration value="military"/>
                        <xsd:enumeration value="active"/>
                        <xsd:enumeration value="inactive"/>
                        <xsd:enumeration value="permanent"/>
                        <xsd:enumeration value="temporary"/>
                    </xsd:restriction>                  
                </xsd:simpleType>               
            </xsd:list>
        </xsd:simpleType>
    </xsd:element>
</xsd:schema>

Then you would have:

<airportType>active civil permanent</airportType>