<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified">

    <xs:element name="FAA">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="airplane" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>


    <xs:element name="airplane" type="airplaneType"/>

    <xs:complexType name="airplaneType">
        <xs:sequence>
            <xs:element ref="altitude" />
        </xs:sequence>
        <xs:attribute name="tailnum" type="xs:ID" use="required" />
    </xs:complexType>

    <xs:element name="altitude">
        <xs:complexType>
            <xs:simpleContent>
                <xs:extension base="xs:integer">
                    <xs:attribute name="unit" type="unitType" use="required" />
                </xs:extension>
            </xs:simpleContent>
        </xs:complexType>
    </xs:element>


    <xs:simpleType name="unitType">
        <xs:restriction base="xs:string">
            <xs:enumeration value="feet" />
            <xs:enumeration value="meters" />
        </xs:restriction>
    </xs:simpleType>

</xs:schema>

