Heathrow Airport Authentication
Heathrow runs a two-layer authentication model, and both layers are closed to anonymous callers. Human access to the developer portal is federated to Microsoft Entra ID (Azure Active Directory) and only issued after a manual enrolment conversation with Heathrow. Machine access to the runtime gateway at api.heathrow.com is gated by mutual TLS - the Azure Application Gateway terminates every anonymous request with "400 No required SSL certificate was sent" before routing, so a client certificate issued by Heathrow is a precondition to reaching any operation. No OpenAPI is published, so no securityScheme could be derived; this profile is taken from the provider's own public prose plus observed gateway behaviour.
Heathrow Airport secures its APIs with mutualTLS and federated-sso across 2 declared security schemes, as derived from its OpenAPI definitions.