may encounter problems that may compromise the aircraft's safety, which are not anticipated or detected in prototype testing stages. The aircraft design is thus compromised. The regulators will now issue an airworthiness directive to the type certificate holder and to all owners globally. The directives normally consists of additional maintenance or design actions that are necessary to restore the type's airworthiness. Compliance is mandatory and thus if an operator does not comply with an AD, then the datum aircraft is not considered airworthy and further operation of the affected aircraft type would be unlawful, making the operator liable to legal action by the relevant national aviation authority, and rendering null-and-void any of the operator's insurance policies relating to the type, such as hull loss and accident third party coverage. ADs may also be raised with changes of the local or global aviation rules and requirements, e.g., the requirement to fit armored cockpit doors for all passenger airliners past 9-11. The certifying authority issues an AD when an unsafe condition is found to exist in a product (aircraft, aircraft engine, propeller, or appliance) of a particular type design. ADs are used by the certifying authority to notify aircraft owners and operators of unsafe conditions and to require their correction. ADs prescribe the conditions and limitations, including inspection, repair, or alteration under which the product may continue to be operated. Service bulletins With increasing in-service experience, the type certificate holder may find ways to improve the original design resulting in either lower maintenance costs or increased performance. These improvements (normally involving some alterations) are suggested through service bulletins to an aircraft owners/operators as optional (and may be extra cost) items. The owner/operator shall exercise their discretion whether or not to incorporate the bulletins and report the decision to the regulatory authority of the state of the aircraft registry. Sometimes SBs can become mandated by relevant ADs. Changes to type certificate Often the basic design is enhanced further by the type certificate holder. Major changes beyond the authority of the service bulletins require amendments to the type certificate. For example, increasing (or decreasing) an aircraft's flight performance, range and load carrying capacity by altering its systems, fuselage, wings or engines resulting in a new variant may require re-certification. Again the basic process of type certifications is repeated (including maintenance programs). However, unaltered items from the basic design need not be retested. Normally, one