A Designated Engineering Representative (DER) is an individual who has been appointed by a regulatory authority, such as the Federal Aviation Administration (FAA) in the United States, to act on behalf of that authority in ensuring compliance with aviation regulations and standards.

A DER is typically an experienced aerospace engineer or technical expert who has been designated by the FAA to perform specific tasks, such as:

1. Reviewing and approving design data, test reports, and other technical documentation related to aircraft, engines, propellers, and other aerospace products.
2. Conducting inspections and audits of manufacturing facilities, quality control systems, and testing procedures.
3. Witnessing tests and inspections to verify compliance with regulatory requirements.
4. Issuing certificates of compliance or airworthiness, such as type certificates, production certificates, or airworthiness certificates.

DERs play a critical role in ensuring that aerospace products meet the regulatory requirements and safety standards set by the FAA or other regulatory authorities. They must have a deep understanding of the relevant regulations, such as the Federal Aviation Regulations (FARs) in the United States, as well as the technical aspects of aerospace design, manufacturing, and testing.

There are several types of DERs, including:

1. Designated Engineering Representative (Structures)
2. Designated Engineering Representative (Systems and Equipment)
3. Designated Engineering Representative (Propulsion)
4. Designated Engineering Representative (Electrical Systems)

To become a DER, an individual must meet specific qualifications and requirements, such as having a degree in a relevant field (e.g., aerospace engineering, mechanical engineering), having a certain amount of experience in the aerospace industry, and completing specialized training and certification programs.