Repair Strategy & Technical Solutions
What Is a Designated Engineering Representative (DER)?
A Designated Engineering Representative (DER) approves the repair data that makes a repair legal. Here is what that approval covers, and where it stops.
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A Designated Engineering Representative (DER) approves the repair data that makes a repair legal. Here is what that approval covers, and where it stops.
A Designated Engineering Representative — DER — is an engineer the FAA appoints to approve technical data on its behalf, within limits the agency sets for that individual.
In component work, that approval is what turns a repair scheme into a legal repair. It does not turn it into a finished part. The distinction decides more than most operators expect.
The gap that sends people here is narrow and specific. A component fails, the manual carries no repair limit for the damage in front of you, and the answer coming back is "replace only".
At that point the question stops being technical. It becomes a question of authorization: who is allowed to say that this repair is airworthy?
That answer has a shape and it has edges: what a DER can approve, what a DER cannot, and where the approval is recognized once the component goes back on a wing you may not own.
DER stands for Designated Engineering Representative: a private individual the FAA designates to examine technical data and find it compliant with airworthiness regulations, acting for the Administrator.
The designation is personal. It is divided by discipline, and it carries written limits. A DER is not a company, a certificate you hold, or a type of repair.
The rule that creates the role spells out how narrow it is. 14 CFR § 183.29 lists the categories one by one: structural, power plant, systems and equipment, radio, engine, propeller and others.
Each one repeats the same formula. The representative may approve information in that discipline "within limits prescribed by and under the general supervision of the Administrator".
Read that twice, because it carries two facts that matter on a shop floor. A DER approves only inside their own category. And a DER approves only inside the written limits attached to their designation, which are not the same for any two of them.
A DER approves data. Specifically, the engineering substantiation showing that a repair or alteration meets the applicable airworthiness standards: the analysis, the drawings, the stress work, the inspection criteria and the as-run process. The document that records it is FAA Form 8110-3, the statement of compliance with airworthiness standards.
What a DER does not approve is your component.
The workmanship, the tooling, the calibration and the return to service belong somewhere else entirely. They belong to the certificated repair station that performs the work.
14 CFR § 145.201 is blunt about the link between the two. A repair station "may not approve for return to service … any article after a major repair … unless [it] was performed in accordance with applicable approved technical data".
So there are two signatures on a component repaired this way, and they sign different things. One says the repair scheme is sound. The other says this unit was built to it. Neither can stand in for the other, and a package missing either one is not a finished package.
One word of caution about the fourth row. "OEM" is a commercial term, and a useful one in a purchasing conversation. It is not a regulatory category.
What the regulations recognize are holders of design and production approvals. Reading a commercial term as a regulatory one is how operators end up mis-drawing the line between what is permitted and what is merely customary.
Physically, often nothing. The same shaft is ground, the same core is restored, the same test is run. What changes is the paper trail and, more to the point, who has to accept it.
Your quality department has a veto and will use it. Some operator procedures require DER repair schemes to be reviewed and accepted in advance rather than introduced mid-repair, which turns a two-day question into a two-week one if nobody asked first.
And if the aircraft is leased, the lease decides before engineering does. Return conditions vary, and some name the data sources that will be accepted at redelivery.
If yours does, read that clause before the component is inducted. Not when the unit is finished and the redelivery inspection is booked.
Where the approval is recognized: your authority, and your lessor
An FAA approval is an FAA approval. Whether it travels depends on the arrangement in force between the FAA and the authority that matters to you, and there is no universal rule that covers every case.
For the European Union the position is stated plainly. In its guidance for design organizations, EASA confirms that repair data approved or accepted under the FAA's system are considered EASA approved, with no separate application required.
That holds for countries with a bilateral agreement in place with the European Union, the United States among them.
The same guidance draws one line, and it is worth knowing before a repair scheme is developed. Repair designs that require the production of new parts amounting to a design change are not eligible for acceptance under the implementation procedures of those bilateral agreements.
That exclusion is not a technicality. If the scheme on the table needs a new part made to a changed design, the bilateral will not carry it across on its own. Ask the question while the scheme is still being scoped, not after.
Outside the FAA–EASA relationship, acceptance follows whichever bilateral arrangement is in force. There is no shortcut: that one has to be checked case by case.
Some component families come back to this route again and again, because the damage they accumulate is repairable in engineering terms and unaddressed in the published data.
-Actuators and servos. Shaft resurfacing, bore and tolerance recovery, and bearing or seal work that falls outside published limits.
-Heat exchangers. Core and tube restoration and fin work, where the alternative is scrapping a unit that still performs.
-Landing gear subcomponents. Axle and bearing housing work that saves the main assembly instead of the fitting. A worked example on landing gear shows how the route runs on a specific assembly.
-Components whose support has been withdrawn. The case where there is no replacement to buy at any lead time, and a repair route is the only route.
That last one is growing, not shrinking. IATA's December 2025 assessment of aerospace supply chain bottlenecks puts the average commercial fleet age at 15.1 years.
Older fleets fly longer, on components whose support window is narrowing. The question of what happens when the manual runs out stops being occasional at that point.
The route matters less than whether it is open for your partnumber. Three questions settle that faster than a quote will.
1. Is my part number inside the station's rating and capability? Capability is specific. "It can be evaluated" is not the same answer as "it is on the capability list". The questions that separate a shop that can from a shop that says it can are worth asking before the unit ships.
2. Does approved data for this damage already exist, or does it have to be developed? These are two different schedules. Asking which one you are on is the single most useful question in the conversation.
3. Which authority, and which lessor, has to accept the release? Answer this first and the other two get easier.
If the route fits your case, the scope of APAS DER repair capability sets out which component families are covered and how a package is assembled.
And if what you are short of is not authorization but time, that is a different problem with a different answer. Where the weeks in a component repair actually go takes that one apart stage by stage.
Is a DER repair the same as a supplemental type certificate? No. A DER finds that specific technical data complies with the applicable airworthiness standards, and records that finding on FAA Form 8110-3. A supplemental type certificate is a design approval the FAA itself issues, with its own application and its own scope. They are different instruments and they are not interchangeable. A repair scheme approved through a DER does not become a design approval, and does not carry the standing of one.
Can any DER approve my repair? No, and this is where schemes stall. Designations are granted by category — structural, systems and equipment, power plant, engine, propeller, radio and others — and each one carries written limits specific to that individual. A hydraulic actuator and an avionics unit do not fall to the same designation. Some data approvals sit outside any DER's delegated authority entirely and go to the FAA directly.
How long does it take to get DER-approved data? It depends almost entirely on one thing: whether approved data for that damage already exists, or whether it has to be developed and substantiated from scratch. Ask the shop to tell you which of the two you are on before you accept a turnaround estimate. A quoted number that assumes existing data, applied to a case that needs new data, is not a delay — it was never the same number.
Is a DER repair permanent, or does it have to be revisited? It depends on the approved data. Some approvals stand for the life of the component. Others carry a defined inspection interval, a life limit, or conditions on subsequent repairs to the same area. That is part of the approved data, not a separate arrangement, and it should be legible in the release package you receive. If itis not clear, ask before the unit is installed rather than at the next shop visit.
