Airworthiness Directives – what, why and how?

I’m sure you have come across the concept of airworthiness directives. Among all the various maintenance requirements for aircraft, airworthiness directives – or ADs, as they are often referred to – seem somehow most important. Many CAA inspectors or leasing consultants will be very keen to take a look at an aircraft’s AD status at the very start of their inspection. Let’s take a look at what ADs are, why they are being issued and how they should be implemented on an aircraft.

Airworthiness Directives

What are Airworthiness Directives?

An Airworthiness Directive is a document issued by an aviation authority. By law (in case of Europe, by the requirements of EASA Part M and Part OPS) any operator is required to comply with the requirements of all ADs which are applicable to their aircraft. An AD is typically prompted by an unsafe condition of the aircraft. The responsible CAA may have learned about a given unsafe condition either as a result of an aircraft incident or accident, or from information obtained from the manufacturer of the aircraft and/or affected component.

An airworthiness directive will typically consists of 3 major parts, telling the operator:

  1. What aircraft (or engines, or other components) are affected by the AD
  2. What actions need to be taken in order to remedy the unsafe condition which has caused the AD (at this point the AD often references specific work instructions, such as a service bulletin)
  3. In what time frame those actions need to be taken, and whether they are repetitive

It is important to note, that an operator needs to follow airworthiness directives from a few sources. First and foremost, there are ADs issued by the aviation authority of the operator. So, if our airline is operating under a European AOC, they need to watch closely all ADs issued by EASA. This is not sufficient, however, as by law each operator must also follow all airworthiness directives issued by the CAA appropriate for the type certificate holder of the given aircraft or engine, or the country of manufacture for other components.

Let me try to explain this, as it may be confusing: each aircraft (or engine) has a document called a type certificate, which is a confirmation that the given aircraft conforms to all certification specifications under a given authority. This document is issued and “owned” by someone – this someone is normally the manufacturer of the aircraft, but this is not always the case (for example when an aircraft manufacturer goes bankrupt, but their aircraft are still flying). Of course, the type certificate holder is registered in a certain country and it is the CAA of this country which will issue ADs related to that given aircraft.

So, let’s imagine that our European airline is flying an Embraer 170 aircraft (Brazilian manufacture, so the type certificate holder is Brazilian) with GE engines (American manufacture, so the type certificate holder as American). The airline now needs to monitor all EASA airworthiness directives (because it’s an airline with a European AOC), all Brazilian airworthiness directives for the airframe (issued by the ANAC) and all FAA ADs issued for the GE engines. Compliance with all of those ADs is mandatory, unless…

…unless they contradict each other. This is rare, as most authorities copy each other’s ADs or simply adopt them as they are. It does happen however, and the Brazilian/FAA vs EASA case is a good example. The Brazilian authority has issued AD 2012-08-04 which asks that all oxygen generators be removed from aircraft lavatories. EASA did not adopt this AD. Hence, complying with this Brazilian AD under EASA regulations would render the aircraft not airworthy (it would not comply to certification specifications, you cannot just remove an aircraft component and keep flying). What should the airline do? Local regulations always take precedence, and our EASA airline will not comply with this particular Brazilian airworthiness directive.

Why do we need airworthiness directives?

Simply: because no one is perfect. Not even aircraft manufacturers. A vast majority of ADs is issued because of a design flaw of the aircraft. This can be a problem with the technical design (a structure element which is corroding faster than anticipated, a wire bundle which is rubbing on a piece of structure and can get damaged, a piece of software which is malfunctioning in certain situations, etc.) or with the maintenance procedure design (the need to perform a lubrication task more often, for example).

The foreseen unsafe condition may also be caused by a manufacturing fault for a certain batch of parts (wrong material used, or improper heat treatment applied).

In general, ADs are a form of constant safety improvement. Aviation authorities look at all aircraft accidents and determine the root cause. Also, in many cases, the unsafe condition is detected and rectified before an accident ever occurs. This is because of airlines reporting damage which should never occur detected during routine maintenance, or manufacturers reporting flaws in the subcomponents they receive from one of their factories.

The unfortunate setback of this process is that it is airlines who need to carry the financial burden of those modifications. Some are not very extreme, but there are others which require a lot of money for new components or cause very significant down time for the aircraft during such unforeseen maintenance. A discussion on whether or not manufacturers should be responsible for those costs is not really in the scope of this article, but it’s certainly worth thinking about for a moment…

How to implement an airworthiness directive?

The main point of an airworthiness directive is for it to be implemented by an airline. There are thousands of issued ADs and new ones become available every day. Sometimes, especially for small operators and start-ups, it can be a challenge to get their head around monitoring and complying with all of them.

Here’s a guide to implementing airworthiness directives in 4 simple points:

  1. Monitoring of all relevant airworthiness directives. This can easily be done via the websites of the FAA or EASA. They also have a feature in which you can sign up to receive all or selected ADs directly to your inbox, as soon as they are published.
  2. Determining which ADs are applicable to your aircraft or engine. If an AD is component related, determining whether this component is (or could be) installed on your fleet.
  3. For all applicable ADs – a thorough analysis of the AD. As a result of the analysis you need to know exactly:
    1. Does this AD affect my specific aircraft, engine, components?
    2. If yes, what action do I need to do?
    3. When is the threshold for doing that action? Do I have a month, or perhaps 200 flight hours?
    4. Are there more requirements than just one? Perhaps an inspection first, and a component replacement later?
    5. Ordering the required maintenance from your MRO.

Sometimes the path above is very easy. However, in some cases, the contents of an airworthiness directive can be very complex. They may require to do a set of inspections and then determine further action based on those inspections. They may offer a terminating action which is voluntary as long as the airline performs a repetitive inspection procedure, etc.

In general, ADs have to be read, and read very carefully. Proper and complete understanding of the requirements of an AD is crucial to flight safety, so this is not to be taken lightly. When in doubt, you can always ask your CAA for advice and explanation.

Any ADs you had particular trouble with?

Posted in Airline Operations Regulations, Airworthiness, Airworthiness Management, Airworthiness of Aircraft, Airworthiness of components, Airworthiness Regulations, Flight Operations, Quality and Safety
10 comments on “Airworthiness Directives – what, why and how?
  1. Hi there, great job, well explained!
    Is AD deferrable? I mean, regardless of the level of ADs (eg, Mandatory, etc), can we comply a certain AD beyond the specific time of compliance stated in the documnet? is there any additional documents or approval needed to be processed before deferring such AD? Thank you.

  2. Mike says:

    Hi Edjohn, apologies for my late reply. I think you may be confusing ADs with SBs (service bulletins). There is no such thing as a “mandatory” AD because every AD is mandatory by definition. In general, you are not allowed to defer an AD by any means. However, you can create a so-called AMOC (alternative means of compliance) if you believe that defering some tasks will not adversely impact flight safety. Such AMOC must be approved by your aviation authority and by means of an AMOC you can change not only the time frame but also the way in which you intend to comply with the AD. However, in general, ADs are not to be deferred and must be complied with at the times specified in the AD.

  3. Ruslan says:

    Dear Mike, what about component ADs? F.e., if some component was manufactured by Chinese company and installed on Airbus aircraft operated by EU-registered airline. Should the operator check Chinese ADs or they will be adomted by EASA? Thank you very much for your reply!

    • Mike says:

      Hi Ruslan,

      Very good questions. I believe you are obliged to follow ADs issued by the state of the type certificate holder. This applies to the aircraft and engines / APUs. Regarding components those would again be ADs issued by the aircraft TC holder state. So no, you probably wouldn’t have to worry about Chinese ADs. Only EASA ADs issued with repect to the given part number.

      But always consult your quality manager, CAA or a lawyer when you’re not certain 😉

  4. chris says:

    what are 4 ways to generate a airworthiness directive

    • Mike says:

      Hi Chris,

      Not sure what you mean? An airowrthiness directive is generated by the aviation authority, EASA or FAA in case of EU or USA operators. No one other than the authority can actually generate an airworthiness directive.

  5. jayanti prajapati says:

    Hello Mike,

    Thanks for detailed explanation.
    Could you write related to structural repairs, it would be great for us.
    Also I have one question
    If repair is performed in AD affected area, do we need to get AMOC for all repairs or only for Major Non-SRM repairs ?
    Singapore aviation authority(CAAS) is asking to provide AMOC for all Major repair performed on the aircraft.


  6. Daniel says:

    Hi Mike,

    Which exactly is the Law that indicates the owner/operator must follow all airworthiness directives issued by the CAA appropriate for the type certificate holder or the country of manufacture for other components??

    Is it an ICAO law?

  7. Mahadeva says:

    Very good Article Mike.

    ADs will be revised during the process?
    ADs can be superseeded,when the solution is applied for serial aircraft?
    In which situation can the ADs can be cancelled?

    • Andrew says:

      Mike I want to know in relation to EASA rejection of the ANAC AD. Why does an operator need to monitor ad of a foreign aircraft when their authority is also monitoring? In other words I’m asking that if an operator receives an AD do the wait for the authority to approve it before accomplishing it?

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