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.

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?