October 22, 2021, ©. Leeham News: Last week, we looked into Continued Operational Safety and there specifically Safety Monitoring and Reporting. Now we look at the role Instructions for Continued Airworthiness (ICAs) play in preventing the air safety issues we talked about last week.
An important part of the Instructions for Continued Airworthiness, ICA, is how to maintain the aircraft in a continued airworthy state during its operational life. The aircraft is airworthy when it rolls out of the factory but it needs to keep this condition for its 25 years or longer productive life. How this is done is contained in the maintenance documents, but there is more to it than just producing a maintenance manual.
For an aircraft OEM, it’s not enough to design and produce the aircraft and sell it to the operator. He’s also responsible to ensure that the aircraft can be maintained in a safe airworthy state during its operational lifetime.
For this, he must develop documents and programs that ensure the aircraft is maintained professionally and that any finding around the aircraft’s safety, which requires changes to the aircraft, is carried out in a timely manner by the operator.
The OEM is required to develop and issue the Instructions for Continued Airworthiness, ICA, under regulations such as 14 CFR 23.1529 and 25.1529. The main ICA documents for the type are:
There will be no Design Type Certificate unless these tasks are completed or there is a clear path to the Regulator on how these will be completed before the first delivery of the aircraft. From a practical perspective, we need to draft these documents for utilization during our testing program to identify gaps and flaws.
Our aircraft is a Part 23 aircraft, where the rules are written for General Aviation aircraft. The standard maintenance program under the FAA jurisdiction is that a yearly inspection+maintenance shall be made combined with a 100 Flight hour inspection/maintenance.
But in certain jurisdictions (e.g. scheduled passenger service in the US) the aircraft operates to Part 121 which demands Part 25 certification. If we want to sell in the US we need to develop for later Part 25 recertification and our maintenance program and documents must fulfill this more extensive requirement.
When we develop the aircraft we have no historical data for our design but there is a wealth of data and know-how on how similar designs have performed during use. This is used in a grouping called Maintenance Review Board when defining how our aircraft shall be maintained. The board contains experts in the field from the regulator, operators, and industry bodies.
The maintenance program that we develop together with the Maintenance Review Board is documented in the Maintenance Review Board Report, which lists the minimum scheduled maintenance requirements for our type.
The Maintenance Review Board Report plus any additional tasks that are part of the type certificate, such as Certification Maintenance Requirements (CMR) and Airworthiness Limitations (AWL) are all condensed into the type’s Maintenance Planning Document, MPD. These items shall b approved by the regulator and furnished to all owners of the aircraft to ensure the product can be maintained in airworthy condition.
The MPD is used by the operator when he develops his maintenance program for the aircraft. Many times he has to add tasks that are required by his local Regulator before he can get the maintenance program approved, but the base is the MPD.
In the MPD each part of the aircraft has its own section and in these, the way to keep this part working without failure is prescribed.
For example, for the structure, there are prescribed inspections at certain Flight Cycle (FC), Flight Hour (FH), and Calendar (CAL) intervals. The MPD also contains a structure corrosion prevention program with inspections and tasks to be performed.
The engines are today maintained on condition, where for our turboprop engine the Exhaust Gas Temperature, EGT, is used as a state parameter to monitor the degradation of our core engine. There are also hard limits on how long highly loaded parts are allowed to serve in the engine, so-called Life Limited Parts, LLPs.
When the engine reaches an EGT or LLP limit it’s taken off wing and a limited hot part restoration is done in case of an EGT reaching its limit, or a complete teardown is done if LLPs must be changed.
Each system in the aircraft has its own maintenance schedule. A lot of systems are maintained on condition, where startup self-tests check if the system is still in operating condition. For others, certain parameters like fuel or hydraulic pressure can decide if it’s time for maintenance.
There is often some hard limit in addition, like for landing gears. With calendar time the gaskets in their oil damped legs will deteriorate and there is typically a 10 year limit for how long they are allowed to stay on the aircraft before a restoration is needed.
Items like tires, brakes, and wheels are inspected before each flight and there are wear marks and pins that show their condition. There are also hard limits on how many landings a wheel is allowed to take before requiring replacement or service. Similarly, if certain operational limitations are exceeded (like Gs in a hard landing) additional inspections may be required by the ICAs to ensure there was no damage sustained.
All this is gathered in the MPD which for a larger aircraft contains maintenance rules that result in thousands of task instructions. An operator uses these instructions as the base to compose a maintenance program for the aircraft that in turn must be approved by the regulator in the country where his operation is registered.
For our 19 seat Part 23 aircraft, we will perhaps have 500 or so tasks in our MPD to prepare it for jurisdictions that have a requirement for Part 25 levels of Instructions for Continued Airworthiness, ICA.