Bjorn’s Corner: The challenges of airliner development. Part 26. Maintenance planning

By Bjorn Fehrm, Henry Tam, and Andrew Telesca.

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. 

Figure 1. The Aircraft Maintenance Manual (AMM) for the Boeing 747, part of ICA for the 747. Source: Boeing.

Instructions for Continued Airworthiness (ICA)

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:

  • The maintenance documents. It should be issued in the form of manuals. In particular, maintenance actions derived from the safety analysis are of particular importance to prevent continued flight with latent failures. 
  • The control and operating instructions for the aircraft and its systems, including special procedures and limitations that prevent damage to the aircraft.
  • Servicing information for the daily and periodical service of the aircraft’s different parts like tank’s capacity, draining points, the fluids to be used in different systems, pressures for systems, lubrication points, tow instructions, jacking and level information, etc.  

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.

Maintenance documentation

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.

10 Comments on “Bjorn’s Corner: The challenges of airliner development. Part 26. Maintenance planning

  1. And people wonder why aircraft are so expensive!

    One that did catch my eye in this regard was the E190 incident (Air Astana 1388) when the maint activity reversed the aileron controls (wires) while the FBW controls (spoilers) were doing what they should).

    Really bad instructions apparently (and the crew did no cross checks on the feedback systems that showed the ailerons were workign backwards)

    they managed to figure out enough of a cobble to get a semblance of control and landed it safely (it never flew again, hull was severely damaged by the stresses of inverted and diving flight)

    • >And people wonder why aircraft are so expensive!
      I’ve been thinking about this when reading Scotts HOTR about Embraer’s potential turboprop.

      It used to be several hundred sales would make a middling successful aircraft. Now Embraer is not sure about launching because they project demand for “only” 2000 to 2400 aircraft of which they may get half. What has changed? Have certification requirements made aircraft development so expensive you need a mega-hit to cover costs?

      • It’s also that airfares are relatively so much cheaper now and part of that comes with lower costs of new planes and longer maintenance periods etc.
        I don’t know what a plane cost in say 1975 but production numbers were lower per month for a successful model which often had lower certification costs but but more time consuming labour intensive production.

        • Adjusted for inflation, today’s price for a new Vickers VC10 is ~£65,000 000 or $87,000 000, about the price of a B737.

          To “jbeeko’s” point, I would contend that yes, certification requirements are such a hurdle now that no commercial entity can afford the expense. Further, I would contend that this is a deliberate ploy and classic protectionism by incumbents. We see it in the auto and big pharma industries amongst others in which regulators are entirely captured by the industries they supposedly regulate and it is most pronounced in the European Union whose politics is so venally corrupted and lacks true democratic legitimacy.

          • “…most pronounced in the European Union…”

            That’s odd: one could argue that the monumental, self-serving, shortcut-taking entanglement between BA and the FAA — with all the accidents, exposés, lawsuits, Congressional hearings and bad press that it has recently generated — would be enough to put the USA firmly in first place when it comes to countries in which “regulators are entirely captured by the industries they supposedly regulate”

  2. Dear Bjorn,

    May I ask if you have had the time to give any thought to Diamond Aircraft’s eDA40 project? They are taking the proven DA40 platform and are equipping it with a seemingly mature powertrain from Electric Power Systems. They aim to be the first FAA/EASA part 23 certified electric airplane. It seems its payload/range envelope will be way better than that of the Pipistrel Velis Electro, so a step forward! And it’s a collaboration of industry veterans rather than start-up companies, so I’m guessing it could be something you might like!

    • Hi JohnB,

      the one area where battery based aircraft has a chance to work is in basic flight training, as the pupil often is exhausted before the battery is (after around 30-45 minutes). But the portraying of lower operating costs for the battery trainer is seldom the truth as in almost all cases the battery costs are not in the equation. You need to change a battery after around 1,000 charges which for a trainer that flies four times a day is every three months.

      The batteries are the most expensive part of the drivetrain so in the end, the operating costs are higher than a piston aircraft.

  3. once read (it may even have been on here) about an avionics manufacture that deigned a new component that would considerably improve the operation of passenger aircraft. The engineers spent some considerable time and the company’s resources on its development and presented it to an enthusiastic reception amongst the potential customers. Certification procedures required the company to then spend the next nine years in passing the necessary tests to enable them to sell the component and then a further number of years to get it into the market by which time the tech was now over fifteen years old and pretty much obsolete. Time had long rendered the business case for the project a write off. The original engineers had retired or moved on and the customers had forgotten all about the component from nine years ago. The executive relating this tale of woe explained somewhat sardonically that while the considerable number of “regulators” prospered on the back of the project his company now takes a different view towards investing in expensive innovation – i.e. they don’t bother!

    • based on your style of writing , I would think you made that up rather than ‘read it here’. The outcome seems so certain but specifics so vague….hmmm

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