Bjorn’s Corner: The challenges of airliner development. Part 3. Certification rules overview.

By Bjorn Fehrm, Henry Tam, and Andrew Telesca.

May 14, 2021, ©. Leeham News: The certification rules for different aeronautical vehicles are specific for each airworthiness jurisdiction. It means each country has its own rules for all the categories we listed in the last article. However, with the support of the International Civil Aviation Organization (ICAO), there is harmonization in many areas, especially around airport and airline operations. For airworthiness rules, most originated from dominant jurisdictions like the US FAA, whose rules have inspired the Canadian and European rules as well as many others. 

As new technologies come to market, or new safety information is learned through testing, accidents, and incidents, the major regulatory agencies of the world constantly adapt with some rules harmonizing across jurisdictions and other rules diverging in different directions. It is an ever-evolving regulatory landscape that supports some of the safest transportation in the world, but it comes at a cost for companies as what was done in the past cannot always be expected to be accepted in the future, and the more global your desired market the more rules you need to understand and the more constraints your design will need to satisfy. 

Figure 1. The EASA certification rules website. Source: EASA.

The main Certification rules

We use FAA as the example for how certification rules are partitioned and distributed and we show how the EU has transposed these into their ruleset, focusing on those areas with greatest impacts to an aircraft manufacturer.

FAA Part EASA Subject Type of Impact Example
23, 25, 27, 29 23, 25, 27, 29 Initial Airworthiness for small airplanes, transport airplanes, small helicopters, and transport helicopters respectively These sections (with only one typically applicable to any single product) govern the majority of the design safety rules to ensure design airworthiness at the time of initial type certification. 25.571 – Damage Tolerance – “An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane. […]”
33, 35 E, P Initial airworthiness of major sub-products (engines and propellers respectively) As with the aircraft level rules, these parts govern the airworthiness of sub-products considered significant & specialized enough to warrant their own full airworthiness certification. 33.49 – Endurance Test – “Each engine must be subjected to an endurance test that includes a total of 150 hours of operation […]”
TSO ETSO Minimum performance specifications for aircraft articles These rules establish minimum performance standards for aircraft articles from cockpit recorders to seats. While not a full certification (meaning they cannot be installed on a specific aircraft without additional airworthiness approvals) these standards support suppliers of aircraft parts/systems with product approvals that help streamline their incorporation onto various products. C63e – Airborne Weather Radar Equipment – This allows for the use of RTCA Document DO-220A to establish the minimum performance standards for such equipment.
21 21 Procedures This is the part that explains what a Type Certification means, which rules need to be applied to receive one, how to control production to achieve airworthiness approval, what your obligations are as a TC/PC holder, and a myriad of other important information. It also governs issues such as when new or changed regulations do/don’t have to be incorporated into a product’s design. 21.17 – Designation of Applicable Regulations – “Except as provided in §§25.2, 27.2, 29.2, and in parts 26, 34, and 36 of this subchapter, an applicant for a type certificate must show that the aircraft, aircraft engine, or propeller concerned meets […]”
34, 36 34, 36 Environmental These parts contain rules for emissions and noise, two key public interest areas for all aircraft designs. A36.2.1.1 – Noise Certification Test and Measurement Conditions – “ This section prescribes the conditions under which noise certification must be conducted and the measurement procedures that must be used. […]”
91, 107, 121, 135 Air Operations
(965/2012), Unmanned (945/2019) 
Operational Rules (general aviation, unmanned systems, airline operations, small commuter operations respectively) These parts contain rules that the operators must comply with depending on how they intend to use aircraft. Many of these rules can only be met with specific equipment or provisions on the aircraft, so while failure to meet these rules won’t prevent receipt of the type certificate, it may result in a failure to meet customer expectations. 91.715, no person may operate a civil aircraft unless it has within it the following: […]” 

121.313(b) – “A windshield wiper or equivalent for each pilot station.”

135.364 – Maximum flying time outside the United States – “[…] no certificate holder may operate an airplane, other than an all-cargo airplane with more than two engines, on a planned route that exceeds 180 minutes flying time (at the one-engine-inoperative cruise speed under standard conditions in still air) from an Adequate Airport outside the continental United States unless the operation is approved by the FAA in accordance with Appendix G of this part, Extended Operations (ETOPS)”


This list is not exhaustive, but it is a useful place to start our coverage.

Other areas that impact product design/production include Part 5 for Safety Management Systems; Part 11, which includes rules on how to seek exemption from the regulations, Part 39 governing airworthiness directives when an immediate safety concern is identified in the current fleet, Parts 43/45 addressing maintenance and part marking, Part 61 covering type ratings for pilots (the influence of which everyone will remember from the 737 MAX), and Part 139 for airports, which can influence everything from wingspan due to separation & gate requirements (ref. the 777X folding wingtip) to takeoff performance requirements and vibration loads induced by runway quality. 

A full listing of regulations can be found at for the EASA and for the FAA.

In the next Corners, we will dive a bit deeper into the most important rulesets for an air vehicle project.

9 Comments on “Bjorn’s Corner: The challenges of airliner development. Part 3. Certification rules overview.

  1. Two of the issue areas that there is (in my opinion) to much latitude and not following up would be the 787 Battery and the PW Fan blade failures

    In the case of Li Ion battery, Boeing was allowed to set its own test standards as there were none before. They could have not done worse (the nail through the battery as a test was absurd as there was no science behind it at all).

    The P&W blade inspection as a new technology was allowed as an interim – But it became a never finalized process and the inspectors were not trained to the echo standard and we had those failures.

    I have no problem with folding wingtips but I don’t want it automatic and I want numerous interlock safeties.

    There also needs to be room for innovation but the correct process is not to let a mfg (any mfg) set the standard but per the Li Ion battery, it was turned over to the Radio Technical Committee to do that and they did.

    The no standards Boeing had vs what the RTC came up with (all valid for a high quality Li Ion battery) was unbelievable in the failures of quality in production alone.

    It was a poster child for how not to do it.

    • Bjorn:

      In regards to Certifying the A400 and the KC767, what is the reason for doing that?

      Its brought up a bit down by Keith but I am not sure he has the right reason cited?

  2. It is a good start – but there are a couple of missed references: should contain the ICAO link which actually links all the regulatory community as most of the countries are part of the Chicago convention – some earlier and some later signatories – and they all pledged to aviation safety –
    Certification is important because in the aviation industry without certification and mostly design of an aeronautical product for certification – that product will not be able to be installed/operated – therefore any and all design engineers in aviation should be aware of the standards they have to meet in order to certify their product – and this is an issue present in most OEMs – there is no understanding of the requirements regulating the product for certification… Aircraft Certification is not only the responsibility of the regulators – but is a shared responsibility of all participants in this industry in order to have a certifiable and operable aeronautical product…

    • While I agree that there are other important references needed for those directly involved in the development and certification of aeronautical products, ICAO, RTCA and SAE Aerospace publications, for example, are not available free of charge and providing a link would not be worthwhile in my opinion.

      The FAA’s Regulatory and Guidance Library (RGL) at is by far the most accessible and useful resource available.

      EU regulations and EASA CS, AMC, etc are a lot more difficult to navigate and I don’t think there is even a useful way to locate deviations, equivalent safety findings or special conditions (if there is, please post a link).

      It is worth noting that the FAA has introduced what I believe will eventually replace the RGL, the Dynamic Regulatory System at It has has certain advantages, particularly when looking for older amendments of the FARs, although the RGL is quicker to navigate.

  3. I do not see the past specs as an issue for the future builds.

    The issue is that you have past designs that live past their era. The updated systems are the future but its what you do with the old ones?

    As the old design does not have a sunset period you are allowed to grandfather in. The 737 is the eye catcher but the 747-8 was base on that as well.

    If you upgrade it then should you not meet the current criteria? And if you can’t, then no upgrade. Or the cost to do so means you come out with a new aircraft instead of the never ending modded 737 (and 747) though clearly they have hit their limit though the 737 MAX is here for some time (unless the -10 is actually build and then a whole lot of changes)

    The 777 is also being scrutinized for is it up to snuff (good thing)

    And while seemingly minder, there should be standardization on system ops like Auto Throttle that works the same across all mfgs.

    Boeing has a poor one that does drop out in the background that NTSB has flat stated should be changed but has not been.

    • Regarding the comment:

      “If you upgrade it then should you not meet the current criteria?”

      The FAA, EASA, Transport Canada, etc have each introduced what is commonly referred to as the Changed Product Rule (CPR) and associated guidance material which is used to classify design changes and determine the appropriate certification basis.

      Recall that design changes are constantly being introduced to meet customer requirements, comply with operating rules or other mandates (ADS-B, CPDLC, ADT, etc), correct deficiencies, replace obsolete parts, reduce manufacturing cost, etc. It would not be practical to adopt the latest regulations for all design changes and mandating such could in fact reduce safety by discouraging OEMs from introducing improvements.

      FAA Advisory Circular 21.101-1B is a useful reference; it is available from the Regulatory and Guidance Library at

      It is worthwhile noting that the operating rules can introduce airworthiness requirements over and above what is required to obtain a Certificate of Airworthiness (C of A). For example, in the United States, commercially operated airliners originally certified with 9g passenger seats must have 16g seats installed if manufactured after 27 October 2009 (reference FAR 121.311(j)).

      • “commercially operated airliners originally certified with 9g passenger seats must have 16g seats installed if manufactured after 27 October 2009”

        Of limited value as the fuselage remains designed for 9g.
        ( lots of papers around on how 16g seating fits 9g frames.)
        IMU the reason why 737 like to “give” just ahead and behind the center wingbox discontinuity. A killer.

        For major revamps ( like from Classic to NG) of an airframe such holdovers should not be condoned.

  4. From my long experience I conclude that ‘certification’ is troubled because it segregates from reality which rewards integration.

    ‘Certification’ is widely regarded as an extra without justification, when in fact safety should be an integral part of design and development – only idiots want a product that does not work reliably.

    They can be shunned, short of that reputation matters, prospective passengers can get advice from independent raters as done with bonds and insurance. Indeed, in my helicopter days oil companies hiring even experienced operators for offshore work audited operators because regulatory approval was not adequate in their

    ‘Certification’ often warps design negatively, the now old regulation fomenting MCAS on the 737MAX may well be one. (It first applied to the 767 in Boeing’s airplane line, a triplex pushover computer system was prepared but not needed as a few vortex generators sufficed.)

    It would be better for a product design effort to get independent review, as Allied military forces may now be doing by requiring civilian certification of transport aircraft as a base. (KC46A and C-130J come to mind, in the latter case Lockheed Georgia now has a good base for a civilian edition. (Military then add unique requirements onto the base and substantiate those somehow.))

    (My experience includes regulatory design approval authority designation in two countries, helping sift through a wreckage, much reading on safety, persevering despite obstruction by jerk or worse managers – sometimes losing jobs for that, design and testing including on civilian Hercules, and airline operations.

  5. Funny how when it gets down to real tech and hard reading the comments go away.

    If it were for bash the MAX comments we would have very few.

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