From Skeptic to Believer: Light Sport TBO is NOT Mandatory!

AUTHOR’s NOTE: This is a long piece and intended for those who have a strong interest in the subject, i.e., you’ve got a stake in this and are hearing what I am and have been about TBO being mandatory for Light Sport. No one has been able to prove to me it was so; so I did a documentation/requirements trace to establish a more solid footing on where the truth laid. Moreover, my CTSW is hitting the Rotax 912 15 year calendar TBO clock and it’s not a small thing to either replace my engine with about 900 hrs to full hourly TBO or take the airplane ELSA, which has financial and use impacts I don’t want. So, here’s what I know and why. If you want to push back on this, feel free but don’t contact me without solid regulatory documentation I can verify.

There have been various misunderstandings since Light Sport was birthed in the U.S. about how much authority the FAA surrendered to ASTM (The American Society for Testing and Materials, which is not an international standards body).   The answer is actually “none”, with the distinction that ASTM standards were accepted by the agency in lieu of aircraft design and manufacturing standards instead of aircraft design standards set forth in the Federal Aviation Regulations (FAR’s). I first became a Light Sport practitioner in 2009 when my wife and I bought a 2006 Flight Design CTSW and I started flying on my driver’s license vice a formal FAA medical.  I still own that same CTSW today; I’ve been a Light Sport pilot ever since and have been a Light Sport instructor since 2012.  So, I’ve been involved both with teaching pilots but certainly as an SLSA owner responsible for maintaining the airworthiness and operational ability of the aircraft.  

When I first bought in, the Rotax maintenance manuals specified you had to be factory trained on any Rotax repair (and that would include FAR owner-approved oil and spark plugs).  Here’s a link to the applicable page in the 2009 version of the Rotax Line Maintenance Manual that states that.

Reading that in a vacuum, you would think that this must be true especially since the common wisdom was at the time (and still reverberates in some places today) that the FAA had deferred all authority over to ASTM and so what Rotax says goes.  But it’s simply not true. (It’s not only because of who the regulator is but the “manufacturer” who determines everything about the aircraft is the aircraft manufacturer, not the engine manufacturer, though what the engine manufacturer says drives what must be done to maintain the engine.)

My proof is a 2015 “Schober” FAA legal opinion which states: “The short answer to your question is no, the special airworthiness certificate remains valid even if a mechanic or repairman has not attended the manufacturer’s specified training program. A manufacturer may not impose additional requirements that are not contained in the regulations on mechanics or repairmen. As we detail below, a manufacturer cannot compel a repairman to complete a specific training program to perform compliant repairs. Therefore, the aircraft’s special airworthiness certificate is not rendered invalid when an appropriately certificated mechanic or repairman performs maintenance on the aircraft without completing a training program specified by the manufacturer in the aircraft’s maintenance instructions.”

The same manual also refers to “time limits” and “Time Between Overhauls” (TBO), the latter being the main subject we’re examining.   In the Time Limit page (Section 05-10-00, page 5), the important part that we’re interested in is the second paragraph entitled “After reaching the time limit” which states: “NOTICE: after reaching this time limit, the engine has to be shipped to an authorized ROTAX overhaul facility.”  There are notes after this that also bear noticing: “NOTES: Regarding engine operating limitations, see the “limits of operations” in the relevant Operators Manual. Maintenance checks and replacement of defined components are required on this engine!  These procedures are described in chapter 05 and are required by the authority in order to ensure Continued Airworthiness”.  Note that the determination of airworthiness and what is required for maintenance lies with “the authority”, which is EASA in Europe and the FAA in the US. ( The latest Line Maintenance Manual Section 05-10-00 page 4 still talks about TBO being mandatory and page 5 still says it has to be shipped out after reaching TBO.)

That lends some weight to the idea that TBO is mandatory here.  But there’s two more things to consider, and the first is from Rotax itself.  Section 04-00-00 page 1 in the Line Maintenance Manual has two relevant inputs.  The first is from a block centered near the top of the page entitled “Approval”.  It states: “The airworthiness limitations section is approved by the European Aviation Safety Agency (EASA) in accordance with Part 21A.31(a)(3) and FAR 33.4”.   We’ll look at what FAR 33.4 says in a minute but there’s nothing in what we just read about approval by the FAA, just EASA. But even that is not as it first appears, because further down the “EASA approved” page is a block listing “Airworthiness Limitations” which says: “None. For the Rotax type engine 912 Series the airworthiness limitations are not applicable.” 

FAR 33.4 concerns “Instructions for Continued Airworthiness” and states: “The applicant must prepare Instructions for Continued Airworthiness in accordance with appendix A to this part that are acceptable to the Administrator. The instructions may be incomplete at type certification if a program exists to ensure their completion prior to delivery of the first aircraft with the engine installed, or upon issuance of a standard certificate of airworthiness for the aircraft with the engine installed, whichever occurs later.”  I will bring up the FAA’s “type certification” instructions for Light Sport later, but for now, just be aware that we need to look at it to see if there is import for TBO (and there is!)

For now, let’s look at another FAA legal opinion that pertains to the required maintenance, i.e., August 21, 2013 opinion answering questions posed by Charles Willette.  Mr. Willette “requested a determination as to whether information contained in the maintenance manuals for aircraft issued special airworthiness certificates in the light sport category (S-LSA) is mandatory.”  Here’s the answer from that letter, including a rather important footnote related to the above: “Part 43 does not mandate that a person specifically perform maintenance, alteration, or preventive maintenance solely in accordance with those instructions specified in a manufacturer’s maintenance manual. It also permits a person to perform such work in accordance with other methods, techniques, and practices acceptable to the Administrator. 

The FAA recognizes that some manufacturers have placed what they deem “mandatory” replacement or overhaul times in their maintenance manuals for S-LSA and that these provisions may be consistent with consensus standards accepted by the FAA. While following the intervals set forth in the maintenance manuals is an acceptable means of maintaining the aircraft under § 43.13(a), a maintenance provider may use another method that is acceptable to the FAA. The intervals specified in maintenance manuals for S-LSA, therefore, are not per se mandatory. Consequently, a maintenance provider may develop an alternative that is acceptable to the FAA and maintain an S-LSA in accordance with those provisions.”

There’s also a footnote to a section referring to another paragraph that is relevant to understanding this: “Section 43.16 refers to Airworthiness Limitations. A person performing an inspection or other maintenance specified in the Airworthiness Limitations section of a manufacturer’s maintenance manual must perform that work in accordance with that section or as otherwise specifically approved by the Administrator. Maintenance manuals for S-LSA do not have an Airworthiness Limitations section to which the provisions of this section would apply.”

So, Willette says that it’s ultimately up to the standards that the FAA sets.  If the FAA standards match the consensus standards, then the consensus standards apply.  If they don’t, then the FAA standards rule.  That’s a key point to remember, and exactly where the misinterpretation of how the consensus standards play in.

At this point, all the pointers are leaning away from the idea that any maintenance items are truly mandatory. That’s not the complete picture, though, and I’ll come back to that in a minute.  Now, there’s always something that gums up the works, and that thing is the Keller legal opinion issued also in 2015 (i.e., in July after Schober was issued in April of the same year).  There are two parts to the Keller answer, and both seem damming until you read them closely and include one more piece omitted from the opinion, i.e., what the actual type certification documentation for SLSA has to say.  The first piece of the Keller question and answer is this (with the important parts excerpted to save space and time):

“1. Is an S-LSA airworthiness certificate rendered invalid when mandatory schedules for overhaul or replacement of components are disregarded and a component or system has exceeded a life limit specified in a manufacturer’s maintenance manual? 

The short answer to your question is no, an airworthiness certificate cannot be “rendered invalid,” though it may be rendered ineffective. Under§ 21.190(b)(l)(ii) and (c) the FAA will issue a special airworthiness certificate in the light-sport category if the aircraft meets the FAA’s requirements. If maintenance on an aircraft is not performed in accordance with 14 CFR parts 43 and 91, the aircraft’s airworthiness certificate is ineffective under 14 CFR § 21.181(a)(3)—this would render the certificate neither appropriate nor current for the purposes of § 91.203(a)(1), which requires that an appropriate and current airworthiness certificate be in an aircraft when it is operated.”

The second piece is even more direct:

“2. Does the FAA consider such an aircraft operated as presumed above to still be airworthy and should the agency take no enforcement action against owners or operators of an aircraft operated with components and/or systems beyond time between overhauls (TBO) and/or outside manufacturer specified life limits as described in the manufacturer’s maintenance manuals.

The aircraft would not be airworthy if operated beyond TBO or outside the manufacturer’s specified life limits. Section 21.181(a)(3)(ii) states that “a special airworthiness certificate in the light-sport category is effective as long as the aircraft conforms to its original configuration, except for those alterations performed in accordance with an applicable consensus standard and authorized by the aircraft’s manufacturer or a person acceptable to the FAA.” An aircraft that is operated after components have exceeded life limits specified in the manufacturer’s maintenance manual or other procedures developed by a person acceptable to the FAA would not comply with § 21.181.”

Seems conclusive when read in isolation, but there are a couple of things to look at here. First, the manufacturer is Flight Design, not Rotax, and nowhere does it say anything in the FD maintenance manual about the Rotax manuals except “refer” to them. FD does not specify any engine life limits or that Rotax life limits must be specifically adhered to. Engine reliability typically follows a bathtub curve, with infant mortality being a major risk during new operation and engine wear accelerating at some point late into the engine’s lifetime. Where Rotax set the TBO on that curve is an unknown and we also do not know what assumptions were behind it, which may have been driven by other factors than reliability and/or safety. The only thing that would make passing the calendar limit immediately unsafe would be if TBO was set at the point where the reliability curve heads for the cliff, and no manufacturer in their right mind is going to set TBO at that point. It makes no sense to suddenly declare that you are no longer in the original configuration just because you have passed TBO if there is nothing in the engine that points to any component needing immediate replacement. Also, if wear results in an alteration, then the airplane is no longer in its original configuration as soon as it is flown and will never return to it. It’s a specious argument, especially when combined with the FAA’s stance that TBO is not mandatory for any Part 91 operator.

There is also another missing piece to consider I referred to earlier, i.e., guidance from the c certification instructions for initial issuance of a special light-sport certificate.

Here’s what the appropriate sections of 21.181 say:

“(1) Standard airworthiness certificates, special airworthiness certificates—primary category, and airworthiness certificates issued for restricted or limited category aircraft are effective as long as the maintenance, preventive maintenance, and alterations are performed in accordance with Parts 43 and 91 of this chapter and the aircraft are registered in the United States.

(3) A special airworthiness certificate in the light-sport category is effective as long as—

(i) The aircraft meets the definition of a light-sport aircraft;

(ii) The aircraft conforms to its original configuration, except for those alterations performed in accordance with an applicable consensus standard and authorized by the aircraft’s manufacturer or a person acceptable to the FAA;

(iii) The aircraft has no unsafe condition and is not likely to develop an unsafe condition; and

(iv) The aircraft is registered in the United States.”

So, the relevant points are that (1) the LSA must be maintained according to parts 43 and 91 of the FAR’s, and (2) the aircraft must conform to its original configuration, unless the manufacturer approves it.

I’ve talked a lot about what Part 43 says, so let’s look now at Part 91.  The regulation in Part 91 that controls maintaining an SLSA for operation is 91.327 (b): 

“(b) No person may operate an aircraft that has a special airworthiness certificate in the light-sport category unless – 

(1) The aircraft is maintained by a certificated repairman with a light-sport aircraft maintenance rating, an appropriately rated mechanic, or an appropriately rated repair station in accordance with the applicable provisions of part 43 of this chapter and maintenance and inspection procedures developed by the  aircraft manufacturer or a  person acceptable to the FAA;

(2) A condition inspection is performed once every 12 calendar months by a certificated repairman (light-sport aircraft) with a maintenance rating, an appropriately rated mechanic, or an appropriately rated repair station in accordance with inspection procedures developed by the  aircraft manufacturer or a  person acceptable to the  FAA;

(3) The owner or operator complies with all applicable airworthiness directives;

(4) The owner or operator complies with each safety directive applicable to the aircraft that corrects an existing unsafe condition. In lieu of complying with a safety directive an owner or operator may – 

(i) Correct the unsafe condition in a manner different from that specified in the safety directive provided the person issuing the directive concurs with the action; or

(ii) Obtain an FAA waiver from the provisions of the safety directive based on a conclusion that the safety directive was issued without adhering to the applicable consensus standard;

(5) Each alteration accomplished after the aircraft‘s date of manufacture meets the applicable and current  consensus standard and has been authorized by either the manufacturer or a  person acceptable to the  FAA;

(6) Each major alteration to an  aircraft product produced under a  consensus standard is authorized, performed and inspected in accordance with  maintenance and inspection procedures developed by the manufacturer or a  person acceptable to the  FAA; and

(7) The owner or operator complies with the requirements for the recording of major repairs and  major alterations performed on type-certificated products in accordance with § 43.9(d) of this chapter, and with the retention requirements in § 91.417.”

So this says that I have to have the aircraft maintained by someone in accordance with Part 43 (i.e., an LSRM or A&P), do a conditional inspection every 12 months, and go to the manufacturer for any alteration and get approval, which for a CTSW means getting an approval from Flight Design via what they call an MRA (Major Repair and Alteration, which is a manufacturer’s permission slip). There’s nothing here about TBO that really relates; it simply points back to Part 91 and 43.  There’s more about Part 91 in a minute.

So, now in bowing to both Keller and 91.327, let’s go see what we can find relating to the initial issuance of a Light Sport airworthiness certificate and if that will tell us anything about whether TBO is mandatory or not. In actuality, what one has to address is what’s in Keller since it seems relatively ironclad (if you disregard the rest of what I’ve presented) and is the latest legal opinion of the ones I’ve gathered.  I started looking for something that could talk to me about that initial Light Sport airworthiness certificate issuance, and I found it in FAA Order 8130.2J dated 7/21/2017 “Airworthiness Certification of Aircraft.  On page 9-4: (10)(c)(1), it states: 

“(c) Maintenance and Inspection Procedures.

1. Verify that the maintenance and inspection procedures address engine/powerplant maintenance. Overhaul procedures for the engine/powerplant are not mandatory. Engine/powerplant procedures may be incorporated entirely within the aircraft maintenance manual or by reference into a separate engine/powerplant manual (such as a manual from the engine OEM). If the engine/powerplant maintenance and overhaul procedures are in a separate manual, verify that the aircraft maintenance manual includes a reference to the engine/powerplant and overhaul manuals; the reference must specify the unique identification information for the manual.”

This definitively states that overhaul procedures for the engine/powerplant are not mandatory for initial issuance of an SLSA certificate then the “initial condition” limits of Keller are satisfied and, by its own arguments, i,e., since overhaul procedures are NOT mandatory for a Light Sport airworthiness certificate, i.e., how can there be a violation 21.181 if they are provided but you don’t use them?  

As to where you find documented evidence that the FAA policy is that Part 91 operators do not have to obey TBO, it’s in National Policy Notice N8900.410 Subject: “Clarification of Inspection and Overhaul Requirements Under Part 91”.  Section 1 (b) states: “Overhauls are Maintenance. By definition, overhauls are a form of maintenance, not inspection, and are not included in an inspection program. Overhauls are part of the maintenance program. Part 91 operators are not required to comply with a manufacturer’s entire maintenance program; as such, overhauls are not mandatory for part 91 operators.”  (That notice was written in 2017 and given a 2018 expiration date because the expectation was that it would be incorporated by then; it hasn’t been because the FAA database is being replaced by the DOT DRS database that doesn’t seem to contain it but where you can verify it was released and which working group did it.)

This policy is why operators of standard category aircraft operate and sell aircraft that have exceeded both calendar and hourly TBO intervals without concern.  (You might want to go look for the articles and presentations Mike Busch has been giving on this subject.). There is no practical reason for Light Sport aircraft to be operated differently than the rest of general aviation, and you can find plenty of folks testifying to Rotax 912’s meeting or exceeding TBO even when operating on 100LL.  It’s all in the maintenance and the real-time condition of the engine.

While I realize that many FAA FSDO’s and personnel are saying differently, I believe this is because most don’t know the history, haven’t looked closely at all the regs and policies, or care enough about Light Sport to really run the issue to ground. There is enough solid documentation and reason to say that TBO is not mandatory for SLSA owners, as I’ve shown. But the reality is that getting a blanket answer out of the FAA that takes into account all the facts has, so far for me, proven to be impossible. You’ve got to dig down into the details and assemble the whole picture or you wind up with nothing but hearsay and innuendo, which is what we have out there today; and unless someone with some political weight weighs in, that is not likely to change. The result will be that owners like me will be forced to either convert their airworthiness certificates to ELSA to avoid trashing a perfectly good engine or spend over $30,000 dollars (ready or not) on getting a new engine to hold onto the SLSA certificate. Taking the ELSA route avoids the immediate expense and gives me control over when I take both the “infant mortality/installation error” engine failure risk (and every major problem I’ve had with my Rotax has been maintenance induced) and when I spend the money to replace the engine. But going ELSA is not without both insurance and use impacts, and it’s not clear that some long term goals I had can still be achieved. Even sadder, the impact on the Light Sport market overall if TBO is mandatory is HUGE; every SLSA owner flying with an engine with a calendar life is working against its clock before having to replace or rebuild (with its inherent risks) a perfectly good engine that still has life left in it. Current LSA owners whose airplane have engines that have or almost have run out the TBO calendar life clock (regardless of engine hours and maintenance) can immediately subtract $25-$30,000 off their market prices; and the new guys will need to realize they may be up against the clock when they spend 150 grand or more for their new ride.  

If TBO is mandatory, then Light Sport is truly brain dead; and it’s only a matter of time before everyone knows it.