New 912 MML Airbox Temp. Check Procedure

Assuming you are able to reliably get this temperature, then what do you do with it? I didn't see any specs on what is good or bad or an acceptable range. Did I miss something?

Disclaimer:

The following information is based on secondhand sources and informal discussions. We do not have firsthand knowledge of the details, and much of the content is speculative in nature, compiled from fragments gathered over time. This information is provided for general context only and may not be accurate or complete. BRP-Rotax does not share or endorse the following views expressed.

It is believed that Rotax and an unnamed aircraft manufacturer are currently in dispute over engine detonation issues. Each party is blaming the other, and while the situation is ongoing, it appears that the problem is isolated to one specific aircraft type and has not become a widespread issue.

Rotax attributes the cause of detonation to several installation and operational related factors, including a restrictive exhaust system with excessive backpressure, a poorly designed induction system, a propeller with too much rotational mass, and overloading the engine by operating at high power settings with low RPM. The aircraft in question also operates in a high-stress environment and is subjected to fairly extreme conditions.

On the other hand, the aircraft manufacturer believes that Rotax is at fault, pointing out that these detonation issues only began appearing around 2018–2019, suggesting that Rotax may have made a change that has resulted in the engine becoming more susceptible to detonation. 

In response to the dispute and issues, Rotax released Service Bulletin SB-912-079, which was initially published in November 2024 and then revised in February 2025. Updates were also made to the Installation Manual and Line Maintenance Manual (LMM) to reflect the new requirements. The bulletin itself explains the reasoning behind these changes:

"Field observations that are carried out as part of our obligations revealed an increased number of unexplained engine power losses, engine vibrations, and piston damage. In-depth investigations on various aircraft models have identified the main contributing factors and causes for the majority of these occurrences. The findings and insights of these investigations are hereby shared with all stakeholders. It is also pointed out that these guidelines and requirements must be observed."

The problem with this bulletin is that its implications extend far beyond the affected aircraft. The SB and resulting manual revisions technically apply to tens of thousands of aircraft that have been operating safely for years. In many cases, compliance with the bulletin is either extremely difficult or entirely impossible due to the nature of older installations.

For example, many aircraft do not use a Rotax airbox or have a carburetor heat system at all. In these setups, each carb typically has an individual air filter, and there is no means of measuring induction air temperature with or without carb heat. Some aircraft use alternative methods for carb heat, such as electrically heated carb bodies or coolant-heated blocks on the intake manifolds, which are not compatible with the monitoring requirements laid out in the bulletin and the LMM.

In the case of Tim, who operates a type-certified aircraft, there are effectively only two options for compliance:

(1) the aircraft manufacturer must issue a corresponding service bulletin requiring compliance, or

(2) Tim must apply for and obtain Alternate Means of Compliance (AMOC) from CASA.

Neither of these options is realistic in his case, which effectively forces him to ignore the bulletin and LMM updates, placing him in technical non-compliance. The best course of action in this scenario would be for Tim to contact his local aviation authority, explain the situation, and seek formal guidance or an exemption.

For owners of existing Rotax-powered aircraft that do not have a Rotax airbox or carb heat system, these new requirements may be signed off as "N/A" (Not Applicable), *subject to the interpretation of the local aviation authority and the aircraft’s operational context*

However, new aircraft manufacturers and builders are still expected to comply fully with the updated bulletin and installation instructions. This involves significant design consideration, testing, and verification, such as fuel flow testing, hot weather operational testing, detonation resistance evaluation, and so on. While this may seem like an additional burden, it is arguably a good thing, as it will help ensure future aircraft are built to a higher standard. Historically, we've seen installations with questionable induction and exhaust layouts, poor airflow management, and inadequate testing in varied conditions.

Turns out, designing and building airplanes isn’t so simple!!!..... However, don’t tell that to the folks on YouTube who are making “helicopters” and “airplanes” out of cardboard, wood, and bicycle parts that completely fall apart when they try and taxi down the "runway/roadway/driveway/dirt road", etc.….. We find those videos very entertaining, and we wish them all the best!

Ultimately, while the current dispute seems to remain, and it’s unclear where the true root cause lies (probably a combination of everything), the updated SB-912-079 serves as both a reminder and an opportunity for the industry/builders to improve safety and consistency in Rotax engine installations.

I fully agree to Rotax Owner. Detonation isn't caused by Rotax engine design which has been proven over many decades, thousands of engines and millions of run hours all  over the world. It's caused by poor Manufacturing, alterations from owners and mechanics that think they know more than Rotax or don't have to follow proper engine protocol and things as simple as following and installation manual  or using an annual checklist. The items listed as the cause in RO's post can certainly attribute to detonation.

I won't say a part can never fail, but most of the time it had help.

All this said I'm still not sure why they want the air temp checked and how it will help owners.

James Ott wrote:

Assuming you are able to reliably get this temperature, then what do you do with it? I didn't see any specs on what is good or bad or an acceptable range. Did I miss something?

As I mentioned in my previous post, The MML refers to the IM for the acceptable values. On Page 8 of Chapter 72-60-00 of the IM, the limit is stated as 8°C (14.4°F) above ambient (outside) temperature, at an atmospheric pressure (QFE) above 950hPa. (28.05 in.Hg)

This atmospheric pressure requirement also means that it becomes effectively impossible to perform a 200h inspection of the engine at an elevation with a pressure altitude above approximately 1500 ft.

For my opinion, referring to "ingest - ambient difference" is completely pointless and ineffective. But I understand that during a "dispute" or a conflict people do not think clearly, sometimes. Engine do not see "ambient". It sees ingested air temperature, its pressure and exhaust backpressure. If the operating temperature, as a combination of oil temp, coolant/head temp, cylinder temp and EGT, are within limits, ambient doesn't matter at all! It doesn't matter that 1in, 1ft or 1yd from the engine there is 20F or 100F!

Therefore, in my opinion, Rotax should mention just a temp and a pressure of the ingested air. Measured on a filter, or in the airbox.

ps. maybe flying at Dead Sea level in the summer would be out of those limits....... even with the mentioned difference being close to zero....

Stefan.