type of device that can be used to check the temperature difference in the intake system

 Excellent question!

We did some digging and discovered the following information:

The P/N listed in the MML on page 18, Chapter 12-20-00, is 222 421; however, this is not correct, as that part number refers to the temperature sensor used on fuel-injected iS engines and does not fit the carbureted airbox.

The 914 does have an airbox temperature sensor, and its P/N is 966 435. However, you will not be able to connect a temperature gauge to it because it uses a dedicated connector designed exclusively for the 914 TCU.

Essentially, Rotax does not offer a temperature sensor for the carbureted airbox, yet they still require you to measure the airbox temperature. This is an oversight in both the MML and SB-912-079 R2 titled “Essential information regarding engine installation, operation, and maintenance for ROTAX® 912 (Series) Aircraft Engines.”

The only way you can currently comply with these instructions is to source a K-type temperature sensor with the same thread pitch and size as the bung on the 912 carbureted airbox. There is no Rotax part number available for this sensor.

Rotax has been made aware of this issue and oversight, and hopefully, a proper solution will be released soon, ideally a sensor that Rotax can provide as an approved part.

Reference: 912ULS

The instruction to "check the temperature difference in the intake system" is very close, if not actual BS.

Common sense needs to be applied.

You can use almost any cheap fridge/weather digital thermometer, with a long lead, to check the undercowl air temperature in the vicinity of your non airbox carburettor inlet/filters. Should the temperature be of concern, increase air flow to this part of the cowling/carburettors. 

You can use the same devise (or a second one) to check the inlet air temperature on your Rotax/Home Build Air Box inlet temperature (which should be the same as ambient).

Assuming the airbox anti ice system (if fitted) is working correctly ie not leaking hot air into air box (a mechanical observation/adjustment, if required) your carburettors will be taking in ambient or near ambient temperature air.

Rotax recommended Max 24 C - 30 C .  I assume the latter for Take-off & Climb.

In certain parts of the World (eg Australia where I live) ambient temperatures can exceed 45 C on the ground with 30+C being almost every day in summer- temperatures at altitude rarely get above 20C  . The only thing you can do about this is to fly early/late in the day when temperatures are usually below 30 C .😈

It is important to recognise the most important areas of icing in a carburator.  The temperature range of ambient and the dew point are the key factors.  At 20 to 25 C and damp air can be a risk.  Flying in Oregon USA or most of the UK presents a much higher risk that a very dry climate.  Flying in clouds or light rain also increases risk.  

Remember, the carburator by design uses a venturi to create a low pressure zone by increasing air speed within the body.  This will drop the ambient temperature to a freezing point if enough moisture is in the air.  

So for Sean his risk area would perhaps be more at altitude and flying into damp air, clouds, anything that raised the dewpoint enough.  That 20C can become 0 inside the venturi, the result is ice.  

There are a  number of devices made for certified aircraft systems to detect ice, most are rather expensive.  The point being know the risky areas and try to avoid them.  My opinion.  The last comment is for all that think it cant happen to them,, think again, I have seen many cases of carb ice in experimental when flown in the wrong conditions.  

Attached is one version of carb ice conditions done by the NTSB, it is in Celsius. 

Cheers

RW,

I am confused by your foray into carburettor icing.

Unless I am very much mistaken, the topic/question is about how to determine the temperature of the air being ingested by the carburettors. This is in relation to Rotax advice (Max 24-30C) to minimise the risk of "..... detonation and pre-ignition.".

For sure, icing is certainly something that all pilots need to be aware of but is it actually part of how to find the carburettor inlet air temperature?😈

Sean, fair point.  The airbox Rotax supplies has a port that can be used for a temp sender.  This reference called out in the MML by CAO-AR

is for the required heat rise to melt ice in the carburetors.  This is not a reference to detonation issues.  

As a note in the type 914 the inlet air temperature from the turbo, when measured at the airbox, will dump boost to prevent detonation from the high airbox temperatures.  (done by the TCU controlling the wastegate controller) The 914 constantly runs much hotter inlet air temps and is not subject to carb ice by the heating from air compression.  

Cheers