SB 18-03-06 Posted

[DeltaRomeo]

RV-12 (Powered with Rotax 912ULS) ALSO SEE Revisions and Changes
Replace the throttle return springs.

http://www.vansaircraft.com/pdf/serv...sb18-03-06.pdf

http://www.vansaircraft.com/public/service-rv12.htm#rev


v/r,dr

[rjtjrt]

I have just received from McFarlane Aviation the new sided springs that are colored Red for L/H spring Mc Farlane p/n 6822 and Blue for R/H McFarlane p/n 7235.
Can anyone confirm these are the same spring now required in SB 18-03-06, and are identical to and interchangeable with
Van?s Part No SPRING-00002-L/R-1?

[Special Delivery]

They are NOT the same spring.
The new spring mounts on the throttle shaft and I expect the new design will all but eliminate the occasional breakages we've all experienced.
Thank you Van's engineering for your continuing efforts to improve the RV-12!

[mike newall]

Shame the original spring is on 50-01 just to confuse.....

[todehnal]

After a quick look at this, it appears that these old pull type throttle springs are being replaced by torsion springs, rather than with the old pull type spring.
Is that correct? If so, this is a fantastic mod!! My guess is that a torsion spring will probably end throttle spring breakage.

Tom

[PilotBrent]

Jeez, just started my annual last weekend, and I'm actually falling behind in SB compliance already!

Actually, I'm very pleased Van's is upgrading these springs. I had two replacement springs fail in 2016 (reported to Van's) so it appears its been an ongoing problem. Glad to see they continue to support us.

[Piper J3]

Originally Posted by todehnal

After a quick look at this, it appears that these old pull type throttle springs are being replaced by torsion springs, rather than with the old pull type spring.
Is that correct? If so, this is a fantastic mod!! My guess is that a torsion spring will probably end throttle spring breakage.

Tom

Correct. These are motorcycle carbs, and when used in that application, a torsion spring on the throttle shaft is used to default the throttle to idle. For Rotax aircraft engine this new torsion spring will default the throttle to wide open.

[rvbuilder2002]

The new spring was conceived at Van's, and then designed under corroboration with the engineering dept of a local spring manufacturer so this should be the end to throttle spring breakage.
As the fleet begins using the new spring, be sure to notify Van's using one of the standard procedures detailed in the maintenance manual, if you have any type of service difficulty with them.

[John C]

How does the new spring force compare to the previous spring?

The original spring force was not sufficient to keep the carbs synced during throttle advance. I did replace the throttle/cables and they start out good, but then start sticking.

I finally resorted to the original Rotax springs. They worked well except reduce the effectiveness of the throttle vernier feature.

Hope the new springs exert more force than the original.

[Piper J3]

Originally Posted by John C

How does the new spring force compare to the previous spring?

The original spring force was not sufficient to keep the carbs synced during throttle advance. I did replace the throttle/cables and they start out good, but then start sticking.

The function of the throttle springs is not to keep the carbs synched, although they do somewhat provide this function. The springs are for safety – they advance power to full if a throttle cable breaks or comes loose.

The safety scenario goes like this… you’re flying along and one of the cables breaks or becomes disconnected. The offending carb now advances to full throttle with its safety spring and the carb that is still controllable follows the pilot’s input. So one carb is full throttle and the other is nearly so. You will probably notice some vibration and even worse vibration if you throttle back because one carb will stay at full power while the other carb is at reduced power. This is the indication that one throttle cable has malfunctioned and now you need to plan your decent to landing. No real problem – just advance throttle to full so both carbs are equal power and find nearest airport. Begin a slow decent to not over-speed the engine too much and then cut power to both ignitions to shut the engine off when you have the runway made. Best to pick long runway…

Because the Rotax 912 is really two separate power sources married to one crankshaft its important to never have a huge imbalance of power between the halves. This is why power is advanced to full instead of idle. Idle would require an immediate decent with the working carb brought back to reduced power to prevent the engine from shaking itself free of the airframe. Full power on both halves of the engine allow the pilot to plan and execute a safe landing.

So what makes the Rotax 912 operate as two separate power sources? The fact that it has two separate carburetors – one for each side of the engine. Lycoming and Continental engines have a single carburetor and so the entire engine follows good or bad.

Sorry for long post…