SB 18-03-06 Posted

[AirHound]

Perhaps a worthy candidate for a POH emergency procedure?

Originally Posted by Piper J3

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?

[John C]

The springs perform two functions.

The first, and probably most rare, is to advance one throttle to full power if the cable breaks.

The second, and day to day, is to take the slack out of the cables so that both carburetors perform in unison.

There is friction in the two cables. If you have a 100-200 hours or so and are using the lightweight springs, advanceing the throttle using the vernier control, you may see that one throttle arm sticks and releases. You can see the effect when using a Carbmate or Tecmate. You can feel it when flying. Replace the lightweight springs with the midweight or Rotax spring, the problem goes away.

After some wear in the throttle cable, the midweight springs do not overcome the friction. The rotax springs keep working.

I replaced the throttle cable and could go back to the lightweight springs for a while. I then had to go to midweight springs followed by Rotax springs.

Thus, back to my question several posts back, where to the new springs fit in the mix?

Try to balance the carbs without the springs. My experience is that the results will be erratic.

[rvbuilder2002]

The springs were designed to closely mimic the springs that have been supplied with the throttle control by McFarlane (hopefully minus the vibration induced fatigue failure).

The RV-12 fleet is pretty good size at this point and other than a higher failure rate of the springs than should be expected, they have generally given good performance.

The cables on the RV-12 throttle control are not much different than those used on motorcycles or bicycles, which have to be maintained to some degree.
If they are noted to be developing some friction that wasn't previously present, they can be lubricated with spray lube, and that usually restores them to the way the were previously.

[RFSchaller]

Beware of assuming throttle cable resistance is just a lube problem. One annual I listed a hard to move throttle as a squawk. Turned out that the cable strands had broke and balled up inside the cable sheath. I only had one or two unbroken strands operating the throttle.

[rvbuilder2002]

Originally Posted by RFSchaller

Beware of assuming throttle cable resistance is just a lube problem. One annual I listed a hard to move throttle as a squawk. Turned out that the cable strands had broke and balled up inside the cable sheath. I only had one or two unbroken strands operating the throttle.

I agree, but this type of problem should be easily detected if the inspection procedure detailed in the Maintenance Manual, and listed on the inspection check list is followed.

[Piper J3]

Originally Posted by RFSchaller

Beware of assuming throttle cable resistance is just a lube problem. One annual I listed a hard to move throttle as a squawk. Turned out that the cable strands had broke and balled up inside the cable sheath. I only had one or two unbroken strands operating the throttle.

Was this on a 12, and if so, at what place along the length of the cable?

[RFSchaller]

No, a Cherokee. I mentioned it only because it was a life lesson in not jumping to conclusions.

[AJSWA]

Just a question out of interest.

To the people operating 3 blade propellers on their RV12 aircraft, has anyone suffered a throttle spring failure?

[dbhill916]

Is there a typo in the SB? The title is SB 18-03-06, but step 4 specifies a logbook entry indicating compliance with SB 17-12-07.

[John Acardo]

What does everyone think about the new style throttle spring?