Carb springs best position options, full or idle?

Hi Paul,

For the clients I see they are just the opposite. Almost all of mine are set to open.

Hi Paul,

Great question! There are a lot of different opinions on this topic, so let’s break it down.

Rotax designs their carburetor springs to default the throttle butterfly to fully open if a throttle cable breaks. The reasoning is that in an emergency, full throttle is better than none, so the engine still produces power.

There’s no specific FAA or EASA regulation that says carburetors must default to fully open if a cable breaks or a control rod disconnects. But in general aviation, many aircraft are built with fail-safe designs, and those vary depending on the aircraft and the manufacturer.

Here’s how different aircraft behave when a throttle cable or control rod fails:

• Most Rotax-powered aircraft and some Cessnas default to full throttle.
• Some multi-surface aircraft (like floatplanes and amphibious aircraft) default to fully closed.
• Certain aircraft, like several Pipers, just stay at the last throttle position.

Now let’s talk about how this plays out in the “real world”, specifically with the Rotax 912ULS engine. This engine uses dual carburetors, each with its own throttle cable connected to a single cockpit throttle lever. If one cable breaks, one carburetor goes to full open, and the other stays where the pilot set it. That mismatch causes serious engine vibration (runs like a bag of hammers).

To stop the shaking, the pilot needs to manually push the throttle to full power to sync the carburetors. But that goes against instinct; when an engine starts shaking, most pilots naturally pull the throttle back, which just makes things worse. If the pilot doesn’t realize a cable broke, they’re now trying to manage a shaking engine and keep flying the plane. That’s a bad combo, especially during takeoff when options are limited.

Here’s an NTSB investigation into an accident that shows what can happen. I’ve attached the full NTSB report, but here’s the summary:

During a solo cross-country flight, the student pilot returned to the departure airport after the engine began to vibrate when power was reduced. She attempted three landings that were aborted due to excessive airspeed. During the fourth landing attempt, the airplane touched down fast and bounced several times on the runway. The student pilot aborted the landing and added full engine power to climb, but the airspeed decreased. The airplane collided with a tree and a building and was destroyed by impact forces.

Post-accident examination of the airplane revealed that one of the two throttle control cables had separated from its carburetor at the control arm. The cable separation from the carburetor resulted in the carburetor’s spring-loaded throttle control moving to full throttle while the second carburetor would have been at a position selected by the cockpit throttle control. When the pilot reduced the cockpit throttle control for landing, the carburetor with the broken cable would have continued operating at maximum throttle regardless of the cockpit throttle control position, which likely resulted in the engine vibration and the excessive airspeed for landing.

Metallurgical examination of the separated control cable revealed that a tightly focused clamping force on the carburetor cable attachment bolt resulted in fatigue failure of the cable.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be: The fatigue failure of the carburetor throttle control cable, which resulted in a partial loss of engine control.

This accident highlights how a carburetor defaulting to full open can complicate a pilot’s response, potentially contributing to confusion and poor decision-making under pressure.

Now, let’s explore the opposite approach: Throttle springs are reverse-positioned to close the throttle butterfly fully in the event of a cable break.

We talked to a SeaRey rep at an airshow who explained it like this: SeaReys are “multi-surface” aircraft; they can land on water or land, so their emergency priorities are different. In their case, full power after a cable break could be dangerous, especially on water. Unlike “land-based” aircraft, water operations don’t have brakes, so a plane that won’t stop could get out of control quickly.

Instead, SeaRey chooses to cut power when a cable breaks. The idea is to give the pilot a safer, more manageable landing and water operation. The rep put it simply: It’s easier to operate a powerless aircraft on water than one that won’t stop!

Their logic sort of makes sense and is hard to argue against, especially if you’ve ever had a stuck throttle on a boat…….No brakes?...Panic sets in FAST!

To give some contrast, here’s another NTSB investigation into an accident, this time involving an aircraft that defaulted to fully closed throttle during a cable failure. I’ve attached the full NTSB report, but here’s the summary:

The flight instructor reported that he and the pilot took off in the airplane to conduct a flight review and proceeded to the practice area. They conducted flight review maneuvers, emergency procedures, and the flight instructor demonstrated a power-on stall. The pilot took over the controls and when he added power, the engine did not respond. He manipulated the throttle several times, but the engine had reduced to idle power and would not change. The flight instructor initiated an emergency landing. The airplane descended to a nearby field and touched down. The nosewheel sank into a small ridge and the airplane flipped upside down and slid to a stop.

A postaccident examination of the throttle/choke splitter assembly revealed the single throttle cable, which runs to the cockpit, separated from the throttle/choke cable splitter assembly. A metallurgical examination revealed a lack of solder material where the input throttle cable connects to the terminal end of the splitter assembly. The lack of solder material left a void in the cable connection with the terminal fitting, which allowed the individual cable strands to fail in a progressive manner, until the cable eventually separated from the terminal fitting. A lack of solder was also noted at the output end of the throttle cable connection with the splitter assembly. The choke cable connections were normal.

The airplane maintenance manual indicates that the throttle and control cables need to be replaced every 5 years. In addition, the manual cautions that “all of the time limits…. must be considered when performing the 100 hour/annual inspection. Removal or overhaul of any these items must be entered into the aircraft technical logbook.” There is no logbook entry that explicitly states that the throttle and choke cables were replaced. Therefore, it could not be determined who produced the deficient solder joints or how long they had been in service.

Probable Cause and Findings:

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A partial loss of engine power due to the failure of the throttle control cable, which resulted in the loss of control of the dual carburetor system. The cable failure was a result of improper soldering at the throttle cable input terminal on the splitter assembly, which led to the progressive separation of the cable strands.

This case shows the downsides of a “fully-closed” throttle position during a control cable failure. The pilot had no power and very little time to respond. If the carburetors had defaulted to full throttle (Rotax factory settings), the engine would’ve kept running, giving them more time and control to plan a safer landing.

So, bottom line?

Whether carburetors default to full power or fully closed, both designs can lead to problems depending on the million different situations and circumstances. There’s no perfect answer.

But there is ONE universal truth!!

Poor maintenance, inspection, and practice are the root cause in every case we’ve seen. You can build in all the safety features you want, but if someone skips inspections or installs a part wrong, it’ll still fail.

You can try to implement a “safety feature” into everything, however someone will always find a way to screw it up!

Hello guys, what happen if  the two throttle cable breaks ? More specifically at the control unit, where the two cables meet. Will the motor start running at full speed? Overheat and eventually fail?  thanks for our advice . Nicolas 

De Nadai,

If the carburetor springs are set to bring the butterflies to the fully open position, the engine will default to full power and continue running at that setting until it either runs out of fuel or the pilot shuts off the magnetos. The actual RPM will be dictated by the propeller pitch.

The maximum continuous RPM is 5800 for up to 5 minutes, but stress testing has shown that the engine is capable of running at that speed for extended periods without failure.

If the carburetor springs are set to close the butterflies, the engine will default to idle.

From now on I am going with the full throttle setting. In our emergency training of engine vibrations, FIND THE THROTTLE SETTING WHERE THE ENGINE IS HAPPY. This would end up with full throttle. Best way to go. Thanks all for detailing out this important question.