Primer Fuel Line Failure

[turbopilot]

In October of 2016 there was a post about the failure of the primer fuel line leading from the gascolator. Here is the link. In that post I suggested it was probably a good idea to cap or completely remove the priming systems from the Carbon Cub, pending a redesign. Unfortunately, I never got around to it.

Yesterday while performing a routine inspection I discovered the fuel line leading from the primer pump to the cylinder had fractured. See image below.



The primer in the Carbon Cub consists of 1/8" copper tubing in two segments: one from the gascolator to the primer pump on the instrument panel and the second from the pump on the instrument panel to aft left cylinder. My failure was in the line from the pump to the cylinder so there was no fuel leak from the brake. The cylinder had a small leak through the primer fitting but it did not change engine performance that I could notice.

The post from October, 2016 involved a failure of the line above the gascolator on the way to the pump. This kind of failure is very serious business as it introduces raw fuel on the firewall near the exhaust.

Today I completely removed the priming system from my 2014 Carbon Cub. The process is very simple all you need are two 1/8" NPT Allen Head Pipe Plugs. Here is a link to aluminum plugs on Amazon. Here are the steel plugs.

I put a steel plug in the cylinder and an aluminum plug in the gascolator then removed the tubing.

In my opinion these failures are safety hazard that clearly call for a mandatory service bulletin to remove this system until a redesign can be undertaken.

I have had a Carbon Cub since 2010 and never required the primer to start the engine. I have heard from folks operating the Carbon Cub in Canada who claim they normally don't need to prime the engine. When you consider the safety hazard associated with running a fuel line to the cockpit and back to the engine for a system that is rarely if ever needed, it would seem prudent to just remove the priming system.

[Snoopy]

Did you ever sell your landing gear/brake set?

[turbopilot]

Did you ever sell your landing gear/brake set?

Nope. Trying to sell everything all together rather than parting it out.

[Dan L]

I’m of the opposite school on priming. On my EX I put primer nozzles on 3 cylinders. The 4th cylinder port goes to the manifold pressure transducer in the electronic ignition system. I pump 4 times prior to start and crack the throttle an inch or so.

I am surprised your line broke with a loop in it. Vibration is the issue. Perhaps a bigger loop would help. Or a dynamic prop balance? Priming with copper lines is very common and it hasn’t been a historical problem in other planes.

[turbopilot]

I’m of the opposite school on priming. On my EX I put primer nozzles on 3 cylinders. The 4th cylinder port goes to the manifold pressure transducer in the electronic ignition system. I pump 4 times prior to start and crack the throttle an inch or so.

I am surprised your line broke with a loop in it. Vibration is the issue. Perhaps a bigger loop would help. Or a dynamic prop balance? Priming with copper lines is very common and it hasn’t been a historical problem in other planes.

Interesting. Been flying the CC340 for 8 years. You are the first person I have heard that uses priming to start. If you need the capability then my thought is the system needs a redesign to avoid vibration fatigue. Maybe stainless steel and/or flexible braided lines.

[Daveembry]

When installing those in new builds I have also always used a good spiral wrap on the entire copper line. Not sure if that helps or not but might help stabilize the vibrations some??? If not, it's good protection against any abrasion that might happen.

Interesting. Been flying the CC340 for 8 years. You are the first person I have heard that uses priming to start. If you need the capability then my thought is the system needs a redesign to avoid vibration fatigue. Maybe stainless steel and/or flexible braided lines.

[Mark Keneston]

When installing those in new builds I have also always used a good spiral wrap on the entire copper line. Not sure if that helps or not but might help stabilize the vibrations some??? If not, it's good protection against any abrasion that might happen.

Agreed, need a much smaller loop and then secure it so there is no vibration. MK

[turbopilot]

Agreed, need a much smaller loop and then secure it so there is no vibration. MK

Can't comment about a smaller loop. My installation was as secure as it could be. One clamp on firewall and one clamp on the engine nearest to the firewall.

This failure appears to be a fatigue fracture of the cooper line. With the rocking of the engine it is just a matter of time before the line fails. Good news is that a failure between firewall and engine is not a significant safety of flight issue. Failure between the gascolator and primer pump is a very big deal.

In any event I have completely removed the primer system from my aircraft. Just dead weight.

[Snoopy]

Been working on aircraft for 40 plus years and have seen very few fail between the primer and the manifold.. I’d suspect the huge loop in your line is the culprit. Even if the line fails...generally it won’t be an issue if the primer is locked. Reduce the loop size down to an 1 1/2” or so and watch it at idle....bend as necessary to reduce movement from vibration and it will be fine. Primer lines like these have been around since the 30’s and have held up pretty good. A flexible line similar to braided brake line would be an improvement for sure...I also prime mine in winter months ....having immediate fuel delivery will save on starter time in the long run.

[turbopilot]

Been working on aircraft for 40 plus years and have seen very few fail between the primer and the manifold.. I’d suspect the huge loop in your line is the culprit. Even if the line fails...generally it won’t be an issue if the primer is locked. Reduce the loop size down to an 1 1/2” or so and watch it at idle....bend as necessary to reduce movement from vibration and it will be fine. Primer lines like these have been around since the 30’s and have held up pretty good. A flexible line similar to braided brake line would be an improvement for sure...I also prime mine in winter months ....having immediate fuel delivery will save on starter time in the long run.

This is a 2014 CC with 140 hours. 95% of those hours below 40% of power. Primer line is virgin factory install. Never been touched.

As I said above if you really need to prime the engine, I guess there are not many options. Been been flying two Carbon Cubs for over 8 years from the Canadian border to the Mexican border and never needed to prime. A couple of times on cold morning two pumps of the throttle did not work. Always worked by doing it again a second time.

If you don't need to prime might as well save a little weight and some degree of risk. Easy to pull out and put back in again if you change your mind.