Hose Debris - A Common Cause
in General Tech Discussion
10 years ago
I cringe at the thought of bringing this subject up again because it takes on a life of it's own and ends up going down the wrong path, but here goes.
After a year's worth of looking at carb debris issues and cutting hose open on engines that had a lot of rubber debris I think I have a reasonable handle on the cause.
Plain and simple: OVER CRIMPING or over tightening of the clamp with barbed fittings.
On all the hoses I have examined the hose between the fittings have all been in good shape. The problem areas have been at the fittings. The hose has a thin liner of Teflon or other inner material. If crimped too hard on a barbed fitting it cuts this layer and then pieces can become dislodged and fuel especially with ethanol can now get under that lining where fuel was not meant to be. Make sure your mechanics have a handle on the type of clamp and size that they will use and do not over crimp or over tighten clamps. The proper size Oetiker clamp can only crimp down so far so it can be ideal, but if it is too small for that particular application it could also cause hose damage.Any type of screw clamp or even the Band-It clamp if used on the fuel hose itself can cut the inner liner when over tightened.
It is obvious that barbed fittings can and have worked if used properly because they are every where in engine use in the world, BUT due to lack of education on fittings, hose and proper clamp use there can be cause for concern over debris that will cause an unwanted and unexpected rpm drop in flight.
The best fix would be to stop using barbed brass hose fittings and use AN fittings with a smooth shank and a flared end which would only be applicable on the low pressure carbureted engines. The new Rotax fuel pumps uses these smooth shank fittings. It would be nice if some aircraft MFG's would issue an LOA (Letter of Approval) so anyone that wanted to switch from a barbed fitting to the AN fittings would have that option.
Back in May 2012 Rotax had a service Alert (http://legacy.rotaxowner.com/si_tb_info/alertbulletins/asb-912-061ul.pdf)out for some hose on the bottom pressure side out of the fuel pump that was actually a bad batch of hose.
Here are a few pictures to show you what over tightening a clamp can do. This hose had a Band-It clamp installed. The long smooth longitudinal cuts where from a box knife opening the hose. These are only one hose that I have examined and some are worse.This particular hose produced 14-18 fairly good sized particles in the carbs. All capable of blocking the main jet.
I hope this helps someone from having an off field landing.
After a year's worth of looking at carb debris issues and cutting hose open on engines that had a lot of rubber debris I think I have a reasonable handle on the cause.
Plain and simple: OVER CRIMPING or over tightening of the clamp with barbed fittings.
On all the hoses I have examined the hose between the fittings have all been in good shape. The problem areas have been at the fittings. The hose has a thin liner of Teflon or other inner material. If crimped too hard on a barbed fitting it cuts this layer and then pieces can become dislodged and fuel especially with ethanol can now get under that lining where fuel was not meant to be. Make sure your mechanics have a handle on the type of clamp and size that they will use and do not over crimp or over tighten clamps. The proper size Oetiker clamp can only crimp down so far so it can be ideal, but if it is too small for that particular application it could also cause hose damage.Any type of screw clamp or even the Band-It clamp if used on the fuel hose itself can cut the inner liner when over tightened.
It is obvious that barbed fittings can and have worked if used properly because they are every where in engine use in the world, BUT due to lack of education on fittings, hose and proper clamp use there can be cause for concern over debris that will cause an unwanted and unexpected rpm drop in flight.
The best fix would be to stop using barbed brass hose fittings and use AN fittings with a smooth shank and a flared end which would only be applicable on the low pressure carbureted engines. The new Rotax fuel pumps uses these smooth shank fittings. It would be nice if some aircraft MFG's would issue an LOA (Letter of Approval) so anyone that wanted to switch from a barbed fitting to the AN fittings would have that option.
Back in May 2012 Rotax had a service Alert (http://legacy.rotaxowner.com/si_tb_info/alertbulletins/asb-912-061ul.pdf)out for some hose on the bottom pressure side out of the fuel pump that was actually a bad batch of hose.
Here are a few pictures to show you what over tightening a clamp can do. This hose had a Band-It clamp installed. The long smooth longitudinal cuts where from a box knife opening the hose. These are only one hose that I have examined and some are worse.This particular hose produced 14-18 fairly good sized particles in the carbs. All capable of blocking the main jet.
I hope this helps someone from having an off field landing.
hose1_2013-11-08.jpg (You do not have access to download this file.)
hose2_2013-11-08.jpg (You do not have access to download this file.)
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell
