ENGINES FOD
In constant displacement pumps, the carbon vanes are relatively soft. Briggs demonstrated to us how “soft” these vanes are as he rubbed a carbon vane against a sheet of paper, and a soft grey shading appeared on the paper. “The carbon in these vanes is about as hard as a hard pencil lead,” explains Briggs. “These carbon vanes are spinning in the pump at several thousand RPM. If any foreign object debris (FOD) is introduced, such as rag pieces, paint chips, ect., it can damage the vanes. The edges of the vanes will break and the pump will lose pressure.”
Briggs adds, “I’ve been there. In my 35 years of fixing and flying aircraft, I’ve been in fuel tanks. It’s an ugly job. But you need to have the mindset right off the bat that you are going to take your time. Cleanliness is next to Godliness when it comes to fuel system maintenance. If you introduce any debris to a fuel tank and don’t get it back out, it’s going to end up going down the tube and will eventually end up in the pump or somewhere else and that’s never a good thing.” Also be mindful of proper fuel tank repair and the age of
your fuel lines. Debris from sloppy fuel tank repairs or from deteriorating linings in old fuel lines can also contribute to FOD damage.
FOD (upper part of photo) and the damaged carbon vanes it caused
Centrifugal pumps don’t have floating carbon vanes that can be damaged. But the impeller disk is not immune to degraded output from FOD. If there is FOD in the fuel system, it can be sucked up though the openings in the center of the impeller. If these openings or the internal vane channels in the impeller become blocked, fuel cannot flow out through centrifugal force, and a loss in pressure results. How do you prevent FOD damage?
Keep it clean! “You would be surprised how even a small piece of debris can damage your fuel pump,” says Lotzer. “If you are doing fuel cell work, make sure the area is clean before closing it up. Also, use clean, lint-free towels to wipe down the fuel cells. Even small chunks of debris left behind from a worn out red rag can damage a fuel pump.”
Debris left behind from worn out red rags can damage fuel pumps
February | March 2014
HelicopterMaintenanceMagazine.com
27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64