• Ben Owen

HOMEBUILT POWERPLANT PRECERTIFICATION INSPECTIONS

by Brian H. Stirm, EAA 106648 W. Lafayette, IN____________

Edited by Ben Owen As a practicing inspection authorized airframe and powerplant technician, as well as an EAA Technical Counselor, I have seen several potentially dangerous situations arise that I would like to address on the subject of homebuilt powerplant pre-certification inspections. I have seen individuals spend many years of hard work and expense to build a fine airframe only to hang on the firewall a powerplant of very questionable origin without benefit of careful inspection. As airframes advance into faster, high wing loading designs (such as the Glasair and SX-300), a very reliable powerplant is required to get from point A to point B. Gone are the days when an engine failure would result in a safe cow pasture landing due to a low and slow "around the patch" design. As more aircraft are used for fast transportation over great distances of trees, mountains and water, the last thing you want is an engine failure. Lately, many builders are purchasing a fast kit plane and then finding a past time between overhaul (TBO) 100-200 hp engine for the least amount of money. While most models of engines are good for at least 300-400 hours past TBO, this is only with 50 hour oil and

airworthiness directive (A.D.) on the oil pump, which mandates replacement with new style of pump impellers at 2000 hours. On the subject of A.D. notes — they all must be compiled with on the powerplant to have a safe thrust generator. Some examples that are often overlooked are: 500 hour inspections of Bendix magneto impulse couplings; 2000 hour coil replacements on some Bendix magnetos; 5 year or 1000 hour mandatory overhauls of many popular Hartzell propellers . . . and the list could go on and on. A.D. notes are there for a reason and should not be ignored. Before a powerplant of questionable past is used on a homebuilt for the first test flight, I would recommend the following: Have a good A&P technician or inspector who works in the day to day business of aviation maintenance give the powerplant a 100 hour inspection and routine maintenance. This would include: 1. Checking the cylinder compression. 2. Changing oil and having it analyzed to check for worn metal (this is a last chance check, as for the oil to be properly analyzed the analysis company needs about 2 or 3 samples at 50 hour intervals to establish a base line wear level. A one time check will only identify a problem if there is a large quantity of metal in it — in other words, if the engine already has started to fail).

filter changes and frequent operation. Most engines from private owner aircraft go 15 years or more between over-

screens to check for metal.

hauls, spend at least 3 months a year idle and are lucky to receive an oil change once a year on the annual inspection. Many Lycoming engines also have an

timing and drive wear. 5. Cleaning, gapping and rotating spark plugs (many aircraft I have looked at that will not start or are hard to start have been due to a too wide plug gap,

3. Removing all oil system filters and 4. Checking magnetos for point gap,

especially true on non-impulse magnetos). 6. Checking plug leads for continuity and high tension shielding leaks. 7. Checking starters, generators and alternators for brush length, drive and wear. 8. Cleaning all fuel system screens, filters and pressure, checking fuel lines. It is amazing how much junk will accumulate in these filters during test running and taxi tests due to filling from dirty 5 gallon cans and items of dirt left in fuel tanks and lines. 9. A fuel flow test at carburetor or fuel injector to determine that enough fuel flow is present for maximum engine power at maximum climb angle. 10. A check of all control travels and safties. 11. Cleaning of air filters. 12. Checking propeller tightness, bolt safety and track. 13. Checking exhaust system for tightness, restrictions and proper exit from engine cowling. Upon completion of these checks, reassembly and cowling of engine, the aircraft should be run to check for static rpm, mag drop, carb heat effectiveness, prop operation, idle speed and mixture rise, instrument function and fuel or oil leaks. Proper limits for this run can be found in the flight or owner's manual of the aircraft that the engine and prop was taken from, or the aircraft's type certificate. If a non-standard prop installation is used, engine power charts and good

engineering practices should be used for rpm limits.

These checks should only cost the builder $100 or less and is a minimal cost to insure that his or her homebuilt will make the first flight, and all flights thereafter, safely.

SPORT AVIATION 59