by Zach Grant L1011jock » Sat Jan 01, 2011 3:45 pm
Cliff,
There is no "official" prebuy list, however the most obvious items are usually the items that cause the most damage, when improperly attended too. These are the Landing gear system, and the fuel system. Twin Comanches are pretty robust and don't have many issues with the airframe (other than possible firewall cracking where the engine mount attaches). There will probably be some small cracks in the upper wing skins. They will not grow once the stress has been relieved (stringers weren't attached to some ribs), and general practice is to leave them alone. Pitch Horn cracking is a non issue with the thick walled torque tube (all twins/400s were built with the thick walled tube). The lower spar corrosion has not been found to be an big issue on the Comanche series due to extensive corrosion proofing at the factory. Pitch trim tab actuation system should have no play in it. Set trim to neutral. Stand aft of the stabilator, push down on trailing edge of stab just in front of trim tab and deliberately move the trim tab up and down with the other hand. A small bit of linkage lash is acceptable, but any large movement, including translation of the jack screw spool (trim drum) in the cage is not acceptable. Also check the solidity of the tail attachment by standing at the tip of the stab, and moving it up down, forward and back while feeling for any movement, listening for any squeek, clunk or grind. This should be done in a quiet atmosphere, as this is very subtle sometimes. Any of these would need to be investigated further. Check the control cable tension. This is often a neglected item.
Landing gear needs to be rigged correctly with no perceptable slop in any bushings or linkages. It needs to be properly rigged with the correct preload set. Limit switches must be set correctly, wiring in good shape and properly routed. The transmission needs to be smooth, serviced and not loose. The AD 77-13-21 is a 3 part AD. Many do not realize this and simply sign it off as complied with when the bungees are replaced. Beware of para. C that requires 1000 hr interval inspection of the entire system. Check that all of the springs are in good condition (many have not been changed in a while. Service manual says to change main gear springs when bungees are changed). All of this is easy to do, but it is best done by someone who knows what to look for.
Fuel system. Bladders leak becuase most people don't fill the tanks all the way up! Capacity is only accurate if the aircraft is leveled (aircraft does not sit level), AND the tank is filled all the way up, as in overflowing. The bottom of the filler neck is about 4 gallons short of full. Fill the tanks up, and you will find the leaks. If you have tip tanks, check the fuel valve solenoid for proper operation. Also check to see if the flexible fuel line from the tip to the hard line in the wing is leaking or has been changed since install (many haven't and leak or will colapse once the tank is about half full and begins to suction feed vs gravity feed). Take very thorough fuel samples. If you find ANY black specks, you will need to change the bladders, and possibly have other fuel system problems such as fuel servo malfunction, and plugging injectors. Rubber dust can find its way through every screen and filter on the plane. Fuel valves. These have a good service history, but after 40 yrs the cad plating is somewhat sketchy. Many have shown internal corrosion where the plating is worn off. You can inspect them by removing the sump bowl, and bypass valve. If you see any corrosion, send them to webco for service. They will replate, rebuild, and it will be almost like new. Also new PMA units made from stainless are available from Parker Hannifin.
These are the biggies. The rest is pretty straight forward Lycoming, Hartzell, and AC-43-13 stuff.
-Zach
"Keep it above 5 feet and don't do nuthin dumb!"