Airworthiness Directive · Amendment 39-16791 · Lycoming Engines Model IO-720-A1B Reciprocating Engines
Lycoming Engines Model IO-720-A1B Reciprocating Engines
This AD applies to Lycoming Engines reciprocating engines, model IO-720-A1B, serial number L-1457-54A and serial number L-1458-54A. These engines were last known to be installed in a Beech U-8F (Queen Air) N51779 and operating in the southern U.S. and Mexico.
We are adopting a new airworthiness directive (AD) for certain model IO-720-A1B Lycoming Engines reciprocating engines. This AD requires a crankshaft inspection for certain parts that may be installed. This AD was prompted by the failure of a crankshaft due to incorrect parts installed. We are issuing this AD to prevent engine crankshaft failure and damage to the airplane.
Comply with this AD before further flight after the effective date of this AD, unless already done.
Remove the four cylinders from one side of the engine. Guidance on removing the cylinders can be found in the Lycoming Engines Overhaul Manual.
(g) Each counterweight has two rollers that should be held in place by washers, Lycoming part number (P/N) 71907. The washers can be identified as having three holes each, with a diameter of 0.185 inch. These washers are located at the front and rear of each counterweight for a total of four P/N 71907 washers per counterweight. The eight counterweights are located at the top and bottom of each crankshaft cheek, totaling 32 washers per crankshaft.
(h) Rotate the crankshaft to inspect the holes in washers at the front and rear of each counterweight as well as the top and bottom of each cheek.
(i) If each hole, in each of the 32 washers, measures 0.185 inch, then no further action is required. Reinstall the cylinders and test the engine. Guidance on reinstalling and testing can be found in the Lycoming Engines Overhaul Manual.
(j) If any of the 32 washers have one or more holes that do not measure 0.185 inch, then remove the crankshaft assembly and replace it with a serviceable crankshaft assembly. Scrap the non-conforming crankshaft.
The aircraft, engines and makers this AD names, by the model designations in its text. A model listed here may still be outside the AD's serial ranges.
