OT Safety Bulletin

Lycoming O-540 engines

The Lycoming O-540-E4C5 engine is a popular choice amongst Territory aircraft owners and operators and is one of the engine types most commonly found on the Britten-Norman Islander and Trislander aircraft.

Due to the varying climates that these engines operate in, there is always the possibility of wear and ultimate failure.

Below is an example of one such failure occurring on a Britten-Norman Trislander, but more importantly, the article is of most interest to owners and operators of Lycoming engines.

The following article was extracted from the Air Accident Investigation Branch April 2013 bulletin.

Lycoming O-540-E4C5 engine stud bolt failure

Engine Description

The O-540-E4C5 is a six-cylinder, horizontally opposed direct drive engine. The cylinders are numbered from front to rear, odd numbers on the right (looking forward), and even numbers on the left. The cylinders are of conventional air cooled construction with the two major parts, head and barrel, screwed together. The piston connecting rods are made from alloy steel forgings. The crankcase assembly consists of two reinforced aluminium alloy castings, fastened together by means of studs and nuts. Double-ended studs run through the crankcase and form two of the eight mounting studs for each of the opposing cylinders. The remaining six studs per cylinder are screwed into fixings in the crankcase using a coarse thread. The cylinders are retained in place on the studs by 'hold down' nuts screwed onto a fine thread. The nuts are torque loaded but have no secondary retaining feature.

The manufacturer recommends an overhaul life of 2,000 hours for this engine type. However, based on a life extension approval granted by the CAA, the operator involved in the incident has increased this life for the engines in their fleet to 3,000 hours.

The No. 2 engine that failed during the incident had operated 996 hours since overhaul and had a time since new of 11,992 hrs.

Initial inspection

On landing, the operator's maintenance provider inspected the aircraft. They reported that a large section of the engine cowling was missing on the left side of the engine.

The remaining cowling and aircraft empennage were heavily stained with oil released from the engine during the failure (see picture above). Following removal of the engine from the aircraft, it was clear that the No. 2 cylinder had released from the crankcase after failure of the mounting studs.

This had resulted in damage to the surrounding crankcase from the flailing connecting rod. The piston head and push rods were missing, as was the inlet pipe from the manifold and the section of exhaust pipe from the cylinder head to the exhaust manifold.

The starter motor had been dislodged from the engine, but remained attached by its power lead and the guide tube for the propeller-feathering unit Teleflex control cable was damaged.

Of the eight studs that secured the cylinder to the crankcase prior to failure, four had been lost with the released sections of the crankcase and cylinder. Two of the 'short' studs remained, as did the two 'through' studs, which ran through the crankcase to the opposing cylinder.

Further occurrence

Two months after the initial incident, a routine maintenance check of the No. 3 engine fitted to another aircraft (G-RLON) from the same operator's fleet, identified another stud failure.

This engine had operated 9,041 hours since new and 460 hours since its last overhaul. Only a single stud had failed and the released section of the stud and 'hold down' nut were found trapped in the baffle between the No 2 and No 4 cylinder barrels.