The ABC's of Engine Mount Inspection & Repair
By Tom Heid
You are faced with the task of removing the engine from your customer's aircraft. It might be routine, scheduled maintenance or it may be a complete engine replacement. Either way, this would be a good time to remove and inspect the engine mount assembly - you know, that tubular, spider web looking framework that remains attached to the firewall after the engine and other power plant accessories have been removed?
Integral part of the airframe
The aircraft's engine mount assembly is an integral part of the airframe. Attached to the aircraft firewall with only a few bolts, the engine mount is the only structure holding the engine and other power plant parts and accessories in place. Letting the aircraft owner head off into the wild blue yonder with a defective engine mount can be a recipe for disaster.
To the untrained eye, looks can, and usually are, deceiving. A cursory once over by your mechanic may not be enough to detect potential flaws lurking under the paint or carbon build-ups. The engine mount structure is exposed to some of the harshest of operational environments. Subjected continuously to extreme heat and temperatures, corrosive gases and load bearing stresses, it is easy to understand how the reliability and structural integrity of the engine mount assembly will degrade overtime.
Engine mounts are subject to abuse every day
In its every day working environment, the engine mount will be subject to pitting and fatigue caused by corrosion, cracks due to stress and vibration, and chaffing from tubing, cabling or moving metal parts such as the landing gear assembly. Additionally, the structure itself may become bent or misaligned due to unnatural stresses or actual flight inflicted damage. With all the potential problems that may occur, one thing is certain - engine mount inspection and repair is not for the novice or inexperienced aircraft mechanic. Unless your FBO has the facilities, equipment, and trained personnel to remove, inspect and repair engine mounts, you may want to consider using an FAA licensed repair station. Let's walk through the recommended inspection and repair process. Afterwards, you can make the call if you want to return the engine mount back to service under your signature.
What is an engine mount?
An aircraft engine mount assembly is actually a weldment constructed from Chromaloy 4130 tubular steel. Why Chromaloy? Chromaloy is a lightweight, high-strength alloy capable of handling the loads required for aircraft engines and supporting components, as well as the stresses encountered during flight operations. Although a very durable material, careful and thorough inspection is required to ensure the structural integrity of the mount remains at 100 percent.
The inspection process
To begin the inspection process, the engine mount must be removed from the aircraft. This is accomplished by removing the bolts located at the engine mount attach points on the firewall. Prior to any meaningful inspection, it is recommended that the engine mount undergo a sand blast operation. Sand blasting serves two very important functions:
1. Removes all paint, carbon and chemical buildups and other discoloration, exposing the bare metal and any potential defects.
2. Cleans the Chromaloy 4130 so that if repairs are needed, good, solid welds will result.
Double check ADs for any additional required procedures
In addition to the sand blasting process, FAA Airworthiness Directives (ADs) require some aircraft models, such as the Beech 58 Baron, to undergo a magnetic particle inspection. Magnetic or mag particle inspection is required because visual and dye penetrate inspection has been determined to be insufficient to uncover defects in the welded joints and seams of these particular aircraft engine mounts. Be sure to double check the ADs for the particular make and model aircraft to ensure the proper procedures will be followed.
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