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.
Inspection points for tubular structural pieces
Now that that the engine mount has been sandblasted, what do we look for. For starters, the tubular structural pieces should be inspected for corrosion and pitting. Tube sections with pitting and voids exceeding 10 percent in relation to the overall tube thickness should be replaced. Weld joints should be inspected for cracks. Follow-on dye penetrant inspection will uncover cracks, voids and pinholes not detected by visual inspection.
Check for chafing and misalignment
Next, the mount should be inspected for visible signs of wear due to chafing. Wear spots resulting from chafing are caused by the contact of cables, wire harnesses, tubing or other moving metal parts will weaken the structural integrity of the Chromaloy 4130 tubing. Tubes displaying any type of chafe marks or wear spots should be replaced.
Finally, the mount should be inspected for bent tubes. The most common causes of bent or misaligned engine mount structures are landing mishaps quite often involving the landing gear.
Performing a fixture check
A fixture check of the engine mount will help determine whether the engine mount is out of alignment or if there may be an interference problem with the aircraft. The use of fixtures is a very important element of the entire repair process. Fixtures ensure that tube replacement and other repairs will be within design specifications minimizing any potential for misalignment. Without the use of the proper fixture designed for the specific aircraft model, the mechanic is taking a huge gamble that the mount will attach correctly to the aircraft and allow for the proper clearances and fit when the engine is reinstalled. Fixture checks are part of the standard inspection and repair process at an FAA licensed repair station.
Assessing needed repairs
Now that the preliminary inspection process has been completed, it is now time to contemplate the repairs that may be needed. The sand blast operation has cleaned the areas requiring repair or replacement. It is very important to note that any tube sections requiring repair must be replaced using Chromaloy 4130 tubing. Do not substitute any other type of steel tubing. Aircraft engine mount assemblies are fabricated using Chromaloy 4130 with tube thickness ranging from .049 to .065 inches.
Never replace a tube section with material having a tube thickness less than the design specification.
Replacement tubing should always have the same or greater tube thickness. It is recommended that Chromaloy 4130 filler rod be used when welding replacement tubes and making other mount repairs. Any other type of filler material, as well as tube material, can degrade the reliability of the engine mount..
Weld repair of aircraft engine mount assemblies is accomplished using the TIG welding method. Engine mount weldments require clean, precise welded joints. TIG welding allows a fine, precision beaded weld seam to be applied to the tube joint sections. TIG welding of engine mounts requires skill and patience. Another reason to consider the services of a FAA repair station licensed to inspect and repair engine mounts.
No preheating required
Engine mount assemblies do not require a preheat operation prior to commencing repair. However, repaired areas requiring holes to be drilled must be annealed, otherwise the point of connection may crack under stress. We also recommend that all firewall and engine attach points be stress relieved.
Dye penetrant inspection
With the repairs completed, the weldment should undergo a dye penetrant inspection to ensure that all weld joints have been made correctly and to detect any hairline cracks, voids ,or pinholes that may not be visible to the eye. Applying the dye and the subsequent inspection should be performed in a well-vented area with good lighting. Dye penetrant inspection is added insurance that the repair has been completed correctly and that a good quality engine mount is being returned to service.
A second sandblast
After the dye penetrant inspection, the engine mount must undergo a second sandblast operation to prep it for painting.
We recommend a good quality zinc chromate primer be applied prior to applying paint. After spending thousands of dollars on an engine replacement, you will want it to be supported with a freshly painted, good-looking engine mount assembly. After painting the engine mount in a well-ventilated paint booth, the mount must be allowed to dry at least four hours before reinstallation in the aircraft.
Following the preceding procedures, we now have an engine mount assembly worthy of supporting that new engine. Although a fairly simple looking structure, the engine mount plays a very important part in the overall airworthiness of the aircraft. Any maintenance organization contemplating the return of an aircraft engine mount to service under their facility's signature must give seriously consideration to ensuring they have:
• Trained personnel with experience in TIG welding Chromaloy 4130
• The facilities to:
b) Perform dye penetrant inspection
• Possess the tooling and fixtures required to perform a detailed inspection and undertake the necessary repairs
If not, you may want to consider sending the mount to an FAA licensed repair station staffed with trained personnel and the facilities to do the job correctly. When considering the overall cost of a major engine replacement, using the services of a licensed repair station can be a cheap form of insurance. With the repair station signing off on the return to status authorization, you can be confident of having an engine mount that won't fail in its assigned task. AMT
The following is a series of questions and answers regarding safety for welding personnel as well as and procedures for the maintenance of welding equipment.
Q: What can I do to avoid electrical shocks?
A: Wet working conditions must be avoided, because water is an excellent conductor and electricity will always follow the path of least resistance. Even a person's perspiration can lower the body's resistance to electrical shock. Poor connections and bare spots on cables further increase the possibility of electrical shock, and therefore, daily inspection of these items is recommended. Equipment operators should also routinely inspect for proper ground connections.
Q: How can I inspect and maintain my wire feeder?
A: Periodically inspect the electrode wire drive rolls. If dirty, remove the drive rolls and clean with a wire brush. Deformed drive rolls should be replaced. Drive rolls should be changed, adjusted or cleaned only when the wire feeder is shut off. In addition, check the inlet and outlet guides and replace if they are deformed from wire wear. Remember that when power is applied to a wire feeder, fingers should be kept away from the drive roll area.
Q: What are some important electrode safety considerations?
A: Welding power sources for use with MIG and TIG welding normally are equipped with devices that permit on/off control of the welding power output. If so, the electrode becomes electrically hot when the power source switch is ON and the welding gun switch is closed. Never touch the electrode wire or any conducting object in contact with the electrode circuit, unless the welding power source is off. Welding power sources used for shielded metal arc welding (SMAW or Stick welding) may not be equipped with welding power output on/off control devices. With such equipment, the electrode is electrically hot when the power switch is turned ON.
Q: How should I store my gas cylinders?
A: Cylinders should be securely fastened at all times. Chains are usually used to secure a cylinder to a wall or cylinder cart. When moving or storing a cylinder, a threaded protector cap must be fastened to the top of the cylinder. This protects the valve system should it be bumped or dropped.
Cylinders should not be stored or used in a horizontal position. This is because some cylinders contain a liquid which would leak out or be forced out if the cylinder was laid in a flat position. Also, welding guns and other cables should not be hung on or near cylinders. A gun could cause an arc against the cylinder wall or valve assembly, possibly resulting in a weakened cylinder or even a rupture.
Q: How can I tell if my regulator is faulty?
A: The following symptoms indicate a faulty regulator:
• Leaks - if gas leaks externally.
• Excessive Creep - if delivery pressure continues to rise with the downstream valve closed.
• Faulty Gauge - if gauge pointer does not move off the stop pin when pressurized, nor returns to the stop pin after pressure release. Do not attempt to repair a faulty regulator. It should be sent to your designated repair center, where special techniques and tools are used by trained personnel.
Q: What are some tips for a safe welding environment?
A: The area surrounding the welder will be subjected to light, heat, smoke, sparks and fumes. Permanent booths or portable partitions can be used to contain light rays in one area. The heat and sparks given off are capable of setting flammable materials on fire. Therefore, welding should not be done in areas containing flammable gases, vapors, liquids or dusty locations where explosions are a possibility.
Metals with plating, coatings or paint that come near the region of the arc may give off smoke and fumes during welding. These fumes may pose a health hazard to the lungs, therefore an exhaust hood or booth should be used to remove fumes from the area.
When welding in confined spaces, such as inside tanks, large containers or even compartments of a ship, toxic fumes may gather. Also, in an enclosed room, breathable oxygen can be replaced by shielding gases used for welding or purging. Care must be taken to ensure enough clean air for breathing. In many companies, it is routine to provide welders with air masks or self-contained breathing equipment.
Q: How should an operator dress for optimum safety?
A: Gloves and clothing should be flame-resistant. Clothing made from a dark-colored, tightly woven material is best suited for welding. Gauntlet-type leather gloves should be worn to protect the hands and wrists. Shirt collars and shirt cuffs should be buttoned, and open front pockets are not advisable as they may catch sparks. Also, operators should never store matches or lighters in their pockets. Pants cuffs are not recommended, as they will also catch sparks. Tennis shoes do not qualify as adequate foot protection. High-top leather shoes or boots are absolutely necessary.
Q: Is there a daily maintenance schedule I should follow?
A: Below (See page 58), is a general engine drive routine daily maintenance schedule, but it should be modified according to a company's specific conditions.
By following a regimen of appropriate and thorough maintenance and safety, a welder can run dependably for decades. Designed to withstand rough use, these machines use high quality components and are tested for durability.
Always refer to the owner's manual for a thorough explanation of safety and maintenance. This article does not give complete coverage of all the maintenance and safety issues in existence.