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ALC-529: Proper Torque (AMT Core Course 2019)
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Slide 1:

Proper Torque

AMT Core

Course 2019

Slide 2: Introduction

Proper torque of fasteners used in aviation is critical to continued safe operation.

If the tools, testing and procedures are not followed disastrous results can occur.

Slide 3: Objective

This course is designed to present you with information to help understand the use of tools, fasteners, manufactures instructions,  and ensure when required torque is properly accomplished.

Slide 4: Purpose

  • Describe Torque
  • Reasons for Torqueing
  • Tool types
  • Tool calibration/standards/frequency
  • Tool maintenance and inspection
  • Improper Torqueing
  • Guidance
  • Hardware types and safeting
  • Human Factors

Slide 5: What is Torque

Torque is a “turning” or “twisting” force and differs from tension, which is created by a straight pull.

Slide 6: Effect on Fasteners

As the nut and bolt are tightened, the two plates are clamped together. The thread angle in the bolt converts the force applied into tension (or stretch) in the bolt shank.

  • A bolt tensioned properly

    works at its optimum
    efficiency and will resist
    coming undone.

Slide 7: Effect on Fasteners

However, if the tension is too low, the nut could vibrate or work loose.

If the tension is too high (overstretched), the bolt could break.

Every bolt has a correct optimum torque/tension figure for each fastening application.

It is important to have these figures available so that the end product will be safe, efficient and economical.

Slide 8: Stretch Bolts

Some styles of bolts may be tightened to a stretched length rather than to a torque value.

Tightening to a stretch length instead of a torque value eliminates the friction variable.

Bolt stretch length is usually measured with a micrometer.

Slide 9: Hardware types and safeting

Maintenance personnel need to reference the manufacturers parts manuals for correct hardware

Cylinder hold down nuts, connecting rod hardware, case halves, through bolts all require specific hardware

Ensure you are using the correct hardware for the application

Slide 10: Reason for Torqueing Fasteners

When proper torque is applied to a bolt or nut, it is designed to keep them from coming loose.

A bolt tensioned properly works at its optimum efficiency and will resist coming undone.

Slide 11: Importance of Torque Control

Its objective is to clamp parts together with a tension greater than any external force trying to separate them.

The bolt then remains under almost constant stress and is immune to fatigue.

The importance of correct torque application cannot be overemphasized.

Slide 12: Torque Wrench

A torque wrench is a tool used to apply precisely a specific torque to a fastener such as a nut or bolt.

The torque wrench is a precision tool consisting of a torque indicating handle and appropriate adapter or attachments.

It measures the amount of turning or twisting force applied to a nut, bolt, or screw.

Slide 13 -16: Torque Wrench Types

  • Beam Type
  • Torque Wrench Types
  • Clicker Type
  • Torque Wrench Types
  • Dial Indicator Type
  • Torque Wrench Types
  • Electronic Type

Slide 17: Torque Extension Adapters

Should a extension adapter be required there are specific instructions and calculations that must be considered.

Slide 18: Torque Standards for Calibration

Such as ASTM E2624

A torque wrench should include a certificate of factory calibration proving the tool has been inspected and tested at the manufacturer’s facility prior to final packaging and is within the stated accuracy range.

Torque wrenches with missing or expired certificates of calibration should never be used.

Slide 19 Certificate of Calibration

Should include:

  • Torque wrench type, manufacturer, model number, and range
  • Calibration equipment used to perform the calibration and its date of last calibration
  • Torque reading clockwise and counter clockwise (if applicable)
  • Calibration date
  • Name of the person who performed the calibration and his/her supervisor
  • A statement indicating that the torque wrench was calibrated to meet the accuracy in a specific standard
  • Wrench was calibrated on a torque standard traceable to a standard

Slide 20: Calibration

Testing machines that apply and indicate torque are used in many industries, in many ways.

They may be used in a calibration research laboratory.

Slide 21: Torque Wrench Maintenance

Good quality torque wrenches are an important investment for any professional user.

Due to their role in ensuring critical parts are properly fastened, proper use and maintenance is required.

Slide 22: Torque Wrench Considerations

Never use a torque wrench with missing or expired certificate of calibration.

Never use a torque wrench to break fasteners loose.

Always perform the preventive maintenance required by the manufacturer.

Never set the wrench at values higher or lower than those indicated on the scale.

When not in use, clean and keep the torque wrench in its storage case at its lowest torque setting.

Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.

Slide 23: Torque Wrench Considerations

Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.

Add the friction drag torque to the desired torque. This is referred to as “final torque,” which should register on the indicator or setting for a snap-over type torque wrench.

Apply a smooth even pull when applying torque pressure.

Handle the torque wrench with care. Dropping a torque wrench will cause loss of calibration.

Never disassemble a torque wrench yourself. Repair and re-calibration should only be done by an accredited laboratory, as per manufacturer’s instructions.

Slide 24: Torque Wrench Considerations

If you buy a used torque wrench, properly calibrate it before the first use.

Never exceed the rated torque of the tool as over torqueing will result in wrench or part failure.

Do not use a cheater bar or any other type of extension on the handle to apply extra torque. If extra torque is required consider a larger torque wrench or the use of a torque multiplier

With proper care and maintenance, a torque wrench should provide reliable service for many years. 

Slide 25: Regulations for Calibration

14 CFR Part 43.13 States in part:

shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. If special equipment or test apparatus is recommended by the manufacturer involved, he must use that equipment or apparatus or its equivalent acceptable to the Administrator.

Slide 26: Regulations for Calibration

For 145 CRS and 135/121 operations the GMM or similar document will specify items such as torque wrench calibrations.

A certificated repair station must ensure all test and inspection equipment and tools used to make airworthiness determinations on articles are calibrated to a standard acceptable to the FAA. (145.109)

Slide 27: Manufacturers Guidance

  • Overhaul Manuals
  • Service Instructions
  • Service Bulletins
  • Other supporting documents

Slide 28: Importance of Manufacturers Guidance

In both Continental Motors and Lycoming engines, the cylinder crankcase through-bolts not only assist in retaining the cylinders on the crankcase, but they also provide the clamping force required to hold the crankshaft main bearings in place in their bearing journals.

Slide 29: Not just Cylinders

Improper torqueing of cylinder through bolts can lead to crankshaft failures

Slide 30: Lycoming Technical Publications

Improper torqueing of cylinder hold down through bolts can lead to crankshaft failures

In this example the cylinder overhead components were lubricated and undamaged. The break-away through bolt torque during disassembly indicated that the cylinder number 1 and 2 through bolts were improperly torqued. The engine was disassembled and the interior surfaces were covered in oil and metal particles. The disassembly revealed the crankshaft separation of the number 2 cheek, aft of the number 2 main bearing journal.

SL-L114 Provides information for accessing Lycoming Technical Publications through subscription services Aircraft Technical Publisher (ATP), T Data or at

SSP-112 Index of Service Bulletins, Instructions and Letters

Slide 31: Manufacturers Guidance

Lycoming – Service Table of Limits and Torque Value Recommendations.

SSP-1776-4 as revised (4/10/18)

This manual contains torque values specifications for various type of hardware used on Lycoming Engines.

Slide 32: Manufacturers Guidance

Lycoming Service Instruction 1029 as amended covers:  Tightening Procedures for Crankcase Thru-Studs and Bolts.

  • To ensure uniform loading on the main bearings, it is necessary to tighten these
    studs and bolts in a sequence

Slide 33: Manufacturers Guidance

Continental Motors technical publication library has moved to Aircraft Technical Publishers (ATP). A current account with ATP is required to access the publications on the ATP web site.

Slide 34: Manufacturers Guidance

Continental Motors Torque Limits.

Continental Motors has moved their torque-related data as well as torque sequence to their Maintenance 0verhaul  Manual.

Cylinder Torque Procedures

Proper cylinder installation requires a multiple step torquing process. Lubricating, preloading torque, and achieving final torque values (not to exceed torque limits). Refer to the applicable Maintenance and/or Overhaul Manual for the proper torquing sequence.

Slide 35: NTSB Safety Alerts

  • Provide information about items/problems that maintenance personnel should be aware of.
  • Emphasize the importance of following written procedures.
  • Site case studies of accidents related to the alert.     

Slide 36: Improper Torqueing

It becomes very costly when failures in a product involve lawyers and lawsuits.

References for Best Practices

Slide 37: References and Resources for best practices:

AC65 handbook canceled, superseded by FAA-H-8083-30A (2018) which included information about torque wrenches and formulas for extension use along with general guidelines.

Slide 38: References for Best Practices

References and Resources for best practices:


Slide 39: Other Best Practices

Torque Seal can be used at completion of the application of Torque to the fastener to verify completion at a later time/date.

Slide40: Other Best Practices

  • Checklist use with second person signoff on critical parts
  • Team concept.  Two people assigned to a task so one can check the other while in process.
  • Use manufacturers checklist guidance.
  • Research AD’s, SL and SB prior to start of task

Slide 41: Contributing Factors for improper Torque

  • Interruptions (Cell Phones)
  • Add picture of mechanic working on A/C while on cell phone.
  • Damaged or out of calibration torque wrench
  • Improper use of adapters/extensions

Slide 42: Contributing Factors for improper Torque

  • Use of improper hardware
  • Use of lock washers when not authorized
  • Using Fiber Lock Nuts in heat areas
  • Failure to install cotter pins
  • Failure to safety wire
  • Use of sealants when not authorized

Slide 43: Contributing Factors for improper Torque

NTSB Safety Alert SA-028 

Ensure that adequate torque is required to spin the nut on the bolt.

Slide 43: What Could Go Wrong

Next we will look at some case studies where it did go wrong!

Slide 44 What Could Go Wrong

Slide 45: Probable Cause and Findings:   

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Bell 206-L4 Failure of

maintenance personnel to

ensure adequate torque of a

tailrotor drive shaft coupling bolt,

which resulted in the partial

disconnection of the drive shaft.

Slide 46: What Could Go Wrong

Probable Cause and Findings:        

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

BE-A36 The failure of an aviation maintenance technician to properly torque and cotter pin the number 2 connecting rod bolts at their attach point to the crankshaft, which resulted in the separation of the connecting rod in flight, and complete power loss.

Slide 47: What Could Go Wrong

Probable Cause and Findings:        

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

PA-32-300 The loss of engine power due to improper installation of the #2 Cylinder by maintenance personnel, which resulted in the separation of the cylinder as a result of fatigue cracking of the cylinder to Crankcase fasteners.

Slide 48: What Could Go Wrong

Probable Cause and Findings:        

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

PA-28-161 loss of engine power due to separation of the propeller due  to under-torqued retention bolts.

Slide 49: What Could Go Wrong

Probable Cause and Findings:        

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

PA-28R-200 A loss of engine power due to the fatigue fracture of the number 2 cylinder attach studs due to improper torque, and the subsequent separation of the number 2 cylinder.

Slide 50: Human Factors

Improper torque is a “latent failure” … a “ticking time bomb”.

Slide 51: Human Factors

Human factors associated with torque-related accidents:

Distractions during work, improper manuals or service documents, environmental conditions, shift changes and communication errors, unfamiliar with task, etc.

Slide 52: Conclusion

The maintenance community needs to be aware of the accident risk associated with  improper torqueing techniques and:

What procedures can be put in place to minimize and/or eliminate the risk.

Training on proper torqueing procedures will help to minimize and/or eliminate the risk to aircraft and people.

Slide 53: Conclusion

Knowledge is key to proper use of a torque wrenches

Have appropriate service information

Inspect tools for calibration

When in doubt, get a second opinion, the owner/pilot is relying on you

Slide 54: Are you Aware of these ?

WINGS Program

AMT Awards Program

General Aviation Awards Program

Slide 55: End of course evaluation

Slide 56: Thank You