Torque settings are suppose to produce a predictable amount of clamp load on the joint being held together by the fastener. There can be large errors in this prediction.

Even the best torque wrench has torque wrench errors - errors are the result in the conditions of torque. 90% of the resistance you feel when you use your torque wrench is caused by friction.* Change the friction by 10% and you get a 100% change in bolt tension with the same setting on your torque wrench.

Considering only the friction portion, 50% of it is created in the threads and 50% is created in the bearing surface under the bolt head. Thus lets say the torque specification states that you should apply a thread lubricant. Do you apply it only to the threads? or do you apply it to both the threads and under the bolt head?

The aircraft mechanic doesn't know unless the engineer who wrote the torque specification specified lubricant type and placement. It's the engineer's responsibility to specify the conditions of torque including where the lubricant that is to be applied. Without this specific information, the resulting joint tension can be almost anything.

Note: Almost all torque specifications published in aircraft repair manuals are deficient in this area. The mechanic cannot achieve proper joint tension with inadequate torque specifications. Joint failures are to be expected and are not caused by improper torque but by improper torque specifications.

For an interesting analysis of the above:

"Failure of bolts in helicopter main rotor drive plate assembly due to improper application of lubricant" by N. Eliaz, G. Gheorghiu, H. Sheinkopf, O. Levi, G. Shemesh, A. Mordecai, H. Artzi, Published in Engineering Failure Analysis #10, 443-451, www.sciencedirect.com http://www.sciencedirect.com/

Surface crushing occurs when the clamping forces exceed the material's compression strength. Torque values depend not only on the bolt strength but also on the material properties of the joint. This is why torque charts based only  on the strength of the bolt are just a starting point in determining the proper torque wrench setting.

Source: "Failure of bolts in helicopter main rotor drive plate assembly due to improper application of lubricant" by N. Eliaz, G. Gheorghiu, H. Sheinkopf, O. Levi, G. Shemesh, A. Mordecai, H. Artzi, Published in Engineering Failure Analysis #10, 443-451, www.sciencedirect.com http://www.sciencedirect.com/

* source "An Introduction to the Deign and Behavior of Bolted Joints" by Bickford.

This last table is the one that is surprising. Evidently there is a lot of friction under bolt / nut head. This table illustrates how the torque specifications should state where the thread lubricant (antiseize in this example) be placed.

To quote from the report: "Thus, it may be concluded that the way in which the antiseize lubricant is applied has significant effect on the magnitude of the axial force that develops in the bolt during torque wrenching."

Good aircraft maintenance requires not only mechanic training but also properly written aircraft maintenance manuals.

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"Thus, it may be concluded that the way in which the antiseize lubricant is applied has significant effect on the magnitude of the axial force that develops in the bolt during torque wrenching."

Applying torque is not just getting the wrench to click - the clamping force must be correct and remain unchanged throughout the life of the product