NTSB Safety Alert SA-056 - Take Time to Torque

SA-056 October 2016

Take Time to Torque

Apply proper torque to prevent accidents

The problem

 From 2009 to 2015, over 45 accidents and incidents have occurred that resulted

from maintenance personnel applying improper torque to engine fasteners (bolts

and nuts) during engine maintenance activities. The application of improper torque

led to internal engine damage and subsequent engine failures and accidents or

incidents (see figures 1 and 2).

 Applying too little or too much torque can cause a bolt to fail and/or the nut and bolt

threads to become stripped, which allo ws the fastening hardware to loosen.

Figure 1. Cylinder with a finger -tightened nut that is

not properly torqued. Figure 2. Internal damage to another engine resulting

from the application of improper torq ue.

Why is it important to apply the proper torque values?

 Allows the structure or assembly the fasteners are holding together to develop its

full design strength.

 Prevents the exceedance of the design limitations of the structure or hardware.

SA-056 October 2016  Reduces the possibility of excessive wear of the fasteners and parts being held.

 Ensures that each fastener carries the load for which it was designed and that the

load is evenly distributed between the fasteners.

REMEMBER — NOT TOO LOOSE, NOT TOO TIGHT:

ENSURE THE TORQUE IS JUST RIGHT!

Related accidents

Unfortunately, the circumstances of many recent accidents are remarkably similar to

those of previous accidents. This suggests that some maintenance personnel are not

taking advantage of the lessons learned from such accidents that could help them avoid

making similar mistakes. The following accident summaries illustrate some common —

and preventable —maintenance -related accident scenarios:

 An in -flight crankshaft failure occurred 231 hours after maintenance p ersonnel had

removed and reinstalled the Nos. 3 and 4 cylinders. The pilot and passenger were

seriously injured during the forced landing. Postaccident examination showed that

the torque on the cylinder flange nuts and through bolts was below factory

speci fications. It was determined that maintenance personnel’s application of

insufficient torque during installation of the engine cylinders led to the loosening of

the components, loss of lubrication, failure of the crankshaft, and the subsequent

loss of engi ne power. ( ERA15LA338 )

 An engine crankshaft failed in flight, which resulted in a subsequent hard, forced

landing, the commercial pilot sustaining serious injuries, and one passenger

sustaining minor injuries. The pilot’s mechanic had replaced four engine cylinders

about 4 month s earlier. Postaccident examination revealed that t he engine cylinder

through bolts on the Nos. 1 through 5 cylinders were significantly undertorqued,

and the crankshaft was fractured . The mechanic’s failure to properly torque the

engine through bolts allo wed the bearings to move and led to the in -flight failure of

the crankshaft. The pilot had observed “an excessive amount of metal” in the oil

filter when changing the oil and filter before beginning the accident flight but

assumed that the debris was chrom e from the overhauled cylinders as a result of

“break -in.” Airport personnel who had observed the pilot perform the maintenance

expressed concerns about the metallic debris, but the pilot chose to finish the oil

change and continue with the accident flight , not realizing the risk of imminent

engine failure. (ERA14LA193 )

 The commercial pilot, who was conducting a skydiving flight with four passengers

on board, was f orced to conduct an off -airport landing to a highway after a total loss

of engine power during climbout. One passenger sustained minor injuries.

Disassembly and examination of the engine revealed that the crankshaft had

fractured near the No. 2 main bearin g surface, and areas adjacent to the No. 2

bearing on the inside of the engine case exhibited rotational scoring, indicating

bearing movement. It was determined that maintenance personnel applied

insufficient torque to the cylinder through bolts during rep lacement of the No. 2

SA-056 October 2016 cylinder 19 days before the accident. The insufficient torque allowed the bearings

to move and led to the eventual failure of the crankshaft. ( ERA15LA263 )

What can maintenance personnel do?

 Apply torque values provided in the applicable aircraft and engine maintenance

manuals. If the engine manufacturer has recommended torque va lues in its engine

specifications , use those specified values.

 Use a calibrated torque wrench to measure the amount of twisting force applied to

a nut or bolt (see figure 3). Verify that your wrench is calibrated and check that the

wrench is not damaged to ensure continued accuracy.

 Follow manufacturer guidance about whether to use l ubricants on fasteners.

 Use smooth, even pulls when applying torque pressure.

 Conduct a postmaintenance check after applying torque to fasteners to ensure that

the correct amount of torque was applied. If possible, have someone else

independently check the torque values using a different wrench.

 Also use torque seal to verify that the applied torque has not changed and to

visually confirm which fasteners have been torqued (see figure 4).

 Beware of leaving hardware that has been only finger tightened. On ce you start

the process, make sure you complete it. Set a reminder if you need to step away

before completing the torqueing process.

Figure 3. Mechanic applying torque to through bolts. Figure 4. Engine on stand showing

torque seal on fasteners.

Interested in more information?

If the engine manufacturer has not provided the specific torque values to be used,

reference Federal Aviation Administration Advisory Circular (AC) 43.13 -1B, “Acceptable

Methods, Techniques, and Practices - Aircraft Inspection and Repair,” chapter 7 ,

section 3, which provides general information about torque application and, in table 7 -1,

shows the recommended standard torque v alues to be used for nuts and bolts.

SA-056 October 2016 Also, figure 7 -2, shows how to calculate the corrected torque values for various torque

wrench adapters.

GA Maintenance Al ert: Safety and Security of Components provides general tips to

maintenance personnel on how they can ensure that airplane components are properly

secured and emphasizes the importance of following written procedures and mitigating

human factors, such as f atigue and time constraints, when performing maintenance

activities. This GA maintenance alert and others can be accessed from the FAA Safety

Team website at www.faasafety.gov .

The Aircraft Owners and Pilots Association article, Torque Time , provides detailed

information about the basic concepts related t o torque, why it is important, and when,

where, and how to apply it properly.

The NTSB’s Aviation I nformation Re sources web page, www.ntsb.gov/air, provides

convenient access to NTSB aviation safety products. The reports for the accidents

referenced in this safety alert are accessible by NTSB accident number from the

Aviation Accid ent Database link, and each accident’s public docket is accessible from

the Accid ent Dock ets link for the Dock et Management System. This safety alert and

others can be accessed from the Aviation Safety Alerts link.