Aw Nuts!

B-nuts to be specific. The NTSB has released Safety Alert 086 titled Mechanics: Ensure B-nuts Are Properly Secured! You can view the Safety Alert by visiting

The Safety Alert starts off saying, “B-nuts are a small piece of hardware that can cause a big problem if they are not properly secured.”

It goes on to discuss “The problem.”

“A B-nut is a common term for a nut that provides the clamping force to create a reliable seal in lines (such as fuel, oil, or air lines on a reciprocating or turbine engine) installed on an aircraft. If a B-nut is improperly secured (either torqued too much or not enough), a loss of engine power or an engine fire could result.

Under- and over-torqued B-nuts could cause fuel, oil or air leaks depending on there the B-nuts are installed; over torqued B-nuts could also result in deformation and damage to a line. Fuel or oil leaked onto a hot engine could result in a fire.

B-nuts are exposed to vibration and thermal expansion and contraction during operations; therefore, it is critical that maintenance personnel ensure that B-nuts are properly secured.”

The Service Alert then goes on to discuss several accidents that were caused by improperly secured B-nuts.

In each of the example accidents in the NTSB’s alert, an improperly installed or torqued B-nut was cited as the cause of the accident. 

“Post-accident examination of the helicopter revealed that the air line that connected the governor to the fuel control unit was not attached and that the fuel control B-nut had ‘backed out’ to the elbow of the air line. These findings indicated that the mechanic had not properly secured the fuel control unit B-nut and that the loose B-nut led to the partial loss of engine power.” 

“The loose B-nut likely led to a loss of fuel pressure and fuel flow to the engine, which resulted in the total loss of engine power.”

“The pilot, who was also a mechanic, performed the helicopter’s most recent annual inspection about 75 hours before the accident, and he had likely not torqued the B-nut on the fuel control according to procedures, which led to the total loss of engine power.”

If you are even remotely aware of Human Factors, you know that the cause of these accidents was likely Human Factors — NOT B-nuts! We are human, and as humans we make mistakes. It’s likely that none of these errors were done intentionally. However, Human Factors likely led to the errors. Distraction can cause us to forget a step in an installation procedure. Fatigue can lead to a lack of concentration. Pressure can cause us to rush a job and possibly lead to an error of omission. The list goes on and on.

Human Factors training CAN prevent errors from happening. You’re “nuts” if you think otherwise!

The FAA has a brochure discussing the twelve Human Factors (known as the Dirty Dozen). It’s basically a short introduction to Human Factors. You can view it by visiting

Thanks for reading, and we appreciate your feedback!

Joe Escobar