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B-nut Fire Safety

CFI1513840

CFI1513840

Well Known Member
I just read about an incident involving a fire in an RV-8. Fortunately, it happened on the ground and both occupants were able to exit the aircraft safely. The fire was extinguished in a timely manner by ground personnel. The problem was traced to a loose B-nut at the firewall.

This brings up the point of safetying B-nuts. Obviously, Vans thought it was necessary for the fuel tank pickup tube and issued a Service Letter, but what about all the other B-nuts?

Are there some special circumstances that make only the fuel pickup tube prone to loosening? Why not safety all B-nuts? Would loctite be a possible solution? I realize that there are thousands of airplanes flying around with unsafetyed B-nuts, but knowing that fact would be little consolation if your airplane is on fire.
 
B nuts tight

My dad and I rescued a member of the US Aerobatic team (Bob Nance) many years ago when the fuel line b-nut loosened and caught his Great Lakes Special on fire after landing. He stomped on the brakes and flipped [yes-even taildraggers can flip. This was NOT an A model} The the plane had just had an engine overhaul.

I once met a missionary mechanic who was was interrupted while re-installing a fuel line. When he returned to the job, a visual did not reveal the loose nut. He later watched the departure takeoff and saw the plane catch fire and crash.
 
Which makes this a really....

Vern said:
My dad and I rescued a member of the US Aerobatic team (Bob Nance) many years ago when the fuel line b-nut loosened and caught his Great Lakes Special on fire after landing. He stomped on the brakes and flipped [yes-even taildraggers can flip. This was NOT an A model} The the plane had just had an engine overhaul.

I once met a missionary mechanic who was was interrupted while re-installing a fuel line. When he returned to the job, a visual did not reveal the loose nut. He later watched the departure takeoff and saw the plane catch fire and crash.
Click to expand...

...good $1.95 safety item!

http://www.skygeek.com/f-900torque-seal-orange-.html

stylespilotshop_2030_93175900
 
Torque Seal

It may be just a question of strength, but from what I understand, torque seal does not "lock" the part in place, but rather provides an indicator that will show if the part has loosened. In other words, a visual inspection would still be required to detect a loosened nut. What I had in mind was a something more like locktite or pro-seal that would positively keep the nut from coming undone.
 
I think Gil's point was that if the missionary mechanic had been in the practice of using torque seal, he would have noticed that the b-nut had not been final tightened.
 
Mel said:
I think Gil's point was that if the missionary mechanic had been in the practice of using torque seal, he would have noticed that the b-nut had not been final tightened.
Click to expand...

This is sort of a mystery to me - why many don't use the inspection lacquer, that is. I am amazed at how many "ready to fly" planes I've looked at that don't have inspection lacquer telling the story of the individual nut's tightening. Another issue are those that tighten ten nuts, then apply the inspection lacquer to all ten. Defeats the purpose. Torque one, mark one, repeat as necessary.

Oh, and I have found more than one loose b-nut on said planes. One loose nut could kill you, why not add 20 minutes' time to the airplane assembly by doing this procedure correctly?
 
AlexPeterson said:
This is sort of a mystery to me - why many don't use the inspection lacquer, that is.
Click to expand...

Last year I made up a few lines on the Mustang I help a friend of mine with. When you have as many lines as these:



Its not uncommon to tighten something up thinking you're done then have to take it apart later. Then you have to clean off the old torque seal and redo it. That can be a major pain. When plumbing the sequence valves here in order to get one line off it often took getting two or three others off to get to it. The other gear well was worse.

Or, you tighten something up, look down for the torque seal, and you end up marking the wrong one. Easy to do if you get distracted.

So I have a simple rule. If you put it on, you tighten it. If you leave it loose, either leave them ALL loose or tag them as I did here with masking tape. When something's tight, pull the tape off. And I always double check every nut before I call anything finished, especially on b-nuts.
 
It still scares me to relate this story...

Many years ago, I flew my former homebuilt after an installation of a new fuel pressure gage. As I climbed out, a strong fuel order was evident. I Immediately reduced power and landed. I rolled to a stop near the pumps and pulled the mixture leaving all other switches alone. Opened both doors and evacuated the plane.

After an hour or so of ventilation, I pulled the plane to my hangar and looked for the problem...a loose b-nut on the fuel pressure line at the firewall bulkhead, aft.

The whole bottom of the fuse was soaked with fuel and any ignition source would have set off an conflaguration which would have mean't a short end to me and my plane.

So, on my new project, all torqued fasteners and components have a torque seal mark...any not marked will be fixed before first flight.
 
Newbie question

OK - I'm a beginning builder and have a question. By "B Nut" are you referring to (in the above posted picture) the blue colored nuts that hold the fuel lines to the different T fittings? I'm only on the empennage kit, but do Van's instructions call for any kind of seal on the outside, or is it just common A&P knowledge?

There's a community college about 20 miles from my home that offers an A&P certificate class -- I may just figure that into the cost of the build, just to be safe :cool:
 
N814P said:
OK - I'm a beginning builder and have a question. By "B Nut" are you referring to (in the above posted picture) the blue colored nuts that hold the fuel lines to the different T fittings? I'm only on the empennage kit, but do Van's instructions call for any kind of seal on the outside, or is it just common A&P knowledge?

There's a community college about 20 miles from my home that offers an A&P certificate class -- I may just figure that into the cost of the build, just to be safe :cool:
Click to expand...

Ralph,

Yes, the B nuts are the blue colored nuts that are used on the fuel lines. A couple of years ago Vans issued a service letter to safety the B nuts on the fuel tank pickup tubes with safety wire. Apparently there were some cases of fuel tubes coming loose. You'll become familiar with this mod when you do the tanks. Otherwise, there is no requirement that I know about, either on homebuilt or certified aircraft, to safety B nuts. They are simply tightened, which is why I asked the question in the first place.
 
B Nut Safety

I think the one on the fuel pickup is subject to rotation as one end is fixed and the other not. Vibs and G forces can turn it like a crank. If both ends are fixed then there is no chance of rotation and no need to safety. Just MVHO.:D
Don
 
N814P said:
OK - I'm a beginning builder and have a question. By "B Nut" are you referring to (in the above posted picture) the blue colored nuts that hold the fuel lines to the different T fittings? I'm only on the empennage kit, but do Van's instructions call for any kind of seal on the outside, or is it just common A&P knowledge?

There's a community college about 20 miles from my home that offers an A&P certificate class -- I may just figure that into the cost of the build, just to be safe :cool:
Click to expand...

Ralph, don't even THINK that your newbie question was stupid. I read this whole thread asking myself, "WHAT'S A FREAKIN' B-NUT??" And I've been building for 12+ years! Never ran across that term before. Now I know.

Thanks for asking! :)
 
"B" Nut nomenclature...

A few years ago, I had the same kind of questions on what exactly was a "B" nut, and why the letter "B". I never did get, what I would consider, a definitive answer, but I did spend some evening down time trying to find an acceptable answer...at least for me.

If you refer to the ACS catalogue, look up p/n 05-16240. There are several types in this group, and they are called Cable End "B" nuts.

So why are "fuel line" type nuts also called "B" nuts? Look up the AN817 Nut/Sleeve coupling. The ACS paragraph indicates this type has been largely superseded by the AN818 nut and AN819 Sleeve.

To my way of thinking, this meant the AN817, the first / original one, became known as the "A" style and the AN818 became known as the "B" style, and the aviation community began differentiating between the two with the term "B" nut. (A was first, B is second,...)

I may be completely wrong on the history of the term, but I do know that the "B" nut term can refer to two completely different mechanical solutions that are not interchangeable.
 
Here's another datapoint. A friend of mine did an extensive condition inspection on an RV-4, which was built in 1993. He found the b-nuts on the fuel selector were only finger-tight. Took them off without a wrench. This was back in 2006 or so, so it flew for 13 years with b-nuts that should have leaked but never did.
 
noelf said:
A few years ago, I had the same kind of questions on what exactly was a "B" nut, and why the letter "B". I never did get, what I would consider, a definitive answer.
Click to expand...

The term ?B Nut? has finally been traced back to the early days of standardization of aviation hardware for the military. For tubing and plumbing, the terminology that was adopted came from the leading supplier of aviation those products at the time, the Parker Appliance Company. Parker published a book at the time called ?Maintenance of Aircraft Tubing Systems?. I?m not sure when the first edition of this book was published, but I am aware of a ?second edition? that was published in 1939, and the copy we have in the EAA library was published in 1943.

There is a section of this book titled ?Flared Tubing Fittings?. It is in this section that we find the references to ?B Nuts?. It is interesting to note that the AN818 coupling fitting commonly referred to these days as a ?B Nut? is not actually a ?B Nut?. It is actually a ?BT Nut?. The AN819 sleeve that is used with the AN818 coupling is a ?T Sleeve?.

There is a ?B Nut? mentioned in the book, and it is also used for flared fittings. However, the ?B Nut? is a male thread nut that is made to be used in a matching female thread receptacle. This combination was called the ?AC810 series fitting and to my knowledge was not included when the move was made to the ?AN? series fittings that we know today. The ?BT Nut? and ?T Sleeve? that became the AN818 and AN819 fittings we use today were referred to as the AC811 series in the Parker book. In the 1943 edition of the book, the AC810 series (including the ?B Nut?) is referred to as ?obsolete?.

So that?s the story of the ?B Nut?. The next time you hear a mechanic refer to an AN818 coupling as a ?B Nut? you can correct him/her with the proper ?BT Nut? nomenclature! The ?B Nut? is obsolete!!

Hope this helps!

Cheers!

Joe
 
OK Joe, we need to get Doug to designate you the "Official VAF Historian"! I just love reading research on history trivia like that - good stuff.

Paul
 
Finally!!!

What a great piece of history! And an explanation! This also sounds like a great writeup idea for the last page of EAA's Sport Aviation magazine...
 
My thoughts on the so called Bnut is the nut may not be loosening, but the fitting itself, flaired line. So checking and tightening on a consistant schedule might be in order. I find its needed on all my bnuts. I know we are suppose to go about a 1/4 turn on them, but I feel better going about 1/2 turn after contact. I think a good experiment would be to tighten a bnut and than mark it, after a year or sooner, check it to find if it's loose, if so, I might be right.
 
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