Safety Value

[F1R]

If any readers are ever wondering about the value of this forum. A number of years ago Ted Chang posted about his RV 10 Fire and the cause being a loose B nut on the pressure side of his fuel pump. The inner voice thought this was a worthwhile item to check on my own 540, where I discovered that finger tip pressure could loosen the pressure side B nut (out put side) on my engine driven fuel pump. There is little doubt in my mind that this forum and Ted's post saved my own ship and possibly a lot more.

So let's keep DR and this forum up and running.

[Tommy123]

B nuts is one item I used torque seal. Not really a seal but a easy way to see if it has loosened.

[Canadian_JOY]

Like you, I recall Ted's story every time I'm looking in between the engine and firewall... Agree - safety value for sure.

[TS Flightlines]

WE get asked ALOT about torque specs and how tight is ok. Ted's story is a great example. Whether its actually 'torqued' or not, have someone put a wrench on every nut and fitting to check. Might save your life.
Torque, well the German spec of 'goodntite' works really well.

Tom

[jrs14855]

Hard to find but the B Nuts are available drilled for safety wire.

[Tommy123]

Originally Posted by jrs14855

Hard to find but the B Nuts are available drilled for safety wire.

Never saw one on a airliner, they typically don't loosen. That example was probably never tightened correctly. The torque is specified in 43:13, and by manufacturer, I forget exactly but something like hand tight and then a flat or two. Not a torque wrench number. Too tight and you'll crack the flare.
That's why I use the torque indicating stuff, I apply it after I know I torqued it, don't have to wonder if I forgot to do it.
Working new airplanes being opened up for the first time at the airlines that stuff was everywhere.

[DanH]

The -6 pump inlet fitting is steel, and the elastomeric line feeding it should have steel ends and nuts. I suspect that use of standard AN torque tables may contribute to loosening. Consider, for example, the Aeroquip page published by Vans:

https://www.vansaircraft.com/pdf/Tor...m_Fittings.pdf

The torque call-out for a -6 is 130 to 180 inch pounds, or 10.8 to 15 ft-lbs. However, those values are for aluminum fittings, not steel. The page tells you to refer to Bulletin JA14A (another Aeroquip publication) for steel fittings, but JA14A only describes the flats method.

There are lots of guides for tightening hydraulic fittings in industry, and the SAE straight thread/o-ring to male flare pump fittings most of us are using are straight from the hydraulic parts bin. Dig around in the industry charts, and you'll see oiled -6 JIC 37 degree flare torque around 16 to 20 ft-lbs, and dry fittings at 20 to 22.

There is another factor which may play into a few loose pump feed hoses. A builder may select a straight, 45 degree, or 90 degree inlet fitting. If he selects a straight fitting, a 90 degree steel tube hose end becomes the most likely choice. The potential problem is that a hose approaching from a fixed point on the firewall can then serve as a lever when the engine moves on its mounts. It is 90 degrees to the fitting axis, in the plane of rotation necessary to loosen the B-nut. Me? I use a 90 pump inlet fitting and a straight hose end. I think the fitting's o-ring seal more resistant to leakage if the clamp nut is loosened, and even if backed off, the fitting itself can't rotate out of the pump.

I previously said the fittings should be steel. As far as I'm concerned, all the firewall forward fuel and oil fittings should be steel. They're fireproof, and far more fatigue resistant, goods reason to use them. However, they're also friendly to the goodntight method Tom mentioned previously. It was a forum contribution by Walt Aronow who convinced me to add a habit to my maintenance; I grab an open end and tug every fuel fitting every time the cowl comes off. No torque wrench, no counting flats, just a firm pull. I know, failing to use a torque wrench isn't kosher, and surly I shall suffer God's own wrath as a result...but I'm sure the fittings are tight.

[Caveman]

Originally Posted by DanH

It was a forum contribution by Walt Aronow who convinced me to add a habit to my maintenance; I grab an open end and tug every fuel fitting every time the cowl comes off. No torque wrench, no counting flats, just a firm pull. I know, failing to use a torque wrench isn't kosher, and surly I shall suffer God's own wrath as a result...but I'm sure the fittings are tight.

Ditto! I'm the world's worst skeptic. I religiously used torque seal during my build and didn't understand how if it didn't crack, a B nut could have loosened. Walt was persistent and I respected everything else he's tried to teach us, so the next time I had the cowl off, I went to testing connections with a wrench. I easily got a flat on the very first one. A few others were not as tight as they should have been. I still don't understand how it happens without cracking the torque seal, but I'm now a firm believer of checking all B nuts with a wrench at my condition inspections.