Fuel Lines

[170C]

In B model 170's ( and I assume on my '56 172), are the aluminum lines that run inside the rear door posts fuel lines? If a leak occured in one, where do they have joints/couplings? Could a section be cut out and replaced with a short piece using some type of coupling?

[GAHorn]

AN 818/819/815 fittings, sleeves, and unions can be used to replace a short section of tubing if it's punctured or damaged.

[170C]

Thanks George, but are the lines in the rear doorposts fuel lines?

Couldn't remember if they were in the rear door posts or in the front door posts.

[c170b53]

Frank, the lines run down the rear posts and right along side of the flap cables. You may be space limited and thus a joint in that location may not be possible. The Cessna manufactured fittings are at the top fwd of the post and below the floor fwd of the aft post. Putting PK screws into the posts can lead to problems as in a leak.

[170C]

Thanks George, that is what I needed to know. It would appear that if a leak developed in the door post area it might require replacement of the entire section of tubing I don't know how much of a bug-a-boo that would be to accomplish. I am not sure what PK Screws are, but I have some upholstry attached to the lower portion of my door post and I am concerned that a screw may have damaged the fuel line. There is a slight indentation on the fuel line. No apparent leak has developed to date, but there is evidence the screw did hit the line. I have applied some J B Weld to the spot and will monitor it.

[flyguy]

Way back when - - - I replaced my windshield and used PKs to replace the retaining strip rivets on the cowl. When the job was finished we took off for "somewhere". After climbing out to the choice of flight level, I told Deana "Hey we got no "airspeed". At least no"indicated" airspeed . The weather Gods were smiling on me, for there was no prediction of low vis or such but I was puzzled about the indicator's failure to show what I knew to be happening. Later, as I began to explore reasons for the problem, I discovered that one of the PK screws had punctured the soft aluminum tubing from the pitot to the A/S indicator. This tube is located just under the boot cowl close enough for that short screw point to drill a little hole in the tubing.

The AN setup George displayed is OK if the tubing is still "soft" enough to accept the "deformation" needed to seal the coupling. Lots of our planes "resident tubing" could have some embrittlement from age and be less malleable. Better to consider new replacement if there is a failure.

I recently had occasion to "inspect" the lowest sump drain on a 172. This is the one below the fuel selector extending through the belly skin. It was apparent that it had held water long enough to cause internal corrosion and the tube itself failed only when I was attempting to remove the plug to drain it. Close inspection showed there were several little spots of corrosion on the exterior exuding from larger areas inside the tube. One might expect fuel loss to occur if one of these spots became a pin hole. The tube I mention is in an area where there is little chance of disturbance but probably hadn't been subject to "inspection" or drained for a long time.

[GAHorn]

Thanks George, that is what I needed to know. It would appear that if a leak developed in the door post area it might require replacement of the entire section of tubing I don't know how much of a bug-a-boo that would be to accomplish. I am not sure what PK Screws are, but I have some upholstry attached to the lower portion of my door post and I am concerned that a screw may have damaged the fuel line. There is a slight indentation on the fuel line. No apparent leak has developed to date, but there is evidence the screw did hit the line. I have applied some J B Weld to the spot and will monitor it.

Frank, that was Jim McIntosh who provided the answer regarding the "yes" about the fuel lines in the rear doorposts. And he is very correct that upoholstery screws and such are famous for poking holes, "Ol' Pokey".

(PK screws are the common, aircraft sheet-metal screws with sharp points. I always admire an interior shop who has the talent to hit a fuel line exactly in the center so the screw punctures the fuel line instead of sliding off to the side.)

And Yes, Jim is also correct that depending upon the exact location of any leak, there is limited space within that doorpost.

IN FACT...space is so limited that I found the cables can chafe against door-latches and the fuel lines. I lined my door post interior with thin teflon sheet in spots where chafeing was a potential in order to prevent it. "Micarta" or phenolic sheet may also be used at such locations and is commonly used in aircraft.
1/16" thickness is available http://www.aircraftspruce.com/catalog/m ... enolic.php

Soft aluminum tubing is available from Spruce also, and you can fabricate your own for this type repair.

Keep in mind that this location is only gravity-fed and extremely low pressure. If the wall was only dented and the diameter not actually reduced from it, then no repair is probably needed. Putting JB Weld on it will not prevent it from leaking , but will hide the damage from inspector's eyes, and make him suspicious. Then he will grind the JB Weld off and wonder how thin the remaining tubing is and then likely require it's replacement..... while a small dent would be obviously still OK.

I once had a friend with a C-175 which has an intake crossover tube that runs below the forward portion of the engine. It had a JB Weld repair on it where the cowling had chafed against it. Since the JB Weld hid the chafed spot from his eyes...he had no way of determining how thin that tubing was. In fact, he had no way of determining if the JB was being used to prevent further chafeing....or was hiding a hole! So he had to remove the JB Weld, and then have the area "built up" by welding. (No salvage induction tubes could be found on this out-of-production system.)

The first welder immediately "blew" a huge hole thru the tube, making it completely useless and maybe condemning it to the junk pile. Fortunately, I knew a very experienced and talented aircraft-certified welder who was able to completely cut out the damaged area and splice new tubing into the induction system. The cost of that little "dab" of JB Weld was about $175, not including the removal/installation of the induction system.

Moral: Be careful of making repairs that may be viewed as "hiding" damage.

[blueldr]

Be advised that PK (sheet metal) screws come in type A and type B, type A being pointed and type being blunt. Any screws terminaing near any other part of the sructure should be type B. The rear door posts are especially vulnerable and any PK screws used therein should be type B.

[cessna170bdriver]

Be advised that PK (sheet metal) screws come in type A and type B, type A being pointed and type being blunt. Any screws terminaing near any other part of the sructure should be type B. The rear door posts are especially vulnerable and any PK screws used therein should be type B.

I can't find any documentation to prove it, but I've always been under the impression that PK screws were a particular type of sheet metal screw, had the letters "PK" as part of the part number, and always have a sharp point. I am also under the impression that the sheet metal screws such as those in our cowling and inspection covers are not "PK" screws, but are something like AN530, and can be specified with either the sharp "A" point or blunt "B" point.

As I said before, I don't know this for sure, and I stand to be corrected if necessary.

As concerns the door trim screws in the 170, I too worry about them chafing the fuel lines. In that application, I use the #8 3/8"-long B-point screws , which aren't long enough to interfere with the lines.

Miles

[GAHorn]

I was taught that "PK" stood for "Parker-Kalon" the company that was the originator/patent-holder of the self-tapping sheet metal screw. The tip is sharp, but other screw-types ("A" or "B") have been assigned to them also. The patent has since run out and the terminology has become commonplace...sort of like talking about a "Zerox copy" when one actually means a "photocopy".

[mike roe]

On this subject.While restoring my 52,I replaced the upper cabin skin.With this remmoved you see where the fuel lines run inside the upper door sill.This plane had a Cessna shoulder harness kit installed.Which required a gusset to be installed right where the fuel lines turn and go down the sill behind the door.Turns out when they drilled for the gusset they hit the fuel line with the drill bit.Putting a good size gouge in it,never knowing it happened.If the headliner was out when you installed the kit you would know the fuel line location.I made a new fuel line,which is another storry.

[blueldr]

Among us really old farts, "P-K" became the generac name for all sheet metal screws but Parker-Kalon was the developer. The same thing is true with those self tapping screws with a "twist drill" type tip for use where the material is too thick for sheet metal screws. They're generally referred to as "TEC" screws and usually have a hex head for accepting more torque for driving through heavier material. TEC screws are not used in aircraft and Ido not believe they have ever had an "AN" desgnation.

[Brad Brady]

Just from experience.....if you remove the data plate from your left hand rear door post for painting or what ever......you might not reintroduce it with #6 PK screws.....I guarantee that you will puncher the left hand main fuel line .....Brad

[Zreyn]

According to the info section of the A/C Spruce catalog the 'a' type pointed sheet metal screws are for self tapping into sheet metal & the blunt "B"type are for use with Tinnerman nuts.
The two dif screws have dif thread pitches & the "B" type lock when tightened the last little bit & the "A" type shouldn't be used with Tinnerman nuts,like on the cowling.