Another Dumb A$$ Mechanic

[Hank Jarrett]

Re: Another Dumb A$$ Mechanic

It's funny what happens to glue and wood with age. I built a model of a DH-4 back in the mid 60s as a kid. Just a little 24" span rubber model. I used to really like the structure so I never covered it and it was in a box stored away for decades. When I found it I hung the bare frame from the ceiling. The earth quake knocked it down and when it hit the floor about half of it disassembled itself! Not one piece of 50 year old wood broken, but the glue joints all just fell apart!
Now I doubt Taylorcraft used Ambroid (remember chewing it off your fingers?) but it was just amazing that when I looked over the pieces there was NOT ONE TRACE of glue anywhere! I KNOW that I wasn't that neat at 10 years old. I wonder what happens to the glues they used on our planes in 70 years?
Hank

[avenger1949]

Re: Another Dumb A$$ Mechanic

I still have a few planes from that era and actually still fly one every so often. Ambroid was my glue of choice back then and I would think perhaps the glues of the 40's would have deteriorated some.

[N96337]

Re: Another Dumb A$$ Mechanic

That's why my Tcraft is apart. ALL the glue joints in the spars were failing. From the outside it looked fine...once the cover was off it was pretty obvious they were gone. Did I submit a m&d report? You betcha! I've never had a problem with "too much information" available, even if it's info that I already know and maybe haven't though of in a while. Never hurts to be reminded.
At any rate, I commend the mechanic that submitted the report and pics in the post... lots of people THINK they know all about wood, and would've looked right past those cracks.
John

[Eagleavn]

Re: Another Dumb A$$ Mechanic

Got this on AMT, must be a cessna mechanic that was suprised to find cracked spars!

Most mechanics on the older fabric aircraft know to how look and where.....IMHO. Tim

http://www.amtonline.com/pdf/alerts/2011_08_Alert.pdf

I think it starts on page 7.

Quote from the good book:


2. Defects Not Permitted.
a. Cross grain. Not acceptable, unless within limitations noted in 1a.
b. Wavy, curly, and interlocked grain. Not acceptable, unless within limitations noted in 1b.
c. Hard knots. Not acceptable, unless within limitations noted in 1c.
d. Pin knot clusters. Not acceptable, if they produce large effect on grain direction.
e. Spike knots. These are knots running completely through the depth of a beam perpendicular to the annual
rings and appear most frequently in quarter-sawed lumber. Reject wood containing this defect.
f. Pitch pockets. Not acceptable, unless within limitations noted in 1e.
g. Mineral streaks. Not acceptable, if accompanied by decay (see 1f).
h. Checks, shakes, and splits. Checks are longitudinal cracks extending, in general, across the annual rings.
Shakes are longitudinal cracks usually between two annual rings. Splits are longitudinal cracks induced by artificially
induced stress. Reject wood containing these defects.i. Compression wood. This defect is very detrimental to strength and is difficult to recognize readily. It is characterized
by high specific gravity, has the appearance of an excessive growth of summer wood, and in most species
shows little contrast in color between spring wood and summer wood. In doubtful cases reject the material, or subject
samples to toughness machine test to establish the quality of the wood. Reject all material containing compression
wood.
j. Compression failures. This defect is caused from the wood being overstressed in compression due to natural
forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs or lumber.
Compression failures are characterized by a buckling of the fibers that appear as streaks on the surface of the piece
substantially at right angles to the grain, and vary from pronounced failures to very fine hairlines that require close
inspection to detect. Reject wood containing obvious failures. In doubtful cases reject the wood, or make a further
inspection in the form of microscopic examination or toughness test, the latter means being the more reliable.
k. Decay. Examine all stains and discoloration carefully to determine whether or not they are harmless, or in a
stage of preliminary or advanced decay. All pieces must be free from rot, dote, red heart, purple heart, and all other


Just sayin.....
LP

[Guest]

Re: Another Dumb A$$ Mechanic

I would not criticize or attack this man for his AMT input that's not what we would want for ourselves.