Accidents: Causes and Contributing Factors.

apkp777 · #1 03-11-2010, 08:23 AM

There's been some discussion on another thread, that belongs in a new thread. I'll get this started.

It seems that it's possible that a VS and Rudder may have separated in flight from a unnamed RV-7.

One of the suggestions to the cause might have been improper over/under torquing of the attach bolts. In my career as an A&P for UAL I had the opportunity to hang many jet engines. On several occasions I would wake up in the night sweating and wondering "did I torque those bolts". Never a good question!

As a mechanic, the question "did I torque?" can only be answered by re-torquing. With only 3 or 4 nuts holding a large turbo fan engine on a wing strut, it's clearly an important question. Fortunately it's easily answered by checking the paperwork and the corresponding sign-offs. Still makes you nervous about the possibility of an oversight. I don't know how you use your builder's manual, but personally I initial every line when it's completed. I even initial the black spaces.

Our little RV's have the same all important fasteners. Torquing them is vital but easily screwed up. My first time torquing something important was on a Queen-Air during my powerplant practical exam. My buddy was holding the wrench but we were partners on the exam. We made the classic mistake of using FT/LBS instead of IN/LBS. Yikes, snap there went the stud for the 1 of 4 studs on the fuel pump (previously airworthy AC). Needless to say, lesson learned.

So yesterday I did a search for torque values for an AN 3 bolt with AN365 tension nut. The first hit came back with a PDF file from Matronics and lists the torque as 20-25 FT/lbs This is WRONG. There is no author of this document and not official in anyway. Still I am sure that individuals have used this "unofficial" document to torque their AN3's. To 12 times the correct torque value of 25 IN/lbs.

AC 43.13-1B should be on the bench of every builder and should be the sole reference document in lieu of manufacturer's specs. I really believe the builder of the aircraft in question did a fine job, and used correct techniques.

Personally, I think it's more likely that an over zealous "snap-roll" is more plausible. Remember the A300 that lost the tail because the pilots were over zealous controlling their engine failure?

RV7Ron · #2 03-11-2010, 08:44 AM

Quote:

AN3 bolts tend to be very fragile at the end of the thread. AN bolts are 125,000 psi rated strength. NAS bolts are 160,000 psi. NAS bolts are cheap insurance in critical areas.

I saw this posted on the other thread and liked the sound of it. From the guys with more experience than me, is there any flaw to doing this that I might be missing? I was taught that when making a repair in a skin (not quite the same, but similar concept) to not make the repaired area stronger than the original or you will introduce a stress concentration in that area. Does this logic hold with stronger bolts or would substituting NAS bolts be ok?

Ron Lee · #3 03-11-2010, 09:01 AM

Good question Ron. I am not a materials guy so this is merely my GUESS.

The bolts are in shear so increasing their shear properties is a possible positive in this application.

Just a guess folks. Don't act based upon this.

apkp777 · #4 03-11-2010, 09:04 AM

The NAS bolts are more expensive and not really necessary in my opinion. 125,000 psi, is ridiculously strong. Especially when you consider that 6 of them (750,000 psi) are loaded in tension on the HS, 6 are loaded in shear. AN bolts are more than sufficient. After all they are common on all light and heavy aircraft. I am pretty sure that unless you damage them by over-torquing, there's no need to worry about them.

NAS bolts are more useful for installations where you can't use multiple fasteners and you need extreme strength of individual bolts. This is the case with engine bolts on large aircraft. Only 3 keep most engine on the airframe. If you remember way back, there was an American airlines DC-10 that crashed over Chicago, after one of those bolts was cracked because of the technique used to hang the engine. So even NAS bolts are not invincible. Still comes down to using proper torque and technique. Not to mention a quality, calibrated torque wrench.

I think when the cause of the RV7 crash is finally released we'll see that it had nothing to do with AN3 bolts.

RV7Ron · #5 03-11-2010, 09:15 AM

Quote:

Originally Posted by apkp777

The NAS bolts are more expensive and not really necessary in my opinion. 125,000 psi, is ridiculously strong. Especially when you consider that 6 of them (750,000 psi) are loaded in tension on the HS, 6 are loaded in shear. AN bolts are more than sufficient. After all they are common on all light and heavy aircraft. I am pretty sure that unless you damage them by over-torquing, there's no need to worry about them.

NAS bolts are more useful for installations where you can't use multiple fasteners and you need extreme strength of individual bolts. This is the case with engine bolts on large aircraft. Only 3 keep most engine on the airframe. If you remember way back, there was an American airlines DC-10 that crashed over Chicago, after one of those bolts was cracked because of the technique used to hang the engine. So even NAS bolts are not invincible. Still comes down to using proper torque and technique. Not to mention a quality, calibrated torque wrench.

I think when the cause of the RV7 crash is finally released we'll see that it had nothing to do with AN3 bolts.

Thanks Tony, good explanation, I appreciate it. And although I have limited aircraft experience I am a Mechanical Engineer and I believe your argument is a good one. I just wanted to hear it from someone who knows about these things.

apkp777 · #6 03-11-2010, 09:25 AM

Quote:

Originally Posted by RV7Ron

Thanks Tony, good explanation, I appreciate it. And although I have limited aircraft experience I am a Mechanical Engineer and I believe your argument is a good one. I just wanted to hear it from someone who knows about these things.

I appreciate that. I am not an engineer, neither did I sleep at a Holdiay Inn last night, but did spend most of my life working on aircraft. Seen lot's of stuff that makes me wonder. Can honestly say I have never seen a failed AN3 bolt. For the record, most A&P's don't use torque wrenches on them 100% of the time. (wait while I duck to avoid all the bullets

).

jrs14855 · #7 03-11-2010, 09:50 AM

I will rephrase what Tony posted to "I believe that many A&Ps do not use a torque wrench on AN3 bolts 100% of the time. In fact I have never seen anyone use a torque wrench on an AN3 bolt. This is exactly my reason for posting the NAS bolt information. I believe that the NAS bolts will stand up to considerably more overtorque than an AN. I also believe that at least SOME AN3 bolts have some manufacturing flaws. I am not suggesting that the much stronger NAS bolt is needed from a structural standpoint, just that the NAS will withstand a lot more "installation abuse".

fl-mike · #8 03-11-2010, 10:28 AM

I found a torque screwdriver to be much more useful for AN3 nuts. With a socket adapter it becomes a torque nutdriver.

Ron Lee · #9 03-11-2010, 10:36 AM

Where is the thread on this incident?

apkp777 · #10 03-11-2010, 10:44 AM

JRS14855 - Yes, I wanted to be careful, truth is, I've never seen anyone use a torque wrench on them either.

A good 1/4" drive ratchet will give you a accurate and consistent torque. Seldom, if ever do I ever use 3/8 drive stuff. 1/2" drive should be banned from Aerospace.

Yes, the NAS would probably be a good alternative. I don't think everyone needs to go out and replace all their AN hardware though.