Flight Control Balancing Instructions

[David Pfeffer]

Alright, I've gone through the maintenance manual twice and can't find the limits or instructions for balancing the PA-30 ailerons and Stabs. Am I blind or is it not there?! I doubt it's going to be on the TCDS, but I don't have a copy yet, so haven't checked there. The surfaces have to be balanced- has anyone found the limits?

I have a structural repair to complete on my R/H Stab upper surface skin that's all the way at the trailing edge. It's gonna hose the balance. Need to rebalance but need the balance weight (washers presumably) part numbers and limits.

Thanks!!

[N3322G]

In the Service Manual I've customized for my 39-10, it is in Part II, structures, 4.1.

In the PA-30 section it is in Structures 4-50

Both have bolt and part #s

FYI. The Service Manual on this website under Tech Tab is searchable. In the find box I put Balancing and up popped the Figure 4.11 for the first reference.

Watched this done after painting so I already knew sort of what to look for.

[Kristin Winter]

Are you aware that any repairs to the stabilator, other than replacement of complete skins, is forbidden by Piper and hence illegal?

From the SM:

4-45. REPAIR OF STABILATOR AND STABILATOR TAB (HORIZONTAL TAIL
SURFACE). Precautions must be taken when performing any maintenance and/or repairs
to the stabilator or stabilator trim tabs. Repairs must be made in a manner that maintains
the original configuration, strength, stiffness and weight distribution. Repairs are limited to
replacement of complete skin sections, ribs, hinges, spars, etc., and paint.

The balancing procedure is in the service manual at or about section 4-50.

[David Pfeffer]

Thanks Kristin. Greatly appreciated. I didn't realize someone had converted the pdf to make it searchable. That's fantastic!!!

Piper's warning should apply to ALL structure. Funny how they mentioned it specifically with the stab. Any repairs need to ensure the original strength is maintained (and that the repair isn't overkill strengthening the area too much, which would lead to fatigue issues). When we don't have loads data to use, such as in the case with nearly all FAA aircraft (the Navy is much better at that), we use a method called equivalent strength to analyze the load lost by the damage and increase in strength with the repair. As the name implies, the repair must be as strong as the full load capability of the original joint, skin, etc. This results in a conservatively strong repair, as no aircraft designer intends for the joint, skin, etc to reach 100% limit load in normal operation.

You're a wealth of knowledge Kristin, greatly appreciate all your help!!

[Kristin Winter]

I suspect that Piper is afraid of changing the stiffness of the stabilator and hence the flutter characteristics. It is also possible that they are afraid that a patch repair to the tail will result in a stabilator that is out of balance.

[David Pfeffer]

Keep in mind too that the manual was written before AC 43.13 was authored. AC 43.13 gave mechs standard structural repairs that were approved using equivalent strength to make them general, applicable to all light aircraft structure. Assuming the repair isnt complete overkill, not significantly changing the aeroelastic properties of the stab, flutter won't be and issue if the stab is balanced.

I'd love to know who dropped a (seemingly) screw driver through the stab though!!!

[Kristin Winter]

I am not sure when the first 43.13 was written, but that is irrelevant as the FAA preamble to AC 43.13-1B is quite clear.

This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions. This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may also be used as approved data, and the AC chapter, page, and paragraph listed in block 8 of FAA form 337 when:

a. the user has determined that it is appropriate to the product being repaired;

b. it is directly applicable to the repair being made; and

c. it is not contrary to manufacturer’s data.


While I don't disagree that repairs to the stabilators can be safely made using AC 43.13, it is flat illegal and anyone signing it off is asking to lose their licenses.

[David Pfeffer]

Kristin, I sincerely love the vast info you bring to the table, but I'm going to have to politely disagree with you on this issue. I do understand your rational though, and I would concur that submitting a 337 for a repair would be the safe way to go, or have it signed by a DER.

[Kristin Winter]

I am not sure what you are disagreeing with. There is really no ambiguity in the FAA's statement about the scope of AC 43.13-1B stating that it cannot be used to override the manufacturer and there is no ambiguity in Piper's statement that repairs involving patches, new seams, etc. are not allowed. However, if you can get an IA to sign off a repair, then they will take the fall if the FAA gets wind of it. Though sooner or later, you are likely to run into an IA on an annual that realizes that such a repair is unairworthy and requires you to have the stabs reskinned.

Best of luck!

[David Pfeffer]

Morning Kristin. My intent was to either use a 43.13 repair for supporting documentation and get a field approval or develop a repair as a Navy DER and send it off to the FAA ACO office for approval. Regardless, my original intentions were hosed last night after discovering additional hanger rash damage (cracks) under the stab tip that have cracked past previous stop drills. Putting a doubler repair under the fiberglass tips would just cause the tips to crack in short time, so now I'm stuck with a skin replacement. I'll just be pulling the stab and performing a simple aft skin replacment now. It'll take longer, but it's not difficult.

I did do some quick research last night on AC 43-13 though. The original document was actually released prior to our PA-30 manuals. I do agree given the warning in the manual that SOMETHING has to be approved to repair the stab. Honestly at this point, if I didn't have other damage to correct, I'd go direct to Piper Engineering with a proposed repair to ensure they had no issues with it prior to looking for FAA approval. I'm scratching my head as an aero engineer wondering why Piper was so concerned. It may have just been the era prior to the development of CFD and other tools that better predict flutter. I'm really not sure, but I am confident a simple surface crack or puncture could be properly repaired. It's just a matter of making it FAA legal.

[Kristin Winter]

I agree that it can be done safely. I am not sure that you could get the FAA to buy off on it, but an approval to make repairs to the stabilators would be a great thing for the community. The most common repair that I have seen is where they just replace this out part of the panel from the tip to the first inbd rib. This uses the same rivet pattern and really looks like it belongs there. I only know of one done legally. It got signed off by the NY ACO in 1960. It has been flying ever since with no problems. I have a copy of that 337, BTW. However, my experience tells me that the FAA would require flight tests before approval of that repair these days. Whether you could get Piper interested is another matter as well. I don't see how they would get a return on their investment necessary to bless the repair and I would guess that the costs for the tests, etc, would grossly exceed the costs of reskinning.

[David Pfeffer]

Piper has a regular engineering department with DERs that can sign field repairs just like Beechcraft and all the other main-line OEMs unless Piper has made more cuts. I personally work with the Beechcraft DERs all the time when my team doesn't have the appropriate stress report to enable our own stress analysis. They charge a fee, but they'll review your repair (or create one) and approve it. Then it's 100% FAA legal, but for that aircraft only (the DER signs off the 8100-9 using the serial number of the aircraft). You can then use that FAA Form 8100-9 approval as supporting documentation to get the FAA to buy off on a 337 for a different aircraft if another requires the exact repair.

I don't personally work with Piper since the Navy doesn't fly any of their aircraft, but have some engineering POCs that could probably pull the right strings if anyone was to ever get into a real bind and needed a repair approval. Keep in mind ANY structural DER can do the same thing though. They don't have to be working for the OEM.