An Archer intermittent front-end shake, and the proper technique for replacing pipe thread fittings

Q: Hi,

I’m new to flying and bought a 1976 Archer II in February. It’s a great airplane, but I have an issue that my mechanics can’t figure out.

The front end shakes badly, but only occasionally. It happens only when braking—and then, only one time in seven or eight. If I get off the brakes and back on, it’s usually okay.

We’ve tightened everything we know to tighten, and recently put in a new scissor kit, which oddly enough, actually made it worse. Please help! Thanks in advance.

—Steve

P.S. I really love your magazine!

A: Hi Steve,

Here’s what I want you and your mechanic to do: remove the nose tire wheel assembly and balance it. Then reinstall it and see if your problem disappears.

Here’s my line of thinking. I suspect the tire/wheel is quite far out of balance and that it is tending to shimmy—but not quite far enough out to shimmy all the time.

Every one out of seven times, it hits a discontinuity in the runway; or, when you apply the brakes unevenly that exacerbates the out-of-balance condition and it starts shimmying.

When you tightened up the linkages it didn’t stop the incipient shimmying, it just better transferred that slight motion to the rudder pedals/airframe.

If your mechanic already has a wheel balancer, that’s great. If your mechanic doesn’t have one, I bought a slick little toolbox-sized one from McFarlane Aviation that I use to balance all my tires/wheels.

Tire Balancer

If he is reluctant to spend $200 on the tool, you can balance the tire using this “bush mechanic” method:

Remove tire/wheel/axle from the airplane. Clean all the grease off the bearings and bearing races. Clean!

Reinstall the bearings in the races and install the wheel assembly on the airplane.

Spin the tire/wheel by hand. Let it spin down until it stops on its own; the heavy point will be down.

By trial and error, place tire weights (get them at an auto parts store) on the wheel until the “down” spot of the tire/wheel assembly is random. When you reach this point, your tire/wheel assembly will be balanced.

Finalize the weight placement, grease the bearings and re-mount the tire.

These actions will very likely solve your shimmy problem. Let me know.

Steve

Q: Hi Steve,

I need to replace the AN fittings indicated in the photo (see page 19).

My [problem] is that new parts get tight before reaching the required angles. Should I apply enough torque to get the alignment, or use thread seal (i.e., Teflon) tape?

Thanks for help, as always.

—Roberto

A: Hi Roberto,

I had to search to find information describing the techniques to install pipe fittings.

The first place I always look for information on general maintenance subjects is AC 43.13-1B, “Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair.” You can download the complete 646-page circular from the internet. (The link is provided under Resources. —Ed.)

However, after a cursory search, I concluded that there is nothing in this circular related to torqueing pipe fittings.

I next searched the Aviation Mechanic Handbook by Dale Crane. It’s an ASA toolbox reference book. Nothing.

I also looked in a Cessna service manual, a Columbia service manual and a Mooney service manual. In fairness, I don’t have copies of the very latest Cessna manuals.

I finally found the information you—and now, we—are looking for in Parker Catalog 4300, under Section S, “Assembly/Installation.” The Parker manual says:

The full thread profile contact of NPT threads is designed to give the tapered threads self-sealing ability without thread sealant. However, variations in condition of mating threads, fitting and port materials, assembly procedures and operating conditions make self-sealing highly improbable. Some type of thread sealant is, therefore, required to achieve proper seal and, in some cases, additional lubricity to prevent galling.

Types of Sealant/Lubricant Sealant/Lubricants

Lubricants assist in sealing and provide lubrication during assembly, reducing the potential for galling. Pipe thread sealants are available in various forms such as dry pre-applied, tape, paste and anaerobic liquid.

PTFE (commonly referred to as Teflon) tape, if not applied properly, can contribute to system contamination during assembly and installation. In addition, because of PTFE’s high lubricity, fittings can be more easily overtightened; and it does not offer much resistance to loosening due to vibration.

Paste sealants can also contribute to system contamination, if not applied properly. They are also messy to work with; and some types require a cure period after component installation, prior to system start up.

For proper performance, sealants and Teflon tape need to be applied to clean and dry components, carefully following the manufacturer’s directions.

Tapered Thread Port Assembly

The proper method of assembling tapered threaded connectors is to assemble them finger-tight and then wrench tighten further to the specified number of turns from finger tight (TFFT).

The following assembly procedure is recommended to minimize the risk of leakage and/or damage to components.

Inspect components to ensure that male and female port threads and sealing surfaces are free of burrs, nicks and scratches, or any foreign material.
Apply sealant/lubricant to male pipe threads if not pre-applied. For stainless steel fittings, the use of Parker Threadmate sealant/lubricant is strongly recommended. (Pre-applied dry sealants are preferred over other sealants). With any sealant, the first one to two threads should be left uncovered to avoid system contamination. If PTFE tape is used, it should be wrapped one-and-a-half to two turns in clockwise direction when viewed from the pipe thread end. Caution: More than two turns of tape may cause distortion or cracking of the port.
Screw the connector (fitting) into the port to the finger-tight position.
Wrench tighten the connector to the appropriate TFFT values referred to below, making sure that the tube end of a shaped connector is aligned to receive the incoming tube or hose assembly. Never back off (loosen) pipe threaded connectors to achieve alignment.
If leakage persists after following the above steps, check for damaged threads and total number of threads engaged. If threads on the fitting are badly nicked or galled, replace the fitting. If port threads are damaged, re-tap, if possible; or replace the component. If the port is cracked, replace the component.

Normally, the total number of tapered threads engaged should be between three-and-a-half and six. Any number outside of this range may indicate either under- or overtightening of the joint or out of tolerance threads. If the joint is undertightened, tighten it further, but no more than one full turn. If it is overtightened, check both threads, and replace the part which has out-of-tolerance threads. As a rule, pipe fittings with tapered threads should not be assembled to a specific torque because the torque required for a reliable joint varies with thread quality, port and fitting materials, sealant used, and other factors.

The TFFT value for the most common NPT sizes are two to three for a 1/8-27 fitting; two to three for a ¼-18 fitting and two to three turns for a 3/8-18 fitting.

I’ve edited the exact text to better answer your question.

The only pipe thread fitting installation I’ve seen that prohibits the use of Teflon tape is where a fitting is screwed into a vacuum power artificial horizon or direction gyro instrument.

Happy flying.

Know your FAR/AIM and check with your mechanic before starting any work.

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, Calif. with his wife Audrey. Send questions and comments to editor [AT] piperflyer [DOT] com.

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