Jam Nuts Redux

By Vic Syracuse, EAA Lifetime 180848

This piece originally ran in Vic’s Checkpoints column in the December 2020 issue of EAA Sport Aviation magazine.

By now everyone knows my top pet peeve is a loose jam nut. Even with all of the visibility I have given this “ailment,” barely a week goes by where I still don’t find one or more, both on flying airplanes and ones presented for an initial airworthiness inspection. So, this month I am going to focus the column on jam nuts and see if we can’t make some more progress. We’ve even trademarked our business logo at Base Leg Aviation — “Are Your Nuts Tight” — and printed shirts. I sure hope eventually we will be successful! I’ve showcased some examples of the worst offenders with pictures in this month’s column. When it comes to jam nuts, a picture is really worth a thousand words.

Let’s start with the definition of a jam nut cobbled together from multiple sources and amended to apply to aircraft. A jam nut is a low-profile type of nut, typically half as tall as a standard nut. It is commonly used as a type of lock nut, where it is “jammed up” against a standard nut or threaded rod to lock the two in place. Its primary purpose is to prevent the unlocking of the joint in a high-vibration environment.

Jam nuts do not have to be tightened with “gorilla” torque. Most of the time it is difficult to access the jam nut, let alone have room for a torque wrench. I know that for those of you who think a torque wrench is required for every fastener, you will find this frustrating, as there are just some places you can’t access easily. Normally, you hold the rod or unit being tightened with one tool and then snug the jam nut tight with another wrench. In some cases, you may even be working in the blind with your hands because the space can be so small. An excellent practice is to follow up with some kind of slippage indicator across the jam nut, such as torque seal or even nail polish. Doing that will make future inspections a breeze, as any movement of the jam nut will show up as a crack in the indicator. Please note that the seal is just an indicator. It does not prevent the jam nut from loosening.

I inspected one aircraft that really impressed me as I walked up to it. There was torque seal on every jam nut I could see. I happened to test one of them for tightness with my finger as I was talking to the builder — and it came loose! Upon further discussion, the builder erroneously thought that the purpose of the torque seal was to hold the jam nut tight, and he had not used a wrench to tighten ANY jam nut anywhere on the airplane!

In another case, I happened to reach in and twist the pitch control torque tube in an RV, and it spun in my hands. Neither jam nut at opposite ends of the tube had been tightened. One of the reasons this happens is that rigging all of the various aircraft control system parts usually takes a lot of time, with many adjustments being done multiple times. This is especially prevalent when adjusting flying wires. Once the task is finished, it can be easy to forget to go back and tighten all of the jam nuts.

You are probably wondering how many jam nuts are in a typical airplane, and why all of the fuss. Well, on some airplanes there are a lot of them. In the most egregious example that comes to mind, I found 12 loose ones on an aircraft that I was certificating before I stopped counting — and stopped the inspection as well! There are a lot of jam nuts on your airplane, everywhere from the engine compartment to the flight controls, trim tab actuators, position sensors, autopilot servos, and even flying wires.

Regarding engine controls, many builders forget that there are usually jam nuts at both ends of those controls. The ones in the cockpit are usually hidden behind the instrument panel or underneath the throttle quadrants. On some controls, and especially on flying wires, the jam nuts will be right-hand threaded at one end and left-hand threaded at the other. It is also important to make sure you have the minimum number of threads into the control rod. Sometimes, but not always, the designer will try to ensure that a particular control rod cannot come undone by turning it too far in either direction if the proper amount of threads are inserted. The control rod will usually stop turning in either direction prior to the rod end bearing falling out. This could be a life-saver in things like control linkages that are directly attached to primary flight controls.

Enough on jam nuts! Please do your part and carefully inspect your own aircraft and apply some indicator to all of the jam nuts.

Many of you may have read my column entitled “The Dreaded Denial Letter” from a few months ago. While it’s certainly not fun or rewarding to receive a denial, it certainly isn’t the end of things. As designated airworthiness representatives (DARs), we have an obligation to ensure an aircraft is safe for flight when we issue the airworthiness certificate. We certainly aren’t there to criticize the design, as that would certainly take the life out of experimental aviation. In the 13-plus years I have been a DAR, I don’t think I ever issued more than one denial in any given year, so they are rare. Most, but not all, have me return for the new inspection. Some shop for another DAR. Their choice. However, I thought I would share how successfully one of the denials did work out.

As you can see in some of the pictures, the workmanship in some really critical areas (like lift struts) didn’t exactly pass muster and certainly wasn’t in conformance with AC 43-13-1 B. That particular advisory circular (AC) should be owned by every aircraft builder and mechanic, as it gives us guidance on acceptable methods, techniques, and practices for aircraft repair, alterations, and inspections. Gee, sure sounds like a really practical book to have around. In this particular case, I spent time explaining why the workmanship would need to improve, and why some things just downright needed to be fixed.

The builder was gracious and really knuckled down over the next eight months and rectified everything, including fabricating and installing new lift struts. He really wanted me to come and redo the inspection. I could tell he was proud of the work. As you can see from the pictures, the change was quite dramatic.

Oh, by the way, I always put on my flight advisor hat after an inspection. In the first visit, I discovered that he was a student pilot and hadn’t flown this type of aircraft in over a year. I politely explained that student solo endorsements are only good for 90 days, so he really couldn’t fly this one if it had passed the inspection.

I am pleased to tell you that not only has the aircraft undergone a fantastic improvement, but the builder is well into his sport pilot certificate as well.

It is a two-seat Challenger aircraft, and he has it based at his own farm. I’m sure he is going to have a lot of fun with it soon!

Now go check those jam nuts.

Vic Syracuse, EAA Lifetime 180848 and chair of EAA’s Homebuilt Aircraft Council, is a commercial pilot, A&P/IA, DAR, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 9,500 hours in 72 different types. Vic also founded Base Leg Aviation and volunteers as a Young Eagles pilot and an Angel Flight pilot.

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