Never Ignore Symptoms: A Massive Failure of Rivets

Friend and airplane guru Jack remarked many years ago, "If you don't find anything wrong during your plane's annual inspection, you haven't looked carefully." We remember his comment every year when we inspect our plane, a Zenith 601HDS. 

This year was no exception to Jack's rule: We looked carefully and found two critical problems.

First, the brake lining had to be replaced for the two main wheels. That wasn't unexpected since last year the wear marks of the lining were still okay but also indicated that replacement was in the near future. 

However, a major shock came as we cleaned the fuselage. The critical support bracket for the nose gear fastened with 13 rivets to the bottom of the fuselage near the firewall, wasn't attached at all: The entire set of 13 rivets had disappeared! 

The repair: We drilled nine of the 13 rivet holes to the next larger rivet size and installed the rivets, then enlarged the remaining four holes to 3/16 in. and used AN3 bolts instead of rivets. These bolts guarantee that the bracket will forever stay in place. Here is a photo.

Bottom bracket of nose gear after repair

How could 13 rivets fail?

Answer: The plane is more than 30 years old. The bracket carries a considerable load, and eventually the 13 rivets failed.

Should we have discovered this earlier?

Answer: Yes, there was a symptom half a year earlier that didn't have an obvious explanation, but that we didn't investigate. The tension of the rudder cables had increased. Not by a lot, but noticeably. 

The fact is: Cable tension generally may get reduced due to wear of bearings and the like. An increase is unexpected and demands a closer look. 

Well, we didn't do that even though the check would have been easy: In the Zenith 601 and 601HDS, the tension of the rudder cables is completely determined by the position of the nose gear. 

We should have reasoned that the nose gear had to have moved forward to increase the cable tension. Yet such movement was impossible unless something had drastically changed. 

And then we would have seen that the bottom bracket was not longer riveted to the fuselage.

The main message of this post:

Whenever an unexpected change shows up, investigate thoroughly until you find a convincing explanation.

A recommendation for Zenith 601 owners: 

You may want to check the rivets of the bottom bracket of the nose gear for wear and damage. Even if all seems well, you may want to drill out four of the rivets and replace them with AN3 bolts as shown in the above photo.

Have any question or feedback about annual inspections? Please share your thoughts in the comments.

Comments

  1. Skywatcher808March 16, 2025 at 9:24 PM

    Although not something you want to find, still, it was something you want to find. And of course obviously before any major incident occurs. Obce the failed rivets were discovered, the repair was not just to replace/correct the problem. The repair was a structural upgrade with the addition of the four 3/16" hardware. Great article Klaus. But even more so, it was an excellent improvement to the integrity of the landing gear. Great to see.
    Aloha!

    ReplyDelete
  2. AnonymousMarch 17, 2025 at 11:03 PM

    It is a wonderful thing when a pilot dodges a bullet like this.

    ReplyDelete

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