During the annual inspection of my Standard Libelle 201B S/N 181, a crack was discovered in the main spar spigot. This happened in March of this year. The following 5 months were filled with repairs and communication challenges with Germany. However the outcome was positive and the Libelle is now repaired, determined airworthy, and has been returned to flight. The following post is intended to benefit other Libelle owners so that they may inspect the area of concern in more detail during their next annual inspection.
The area of concern is on the left wing main spar spigot : See Figure 1.
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| Figure 1 |
A detailed drawing, shown in Figure 2, of the welded steel assembly of concern was provided by Glasfaser-Flugzeug-Service.
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| Figure 2 |
The crack was found in the weld surrounding the main pin, shown in Figure 3. The crack was difficult to see in the picture so I outlined its general shape in red. It is important to note that the main pin is not welded to the rest of the assembly, the weld attaches a thin walled tube that accepts the main pin to the rest of the assembly. The main pin is machined separately and is not heat treated nor hardened.
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| Figure 3 |
Figure 4 shows the area with paint removed and a stir stick inserted between the rapping plies and steel assembly. This is the area of concern regarding corrosion due to trapped moisture.
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| Figure 4 |
Figure 5a shows the assembly after the plies that rap the assembly have been removed. Figure 5b shows the removal of the pins that attach the assembly to the upper and lower spar caps. The best way to remove these pins is to use a pin of slight smaller diameter, a large C-clamp, and a large diameter deep socket. Use the pin and C-clamp to press the pin out with the socket as a backing plate that allows the pin to slide thru.
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| Figure 5a and 5b |
Once the steel assembly has been removed the thickened resin used to bond the assembly in place was removed, shown in Figure 6. The important aspect of the step is to remove the non-load bearing thickened resin only, no fibrous spar cap material can be removed, or the structural integrity of the spar will be compromised.
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| Figure 6 |
At this stage I decided not to have a certified aircraft welder try to re-weld the steel assembly so I ordered a new assembly from Glasfaser-Flugzeug-Service. When it arrived I discovered that it was made from much thicker steel and fit slightly differently than the original. I was not concerned about alignment because the pins and width of the spar caps filly constrained the new part in the same position as the old part. I later found out that the new assembly is slightly different than the original because it is the assembly used on the Kestrel and the one part has been approved as the replacement for both the Kestrel and Libelle. Figure 7 shows the new part bonded in place. Note the squeeze out of the MGS 285/cotton flox mix. It is important to get lots of squeeze out during the bonding process to ensure there are no voids. Also note that the joint is design so that all loads go thru the pins, the bond between the assembly and the spar caps and shear webs is just additional margin; same is true for the rapping plies. Figure 7 also shows the pins installed, I chose to fill the thru holes in the pins with steel filled resin to prevent moisture collection in the future.
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| Figure 7 |
The final operation is to cover the area in fiber glass, and post cure the area to 130deg F. Figure 8 shows the spar rapping plies before the resin has cured. The tech note covers the detailed ply layout and position. The post cure was achieved by creating a tent around the spar end and using a space heater and digital thermometer from Home Depot.
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| Figure 8 |
The entire process was oversaw by an experience composite repair technician, a certified aircraft inspector, and myself. When we agreed all work was done in accordance with TN201-31, the IA filled out a 337 and submitted it to the FAA and signed the aircraft logs to return the glider to airworthy condition. For the first flight after the repair I planned a short flight to expand the g envelope in a safe environment. For the flight I replaced my compass with a g-meter and planned a +2.0g wind-up turn followed by a -.5g pushover. I used a go-pro camera to record the data. A screenshot of the g-meter is shown in Figure 9. I have since open the g envelope to +3/-.5g.
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| Figure 9 |
My first cross country flight in the Libelle was a 650km out and return flight along the infamous Owens Valley. The flight trace can be found here. The Libelle is truly and amazing glider and I'm very happy to be flying it again, however it is true that I purchased a Schempp Hirth Discus A during the 5 months my Libelle was down for repairs. I plan on tuning the Discus to the same caliber as the Libelle, but the Libelle will always be my favorite.
Until the next repair, fly while you can, whatever you can.
Keep Soaring,
Michael BK and 2Y
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