Wing Spar Failure on a BushCaddy L-164

December 2019 - James Eby and I built C-FZGG from a kit manufactured by BushCaddy (aka CLASS). We started in 2005 and completed it in 2017. The aircraft is an all-aluminum single-engine four-seater with a tail wheel. It is a high-wing monoplane with strutted wings and a wingspan of 36 feet. The powerplant is a LOM M337C, an inverted six-cylinder inline engine built in Prague, Czech Republic. The engine delivers 210 hp and is equipped with a supercharger boosting it to 250 hp. The max gross weight of this airplane, according to the manufacturer's data, is 2,500 pounds.

Soon after our assembly started, an NTSB report came out in 2006 about a U.S.-registered L-164 experiencing a wing spar failure in flight that did not result in an accident — the pilot made a successful precautionary landing. The report cited a sudden aileron flutter that started at 135 mph in calm air and subsided as the pilot reduced speed. After the landing, the pilot inspected the wings and found out that one of them was slightly bent upward and top skin was buckled just inboard of the strut attachment points.

The manufacturer claimed at the time that the incident was due to an improper assembly procedure. They did, however, increase the front spar material thickness from 0.063 inch to 0.080 inch, and the rear spar from 0.050 inch to 0.063 inch after this occurrence. We were not informed of this change.

We did not make any further inquiries then and kept working on the plane — in hindsight, a naïve and potentially unsafe decision.

C-FZGG at Bromont, Quebec.

Our aircraft, C-FZGG, was completed by 2017. It went through the inspections by Transport Canada, was flight-tested, and obtained a special certificate of airworthiness in the fall of 2018. By the spring of 2019, the plane had flown a total of 35 hours.

On May 20, 2019, we took off from Bromont, Quebec, heading to Sterling, Ontario — an approximately 200 nm cross-country flight. The plane carried full fuel and two people, so its takeoff weight was about 2,200 pounds. It was a clear day with a 25- to 30-knot headwind, so we stayed below 2,000 feet to maintain a better groundspeed. At this altitude, we were experiencing constant light to occasional moderate turbulence. After two hours en route, the left aileron suddenly went into a violent flutter, causing the left wingtip to move up and down 4 to 5 inches. We were at the time near Gananoque, Ontario, flying level at 1,000 feet AGL at 100 mph. Slowing the plane to 70 mph stopped the vibration. The aileron operation was affected — the stick deflection to the right was somewhat restricted, and the plane had a tendency to bank left, though this could be countered with full right aileron and some right rudder. Being unsure of the structural condition of the aircraft, we decided to make a precautionary landing at the Gananoque airport, which was in sight at the time, and carried out a three-point landing into the wind that was gusting at over 30 knots.

A quick walk-around showed that the right wing was bent near the strut attachments, with some buckled skin visible on top.

Visible wing buckle.

We guessed that the deformed right wing had pulled the aileron linkage — an aluminum tube — and bent the left aileron bellcrank, limiting the aileron motion and causing the flutter.

Bent aileron bellcrank.

As soon as we opened the top skin, we saw that both spars (main and rear) were kinked from a compression force and cracked a few inches inboard of the strut attachment points. They both failed downward from the lift of the outboard portion of the wing. The wing was bent slightly, just enough to stretch the bottom skin that took most of the load once the struts got bent.

A kink was visible in both spars.

The symptoms of this failure were similar to the occurrence described in the 2006 NTSB report, except in our case, only the left aileron fluttered.

A Google search for "BushCaddy wing spar failure" delivered a very informative blog by an Australian pilot and builder, Ian Swinbourne. He bought an L-164 kit in 2010 but seriously questioned the wing strength issue on this aircraft after the 2006 NTSB report and a similar occurrence in Canada in 2009. This occurrence prompted the TSB engineering report with a conclusion that the L-160 (with a maximum gross weight of 2,200 pounds rather than the 2,500 pounds for the L-164) wing would fail at a 3g overload. At that time the L-160 and L-164 used the same spars. Ian consulted an aeronautical engineer to adequately reinforce the wing spars.

We can blame only ourselves for being too trustworthy and not looking any deeper into the issue at the time.

It is, however, appalling that Transport Canada did nothing after analyzing the 2009 incident and calculating the ultimate wing loading of only 3g. As far as we know, the report was never made public, and no further action was taken either by Transport Canada or the kit manufacturer to warn existing BushCaddy owners and builders of this issue, although subsequent spars were strengthened significantly.

At this point, there is little doubt that the original BushCaddy wing spars were too weak and can lead to a catastrophic in-flight failure in ordinary flight conditions — cruise speed and light turbulence. Any owner of such aircraft should immediately ground the plane and consider strengthening the wings.

APPENDIX: NTSB Report on a U.S.-based L-164

NTSB Identification: CHI06LA260

HISTORY OF FLIGHT

On September 11, 2006, about 1015 Eastern Daylight Time, an experimental amateur-built airplane, registered as Endless Skies Aviation Inc. L-164, N2627J, sustained substantial damage when wing spars failed during cruise near Durand, Michigan. The pilot executed a forced landing to a nearby field. The personal flight was operating under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot and pilot-rated passenger reported no injuries. The flight originated from the Livingston County Spencer J. Hardy Airport, near Howell, Michigan, about 1000 hrs., and was destined for the Bismarck Municipal Airport, near Bismarck, North Dakota.

The pilot's accident report, in part, stated:

When the aircraft reached 135 mph indicated and was at that airspeed for a minute or two in calm stable air, it suddenly developed an aileron flutter. Both ailerons fluttered. [The pilot immediately] pulled the power back to slow the aircraft. [The passenger] noticed the flutter before [the pilot] because the stick did not shake. The flutter started at the trailing edge of the wing and progressively got worse. Wingtips traveled 4 to 5 [inches] up [and] down. [The pilot] saw the right wing fail in an up position. The flutter stopped at approximately 120 mph. The nearest airport was 8.5 miles away. That was too far away. [The pilot] elected to land in a farmer's field straight ahead before we experienced further wing failure. The landing was uneventful.

PERSONNEL INFORMATION

The pilot held a commercial pilot certificate with single-engine and multiengine land airplane ratings. His most recent FAA second-class medical certificate was issued on February 20, 2006. The pilot reported having accumulated 7,950.9 hours of total flight time, 200 hours of flight time in the 90 days prior to the accident, and 80 hours of flight time in the 30 days prior to the accident.

AIRCRAFT INFORMATION

N2627J was registered as an experimental amateur-built Endless Skies Aviation Inc. L-164 airplane, serial number CA004-90. The airplane was reported to be powered by a 180-hp ECI Titan O-360 engine. The airplane was constructed from a BushCaddy L-164 kit. The airplane had accumulated 40.6 hours of flight time.

According to the kit manufacturer's data, it was a four-seat, externally braced, high-wing tailwheel airplane of all-metal construction with a fiberglass nose bowl. The airplane was designed with a 2,500-pound gross weight and had two main doors. The cabin construction started from a cage, which was built from 6061-T6 aluminum angle and gusset reinforcements and was pre-jigged and pre-drilled. The airplane's fuselage was reinforced with stringers that span the entire length of the bulkheads to the tail section. Rear bulkheads were designed for tail wheel and float operation. All four corners of the fuselage were reinforced with interior and exterior sandwich construction. The leading edge of the vertical stabilizer was designed with a forward pitch, eliminating the need for strakes. Elevators were trimmed from within the cockpit by means of a spring mechanism. The high-lift wing design is a NACA 4412 modified airfoil. The 63-inch constant chord wings were designed with both dihedral and washout. Landing gear was constructed from welded 4130 steel tubing, and the airplane was equipped with Frise ailerons and flaps. Aileron hinge brackets for this airplane kit were not pre-drilled by the kit manufacturer.

METEOROLOGICAL INFORMATION

At 0953, the recorded weather at the Bishop International Airport, near Flint, Michigan, was this: Wind: 080 degrees at 8 knots; visibility: 10 statute miles; sky condition: overcast 9,500 feet; temperature: 12 degrees C; dew point: 8 degrees C; altimeter: 30.29 inches of mercury.

WRECKAGE AND IMPACT INFORMATION

FAA inspectors performed an on-scene examination of the airplane. The examination revealed that the front and rear spars failed and that the aileron hinge brackets were not attached to the ailerons as depicted in the kit drawings for the airplane. Also, the bracket's fastener holes were drilled at spacing different from depicted.

ADDITIONAL DATA/INFORMATION

The pilot's safety recommendation, in part, stated:

The cause that led to the aileron flutter was incorrect placement of four holes that bolted the control lever to the aileron spar which allowed it to flex. … There was a S.B. [service bulletin] on the install of this part but [the kit builder] missed it. I also missed seeing it on the manufacturer's website. … [The kit builder] … had the airplane inspected as per [Canadian] rules, and no squawks were found. He also worked with his local [Experimental Aircraft Association] in Michigan and [an airframe and powerplant mechanic]. All parties missed it. The factory [kit manufacturer] now pre-drills these holes to prevent builder error.

Subsequent to the accident, the kit manufacturer issued SB-2. That SB is available from the www.bushcaddy.com website. SB-2, BOLT HOLE CRITICAL DIMENSIONS, in part, stated:

As of January 2004, Parts L370 R & L are delivered to customer pre-drilled for bolt holes as per attached drawing. Compliance method: Customers who have received the above parts un-drilled should note that the center to center distances of the bolt holes are critical. Particularly those which are for the bolts holding the assembly to the aileron spars. The attached drawing dimensions must be respected.

The FAA was a party to the investigation.