The Loss of N101LX

17 June 2008

The following contains references to discussions with several persons, including the NTSB investigator and a pilot-relative of the plane's owner, as well as the author's own speculations.  Where statements and information were received from other parties, I have tried to make it clear.  Readers should understand that I have no accident-investigation expertise.  While I am an engineer, I am an electrical engineer, with no training in aeronautical engineering.

On April 4, 2007, Fly Baby N101LX lost a wing while flying in Arizona.  The pilot was killed.  The NTSB report ruled that the Probable Cause was "The pilot exceeding the design stress limits of the aircraft while maneuvering in intentional aerobatics."

The airplane had a number of modifications, including solid bracing wires and the split landing gear.  While this immediately focused my attention, my suspicions were unfounded.  The NTSB found no problems with them, or the wing bracing anchor plates.  The owner of the aircraft was well-aware of the previous problems. According to a relative, the pilot had owned the airplane for more than fifteen years, and the previous year he had removed the wings and had the appropriate fittings "X-Rayed and chemically checked."

The failure mechanism was frankly unusual.  The wing anchor plate bolts themselves failed.  To quote the NTSB report:

"The right wing forward and aft solid flying wires, turnbuckles and respective anchor attachment brackets remained connected to one another, however, the attachment brackets were separated from the forward and aft spar assemblies. The four through bolts (AN3 3/16-inch-diameter) associated with the forward bracket were fractured along the bolt shank near the bolt head. The bolts necked down and were bent near the fracture points; the fracture surfaces were angular. Only two of the four through bolts associated with the aft bracket were recovered and similar findings were noted; the bolts necked down and the fracture surfaces were angular."

Wing Blocks

I spoke to the NTSB investigator several times about this accident, and the failure of the bolts.  No previously-existing problems were found with them... no evidence of fatigue, corrosion or other deterioration.  They failed in overtension...being pulled too hard.  With no other evidence of casual events, the investigator's finding was that the bolts themselves failed.

(As an aside, I had a bolt fail in my tail spring mounting system a few years ago.  Under a microscope, you could easily see a track of corrosion from some sort of surface flaw, that let the bolt gradually get eaten through from the inside.  If the bolts on N101LX had a flaw that led to their failure, the NTSB would have found it.)

We don't know why the bolts were overstressed.   It may have been an aerobatic maneuver gone wrong.  It may have been the sudden onset of a medical emergency (the drug Amlodipine mentioned in the NTSB report is a high blood-pressure medicine), but one would assume the autopsy would have uncovered it.  It may have been some other sort of failure that precipitated a high-G reaction, although there is apparently no evidence of other causes.

What I find unusual about this case is that the bolts failed at the head.  To me, logic says that failures at the thread end would be more likely 

Wing Bracing Cross Section

The diagram above is a cross-sectional view of the wing spar at the point where the wing wires attach.  Standard practice for installing bolts on aircraft is to place the head on the upper side, so that if the nut comes off, gravity will keep the bolt in place.  When undergoing positive Gs, the Landing Wires atop the wing aren't under any any Fly Baby can testify, they're a bit slack and usually flopping in the wind.    All the flight load is carried by the Flying Wires under the wing.   The Flying Wires' tension is transmitted to the lower anchor plates, which the vertical bolts hold in place.

However, because of the way the plates attach, it seems to me that the plates would be applying a bit of side force to the bolts, as well as tension.    To me, then, the friction of the bolts in their holes would relieve some of the tension on the heads and make failure at the threaded end more likely.  The NTSB investigator said that the bolt holes in the pads were sleeved,  however, so there wouldn't be as much friction (Sleeving these bolt holes is per plans, although Pete says it isn't required).

With some of the load on the bolt head relieved by the friction of the side load, it seems likely that a failure of the head end of the bolts had to be precipitated by negative Gs...the top anchor plate had to be pulling hard on the bolt heads.

However, it doesn't seem logical that the wing was subjected to a failure level of negative Gs.  Flying upside down is one thing, but packing on multiple negative Gs is another thing entirely.  This is not "normal" aerobatics, unless one is heading towards the competition/airshow world.  And it doesn't seem likely someone would be practicing such in a 65-HP Fly Baby.

However, Steve Garrison, a participant in the Fly Baby email list, had an interesting insight.  Remember, the landing wires atop the wing (which support the loads under negative Gs) are usually a bit slack in positive G flight.  This is "the nature of the beast" for a monoplane Fly Baby; there's no cross-bracing to tighten against each other like biplanes have).  If one were pulling positive Gs in a Fly Baby, then suddenly "pushed over" into a negative-G maneuver, the wing will jerk downward as it shifts to having the load taken by the Landing Wires instead of the Flying Wires.  This is certainly not ideal; the abrupt stop will cause a momentary spike in the stress in the bracing system.  If the pushover was hard enough, it seems like you could certainly exceed the limitations of the plates, wires, turnbuckles, or bolts.

On Page 8-1 of the plans, Pete calls for using AN3 (3/16" bolts) for the anchor plates,  but adds "If you plan extensive aerobatics, drill for 1/4" [AN4] bolts."   AN4 bolts are rated at 4,080 pounds tensile strength, vs. 2,210 for AN3 (are nearly twice as strong as AN3 (per Table II in Mil-B-6812E). 

It is unknown if the owner of the accident airplane was aware of this recommendation; he was not the original builder of the aircraft.  In any case, N101LX had the stock bolts.  Having bolts with twice the strength might have made a difference.

One point which should be made.  I've received email from a cousin of the accident pilot (who is also a pilot and is building a Fly Baby).  He describes the man as an acccomplished aerobatic pilot who had previously owned a Bucker Jungmeister.  The cousin goes on to say that the pilot was gradually increasing the level of aerobatics in his Fly Baby...he didn't just go up and start throwing it around.  This was not a case of  an inexperienced pilot trying to teach himself aerobatics.  He was very qualified to perform the maneuvers.

 Ron Wanttaja.