NTSB Identification: LAX99MA251.
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Accident occurred Tuesday, July 13, 1999 in MINDEN, NV
Probable Cause Approval Date: 09/30/2002
Aircraft: Schempp-Hirth NIMBUS 4DM, registration: N807BB
Injuries: 2 Fatal.
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft accident report.
The glider broke up in flight during the recovery phase after a departure from controlled flight while maneuvering in thermal lift conditions. Airborne witnesses in other gliders who saw the beginning of the sequence said the glider was in a tight turn, as if climbing in a thermal, when it entered a spiral or a spin. With a 45-degree nose down attitude, the speed quickly built up as the glider completed two full rotations. The rotation then stopped, the flight stabilized on a northeasterly heading, and the nose pitched further down to a near vertical attitude (this is consistent with the spin recovery technique specified in the Flight Manual). The glider was observed to be pulling out of the dive, with the wings bending upward and the wing tips coning higher, when the outboard wing tip panels departed from the glider, the wings disintegrated, and the fuselage dove into the ground. Several witnesses estimated the wing deflection reached 45-degrees or more before the wings failed. Examination of the wreckage disclosed that the left and right outboard wing sections failed symmetrically at 2 locations.
The glider is a high performance sailplane with an 87-foot wingspan and is constructed from fiber reinforced plastic (FRP) composites. The manufacturing process uses a hand lay-up of carbon and glass materials with applied epoxy resins. The glider is certificated in the normal category in Germany under the provisions of the European Joint Airworthiness Regulations.
Pilots with experience in the Nimbus 4 series gliders stated that the glider was particularly sensitive to over input of the rudder control during turns due to the 87-foot wingspan, with a resulting tendency for unwanted rolling moments. The manufacturer reported that to avoid undesired rolling moments once the bank is established the ailerons must be deflected against the bank.
Maneuvering speed (Va) is 180 km/h (97 kts) and the AFM notes that full control surface deflections may only be applied at this speed and below. Never exceed speed (Vne) is 285 km/h (154 kts) and control deflections are limited to one third of the full range at this speed and a bold print cautionary note reads, "Avoid especially sudden elevator control movements." The manufacturer reported that design dive speed (Vd) is 324 km/h (175 kts). The manufacturer also said that, assuming a 45-degree nose down attitude with airbrakes closed, the glider would accelerate from stall speed to Vne in 8.6 seconds, with an additional 1.8 seconds to accelerate from Vne to Vd. While no specific information on stick force per 'g' was available, certification flight test data showed that the elevator control stick forces were relatively light, with only 11.9 pounds of force (nose down) required to hold a fixed attitude at Vne versus the neutral stick force trim speed of 135 km/h (72.89 kts).
Detailed examination of witness marks and other evidence in the wreckage established that the pilot extended the airbrakes at some point in an attempt to slow the glider during the descent prior to the break-up. Concerning limitations on use of the airbrakes, the AFM notes that while airbrakes may be extended up to Vne they should only be used at such high speeds in emergency or if the maximum permitted speeds are being exceeded inadvertently. The manufacturer noted that the airbrakes function like spoilers and have the effect of shifting the aerodynamic loads outboard on the wings. The control linkages for the airbrakes and flaps are interconnected so that when full airbrake deployment is achieved, the flaps are extended to their full down limit.
The maximum maneuvering load factor limits (in units of gravity or g's) change with variations in glider speed and flap/airbrake configuration. From a "flaps up" configuration at Va to the condition of airbrakes and flaps extended at Vne, the maximum maneuvering load factor limits decrease from positive 5.3 to a positive 3.5.
The pertinent certification regulations require a minimum safety margin of 1.5 above the design limit load, which is defined as ultimate load. Review of the manufacturers data on safety margins in the wing spar disclosed that in the area of the primary wing failures, the structural design safety margin ranged between 1.55 and 1.75.
The manufacturer supplied data of the wing deflections under various load and aerodynamic conditions. At the design load limit (3.5g's) with airbrakes extended and at Vd, the wings were deflected to a 31-degree angle. At the ultimate load limit, the deflection was 46.5-degrees, similar to the witness observations of the wing deflection just prior to the break up.
An extensive series of scientific investigations were undertaken to establish: 1) if the structure as built conformed with the approved production drawings; 2) that the wing design met pertinent certification standards for strength safety margins; and 3) whether or not the failures occurred in overload beyond the ultimate load limits of the structure. While production control type discrepancies were found in the structure that differed from drawing specifications, none contributed to the failures. The testing established that the structure as built exceeded the minimum safety margin requirements. All the wing failures were overload in character and occurred at loadings well above the ultimate design load limits.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's excessive use of the elevator control during recovery from an inadvertently entered spin and/or spiral dive during which the glider exceeded the maximum permissible speed, which resulted in the overload failure of the wings at loadings beyond the structure's ultimate design loads. Full narrative available
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