SCHEMPP-HIRTH NIMBUS 4DM, MINDEN, NEV., JULY 13, 1999– The National Transportation Safety Board has issued its final report on the crash of a Schempp-Hirth Nimbus 4DM glider that killed former FAA Administrator Donald Engen and friend Bill Ivans, who owned the powered glider. It is believed that Engen was sitting in the passenger seat behind Ivans, who would have been flying the aircraft. At the time of his death, Engen was director of the Smithsonian Institution National Air and Space Museum. He had previously been a member of the NTSB and the executive director of the AOPA Air Safety Foundation.
The glider broke up in flight during the recovery phase after a departure from controlled flight while maneuvering in thermal lift conditions. Other glider pilots in the area who saw the accident told investigators that Ivans’ glider was in a tight turn, as if climbing in a thermal, when it entered a spiral or spin. With a 45-deg nose-down attitude, speed built quickly as the glider completed two full rotations. The rotation then stopped, the aircraft stabilized on a northeasterly heading, and the nose pitched further down to a near-vertical attitude.
The aircraft was seen to be pulling out of the dive, with the wings bending upward and the wingtips coning ever higher, when the outboard wing panels departed from the glider, the wings disintegrated and the fuselage dived into the ground. Several witnesses estimated the wing deflection reached 45 deg or more before the wings failed. Examination of the wreckage disclosed that the left and right outboard wing sections failed symmetrically at two locations.
Pilots with experience in the Nimbus 4-series gliders told investigators that the type is particularly sensitive to overinput of rudder during turns due to its 87-ft 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 97 kt, and the aircraft flight manual (AFM) notes that full control-surface deflections may be applied only at that speed and below. Never-exceed speed (Vne) is 154 kt, and control deflections are limited to one-third of the full range at that speed. A bold-print cautionary note reads, “Avoid especially sudden elevator control movements.” The manufacturer reported that design dive speed (Vd) is 175 kt.
The manufacturer also told the NTSB that assuming a 45-deg nose-down attitude with air brakes closed, the glider would accelerate from stall speed to Vne in 8.6 sec, with an additional 1.8 sec 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 lb of force (nose down) required to hold a fixed attitude at Vne versus the neutral stick-force trim speed of 72.89 kt.
An examination of the wreckage established that the air brakes had been extended at some point in an attempt to slow the glider during the descent before the break-up. Concerning limitations on use of the air brakes, the AFM notes that while air brakes may be extended up to Vne they should be used at such high speeds only in an emergency or if the maximum permitted speeds are being exceeded inadvertently. The manufacturer further noted that the air brakes function like spoilers and have the effect of shifting the aerodynamic load outboard on the wings. The control linkages for the air brakes and flaps are interconnected so that when the air brakes are fully deployed, the flaps are extended to their full down limit.
The maximum maneuvering load factor limits change with variations in glider speed and flap/air-brake configuration. From a “flaps up” configuration at Va to the condition of air brakes and flaps extended at Vne, the maximum maneuvering load-factor limits decrease from +5.3 g to +3.5 g. The pertinent certification regulations require a minimum safety margin of 1.5 times the design limit load, which is defined as ultimate load. Review of the manufacturer 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 g and 1.75 g.
Investigators found some production control discrepancies in the structure that differed from drawing specifications but determined that none of them contributed to the failures. The testing established that the structure as built exceeded the minimum safety margin requirements. The wing failures were overload in character and occurred at loadings well above the ultimate design load limits.
The NTSB determined the probable cause 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, resulting in the overload failure of the wings at loadings beyond the structure’s ultimate design loads.