Tail Section Detaches from Motor Glider During Flight Test

Casualties unknown • Nahe Lenningen, DE

The tail section of a Schempp-Hirth Nimbus 4-M broke off during a flight test near Lenningen, Germany, forcing the pilot to bail out.

What happened

On April 22, 2010, a Schempp-Hirth Nimbus 4-M motor glider was conducting flight testing near Lenningen, Germany. The purpose of the flight was to practically confirm flutter stability following a design change intended to increase the maximum takeoff weight by 50 kg. The flight involved intentional maneuvers at various speeds and weights, including descents to the design dive speed (VD) of 308 km/h.

During the testing, the pilot noticed unusual control behavior at approximately 140 km/h, describing the rudder as feeling "flat, vague, and soft." Upon looking back, the pilot observed the elevator edge moving erratically. A second pilot flying nearby observed the entire tail unit oscillating with an increasing angle of tilt, estimated at 45 degrees. Shortly thereafter, the tail section detached from the fuselage. The aircraft entered a nose-down pitch, prompting the pilot to activate the emergency canopy release and bail out. The pilot successfully deployed an automatic parachute and landed safely on a meadow without injury.

The investigation

The BFU examined the wreckage, which included the main fuselage, the detached tail unit, and the discarded canopy. Investigators reconstructed the broken fuselage area and analyzed the fracture surfaces of structural components, specifically bulkhead 11 and bulkhead 12. Microscopic analysis of the fracture surfaces revealed that while bulkhead 12 showed fibers protruding from the resin (indicating a spontaneous tear), the surface of bulkhead 11 appeared smoothed and matte. This suggested that bulkhead 11 had already become detached from the internal fuselage structure prior to the final failure, and its movement within the fuselage had abraded the fibers.

Findings

  • The structural integrity of the fuselage was compromised by a history of extraordinary operational stresses, including previous ground incidents and repairs.
  • A previous repair to bulkhead 11, performed after a ground collision in 2001, had altered the original structural properties.
  • The flight test program involved repeated, intentional provocations of flutter and excursions to the maximum design dive speed, placing the aircraft at its absolute structural limits.
  • The detachment of the tail was preceded by the failure of bulkhead 11, which eventually led to the complete separation of the tail unit from the fuselage under torsional loading.

Probable cause

The structural strength of the aircraft's fuselage was weakened by previous high-stress operational events and repairs, which led to the failure of bulkhead 11 and the subsequent detachment of the entire tail unit during high-speed flight testing.

Frequently asked questions

What happened in the 2010-04-22 Schempp-Hirth Flugzeugbau Nimbus 4-M accident near Nahe Lenningen, DE?

The tail section of a Schempp-Hirth Nimbus 4-M broke off during a flight test near Lenningen, Germany, forcing the pilot to bail out.

What aircraft was involved and where did it happen?

The accident on 2010-04-22 involved a Schempp-Hirth Flugzeugbau Nimbus 4-M, at Nahe Lenningen, DE.

What was the probable cause of the accident?

The structural strength of the aircraft's fuselage was weakened by previous high-stress operational events and repairs, which led to the failure of bulkhead 11 and the subsequent detachment of the entire tail unit during high-speed flight testing.

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