Pilot Debrief

Urban Air SRO UFM-13 near Bourne Stage, TX — 2009-06-23

Final reportCEN09LA379
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Date
2009-06-23
Location
Bourne Stage, TX, USA
Aircraft
Urban Air SRO UFM-13
Registration
N17UA
Category
Glider
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

The pilot’s exceedance of the aircraft’s placarded never-exceed speed (Vne), resulting in aerodynamic flutter in the empennage and the subsequent in-flight structural failure of the aft portion of the fuselage.

NTSB narrative

The pilot noticed a light fluttering of the rudder pedals that stopped when he applied gentle foot pressure. The motor glider was in level cruise flight when it suddenly began to shake violently, throwing the pilot back and forth against his restraints. He said that the airspeed was about 80 KIAS (knots indicated airspeed); however, data available from the airplane’s on-board Global Positioning System (GPS) indicates that the aircraft was flying approximately 110 KCAS (knots calibrated airspeed). The pilot deployed the emergency ballistic parachute and the aircraft landed gently on top of small trees, sustaining additional damage. Postaccident examination of the motor glider revealed the empennage had separated from the fuselage, but remained attached by control cables and tubing. GPS data showed the aircraft’s airspeed had gradually increased to about 110 KCAS when the in-flight breakup occurred. After a similar accident in the Czech Republic, the never-exceed speed (Vne or redline) was lowered from 119 KCAS to 81 KCAS. The accident airplane’s airspeed indicator had two redlines: one was at 119 knots and the other was taped onto the instrument glass at 81 knots and was in the middle of the green arc. The pilot said that he was not aware that the Vne speed was 81 KCAS, but that the change was a temporary flight recommendation from the manufacturer. He said that he had received the memorandum containing the temporary restriction on airspeed not to exceed 81 KCAS in any configuration, but that he “did not think the 81 knots was a Vne speed.” The manufacturer had lowered the Vne as a temporary measure because of a previous accident where an in-flight structural failure of the aft portion of the fuselage occurred. The investigation of the first accident by Air Accident Investigations Institute, Republic of Czechoslovakia, determined that the separation was the result of aerodynamic flutter in the empennage. According to the NTSB’s Materials Laboratory report, the fractures were consistent with compressive loads and tension or tearing from local bending. No evidence of manufacturing errors or preaccident damage was noted.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot did not adhere to the reduced Vne of 81 KCAS despite receiving the memo.

NTSB coding

Evidence available

  • Photos
  • 18 docket documents
View NTSB final reportView NTSB docket

Docket documents18

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