Accident MBB BK 117B-2 N110HH,
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ASN Wikibase Occurrence # 34906
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Date:Saturday 17 July 1999
Type:Silhouette image of generic BK17 model; specific model in this crash may look slightly different    
MBB BK 117B-2
Owner/operator:Hermann Memorial Hospital Systems
Registration: N110HH
MSN: 7122
Year of manufacture:1987
Total airframe hrs:7815 hours
Engine model:Honeywell LTS101-750B-1
Fatalities:Fatalities: 3 / Occupants: 3
Aircraft damage: Destroyed
Location:Fresno, TX -   United States of America
Phase: Approach
Departure airport:Mag Drop Airport, TX (20TA)
Destination airport:Life Flight/Refuel Heliport, TX (TE11)
Investigating agency: NTSB
Confidence Rating: Accident investigation report completed and information captured
On July 17, 1999, at 1231 central daylight time, a Messerschmit-Bolkow-Blohm BK-117-B2 helicopter, N110HH, operating as LIFEFLIGHT 1, was destroyed when it impacted terrain following the separation of a main rotor blade during an approach to an unmanned fueling site near Fresno, Texas. The airline transport rated pilot and two medical crewmembers were fatally injured. The helicopter was registered to and operated by Hermann Memorial Hospital Systems, of Houston, Texas. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 positioning flight, for which a company VFR flight plan was filed.

The helicopter was making a VFR approach to a fueling site when witnesses observed components of the main rotor system separate from the helicopter. Subsequently, the helicopter entered an uncontrollable descent and impacted the ground. Examination of the helicopter revealed that one of the four main rotor blades separated due to the failure of the tension-torsion strap (T-T strap), which secured the blade to the main rotor head. Examination of the failed T-T strap revealed that the polyurethane protective coating had debonded from a portion of the strap allowing corrosive agents, including carbon, sulphur, and chlorine into the strap. The strap separated as a result of fatigue cracking of the wires making up the strap. The fatigue cracking emanated from corrosion pits and deposits that were introduced to the strap when the polyurethane protective coating debonded. The T-T strap had accumulated a total of 7,815.3 flight hours and 35,971 flights at the time that it failed. It had been in service since the helicopter was manufactured in 1987. Following the accident, a life limit of 120 months or 25,000 flights (whichever comes first) was established for the T-T straps.

Probable Cause: the loss of control during approach due to the corrosion of the T-T strap which resulted in fatigue cracking and subsequent separation of the strap and main rotor blade from the helicopter.

Accident investigation:
Investigating agency: NTSB
Report number: FTW99FA192
Status: Investigation completed
Duration: 1 year and 9 months
Download report: Final report




Photo: NTSB

Revision history:

24-Oct-2008 10:30 ASN archive Added
17-Feb-2010 11:23 TB Updated [Other fatalities, Phase]
24-Jul-2010 12:37 Alpine Flight Updated [Aircraft type, Source]
20-Aug-2010 03:11 TB Updated [Aircraft type, Operator]
20-Aug-2010 04:54 TB Updated [Operator]
22-Nov-2012 09:14 TB Updated [Time, Source, Narrative]
25-Apr-2016 21:12 Aerossurance Updated [Source, Narrative]
14-Dec-2017 08:40 ASN Update Bot Updated [Time, Operator, Source, Narrative]
13-Oct-2022 05:37 Captain Adam Updated [Operator, Departure airport, Destination airport, Narrative, Accident report, Photo]

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