FTW99FA192 · MBB BK-117-B2
MBB BK-117-B2 · Accident: rotor failure/malfunction on approach
From Mag Drop Airport (20TA) to Life Flight/Refuel Heliport (TE11)
Event
- NTSB case
- FTW99FA192
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Accident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 3 · all aircraft and ground
- Ground fatalities
- 0
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- General aviation
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
MBB BK-117-B2
- Aircraft type
- MBB BK-117
- Category
- Helicopter
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N110HH
- Operator
- Unknown
- Onboard fatalities
- Unknown
- Route
- From Mag Drop Airport (20TA), Houston, TXTo Life Flight/Refuel Heliport (TE11), Fresno, TX
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 91: General Aviation
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- Approach · VFR pattern base
- First occurrenceThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Rotor failure/malfunction
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Destroyed
Cause areas
- Rotor system, main rotor tension torsion bar › Corroded
- Rotor system, main rotor tension torsion bar › Fatigue
- Rotor system, main rotor tension torsion bar › Separation
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
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.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record FTW99FA192
- Event ID
- 20001212X19258
- Case number
- FTW99FA192
- Dataset
- historical-pre2008
- Source SHA-256
- 89e2df6d848e7ab3b42e7d7c7a03ff403303057a26ca04c95b3ea67201c3ce74
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: investigation status. Values present in the bulk record are kept.
- The date is the local date, which is the same as the UTC date the NTSB stores.
- API snapshot SHA-256: fabab617e24652aa53bc0437042a75475734871313444f2c0cb4ccdac7fee690; retrieved 2026-09-20T21:57:31.373Z.