Hiller UH-12E accident near Wenatchee, Washington, May 26, 2013
On May 26, 2013, a 1962 Hiller UH-12E (helicopter), registered N99090, was substantially damaged in an accident during enroute (cruise) near Wenatchee, Washington (Pangborn Memorial Airport). It was a personal flight under general aviation rules (Part 91). 1 person had minor injuries; 1 other was unhurt. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
Fatigue failure of the six bolts that secured the bevel gear to the first stage planetary carrier, which caused the transmission to seize while the helicopter and main rotor were still operating.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- May 26, 2013
- Place
- Wenatchee, Washington · Pangborn Memorial Airport · map
- Type
- Accident
- Injuries
- 1 person had minor injuries; 1 other was unhurt.
- Weather
- visual conditions (good weather)
- Aircraft
- Hiller UH-12E, built 1962 · all UH-12Es on the register
- Registration
- N99090 · no longer on the register · serial 1395
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
The pilot reported that, after takeoff, when he was 3 miles from the airport, he heard a noise above and behind him. The pilot immediately turned the helicopter back toward the airport; when about 1 mile away, he heard a second noise. The pilot continued to the airport and was hovering the helicopter about 4 feet above the ground when the main rotor head and blades separated from the helicopter, flew up into the air, and impacted the ground about 300 feet away. The helicopter impacted the ground hard, and the tailboom separated about midspan. Postaccident examination revealed that the main transmission housing exhibited multiple fractures on its second stage planetary gear housing; the fractures were primarily orientated vertically and were through the thin-walled section of the housing. Analysis of these fracture surfaces revealed that the cracks were consistent with overstress from radial stresses. The transmission was disassembled and substantial damage was observed to the first stage planetary gear teeth and the input gear teeth. A high quantity of metallic chips and flakes were discovered at the bottom of the lower housing and throughout the first stage planetary assembly. All six bolts securing the bevel gear to the first stage planetary carrier were fractured. Only one of the six separated bolt heads remained secured with safety wiring. The six bolts were removed each bolt exhibited fatigue fracture features consistent with overstress, such as crack arrest and beach marks, that surrounded a center region. The fatigue cracks initiated along the thread roots of the bolts; these cracks were generally positioned about 180 degrees from each other, and were consistent with failure from reverse bending fatigue. After the bolts fractured, the bevel gear was unconstrained, leading to overstress fracture of the adjacent components. The fracture surface of the main rotor shaft was analyzed; the fracture surface was orientated 45 degrees from the longitudinal direction of the shaft, and the shaft was deformed to one side. All features were consistent with overstress failure in tension. A complete set of maintenance records were not available for review, thus, it could not be determined if the bolts met installation requirements or if regular inspection of bolt tightness was met.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Powerplant sys/comp malf/fail during enroute (cruise) defining event
- Part(s) separation from AC during maneuvering (hover)
- Off-field or emergency landing during maneuvering (hover)
The NTSB's findings
- cause Aircraft › Aircraft propeller/rotor › Main rotor system › (general) › Fatigue/wear/corrosion
- cause Aircraft › Aircraft power plant › (general) › (general) › Fatigue/wear/corrosion
- cause Aircraft › Fluids/misc hardware › Misc hardware › (general) › Not specified
Pilot
- Certificate: flight instructor, commercial pilot
- Ratings: multi-engine land; instructor: helicopter; instrument: airplane; rotorcraft: helicopter
- Flight time: 1,555 hours in all; 928 in this make and model; 2 in the last 90 days; 1 in the last 30 days; 1,286 as pilot in command; 421 on instruments
- Last flight review: July 6, 2011
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: left
- Injury: no injuries
The aircraft
- Airframe total time: 10,568 hours
- Last inspection: 100-hour inspection, November 1, 2012
- Maximum gross weight: 2,750 lb
- Seats: 3
- Landing gear: fixed
- Engine: Lycoming VO-540 SERIES (piston); 0 hours total
The flight
- Departed from: EAT Wenatchee WA
- Destination: EAT Wenatchee WA
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 080° at 5 knots
- Visibility: 10 statute miles
- Sky: broken clouds at 10,000 ft; clear
- Temperature: 64°F (18°C), dew point 37°F (3°C)
- Altimeter: 29.93 inHg
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 | |||
| Passengers | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.