CEN21LA346 · HpH, Spol. S.R.O. Glasflügel 304S
HpH, Spol. S.R.O. Glasflügel 304S · Accident: flight control system malfunction/failure during takeoff
Event
- NTSB case
- CEN21LA346
- 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
- Event fatalities
- Unknown
- Ground fatalities
- Unknown
- 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
HpH, Spol. S.R.O. Glasflügel 304S
- Aircraft type
- HPH GLASFL GEL 304S
- Category
- Glider
- 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
- N304AB
- Operator
- Unknown
- Onboard fatalities
- Unknown
- Route
- From not recordedTo not recorded
- Aircraft age
- About 13 years (built 2008)
- 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
- Takeoff
- Defining eventThe 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
- Flight control system malfunction/failure
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Substantial
Cause areas
- Aircraft › Aircraft systems
- Personnel issues › Task performance
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
The pilot was departing on a local flight in the experimental glider when the glider unexpectedly separated from the towline shortly after liftoff. The glider then entered a left turn and landed in a wings-level attitude. The tailboom was substantially damaged during landing. The tow airplane and the towline were not damaged during the event.
Postaccident examination determined that the elevator control tube installed in the vertical stabilizer was corroded along the entire length of its inner surface, reducing its wall thickness. Water likely entered the control rod, either through a witness hole near the upper end of the control tube or as moisture carried in by humid air. There was no drain hole at the bottom end of the control tube and, as a result, there was no way for liquid water to drain out of the control tube.
The wall thickness eventually thinned sufficiently to cause the tube to burst in the longitudinal direction near its upper end. After the control tube burst, the resulting hole on the side of the tube allowed for the easy ingress of water that made its way past the boot seal. The corrosion product and standing water at the base of the tube eventually reduced the tube wall thickness to a point where it could no longer withstand the typical operational loads and subsequently fractured in overstress near the clevis fitting during the accident flight. The overstress failure of the control tube prevented the pilot’s control of the elevator during the accident flight.
The last condition inspection of the glider was completed 24 days before the accident. The corresponding logbook entry noted that the flight controls were inspected, and that the glider was in an airworthy condition. The longitudinal fracture near the upper end of the elevator control tube would have been readily visible with the rubber boot removed and, as such, it is likely the mechanic did not remove the rubber boot to adequately inspect the elevator control tube during the last condition inspection.
Probable cause
The overstress fracture of the elevator control tube due to reduced wall thickness from water intrusion and subsequent corrosion. Contributing to the accident was the lack of a drain hole at the bottom of the elevator control tube, which allowed the tube to collect water, and the mechanic’s inadequate inspection of the elevator control system during the recent condition inspection.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record CEN21LA346
- Event ID
- 20210801103600
- Case number
- CEN21LA346
- Dataset
- full-current
- Source SHA-256
- 5cf380f0061817c0331a6b2d8cc7e0ee3a79bea469a1001dc5c10e56f35f5ab3
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, cause areas. 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: c6529e276f03829aac107397dcc8aeb7854fed38441cce7e5865d132b0545f31; retrieved 2026-09-16T14:21:30.759Z.