United Airlines Flight 256
Boeing 777-222 · Accident: uncontrolled altitude deviation en route
From Minneapolis–Saint Paul International Airport / Wold–Chamberlain Field (MSP) to Chicago O'Hare International Airport (ORD)
Event
- NTSB case
- CHI99LA184
- 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
- 0 · 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
- Airline
- 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
Boeing 777-222
- Aircraft type
- Boeing 777
- Category
- Airplane
- 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
- N781UA
- Operator
- United Airlines
- Onboard fatalities
- Unknown
- Route
- From Minneapolis–Saint Paul International Airport / Wold–Chamberlain Field (MSP), Minneapolis, MNTo Chicago O'Hare International Airport (ORD), Chicago, IL
- 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 121: Air Carrier
- 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
- En route · descent
- 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
- Uncontrolled altitude deviation
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- None
Cause areas
- Pilot in command › Weather evaluation, inadequate
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
While en route from Minneapolis to Chicago the flight encountered turbulence at 19,680 feet msl while in a descent. Four passengers and four flight attendants received minor injuries. One passenger received a broken ankle. The flight had diverted to Minneapolis due to thunderstorm activity in the Chicago area. The captain reported the flight was smooth up until the unexpected turbulence was encountered. The seat belt sign was not on at the time of the encounter. The flight crew received a Weather Briefing Message (WBM) prior to departing Minneapolis, which contained convective sigmets for Wisconsin, Illinois, and Indiana. The accident took place ahead of a stationary front which extended northeast-southwest through central Wisconsin and Iowa which had been creating shower and thunderstorm activity throughout afternoon and evening on the day of the accident. A review of the data from the Quick Access Recorder revealed the flight experienced a change in G-loading from .0843 g's to 1.9375 g's over a 2-second time period while at an altitude of 19,680 feet msl.
Probable cause
the pilot-in-command's inadequate evaluation of the weather conditions. Factors associated with the accident were the turbulence encountered and the seat belt sign was not illuminated.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record CHI99LA184
- Event ID
- 20001212X18945
- Case number
- CHI99LA184
- Dataset
- historical-pre2008
- Source SHA-256
- 89e2df6d848e7ab3b42e7d7c7a03ff403303057a26ca04c95b3ea67201c3ce74
Source notes (5)
- 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: local date, investigation status, damage. Values present in the bulk record are kept.
- The date is the local date; the NTSB stores the UTC date 1999-06-12.
- API snapshot SHA-256: a7b98f563b112e2cde2fd6cfb91a8c22a1dcd37ba74b6edb308713d0fae05bcf; retrieved 2026-09-18T16:55:52.983Z.
- The NTSB's operator name is blank for N781UA; the operator shown is the name the NTSB records it doing business as.