United Airlines Flight 531
Boeing 737-322 · Accident: abrupt maneuver en route
From Boston Logan International Airport (BOS) to Chicago O'Hare International Airport (ORD)
Event
- NTSB case
- CHI00LA269
- 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 737-322
- Aircraft type
- Boeing 737
- 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
- N313UA
- Operator
- United Airlines
- Onboard fatalities
- Unknown
- Route
- From Boston Logan International Airport (BOS), Boston, MATo Chicago O'Hare International Airport (ORD), 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
- Abrupt maneuver
- 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 › Evasive maneuver, performed
Approximate · Estimated position: the centre of Chicago, IL, the place the record names. Not the accident site.
NTSB narrative
A passenger sustained a serious in-flight injury during an evasive maneuver following a Traffic Alert and Collision Avoidance System (TCAS) warning approximately 20 minutes prior to landing. The weather did not support the presence of clear air turbulence. In a written statement, the copilot stated, "However, shortly thereafter while continuing our descent into Chicago, the Traffic Alert and Collision Avoidance System (TCAS) alerted us to a potential conflict with an aural 'TRAFFIC, TRAFFIC' warning, the EHSI [Electronic Horizontal Situation Indicator] displayed a visual warning as well. ... Almost immediately after the initial Traffic Advisory (TA), a subsequent 'CLIMB, CLIMB, CLIMB' aural warning was issued at approximately 14,000 feet as well as visual warnings on the EHSI and the EADI [Electronic Attitude Director Indicator]. ... The TCAS Resolution Advisory 'RA' commanded a pitch attitude of approximately 5 degrees nose up. Within one minute the TCAS 'CLEAR OF CONFLICT' aural warning was issued and the captain reestablished a descent while I notified ATC [Air Traffic Control] of the RA and our resumption of the descent. ... The captain contacted the flight attendants to ascertain the condition of the passengers and cabin crew. He was informed that one passenger hurt his ankle during the avoidance maneuver, but was not in need of immediate medical assistance." The doctor who treated the passenger stated, "Radiographs were take[n] 01 August 2000 and were significant for a left fibular fracture."
Probable cause
the evasive maneuver performed by the pilot. A factor was the activation of the collision avoidance system.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record CHI00LA269
- Event ID
- 20001212X21420
- Case number
- CHI00LA269
- 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 2000-08-01.
- API snapshot SHA-256: 399cd75ed292042561df5a171a2017637c54219d90c86c84128d61911de6f657; retrieved 2026-09-18T16:30:27.394Z.
- The NTSB's operator name is blank for N313UA; the operator shown is the name the NTSB records it doing business as.