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United Airlines Flight 409

23 Apr 1993 · Denver, CO, United States

Boeing 737-522 · Incident: vortex turbulence encountered on approach

From San Antonio International Airport (SAT) to Denver International Airport (DEN)

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Event

NTSB case
DEN93IA044
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
Incident
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 737-522

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
N905UA
Onboard fatalities
Unknown
Route
From San Antonio International Airport (SAT), San Antonio, TXTo Denver International Airport (DEN)
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
Approach · VFR pattern final
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
Vortex turbulence encountered
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 › Visual separation, inadequate
  • Pilot in command › Wake turbulence, encountered

Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.

NTSB narrative

ON A VISUAL APPROACH TO RUNWAY 26L, UNITED FLIGHT 409, A BOEING 737, EXPERIENCED AN UNCOMMANDED ROLL OF ABOUT 23 DEG AT AN ALTITUDE OF 882' AGL. THE CREW WAS ABLE TO CORRECT THE ROLL & MADE A GO-AROUND TO AN UNEVENTFUL SECOND APPROACH & LANDING. THE BOEING 737 WAS FOLLOWING A BOEING 757, WHICH WAS ON A VISUAL APPROACH TO RUNWAY 26R. THE TWO RUNWAYS WERE 900' APART FROM CENTERLINE TO CENTERLINE & THE THRESHOLD OF 26L WAS OFFSET ABOUT 1300' EAST OF 26R. ACCORDING TO DATA FROM THE FLIGHT RECORDER, THE WIND AT THE ONSET OF THE INCIDENT WAS FROM THE NORTHWEST AT 5 TO 10 KNOTS. THE SURFACE WIND WAS FROM 360 DEGREES AT 10 KNOTS. THE TWO AIRCRAFT WERE 1.32 NAUTICAL MILES APART IN AN OFFSET TRAIL POSITION. BOTH AIRCRAFT WERE ON 3 DEG GLIDE SLOPES. THE WAKE VORTICES HAD SETTLED ABOUT 100' & MOVED LATERALLY TOWARD THE GLIDE PATH OF 26L. WEATHER DATA SHOWED EVIDENCE OF POSSIBLE INTERACTION BETWEEN WAKE TURBULENCE FROM THE 757 & AN OUTFLOW BOUNDRY, WHICH WAS MOVING THROUGH THE AREA.

Probable cause

THE PILOT-IN-COMMAND'S FAILURE TO MAINTAIN ADEQUATE SEPARATION FROM THE BOEING 757, AND HIS FAILURE TO STAY ABOVE THE PROBABLE PATH OF THE BOEING'S WAKE VORTICES, WHICH RESULTED IN THE VORTEX ENCOUNTER. FACTORS RELATED TO THE ACCIDENT WERE: THE CROSSWIND, AND AN INADEQUACY IN ATC PROCEDURES CONCERNING VISUAL APPROACHES OF AIRCRAFT, BEHIND AND DOWNWIND FROM APPROACHES OF HEAVIER AIRCRAFT TO PARALLEL OR ADJACENT RUNWAYS.

Verbatim NTSB analysis and probable cause from the NTSB dataset

Sources

NTSB record DEN93IA044
Event ID
20001211X12116
Case number
DEN93IA044
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 1993-04-24.
  • API snapshot SHA-256: 74cddd12e8bd9164c2911f0f9c3abe277805de470ba01f66ef5a62a94fdd3529; retrieved 2026-09-18T18:38:16.340Z.
  • The NTSB's operator name is blank for N905UA; the operator shown is the name the NTSB's other records give its FAA operator code, UALA.