# NTSB DEN93IA044: Boeing 737-522, N905UA, Denver, CO, April 24, 1993 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: April 23, 1993, 20:20 local (Apr 24, 1993, 02:20 UTC) - Place: Denver, CO - Coordinates: 39.759849, -104.919342 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=39.7598&lon=-104.9193&mi=10 - Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 133 uninjured - Status: Completed, report September 20, 1994 - Report: Approved by Office Director - NTSB case key (mKey): 11087 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=DEN93IA044 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/11087/pdf - Page: https://openairdocs.org/ntsb/DEN93IA044 ## 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. ## Analysis 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. ## Factual narrative On Sunday, April 23, 1993, at 2020 mountain daylight time, a Boeing 737-522, N905UA, operated by United Airlines as flight 409 experienced an uncommanded roll at approximately 1,000 feet above ground level (AGL) while conducting a visual approach to runway 26 left at Stapleton International Airport, Denver, Colorado. There were no injuries to the 2 pilots, 3 flight attendants, or 128 passengers, and the aircraft was not damaged. The aircraft was being operated under the provisions of 14 CFR Part 121 when the incident occurred. Visual meteorological conditions prevailed and an IFR flight plan was on file. The flight originated from San Antonio, Texas, at 1830. At the onset of the roll, according to the crew, maximum power was applied and a go around initiated. The roll was corrected by the crew and an uneventful landing was made on the next approach. Radar data provided information that the incident aircraft was following a Boeing 757 which was conducting a visual approach to runway 26R. The two runways are 900 feet apart centerline to centerline and the incident aircraft was 1.34 miles (32 seconds)in trail behind the B757. According to flight recorder data from both aircraft, wind at the time of onset of the occurrence was from the northwest at 5 to 10 knots. The wind values changed to the northeast on the incident aircraft, during the occurrence, and reached a peak value of 30 knots. According to flight recorder data, the incident aircraft achieved a maximum deflection of 23.027 degrees left wing down during the uncommanded roll. This occurred at a radio altitude of 882 feet above ground level. A flight recorder factual report and radar data is attached. In addition, research was conducted by NOAA in Boulder, Colorado, in the area of the interaction of wake turbulence with an outflow boundary. The research provided information that wake turbulence can be lifted up as an interaction with the boundary, and that an outflow boundary was moving through the area at the time of the incident. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Vortex turbulence encountered, approach - vfr pattern - final approach - visual separation - inadequate, pilot in command (cause) - weather condition - crosswind (contributing factor) - wake turbulence - encountered, pilot in command (cause) - procedure inadequate, faa(other/organization) (contributing factor) ## Aircraft - Registration: [N905UA](https://openairdocs.org/deregistered/N905UA-2008-10-29), registration cancelled 2008-10-29, S/N 25005, built 1991 (OpenAirDocs) - Reported make and model: BOEING 737-522 - Registry model: [Boeing 737-522](https://openairdocs.org/model/boeing-737-522) (registry code 138449F) (OpenAirDocs) - Registry engine: [CFM International CFM56 series](https://openairdocs.org/engine/cfm-international-cfm56-series) (registry code 13802, turbo-fan) (OpenAirDocs) - Category: Airplane - Serial number: 25005 - Engines: GE CFM-56 20000 lbs thrust turbo fan - Damage: No damage - Flight: Part 121 - Registered owner: United Airlines - Departed: SAT, SAN ANTONIO, TX - Destination: DEN - Flight plan: IFR - Airport: DEN, Stapleton International, runway 26L, 10,004 × 150 ft ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 47; commercial, airline transport, flight engineer; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, valid medical--no waivers/lim.; seat: left; flight hours: 8,428 total, 1,731 in this make and model, 1,731 as pilot in command, 198 in the last 90 days, 73 in the last 30 days, 5 in the last 24 hours ## Weather - Conditions: Visual (VMC), Dusk - Wind: 360° at 10 knots, gusting 16 - Visibility: 50 statute miles - Lowest ceiling: 27,000 ft broken - Temperature: 59 °F, dew point 19 °F - Altimeter: 29.00 inHg - Observed at: DEN ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 19930423017489I](https://openairdocs.org/incident/19930423017489I), Apr 23, 1993: Boeing 737522, Denver, CO. Pilot firewalled engines on final. Encountered windshear. Aircraft rolled left. Landed safely. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.