Report: United Airlines Flight 585
By: Darryl Morrell
For the flight crew of Captain Harold Green and
his First Officer, Patricia Eidson, Sunday March 3rd began just like any
other day. They were on a second day of a busy schedule flying Boeing 737s
on a number of domestic flights throughout the western United States. The
evening before, they checked into a hotel in Denver, ready to take over the
flight from another crew early the next morning. On the morning of the 3rd
they were rostered to take over from another crew, Boeing 737-200, N999UA.
The aircraft was operating as flight 585 from Peoria, Illinois, to Colorado
Springs, via Moline and Denver. The aircraft departed Peoria on schedule
at 5am. The flight to Moline and on to Denver was completely uneventful and
it touched down ahead of schedule in Denver. The previous crew handed the
aircraft over to Green and Edison, reporting no problems on the trip to Denver.
The aircraft was checked by the dispatch engineer and apart from the avionics
door latch being out of its stowed position, he reported no problems with
the aircraft and the crew were told that it was fully servicable to carry
on the next sector of the flight.
|The wreckage of United Airlines 585. (File Photo)
weather center reported good conditions for the next leg, with visibility
at 100 miles and the temperature at 49° F. On board the aircraft there
were 20 passengers, with 3 flight attendants to look after them.
The aircraft took of as normal with the Captain
flying and the First Officer handling the communications. The flight to Colorado
Springs was expected to take about 20 minutes. The departure controller vectored
them on to a heading of 140° ready to intercept the designated
airway, V81, to Colorado Springs. Three minutes later the crew were cleared
to maintain 11,000ft. The aircraft called Approach control reporting their
present altitude and that it had copied "Information Lima" from the ATIS
transmission. This information current for the last 40 minutes was reporting
"...wind 310 at 13 knots, gusting to 35, low level wind shear warnings are
in affect, occasional severe turbulence reported by numerous aircraft between
FL180 and 380. Local aviation wind warning in effect calling for winds out
of the northwest, gusts to 40kt and above."
Approach instructed 585 to proceed to the VOR,
then leave it on a heading of 165° in preparation for being vectored
to Runway 35 for a visual approach. The controller reported current winds
as 320 at 13kt, gusting to 23kt.
Several minutes later ATC cleared the 737 to
descend to 10,000 feet and 3 minutes later requested they descend to 8,500 feet.
When the aircraft reported "airport in sight" they were instructed to maintain
8,500 feet until on base leg, then they were cleared for a visual approach to
runway 35, and to contact the Tower on 119.9Mhz. On final the Captain was
flying the aircraft, the first officer was handling communications, the runway
was in sight, the aircraft was configured, and the checklist was complete.
The aircraft was turning from its 45° intercept to the extended runway
centerline, and the first officer called "we're at 1,000 feet."
Dozens of witnesses in the community directly under
the extended centerline watched as the airplane levelled off momentarily
on the runway heading, 3.5 miles from the threshold. Then, it rolled to the
right, pitched down until reaching a nearly vertical attitude, and compacted
itself into a 39-foot wide, 15-foot deep crater in an area known as Widefield
Park. All onboard were killed.
The Cockpit Voice Recorder (CVR) tape for the last
12 seconds of the flight indicated the crew was completely surprised by the
upset and attempted corrective measures. Two seconds after calling "1,000
feet," the first officer exclaimed, "Oh, God." The captain called, "15 flaps,"
an indication that he was initiating a go-around. The first officer responded
"15." The following five seconds contained only exclamations as the crew
tried to regain control.
|An aerial view of the United 585 crash site. (File Photo)
For the last 20 seconds, the Flight Data Recorder
(FDR) showed the rate of heading change consistent with a 20-degree (right)
bank angle and a turn for alignment with the runway. Sixteen seconds prior
to the crash, the thrust was increased to about 6,000 pounds per engine (from
3,000 pounds). As the thrust was increasing, the first officer made the "1,000
Within the next four seconds (about nine seconds
prior to the crash), the heading rate increased to about five degrees per
second to the right, nearly twice the rate of a standard turn. The first
officer said, "Oh, God," - the altitude decreased rapidly; the indicated
airspeed increased to over 200 knots; and the normal acceleration increased
to over 4g.
"The two most likely events that could have resulted in a sudden uncontrollable
lateral upset," said the Safety Board, "are a malfunction of the airplane's
lateral or directional control system or an encounter with an unusually severe
atmospheric disturbance. Although anomalies were identified in the airplane's
rudder control system, none would have produced a rudder movement that could
not have been easily countered by the airplane's lateral controls. The most
likely atmospheric disturbance to produce an uncontrollable rolling moment
was a rotor (a horizontal axis vortex) produced by a combination of high
winds aloft and the mountainous terrain. Conditions were conducive to the
formation of a rotor, and some witness observations support the existence
of a rotor at or near the time and place of the accident. However, too little
is known about the characteristics of such rotors to conclude decisively
whether they were a factor in this accident."
During the course of the investigation, NTSB personnel
interviewed several sailplane pilots who spend their spare time chasing thermals
along Colorado's Front Range. These aviators probably know as much (if not
more than) the region's professional meteorologists about the wind flows
around the peaks and through the passes.
The sailplane pilots seemed to agree that rotors
are a fairly common occurrence and that they sometimes touch the ground on
the lee of the mountains. They often are present along with mountain waves
and lenticular clouds.
One of the Safety Board's recommendations stemming
from this investigation is that the FAA develop a meteorological aircraft
hazard program to include airports in or near mountainous terrain. This program
would be based on research conducted at Colorado Springs to observe, document
and analyze potential meteorological aircraft hazards with a focus on the
approach and departure
HISTORY OF THE AICRAFT
The 737-200, built in Renton in 1982, started out
as part of Frontier Airline's fleet. Four years later, Frontier sold the
plane to United and it was assigned tail number N999UA.
On a Feb. 25, 1991, flight, N999UA's rudder deflected
inexplicably to the right. The problem went away when the pilots switched
off the yaw damper, a device that automatically commands small rudder adjustments
during flight. Mechanics replaced a part called the yaw-damper coupler and
returned the plane to service.
Two days later, a different flight crew reported
N999UA's rudder again moving to the right. The new coupler evidently had
made no difference. This time mechanics replaced a valve in the yaw damper
and returned the plane to service.
Four days later, on the blustery morning of March
3, 1991, Captain Harold Green and First Officer Patricia Eidson were bringing
N999UA down for a routine landing in Colorado Springs. At 1,000 feet, the
jet suddenly flipped to the right and dived straight down, smashing into
a city park and killing all 25 on board.
The pilot of a Cessna flying near the airport called
the tower with a bird's-eye account: "We just saw the plane . . . uh just
suddenly a complete downward dive."
From the control tower, air-traffic controller
Kevin Ford reported from another perspective: "It looked like a dropped pencil
going straight down."
It didn't take long for errant rudder movement to surface as a possible cause
of the crash. Witness reports and readings from the plane's flight-data recorder
confirmed that the 737 had traced a classic aerobatic maneuver, known as
a "split-S," into the ground.
A split-S results from radically altering the symmetry
of flight. Such a quick or severe change would be consistent with the right
engine or right wing falling off, but that had not happened. The pilots could
have moved the rudder to the extreme right, but to do so within 1,000 feet
of the ground would be suicidal. The other possibility was that the rudder
had moved on its own.
Investigators with the National Transportation
Safety Board were unfamiliar with the make up of the plane's rudder-control
system when they arrived in Colorado Springs to comb through N999UA's wreckage.
Discovery of the PCU (power control unit) recovered
from the wreckage showed wiring to the solenoid was loose and the circuit
intermitent, this could have been the cause of the uncommanded rudder yaws
in previous flights. Also the weather conditions in mountain ranges were
A test flight the next day in the area reported severe windshear in the
The NTSB, in early-2001, cited a rudder system malfunction, leading to a loss of rudder command by the crew, as the primary probable cause of this accident.