On March 3, 1991, a United Airlines Boeing 737-200 departed from the now-defunct Stapleton International Airport (DEN) in Denver, Colorado, on a routine connection to Colorado Springs, Colorado. United Airlines Flight 585 was a unique connection: it departed from Peoria, Illinois, just outside of Chicago, stopped in Moline, Illinois, and before its final landing in Colorado Springs, it also landed in Denver to make a crew change, according to a National Transportation Safety Board (NTSB) report.

The first three legs of the flight were nothing out of the norm, as pilots reported no issues in the maintenance log of the aircraft. Before the Boeing 737 departed for its last leg, the weather in Colorado Springs was clear, with great visibility, but a bit windy: 23 knots, with gusts up to 33 knots.

Flight 585 departed from Stapleton International (DEN) at 09:23 Local time, with no issues. It was scheduled to arrive in Colorado Springs Airport (COS) at 09:42. The 737-200 carried 20 passengers, including three flight attendants and two pilots on its last leg to the city, dubbed as Olympic City USA.

At 09:41, Air Traffic Control in Colorado Springs told the flight crew on board Flight 585 to “hold short of runway three zero for departing traffic on runway three zero,” with the first officer confirming that United 585 heard the instructions and repeated them to the controller, a standard procedure.

However, that was the last active contact United Airlines Flight 585 had with the controller: shortly after, the Boeing 737 plunged into the ground, just short of Colorado Springs Airport’s runway 35.

What started as an accident in 1991 would eventually end in 2008, when around all Boeing 737s would be eventually retrofitted with a new rudder control system.

From the 727 to the 737

The issue, which crossed the NTSB’s roads for the first time in 1991, could be back-tracked to the 1960s when the commercial aircraft market was much more competitive: while Airbus was still not conceived, in the United States alone, three manufacturers were competing for airlines’ attention: Boeing, McDonnell Douglas and Lockheed.

Lockheed focused on a wide-body, the L-1011 TriStar.

McDonnell Douglas released and enjoyed relative success with the DC-9, a short-haul narrow-body that seats up to 140 passengers in a single-class configuration, while Boeing was busy with the design of the 747. At the end of February 1965, the DC-9 attracted 112 orders.

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The same month, Boeing decided to launch their answer to the DC-9: the Boeing 737. Lufthansa became the launch customer of the 737, with a firm order of 21 aircraft, signed on February 15, 1965.

The Boeing 727 and the Boeing 737 were eerily similar: except for the fact that the Baby Boeing had two engines placed under the wings, and the aircraft’s horizontal stabilizer was also placed on the aft fuselage, unlike on the 727, which had its horizontal stabilizer on its tail. The tail had another major difference between the two aircraft: while the 727 had a split-rudder, the Boeing 737 emerged from the design sheets with a single-panel rudder.

Theoretically, if one of the rudders on the 727 would encounter an issue, the other could be used to offset the issue, as they were controlled by separate Power-control units (PCU). The same luxury was not available on the 737.