Accident at Pueblo, Colorado, 23 Aug 1958

Accident narrative

On August 23, 1958, shortly after noon, Frontier Airlines Flight 73 made an emergency gear-up landing two miles west-northwest of Pueblo Memorial Airport in Colorado. The DC-3C had just taken off when a false fire warning prompted the crew to feather a propeller. Unable to maintain altitude, the aircraft struck the ground and slid nearly 1,000 feet. Several of the 19 passengers and three crew members received minor bruises, but there were no serious injuries. There was no fire.

### The flight and the emergency

Flight 73 was a daily scheduled service originating in Denver and terminating in Grand Junction, Colorado, with intermediate stops including Pueblo. The aircraft departed Denver and arrived in Pueblo. It departed Pueblo on a visual flight rules (VFR) flight plan. Aboard were 19 passengers and a crew of three: Captain James R. Langhofer, First Officer Donald L. Lockwood, and Stewardess Joanne V. Lohse. The aircraft's takeoff weight was properly distributed and below the allowable maximum of 24,900 pounds.

The first officer made a normal takeoff from runway 30. Immediately after the aircraft became airborne and gear retraction began, the captain observed the cockpit fire warning light for the left engine. Following the company operations manual, he tested the system using the fire panel "opposite" switch. This switch transfers the circuit to check for a relay malfunction; when actuated, the right warning light came on, indicating the left relay system was functioning and a fire was probable. The captain feathered the left propeller, advised the Pueblo control tower that the flight was returning, and took over control.

Witnesses at the end of the runway observed the left propeller completely feathered and the landing gear fully retracted as the aircraft passed over them at an altitude of 50 feet. The aircraft gained altitude very slowly on one engine, reaching approximately 100 feet.

Because of higher terrain ahead and to the right, the captain initiated a shallow left turn at an altitude of 50 to 75 feet and an indicated airspeed of 95 knots. The flight could not maintain this altitude and airspeed. After passing under a power line, the airspeed dropped to 75 knots, and the captain cut power to the right engine.

The aircraft contacted the ground almost immediately on a heading of 190 degrees magnetic. It skidded on the underside of its fuselage for nearly 1,000 feet, passing over three shallow ditches. At the third ditch, the aircraft swung approximately 115 degrees to the right, and the right engine separated completely at the firewall. One blade of the right propeller punctured the right side of the cockpit, severely damaging the right pilot seat and severing a carbon dioxide line. Evacuation began immediately and was completed in approximately one and a half minutes.

### What the investigation found

The Board found no evidence of airframe or powerplant failure prior to ground impact, and a detailed examination of the left engine showed no evidence of fire or of any hot spot that could have actuated the warning signal.

Investigators reviewed pilot flight reports and found that the aircraft had experienced five false fire warnings for the left engine between July 9 and August 19, 1958. The accident crew was unaware of these reports. In each previous instance, pilots had written up the false warnings, but base records did not indicate that corrective action was taken at the Denver base to eliminate the recurring issue.

Bench tests proved the fire-warning relays and test switch were operating normally. However, investigators examining the aircraft's wiring found an irregularity in a flexible conduit extending from the firewall junction box to the inboard nacelle junction box. Inside the conduit, they found damp oxidized aluminum, oil, and dirt. Two wires in the bundle were unnecessarily long and criss-crossed other wires. Moving one of the thermocouple wires in this conduit produced a variable resistance, indicating an electrical fault capable of introducing the minimal current required to trigger a false warning.

The Board analyzed the aircraft's performance capabilities. With a temperature of 75 degrees Fahrenheit, the aircraft should have been capable of a rate of climb of 282 feet per minute on one engine. Its best single-engine climb speed was 92 knots. Although the captain had 95 knots after feathering, the Board noted the aircraft could not have cleared the high terrain ahead even if it achieved its theoretical performance. The necessary left turn away from the higher land, combined with gustiness and turbulence from the high ground to windward, was a factor in the loss of airspeed and altitude.

The Board noted that a glance back from the cockpit toward the left engine would have disclosed no evidence of fire, which could have delayed the feathering and averted the accident. However, the captain was following the emergency procedures in the company manual, which did not call for a visual check prior to feathering. As a result of the accident, the carrier revised its manual to allow pilot judgment regarding immediate feathering without a visual check.

The Board concluded that the aircraft's log sheets were not properly monitored, that maintenance personnel failed to take adequate corrective action for the recurring false fire warnings, and that supervisory personnel were lax in not detecting this inadequacy.

### Probable Cause

The Board determines that the probable cause of the accident was a false fire warning during climb-out toward rising terrain, followed by the immediate feathering of a propeller. The resulting aircraft performance under the existing conditions necessitated an off-airport landing. The false fire warning was due to inadequate maintenance.