Douglas DC-4 accident at Annette Island, Alaska, 26 Oct 1947

Accident narrative

At approximately 1345 on October 26, 1947, Pan American Airways Flight 923, a Douglas DC-4, crashed on the north side of Tamgas Mountain near Annette Island, Alaska. The aircraft was destroyed, and all 13 passengers and five crew members were killed. The flight had abandoned its approach to the airport due to extreme turbulence minutes before the crash. The Civil Aeronautics Board could not determine the probable cause of the accident.

### The flight

Flight 923 departed Seattle, Washington, at 1030 for Juneau, Alaska, with an intermediate stop scheduled at Annette Island. The airplane carried 13 passengers, including an infant, 822 pounds of cargo, and 2,500 gallons of fuel. The load was properly distributed and within allowable weight limits.

The crew consisted of Captain Alf N. Monsen, First Officer Laurence A. Foster, Flight Engineer C. L. Dunwoody, Check Purser Mary E. Chidiac, and Purser Helen H. Darrah. Both pilots were highly experienced: Captain Monsen had logged 13,565 hours, including 514 in DC-4 aircraft, and had flown as a pilot in Alaska since 1928. First Officer Foster had logged 12,412 hours, with 359 in the DC-4.

Company weather information provided before departure forecasted an 800-foot overcast at Annette Island, with lower broken clouds, two miles of visibility, light rain, and an 18-knot surface wind from the south-southeast. A cold occlusion or front was expected to pass Annette Island before the flight arrived.

### The approach

Proceeding on an instrument clearance at 9,000 feet, the flight arrived over Annette Island at 1338. Earlier, at 1326, the company radio at Annette Island transmitted a weather observation reporting an indefinite 1,400-foot ceiling, overcast, lower broken clouds, three miles of visibility, rain, and southeast winds at 28 miles per hour with gusts to 40 miles per hour. The flight acknowledged this information.

Airway Traffic Control cleared the flight to pass over the radio range station, located northwest of the airport, at 7,000 feet, and to proceed with a normal instrument letdown and approach. The flight reported its position over the radio range station at 7,000 feet at 1339. The normal instrument procedure required the flight to remain west of the south course of the range while on instruments, avoiding the mountainous terrain to the east and northeast that rises to 3,596 feet on Tamgas Mountain.

At 1343, five minutes after reporting over the range station, the flight advised Annette Radio that it was proceeding to Juneau because of extreme turbulence. This was the first indication the flight was encountering difficulty. Annette Radio asked the flight to report its altitude, but received no reply. After repeated calls, an alert was issued at 1401.

### What the investigation found

Adverse weather retarded the search, and the wreckage was not located until October 31. The airplane had crashed on the north slope of Tamgas Mountain at an elevation of approximately 3,400 feet, 196 feet below the summit. The site was approximately eight miles east of the Annette Island Airport and eight miles east of the south course of the Annette radio range.

Because the wreckage was scattered over 20,000 square feet and buried in snow, a detailed examination was impossible until the summer thaw in late August 1948. Propeller marks on a rock wall 50 feet above the wreckage showed the aircraft struck at a 45-degree angle in a 20-degree climbing attitude. The true course at impact was estimated at 145 degrees, which was corroborated by the Fluxgate compass, found jammed at 116 degrees (which, with a 29-degree easterly deviation, results in a 145-degree true course).

The engines were relatively intact, except for one that was badly shattered, and the degree of propeller breakage indicated none were feathered. A flash fire occurred at impact, but there was no evidence of an in-flight fire. All flight instruments except the compass were battered beyond readability, and the positions of the flaps and landing gear could not be determined. The Board found no evidence of structural or mechanical failure prior to the crash, and maintenance records indicated the aircraft was airworthy.

A post-accident check of the Annette Island Radio Range found the northwest course was in error by 3 1/2 degrees clockwise. The Board determined this displacement was not contributory, as the course would still remain at least 10 miles from the mountains. An Army pilot who landed 23 minutes before Flight 923 reported the range operation was substantially normal.

An aftercast of the weather showed the company forecast was substantially correct except for the high winds and turbulence over Annette. Winds had progressively increased to 50-60 miles per hour, producing marked turbulence and downdrafts over the mountainous terrain. The Board noted that the company meteorologist should have forecasted much more severe turbulence at low altitudes based on available data. A revised forecast predicting stronger winds was dispatched at 1229, but there was no record that it reached the flight. The front itself was actually still approximately 100 miles west of Annette Island at the time of the accident.

Other aircraft in the area confirmed severe conditions. The Army pilot who landed just before Flight 923 encountered slight turbulence at 6,000 feet that grew progressively more severe down to 400 feet, along with light rime ice. A non-scheduled flight arriving three hours later encountered moderate ice and turbulence so severe it almost tumbled the gyro flight instruments. Another flight departing six hours later encountered extreme turbulence and severe icing that forced a return to Annette Island.

### Probable cause

The Board noted several possible explanations for the accident, including that severe turbulence or severe icing may have caused a loss of control. However, the Board stated it remained impossible to explain why the flight was unable to stay on the proper side of the radio range and why it was flying a heading of 145 degrees.

The Board concluded: "There is not sufficient evidence to determine the probable cause of this accident."