crashes, Pedro Bay, AK-39

lornajarrettblanchard December 2, 1968

–39 Aviation Safety Network. Accident Descrip. Wien Consolidated Air Flight 55, 2Dec1968.

–39 Liefer, G. P. Broken Wings: Tragedy & Disaster in Alaska Civil Aviation. 2003, p. 180.

–39 NTSB AIR. Wien Consolidated Airlines…N4905, Pedro Bay, Alaska, Dec 2, 1968.

Narrative Information

NTSB Synopsis:

“A Wien Consolidated Airlines, Inc., Fairchild F-27B, N4905, crashed at Pedro Bay, Alaska, at approximately 0936 Alaskan standard time, December 2, 1968. The 36 passengers and three crewmembers died in the accident and the aircraft was destroyed.

“The aircraft was operating as Flight 55 from Anchorage to Dillingham, Alaska, with en route stops at Iliamna, Big Mountain, and King Salmon, Alaska. Flight 55 had been cleared for an approach to Iliamna Airport and as it neared the vicinity of Pedro Bay, witnesses on the ground observed a large cloud of black smoke and fire behind the aircraft. They stated that shortly after that, they saw pieces separate from the aircraft and the aircraft descend in a spin. The weather at the time of the accident was clear with good visibility. High winds were reported on the ground in the Pedro Bay area.

“Investigation showed that the right outer wing, the empennage, portions of the left wing, and other components of the aircraft structure had separated from the aircraft in flight.

“The Board determines that the probable cause of this accident was an in-flight structural failure caused by an encounter with severe-to-extreme turbulence. This turbulence was not forecast and its presence was not known to the flightcrew. The failure occurred in an area of the right wing (WS 197) which had been weakened to an indeterminate degree by pre-existing fatigue cracks.

“The Board recommended to the Administrator, Federal Aviation Administration, that all F-27 aircraft which had more than 5,000 hours in service be inspected for possible fatigue cracks in the wings. This recommendation was carried out by the Administrator and a total of 13 cracks were found in eight of the 67 aircraft inspected.

History of the Flight

“A Wien Consolidated Airlines, Inc Fairchild F-27B, N4905, crashed at approximately 0936 A.s.t., December 2, 1968, at Pedro Bay, Alaska. The 36 passengers and three crewmembers were killed in the accident and the aircraft was destroyed by in-flight breakup and ground impact.

“The aircraft was being operated as Flight 55 in scheduled domestic passenger service between Anchorage and Dillingham, Alaska, with en route stops at Iliamna, Big Mountain, and King Salmon, Alaska.

“Flight 55 departed from Anchorage International Airport at 0846 on an instrument flight plan for Iliamna Airport. The flight was cleared to cruise at 16,000 feet mean sea level. The weather at Iliamna was reported to be clear, and the visibility was 15 miles at the time of the flight’s departure from Anchorage.

“The flight proceeded toward Iliamna without reported difficulty, and at 0925:29, the first officer requested a clearance for an approach to Iliamna. This request was approved just prior to 0926. No further communication was received from the crew.

Ground witnesses in and around the Pedro Bay area reported that they saw a fireball and a large cloud of black smoke which appeared to be behind the wing of the aircraft. The aircraft appeared to continue on course for a short period of time, then pieces of the aircraft were seen falling, and the aircraft entered a spinning descent. The aircraft disappeared from the view of the witnesses, and no fire or explosion was observed after the initial fireball.

“The major portion of the wreckage was located on the southern fuselage was located at latitude 59″ 46′ 17″ N. and longitude 154″ 08′ 28″ W. shore of Foxies Lake at an elevation of approximately 220 feet. The accident occurred in bright daylight…..

Aircraft Information

“….An examination was made of a number of X-ray radiographs supplied by the carrier. These radiographs had been prepared, in compliance with Fairchild Hiller Service Bulletin 51-2, during periodic radiographic inspections of the aircraft structure in the vicinity of the No. 1 access doors in both wings.

“The examination of these radiographs, subsequent to the accident, revealed that cracks were present before the accident in structural components of both wings near the No. 1 access doors at Wing Station (WS) 197…. aircraft maintenance records indicated that these cracks were not detected by the persons interpreting the radiographs for the carrier….

Meteorological Information

“….A local pilot from Iliamna was sent out in a Cessna 180 to search for the wreckage location. He reported that the rough air between Iliamna and the accident site required him to reduce his airspeed about 40 miles per hour and put down some landing flaps. The closer he got to Pedro Mountain the worse the turbulence became because of the winds ‘burbling over the mountain as north-northwest, it was coming right over the top of the mountain and partly around the mountain….’ At an altitude of about 1,500 feet the turbulence was “. . . to a point where there was no positive control whatsoever. The aircraft just went where it felt like going.” The time of this observation was about 1000 or 1015. He estimated the wind velocity to be 45 to 50 knots with stronger gusts in the vicinity of Pedro Bay. He reported the sky to be clear with unrestricted visibility above 2,000 feet, but the fog off the lake and the blowing snow was from 1,500 to 2,000 feet high. He also reported blowing snow off the mountain peaks and a cap on the higher peaks. This pilot had lived in the area all his life and he couldn’t recall flying ‘in this rough of weather before’….

Flight Recorders

“….The flight recorder data graph covered the entire flight from Anchorage to the end of the recorded traces, a time period of 46:35 minutes, with the exception of the period between 19 and 38 minutes after takeoff which represented cruise flight at a relatively stable altitude and airspeed. Only the last 5 minutes of recorded binary traces were plotted.

“Thirty-nine minutes after takeoff (0925), the indicated airspeed (IAS) was approximately 180 knots and the altitude, corrected to an altimeter setting of 29.98 inches Hg, was approximately 16,300 feet and was slowly decreasing. One minute later (0926), the airspeed had increased to approximately 210 knots IAS and the altitude was approximately 16,250 feet. A few seconds later, the altitude began to decrease, and the airspeed increased further to approximately 215 knots IAS. The rate of descent averaged approximately 975 feet per minute until reaching 11,650 feet at 0931:42. At this time, the rate of descent increased suddenly to 4,100 feet per minute and the airspeed increased from approximately 220 knots to 340 knots IAS over a period of 14 seconds.

“ Prior to this sudden change in rate of descent, the airspeed had varied between 210 and 230 knots IAS. The recorded traces terminated at 0932:32 or 46:32 minutes after takeoff from Anchorage. At this time, the indicated altitude was approximately 4,700 feet and the indicated airspeed was approximately 260 knots….

Wreckage

“….There was no evidence of an explosion in any of the examined wreckage. The fractures of the empennage surfaces were all consistent with a counterclockwise (viewed from the rear) separation of the surfaces. The fin failed to the left, the left stabilizer failed downward, and the right stabilizer failed upwards. All of the observed fractures were typical of bending overloads, with some evidence of torsion observed in the fractured fin. No evidence of fatigue was seen in any of these fractures….

“This was a nonsurvivable accident. All the cabin seats were separated from their attachment points, but all the passengers’ bodies except two were found in the cabin area. These two were found outside the cabin within 20 feet of a break in the fuselage.

Analysis and Conclusions

“It was established early in the investigation that the aircraft had broken up in flight, and the major thrust of the investigation was an attempt to learn what factor or factors had led to the breakup….

“Examination of the recovered wreckage revealed no evidence of an in-flight fire or explosion. The fire, observed by witnesses while the aircraft was in flight, left no evidence on the examined wreckage. The Board believes this fire resulted from the ignition of fuel escaping from the severely cracked wing skin….

“The Board’s analysis of the weather indicates that, in the area of the accident, there was a potential for severe to extreme turbulence from the surface to the tropopause at approximately 21,500 feet. This condition existed because there were present:

a strong mountain wave generated by an intense windflow over the mountains west and north of Iliamna;
a sharp trough aloft with its associated horizontal wind shear;
the vertical wind shear between 9 and 12 thousand feet of approximately 6 knots per thousand feet; and
the strong thermal gradient which existed in the accident area.

“….Based on the analysis of the available evidence, the Board believes that the aircraft encountered severe to extreme turbulence at an altitude of approximately 11,500 feet at an indicated airspeed of approximately 220 knots [113 mph]….

“In summary, the Board believes that this aircraft was subjected to severe-to-extreme turbulence while descending to land at Iliamna. This resulted in incremental load factors being imposed on the wing that were in excess of the ultimate load carrying capability of the cracked wing and the structure failed at its weakest point, near WS 197. The right wing failed, releasing fuel which ignited, and the wing subsequently separated from the aircraft.

“The turbulence that was encountered by the aircraft was not forecast and was not detectable by the crew….

“The Board determines that the probable cause of this accident was an in-flight structural failure caused by an encounter with severe to extreme turbulence. This turbulence was not forecast and its presence was not known to the flightcrew. The failure occurred in an area of the right wing (WS 197) which had been weakened to an indeterminate degree by pre-existing fatigue cracks.

Recommendations

“On December 23, 1968, the Board advised the Administrator, Federal Aviation Administration, that our investigation of this accident had revealed the presence of fatigue cracks in the wing of the accident aircraft. The Board had previously recommended that the Administrator initiate a special inspection of F-27 and FH-227 aircraft to determine whether such cracks might exist in other aircraft.

“The Administrator issued a telegraphic Airworthiness Directive regarding the recommended inspection and reported to the Board that 59 air carrier and eight general aviation aircraft had been inspected. Eight aircraft were found to have cracks in the suspect area. The Administrator also indicated that he was continuing a reassessment of design and inspection data in order to determine what additional actions might be needed to assure adequate structural integrity of these aircraft…

“ Finally, the Board advocates the program incorporated in the Federal Plan for Clear Air Turbulence described in Department of Commerce Publication FCM 69-2, dated November 1969. In this connection, the Board, on March 26, 1968, recommended, inter alia, to the Administrators of the Federal Aviation Administration and the Environmental Science Forecasting Center similar to the Weather Bureau Severe Local Storms Services Administration, the establishment of a Clear Air Turbulence Unit at Kansas City, Missouri….” (NTSB-AAR-70-16)” (Aviation Safety Network. Accident Descrip. Wien Consolidated Air Flight 55, 2Dec1968.)

Sources

Aviation Safety Network. Accident Description. Wien Consolidated Airlines Flight 55, 02 Dec 1968. Accessed 3-2-2009 at: http://aviation-safety.net/database/record.php?id=19681202-0

Liefer, G. P. Broken Wings: Tragedy & Disaster in Alaska Civil Aviation. Blaine, WA: Hancock House, 2003.

National Transportation Safety Board. Aircraft Accident Report. Wien Consolidated Airlines, Inc., Fairchild F-27B, N4905, Pedro Bay, Alaska, December 2, 1968 (NTSB/AAR-70/16; File No. 1-0038). Washington, DC: NTSB, Adopted July 22, 1970, 48 pages. Accessed at: http://www.airdisaster.com/reports/ntsb/AAR70-16.pdf Also accessed 12-30-2017 at: http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR70-16.pdf