Departure and Flight
Pan American's Flight 151 departed Johannesburg at 0812 on June 21, 1951. After a routine flight and a scheduled stop at Léopoldville, Belgian Congo, the aircraft arrived at Accra, Gold Coast, at 2125. Following a mechanical delay requiring the replacement of several spark plugs and a set of magneto points, the flight was dispatched and cleared to Roberts Field, Monrovia, on an instrument flight plan at 16,500 feet, with Dakar and Accra as alternates. Take-off from Accra occurred at 2352Z. The gross weight at departure was 89,255 pounds, including cargo, mail, 3,340 gallons of fuel, 31 passengers, and a crew of nine. The weight and load distribution were within allowable limits.
Communications and Descent
The en route communication system between Accra and Roberts Field used high frequency radio-telephone with ground stations at Accra and Roberts Field as primary guarding stations. Normal communication was maintained as the flight progressed westward. At 0057, a position report was given over Abidjan, 258 miles west of Accra, and arrival over Cape Palmas, Liberia, 265 miles farther west, was estimated at 0156. At 0156, Flight 151 reported over Cape Palmas at 16,500 feet MSL, on instruments, and estimated arrival at Roberts Field at 0246. A plantation employee in the Cape Palmas area reported hearing an airplane inland and very high at approximately 0220, with a bright moon and clear sky; no other aircraft was known to be in the area, so this was presumed to be Flight 151.
At 0220, Flight 151 requested clearance to descend. Roberts Field radio cleared the flight to descend to 3,000 feet and advised that at 0225 the tower would establish contact on VHF. Clear two-way contact was made at 0225 on 118.1 mcs, during which the tower provided local weather, altimeter setting, cleared the flight to descend IFR over Roberts Range Station, and indicated Runway 05 was in use. At 0237, the flight was given local weather: cloud base estimated 1,000 feet, broken, light drizzle and haze, visibility 3 miles. At 0241, wind was given as W-WNW variable 7 miles per hour. All messages were acknowledged.
Loss of Contact
At 0255, nine minutes after its estimated time of arrival at Roberts, Flight 151 was heard calling Roberts Field on VHF 118.1 mcs. The tower responded, repeating the call three times without acknowledgment. The tower then switched to 3270 kcs and requested the flight's current position, but received no reply. Immediately thereafter, the Roberts Field high frequency radio-telephone facility contacted the flight, advising that they were unable to read it on 118.1 mcs and that the flight should reply on 3270 kcs. This message was acknowledged at 0301. At 0305, the flight contacted Roberts tower on 3270 kcs, advising that the Dakar radio beacon was interfering with the Roberts Field beacon and that they would "be back in 15 minutes." The tower advised that Dakar would be requested to turn off the beacon, and this was acknowledged (the Dakar beacon was not turned off until 0410 due to incoming traffic). At 0315, Flight 151 called Roberts tower on 3270 kcs, and the tower transmitted the latest weather. The flight did not acknowledge this transmission on 3270 kcs but called on 118.1 mcs; the tower replied on 118.1 mcs but received no acknowledgment. Thereafter, the tower repeatedly called Flight 151 on both frequencies without response. The incomplete contact at 0315 was the last transmission received from the flight.
Wreckage and Fatalities
The wreckage was found a day later near the village of Sanoyie, about 91 km northeast of Monrovia-Roberts Airport. The aircraft was destroyed by impact forces and a post-crash fire. All 40 occupants were killed.
Probable Cause
The Board determined that the probable cause of this accident was the action of the captain in descending below his en route minimum altitude without positive identification of the flight's position. The finding noted that the aircraft never overflew the Roberts Field range station and a let-down was made without positive determination of the flight's position.