Tag Archives: 5th Composite Group

27 December 1935

Mauna Loa viewed from Hilo, Hawaii. (Donnie MacGowan, Lovingthebigisland’s Weblog)
Advancing lava flow, December 1935. (USGS)

27 December 1935: When an eruption of Mauna Loa, a volcano on the Island of Hawaii (ongoing since late November) threatened the town of Hilo on the island’s northeastern coast, a decision was made to try to divert the flow of lava by aerial bombing. (The population of Hilo in 1935 was 15,633.)

Until recently, Mauna Loa was thought to be the largest volcano on Earth, but has been downgraded to second-place status by the Tamu Massif in the northwestern Pacific Ocean. It is a shield volcano, meaning that it was built up of fluid lava flows, as opposed to a stratovolcano, such as Vesuvius, which is created by the build up of solids like ash and pumice. The summit of Mauna Loa is 13,679 feet (4,169 meters) above Sea Level, but the volcano actually rises 30,085 feet (9,170 meters) from the floor of the Pacific Ocean.

“Lava flows from Pu’u ‘O’o Crater on Kilauea,” one of five active volcanoes on the Island of Hawaii in the Hawaiian Islands. (USGS)

The mission was planned by Lieutenant Colonel George S. PattonThe U.S. Army Air Corps’ 23d Bombardment Squadron, 5th Composite Group, based at Luke Field on Ford Island, Oahu, Territory of Hawaii, sent three Keystone B-3A and two Keystone B-6A bombers. The five airplanes dropped twenty 600-pound (272.2 kilogram) Mark I demolition bombs, each containing 355 pounds (161 kilograms) of TNT, with 0.1-second delay fuses.

A Keystone bomber flying over the Ko’olau Range on the island of Oahu, Territory of Hawaii. (U.S. Air Force)

Five of the twenty bombs struck molten lava directly; most of the others impacted solidified lava along the flow channel margins. . . Colonel William C. Capp (USAF, ret.), a pilot who bombed the lower target, reported direct hits on the channel, observing a sheet of red, molten rock that was thrown up to about 200′ elevation and that flying debris made small holes in his lower wing. Bombs that impacted on solidified, vesicular pahoehoe along the flow margin produced craters averaging 6.7 m diameters and 2.0 m depth. . .

Pilots observed that several bombs collapsed thin lava tube roofs, although in no case was sufficient roof material imploded into the tube to cause blockage. The extrusion of lava ceased within a week, however, and Jaggar wrote that the bombing caused the fluid pahoehoe to thicken and block the vent by the process of gas release. . . .

Diversion of Lava Flows by Aerial Bombing — Lessons from Mauna Loa Volcano, Hawaii, by J.P. Lockwood, USGS, and F.A. Torgerson, USAF, abstract.

A flight of three Keystone B-3A bombers of the 23d Bombardment Squadron take off at Luke Field, Territory of Hawaii. Diamond Head is visible in the background. (U.S. Air Force)
A flight of three Keystone B-3A bombers of the 23d Bombardment Squadron take off at Luke Field on the island of Oahu, Territory of Hawaii. Diamond Head is visible in the background. (U.S. Air Force)

Eventually the lava turned to follow the natural drainage toward Hilo, instigating a crisis. On December 26, the flow was moving 1.6 km per day (1 mile per day), and at that rate scientists calculated the flows would reach Kaumana Road by January 9 (disrupting mochi-pounding parties). A suggestion to bomb the eruption was made. The U.S. Army Officer who planned the bombing operation was then Lt. Colonel George S. Patton, who would go on to WWII fame.

Three Keystone B-6As of 20th Bombardment Squadron, 2d Bomb Group, release their bombs on a practice mission. (U.S. Air Force)

On December 27, U.S. Army planes dropped bombs, targeting the lava channels and tubes just below the vents at 2,600 m (8,600 ft). The object was to divert the flow near its source. The results of the bombing was declared a success by Thomas A. Jaggar, Director of the Hawaiian Volcano Observatory.

Jagger wrote that ‘the violent release of lava, of gas and of hydrostatic pressures at the source robbed the lower flow of its substance, and of its heat.’ The lava stopped flowing on January 2, 1936.

Hawaiian Volcano Observatory, Volcano Watch Archive, November 27, 1997: http://hvo.wr.usgs.gov/volcanowatch/archive/1997/97_11_28.html

“Aerial view of a bomb detonating on Mauna Loa near the 8,500-foot elevation source of the 1935 lava flow on the morning of Dec. 27, 1935. This was one of twenty 600-pound bombs dropped on the lava flow that morning by the Army Bombing Squadron from Luke Field, O’ahu. Photo by Army Air Corps, 11th Photo Section.” (Big Island Now)

The Keystone B-3A was a twin-engine two-bay biplane bomber, among the last biplanes used by the United States Army. It was operated by a crew of five. The B-3A was 48 feet, 10 inches (14.884 meters) long with a wingspan of 74 feet, 8 inches (22.758 meters). The maximum gross weight was 12,952 pounds (5,875 kilograms).

The B-3A was powered by two air-cooled, supercharged, 1,690.537-cubic-inch-displacement (27.703 liters) Pratt & Whitney Hornet A1 (R-1690-3) single-row 9-cylinder radial engines  with a compression ratio of 5:1. The engine was rated at 525 horsepower at 1,900 r.p.m., and turned two-bladed propellers through direct drive. The R-1690-3 was 3 feet, 8.88 Inches (1.140 meters) long, 4 feet, 7.44 inches (1.408 meters) in diameter and weighed 800 pounds (363 kilograms).

The B-3A had a maximum speed of 114 miles per hour (184 kilometers per hour) at Sea Level. Cruising speed was 98 miles per hour (158 kilometers per hour) and the service ceiling was 12,700 feet (3,871 meters) —nearly 1,000 feet (305 meters) lower than Mauna Loa’s summit.

Armament consisted of three .30-caliber machine guns and 2,500 pounds (1,133.9 kilograms) of bombs. With a full bomb load, the Keystone B-3A had a range of 860 miles (1,384 kilometers).

63 Keystone B-3As were built for the Air Corps and they were in service until 1940. The 2nd Observation Squadron at Nichols Field, Philippines, was the last unit equipped with the B-3A.

Keystone B-3A, Air Corps serial number 30-281, the first B-3A built. (U.S. Air Force)
Keystone B-3A, Air Corps serial number 30-281, the first B-3A built. (U.S. Air Force)

The Keystone B-6A was a re-engined B-3A. There was a change to two 1,823.129-cubic-inch-displacement (29.875 liter) air-cooled, supercharged Wright Aeronautical Division Cyclone 9 R-1820E single row 9-cylinder radial engines turning three-bladed propellers. The R-1820E was rated at 575 horsepower at 1,900 r.p.m. The engine weighed 850 pounds (386 kilograms).

Maximum speed increased to 120 miles per hour (193 kilometers per hour) at Sea Level with a cruising speed of 103 miles per hour (166 kilometers per hour). Armament and bomb load remained the same but the service ceiling increased to 14,100 feet (4,298 meters). The range decreased to 350 miles (563 kilometers) with a full bomb load.

39 Keystone B-6As were built and they remained in service until the early 1940s.

A U.S. Army Air Corps Keystone B-6A bomber. (U.S. Air Force)
A U.S. Army Air Corps Keystone B-6A bomber. (U.S. Air Force)

Newsreel footage of the bombing is available at Critical Past:


© 2018, Bryan R. Swopes