AF Jets Cut Speed to Reduce Sonic Booms : Pilots Slow Down to Be Good Neighbors

Three ear-shattering booms mark the flight of an F-111 thundering at supersonic speed just 200 feet above a test range in the Florida Panhandle.

The fighter-bomber drops an experimental bomb, then the pilot yanks the throttle back to idle, cutting the jet’s thrust 90%.

In seconds, the plane’s speed drops from 940 m.p.h. to just under 600 m.p.h.--a radical maneuver designed to limit noise and damage from sonic booms.

Without it, windows might have shattered in Holley and Navarre, which lie near the end of the range; with it, residents might not even hear the booms.

Thrown Forward

“It’s such a violent deceleration we are actually thrown forward in the cockpit, held back by our seat straps,” Maj. Dan Isbell said. “We try our best to be good neighbors. We don’t want to go around breaking people’s windows.”

Isbell, a test pilot from Chattanooga, Tenn., was describing a typical mission at Mach 1.2, or just over the speed of sound, at the Air Force’s only supersonic test range in the eastern United States. The special low-level missions test non-nuclear weaponry being developed here, including so-called smart bombs that glide to their targets with pinpoint accuracy.

Col. Harvey Greenberg, Eglin’s chief claims officer, said the Air Force nationwide received 265 claims for $465,138 in the 1987 budget year, but paid only $67,003 for 160 claims deemed valid while denying 80. Some claims carry over to the next year.

Eglin accounted for 27 of the valid claims and $4,931 in damages, records show.

One Boom Costs $15,865

But it has been worse in the past. A single boom in 1984 produced 34 claims that cost Eglin $15,865.

The only other places where the Air Force permits low-level supersonic flights are Edwards Air Force Base, Calif., and certain other desert areas in the West, Isbell said.

But none is as close to populated areas as Eglin’s Range B-70. Holley and Navarre lie just 5 to 7 miles to the west.

The 13-mile-long range is in the western half of a 720-square-mile military reservation, about 10 miles northwest of Ft. Walton Beach, once billed as the “Home of the Sonic Boom.”

Local newspapers and radio stations carry Air Force news releases about impending supersonic flights so residents will not be startled if they hear the booms, said George Roberts, a civilian spokesman in Eglin’s public affairs office. He said missions have been canceled when the releases failed to make the newspapers.

Inexact Science

Predicting the strength of sonic booms is an inexact science that depends on such variables as aircraft size, speed, altitude and weather factors including wind, air pressure and temperature, Isbell said.

Temperature inversions are the most serious complication, capable of bouncing the sonic shock waves great distances. Isbell said inversions have been suspected of breaking windows as far away as downtown Pensacola, about 30 miles to the west.

The F-111 is the largest of several types of aircraft used in low-level supersonic testing and thus produces the loudest and most damaging booms.

An aircraft flying faster than the speed of sound generally creates three shock waves, one by its nose, another by its midsection and the third by its tail, Isbell said.

Air Molecules

Shock waves are caused when air molecules cannot get out of the way of a plane flying faster than the speed of sound.

The booms, however, dissipate rapidly over distance. Those produced at high altitudes rarely are heard on the ground, and supersonic flight above 30,000 feet, or almost 6 miles up, is unrestricted, Isbell said.