Released: 22 Feb 2000
SCOTT AIR FORCE BASE, Ill. (AFPN) -- Air Mobility Command C-17s will soon be able to airdrop a brigade of troops and equipment within 30 minutes, meeting the Army's goal for how long the airdrop should take.
The requirement, called Strategic Brigade Airdrop, is currently met by a mix of AMC's C-141 and C-17 fleets, but will be accomplished solely by the C-17 fleet when the C-141 is retired. The following three initiatives will allow this to happen: reduce the spacing between aircraft during personnel airdrops, install a dual-row airdrop capability, and install new equipment that will allow the aircraft to fly in tighter formations during inclement weather.
An Army brigade, which contains about 3,250 troops and 3,450 tons of equipment, is airdropped and airlanded in two phases. During the first phase of the SBA, the aircraft must be able to drop roughly 2,500 troops and 1,350 tons of equipment within a limited amount of time. During the second phase of the operation, the remaining 750 troops and 2,100 tons of equipment are delivered to a landing zone.
Had none of the initiatives currently underway taken place, it would take a C-17 SBA formation about 25 minutes longer than the Army's requirement for the airdrop portion.
"As soon as we realized that we wouldn't be able to meet the Army's requirement, AMC started working to change that," said Maj. David Kasberg, chief, C-17 tactics. To get to the time requirement, AMC decided on its three-pronged approach to fix the problem.
The first step was to reduce aircraft spacing. Only 12,000 feet was required between lead aircraft airdropping personnel with the C-141. However, vortices created by the C-17 means a larger spacing is needed to ensure jumper safety.
"The C-17 has the same length and wingspan as the C-141, but a much wider cargo compartment, and is heavier overall," said Kasberg. "The heavier airplane, especially with the same length wings, causes a lot more wake turbulence. It's a physics problem we're trying to overcome."
Using computer models, the command began investigating 15,000 feet spacing between element leads and gradually increased the spacing to the current 40,000 feet to decrease the number of interactions between the aircraft's vortices and the jumpers.
"I got involved in building the model trying to come up with a better way of predicting where the vortex would be and how it would interact with the jumpers," said Lt. Col. Hans Petry, mobility analyst. Petry was a student at the Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, when he worked on the model.
Using a Wright Lab engineer's static model, Petry and a fellow student made the model move, added wind effects and random variation of the jumpers' trajectories, and snaked the vortices behind the aircraft instead of in a straight line. This created a much more realistic picture of what was actually happening around the aircraft in flight.
"We went over a bunch of iterations and cases in trying to figure out anything that could possibly happen, and came up with some predicted percentages of encounters and ranges of spacing," Petry explained. "At 28,000 feet, we predicted that we would have an acceptable range -- minor encounters that would keep anybody from getting hurt -- and that's where they started testing."
The testing was completed in December at Pope AFB, N.C. Despite the predictions, at 28,000 feet there were still several major encounters between the mannequins and the vortices, so the spacing was increased to 32,000 feet.
Before progressing to using real paratroopers, AMC dropped 712 mannequins with element lead aircraft spaced 32,000 feet apart to ensure the spacing was safe, said Lt. Col. Pete Livingston, AMC C-17 operational test manager. Then 60 test jumpers, followed by 302 paratroopers from the Army's 82nd Airborne Division at Fort Bragg, N.C were airdropped to complete the test of the reduced spacing. All test events were completed successfully and safely.
"We feel that the risk that hasn't increased by moving the aircraft from 40,000 feet apart to 32,000 feet apart and it meets the Army's pass time requirements," Kasberg said. "For safe training and during contingencies, 32,000 feet is right for our standard spacing."
More time is shaved off the airdrop time by developing a dual-row airdrop capability in the C-17. The dual logistics rails allow two rows of equipment to be airdropped from the C-17. This more than doubles the capacity of each C-17 and cuts in half the number of C-17s required to airdrop the heavy equipment portion of the SBA.
"Right now, the airplanes rolling off the assembly line have dual-row capability and we will have enough dual-row airplanes by July to meet the Army's SBA requirement," Kasberg said.
Current station-keeping equipment, or SKE, is in use throughout the Air Force on most mobility aircraft. Using radio wave frequencies, it allows aircraft to fly in formation during inclement weather. A new version of this equipment called SKE Follow-On is currently in development. It will continue to allow C-17s to fly in poor weather conditions while reducing the amount of space needed between aircraft.
"The SKE allows us to fly in formation in the clouds (when you can't see the airplane in front of you)," Kasberg said. The current SKE causes interference if formations use the same radio wave frequency in close proximity to each other. This interference is not a factor with the new SKE. It is expected to be installed in the aircraft by the fourth quarter of fiscal 2004. (Courtesy of AMC News Service)
* C-17 Globemaster III
* C-141 Starlifter
* Air Mobility Command
* Scott Air Force Base, Ill.
* Pope Air Force Base, N.C.
* U.S. Army
* Wright-Patterson Air Force Base, Ohio