After we learned that the Australians were not pleased with the performance of 10 Tanker Air Carrier’s DC-10 air tanker in their tests earlier this year, we became more interested in the drop patterns of air tankers. Specifically, we wondered if it was common for the patterns to have gaps in the coverage as pointed out by the tests in Australia.
We contacted the United States’ Interagency Air Tanker Board (IATB) that evaluates the performance of air tankers and requested the results of the tests that were conducted for them by the U. S. Forest Service’s San Dimas Technology Development Center. The chart below compares the drop pattern characteristics of seven different air tankers and one drop from a helicopter’s Bambi Bucket. The darkest color represents the heaviest coverage, at 8 gallons per 100 square feet. Lighter colors have less retardant coverage. Click on the image to see a larger version.
I found the differences between the drop patterns of the DC-10, 747, and the MAFFS II to be interesting. The 747 was pretty uniform while the DC-10’s pattern had more variability. The MAFFS II, the latest version of the slip-in system for the military-operated C-130 aircraft, had a much more narrow pattern and more areas of lighter coverage. But the C-130 dropped about 3,000 gallons while the DC-10 and 747 each dropped about 12,000 gallons in the tests.
One of the factors affecting the uniformity of the patterns is the design of the tank system on the aircraft. Both the MAFFS II and the 747 have tanks that are pressurized by compressed air. The pressure forces the retardant out of the tanks through nozzles at a constant flow, while the DC-10 has three tanks attached to the bottom of the aircraft that are modified versions of the tanks produced by Erickson for their Air Crane helicopters. Doors open on the bottom of the DC-10 tanks and gravity pulls the retardant out.
The BAe-146 jet-powered air tanker being developed by Neptune Aviation in Missoula, Tanker 40, appears to have a pressurized constant flow system. Neptune conducted some drop tests of the aircraft in Missoula this summer. Wildfire Today has learned that the aircraft was not able to obtain adequate line lengths for the higher coverage levels. The company has gone back to the drawing board in order to modify the system to increase the flow rates in order to meet the standards.
Minden Air Corp. at Minden, Nevada is also converting a BAe-146 into an air tanker, Tanker 46, but they have not scheduled drop pattern grid testing for the aircraft.
Grid testing involves setting up 800 to 3,000 cups on the ground. After the air tanker or helicopter drops over the cups, the amount of retardant or water in each cup is measured. The cups in the photo below, which were set up for a test, were overrun by cattle. It took several days to straighten the stakes and replace the cup holders.
We were cautioned by the IATB, regarding the tests, that:
- A direct comparison is not always intuitive from these recorded patterns. Close study is required to understand continuity and density for the pattern and determine if the pattern meets the established standards.
- The patterns displayed from the grid testing provide a comparison to the standard set by the Interagency Airtanker Board but do not always translate directly to how the system performs in the field. How the fluid exits the aircraft (under pressure, by gravity, in a block or in a column) can be a significant reason for differences in performance of tanking systems.Terrain, cross wind, gusts, drop height, speed of the aircraft and other conditions may also affect the retardant as it travels to the ground and into the vegetation.