The Doctor Will Scan You Now
When the Forest Service launches its heavy aircraft to fight forest fires this summer, they will be doing it with retardant tanks safety-checked with the aid of unique scanning technology at the University of California — Davis.
The Forest Service uses Lockheed C-130 aircraft on loan from the U.S. Air Force to drop heavy loads of retardant on forest fires. The tanks, called Modular Airborne Firefighting Systems, belong to the Forest Service. The MAFFS containers have been in use for decades.
In the early 1970s, Congress established the MAFFS program after a major fire burned into Long Beach, Calif., destroying hundreds of homes and overwhelming the civilian air tanker fleet's ability to respond.
The pressurized 3,000-gallon MAFFS tanks are rolled into the cargo hold of the C-130s on pallets, five to an airplane. Retardant or water is dropped out of the tank in under five seconds through two tubes at the rear of the plane. This load can cover a swath one quarter of a mile long and 60 feet wide to act as a fire barrier. The objective of the MAFFS program is to provide emergency capability to supplement the existing commercial air tanker support on wildland fires.
Sensing the MAFFS tanks could use some maintenance attention, the Forest Service hired Dyncorp Inc. to safety-check the tanks for corrosion and potential leaks. Dyncorp turned to the neutron radiography facility at UC-Davis' McClellan Nuclear Radiation Center to scan the tanks for flaws.
MNRC, an integral component of the UC-Davis Healthy Aircraft program, is well-known in aviation circles. The facility, in operation for the past 15 years or so, has been used before to detect moisture and corrosion in aircraft components, predominantly the honeycomb areas of control surfaces.
“You can't get inside the tanks to inspect them, and a camera only gives a surface view,” said UC-Davis radiography supervisor Hal Egbert, who conducts the tests. Neutron radiography allows inspection of the inside of the tank in three dimensions to see where any material has been lost.
Neutron radiography is like a routine medical digital X-ray using computed radiography, in that the neutron scans are recorded on digital recording plates, then analyzed with computer software similar to the computer-aided diagnosis systems used in health care. But while X-rays see through water and are stopped by metal, neutrons easily penetrate metal but are stopped by water and corrosion.
Results of the neutron scans on the tanks are encouraging. Egbert was able to find flaws in the tubes leading out of the tanks, which could have failed in use over a fireground. He also was able to show that other apparent flaws seen on visual inspection were not significant. The images allowed Dyncorp to then repair the damage.
“We do not perform 100% neutron inspection of the tanks,” Egbert said. “We only look at the bottom profile of these units.”
Even so, that much takes about three hours to complete.
The scanning process at the radiation center is not that much different than what happens when you get a routine chest X-ray at your local radiology clinic, although these patients arrive on a flat-bed truck and are removed by a fork lift.
After completing a short check-in process, the tank is positioned on a wheeled radiography table, which is then pushed onto the radiography robot — a six-axis positioning system in the radiography bay.
The computed radiography image plate is placed at the desired location near the tank. The bay is then cleared of all personnel, and the large rolling shield door is closed. From the radiography control room, the personnel shield is opened. Then the exposure control shutter is opened to expose the image plate.
At the completion of exposure, the shutters are closed, the bay entered and the image plate retrieved. The image plate is processed in the image reader, and the radiograph is transferred to the viewing station. The tank is then released to Dyncorp.
