A look into this unique process
By Joe Escobar
The company jet is sitting on the ramp tied down. The pilots are waiting for a storm to pass before taking off to their final destination. All of the sudden, the storm intensifies and the aircraft is subjected to hail damage. Several large dents are visible on the skin, some of them beyond limits. Time to do some major sheet metal repairs. Or is it? Boeing Maintenance Services RAMS/EMF has an Electromagnetic Dent Removal (EDR) operation that can offer time and money savings to some operators faced with similar situations.
How it started
The Boeing Company developed RAMS in the 1980s. The realization that this technology could be used as a process to remove dents was sort of an accident. Boeing was looking for a machine to proof load honeycomb material such as floorboards. It needed something that was going to pound on the the floorboard with a certain amount of pressure in order to perform the proof loading tests. It developed an electromagnetic tension system in order to perform these nondestructive strength inspections of the structures. But Boeing soon discovered that this equipment could pull dents out of the skins. Boeing and McDonnell Douglas started using EDR in 1986 to remove dents from production aircraft. The service is now available to all aircraft owners and operators worldwide.
How it works
The process of removing dents electromagnetically is quite simple. Upon sustaining damage to the aircraft, the Boeing Maintenance Services RAMS/EMF division is contacted. After consultation with them on the location and extent of the damage, it is determined whether or not the damage can be repaired using EDR.
If repair is possible, an EDR technician is dispatched to your location. He sets up the EDR equipment near the aircraft. Depth measurements of the damage are taken to assess the damage. A mylar sheet is then placed on the metal surface where the dent is. The machine takes a while to power up. The technician then places the coil of the unit over the mylar sheet and pushes the button. A loud pop is heard. This process may be repeated several times depending on the damage. Depth measurements are taken throughout the process until the dent is gone or within limits. A logbook entry is made and the aircraft is good to go.
The technology behind EDR
The science behind the process of EDR is quite intriguing. It uses the same basic principles as eddy current. Electromagnetic fields produce the pulling effect on the metal.
Basically, the process relies on the fact that the magnetic permeability in metals is much lower than in air. In other words, it takes longer for a magnetic field to penetrate through a metal than it does in air. When the EDR unit's coil is placed over the damaged area, the capacitors in it release 36,000 amps of electric current in 1 1/2 milliseconds to produce a magnetic field on both sides of the dent. A second set of capacitors then collapses the nearest magnetic field by instantly discharging half the energy generated by the first set of capacitors. Because of the low permeability of the metal being repaired, the magnetic field behind the dent is kept relatively constant. This imbalance is what pushes the dented metal outward toward the face of the EDR coil.
Typically, an EDR technician can be on location within 24 hours. There are two teams that are on call and can be dispatched at a moments notice.
EDR works best on aluminum, typically 2024 T3. It is also valuable for use on honeycomb structure or other assemblies with access to only one side. In fact, this is a big plus. Access to the back side is not necessary during this process.
There are some limitations to being able to remove a dent through EDR. It is not effective on other hard metals. Dennis Pinto, an EDR representative for Boeing Maintenance Services RAMS/EMF explains. "We can't use EDR on titanium, inconel, steel, or other hard metals. The magnetic field has to go through the structure for it to work."
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