RAA Convention 2006: CRJ fires traced to change in terminal insulation material
A subtle change in the material used in an electrical component caused seven fires in Bombardier CRJs, according to an NTSB recommendation released on March 30. Tyco Hartman, the manufacturer of the component, changed the material and didn’t notify Bombardier that the new part was coated with a different material that turned out to be susceptible to moisture-induced arc-tracking.
The component is the base terminal of the CRJ’s 1K4XD electrical contactor, which delivers electrical power to a utility bus used during ground operations. The base terminal is coated with an insulating material called Ultem 2200, which replaced the previous laminate of NP509 woven glass fabric and melamine resin.
Both types of base terminal are approved for installation on the Bombardier CRJ200, according to the NTSB. Ultem 2200 is a polyetherimide, which is similar to Kapton, an aromatic polyimide electrical wire insulation that is susceptible to arc-tracking, a problem that can cause fires to occur.
In each of the seven fire incidents, NTSB investigators determined that moisture in the avionics compartment bridged terminals on the 1K4XD contactor, causing a short circuit. Precipitation before each incident flight resulted in moisture ingress, likely due to open main cabin doors allowing rain onto the cabin floor above the avionics compartment where the contactor is located. According to the NTSB, “Pulling the main entry door into the closed position may also result in water draining into the cabin area and subsequently into the avionics compartment.”
During tests at Tyco Hartman, NTSB investigators observed that the short-circuit causes the moisture to evaporate, but, according to the NTSB recommendation letter, “The Ultem 2200 material gradually turned to a carbon char for the duration of power application. After the water evaporated due to the short-circuit, the carbon char provided a semiconductive path between terminals, and intense flames and arcing extended more than a foot from the contactor.”
During these tests, one fire lasted for more than 100 seconds before the wires burned enough to disconnect power from the circuit. Tests were done on the NP509 fabric melamine laminate base terminal, and the result was no arcing or fire, just water steaming off the contactor.
For pilots in two of the CRJ200 fire incidents, the fire’s effect on cockpit instruments was confusing and dangerous. Pilots flying Comair Flight 5029 on December 8 last year from Cincinnati temporarily lost all electronic flight instruments and used a battery-powered standby instrument until they were able to engage backup electrical power.
Nearly a week later, pilots in an Atlantic Southeast Airlines CRJ200 descending to Atlanta experienced a cascading series of failures, according to the NTSB, including loss of electrical power, the smell of electrical smoke, loss of electronic displays, high cabin temperature and deployment of the air-driven generator. In both incidents, the fire burned the captain’s oxygen supply line, which could have caused a catastrophic fire. In four of the seven incidents, pilots reported loss of electronic displays.
Bombardier issued an all-operators message on December 22, suggesting that airlines make a simple wiring modification that separates power sources for cockpit instruments, allowing one pilot’s EFIS to remain online if a contactor fire occurs. Approximately 700 CRJs have the Ultem base terminals installed, according to a Bombardier spokesman. “We will be working closely with [Transport Canada and Tyco],” he said, “identifying the root cause and taking remedial action as soon as we can.” Meanwhile, Bombardier issued another all-operators message in March, to keep operators current on its work with Tyco.
A Tyco spokesman, responding to questions from AIN, wrote that “Tyco Electronics has been fully cooperating with Bombardier and the NTSB during the investigation, and we continue to work with Bombardier to assist them in addressing the NTSB’s safety recommendations. We are exploring a number of alternative solutions.”
The Tyco spokesman confirmed that it switched to Ultem 2200 on the terminal bases without notifying Bombardier. “We followed our well established procedures regarding changes in materials,” he wrote. “This was a change that did not affect form, fit or function or qualification status of a released product, so no notification was needed.”
The FAA had not responded to the NTSB recommendations as of early last month and has until June 30 to provide a response. The NTSB listed seven recommendations, four of which it considers urgent, including:
• Require operators to separate the electrical sources for the EFIS displays.
• Require Bombardier to develop a way to protect electrical terminals on Ultem terminal bases from moisture-induced short-circuits.
• Require operators to install the above modification as soon as possible.
• Require Bombardier to evaluate CRJ200 abnormal and emergency procedures to make sure they adequately address the fire hazard when the 1K4XD contactor catches fire and provide flight crews with additional guidance.
The NTSB also wants Bombardier to replace the contactor with one that isn’t susceptible to the short-circuiting problem; prove that electrical components can safely withstand exposure to moisture under full electrical load; and have aircraft manufacturers check if parts made with Ultem 2200 are installed and determine how to protect or replace them with safer parts.