Thursday, July 30, 2009

Circuit Breaker Safety

I recommend the Flight Safety Information newsletter sent out by e-mail each day from Curt Lewis & Associates, LLC. They are a consulting organization based in Arlington, TX and specialized in aviation and industrial safety. You can and should subscribe to their useful and interesting newsletter at the FSInfo.org web site.
The other morning the newsletter drew my attention to a recent article from Business and Commercial Aviation. It commented on conclusions drawn by the NTSB after their investigation of the fatal July 2007 crash in Sanford, FL of a Cessna 310R. This aircraft was owned and operated by the NASCAR organization. The newsletter item led me to find and read the NTSB Final Report on the accident.
The report discusses a number of lessons learned from this accident. The one that I took special note of related to Circuit Breaker (CB) Safety. It's widely accepted in the general aviation world that when a CB trips it is acceptable practice to re-set it once. The rationale, of course, is that if the cause of the CB's tripping was a transient event all will be well, and if the cause persists the CB will trip again -- and all will be well! The NTSB demurs.
The report makes for interesting reading, but let me here offer a few pertinent quotes:
  • Circuit breakers are installed on aircraft to protect wiring. [Emphasis added] When current flow in a system exceeds a predetermined value for a period of time, the circuit breaker activates, or “trips,” to stop current flow through that system by breaking the electrical circuit. To use the system after a circuit breaker trips, a pilot must reset that circuit breaker manually. Historically, it has been common practice to reset a circuit breaker on an airplane one time after the breaker trips. ... However, this practice does not consider the cumulative nature of wiring damage and that the removal of power only temporarily stops the progression of the damage. The aviation industry has begun to recognize the potential hazards of resetting noncritical circuit breakers even once.
  • Quoting the FAA's 2004 AC 120-80 on "In-Flight Fires": Crewmembers may create a potentially hazardous situation if they reset a CB without knowing what caused it to trip. A tripped CB should not be reset in flight...unless, in the judgment of the captain, resetting the CB is necessary for the safe completion of the flight. [Emphasis Added]
  • [M]any Part 91 pilots and operators have not yet made changes to address current guidance about circuit-breaker resets. ...[M]any general aviation pilots, mechanics, and operators may not have reviewed AC 120-80. ...the guidance contained in manuals provided by general aviation airplane manufacturers often directly conflicts with the guidance contained in AC 120-80. ...general aviation pilots, mechanics, or operators who did review the AC might not have perceived its relevance to their operations.
  • The Safety Board concludes that existing guidance in manuals provided by general aviation airplane manufacturers regarding the resetting of circuit breakers often does not consider the cumulative nature of wiring damage and that the removal of power only temporarily stops the progression of such damage. ...if general aviation pilots, maintenance personnel, and operators had a more thorough understanding of the potential hazards of a reset circuit breaker (as outlined in AC 120-80), they would be less likely to reset a tripped circuit breaker without knowing what caused that circuit breaker to trip. ...the FAA should develop a safety alert for operators (SAFO) informing general aviation pilots and maintenance personnel of the circuit breaker policy contained in AC 120-80. ...the FAA should require that the contents of the SAFO ... be included in initial and required biennial training for general aviation pilots and maintenance personnel.
Going forward, I do not believe that I will re-set a tripped CB in flight even once unless I am really sure that I know why it tripped (and I judge that cause to be benign) or I really, REALLY need electrons to continue to flow to the device down-wire from that CB...and, frankly, it's pretty hard to conceive of a situation on N631S where that would be the case.
Both the NTSB report and the In-Flight Fires Advisory Circular (linked above) are recommended reading.

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