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Aircraft electrical systems use two types of electricity:

  • Direct current (DC),1 which always travels in the same direction (can be supplied directly from the aircraft’s battery, if necessary, but usually obtained from the AC electrical system through a rectifier).
  • Alternating current (AC), which reverses direction many times every second (usually supplied by the engine-driven generators, or, sometimes, the APU-driven generator, but, in an emergency, can also be obtained from a ram-air-turbine-driven generator or by passing the battery’s DC output through an inverter).

Most of a typical aircraft’s AC electrical system uses single-phase AC, with just one set of wires; however, a few electrical components (primarily heavy motors) require three-phase AC, with three sets of wires. Like any other electrical circuit, each of those three phases is protected by a circuit breaker.

Accepted engineering practice for three-phase circuit breakers calls for mechanically linking together the breaker mechanisms for each of the three phases, so that a fault with any one of the phases trips the other two offline as well (preventing electricity from the non-faulted phases from going places it shouldn’t). Indeed, googling “3 phase aircraft circuit breaker” or the like turns up images and descriptions of breakers with all three phases sharing a single mechanical toggle.

This standard practice was apparently not followed, however, for the three-phase breakers on the DC-9 (or, at least, one particular DC-9; all emphasis and [comments] are mine):

At 1851:14 eastern daylight time [...] the three circuit breakers associated with the aft lavatory’s flush motor and located on a panel on the cockpit wall behind the captain’s seat, tripped in rapid succession. (The motor is a three-phase alternating current (a.c.) motor; each phase incorporates a circuit breaker for protective purposes.) After identifying the circuit breakers, the captain immediately made one attempt to reset them; the circuit breakers would not reset. The captain assumed that the flush motor had probably seized and took no further action at this time. About 1859:58, the captain again tried unsuccessfully to reset the three circuit breakers. According to the cockpit voice recorder (CVR), he told the first officer that the circuit breaker(s), “Pops as I push it.” [Page 7, numbered as 2.]

[...]

The 5-ampere phase-A, phase-B, and phase-C flush motor circuit breakers were removed from the cockpit and were X-rayed, revealing no internal damage. During a functional test, all three circuit breakers tripped when the electrical load exceeded the 5-ampere rating. The circuit breakers were connected to a power source and load and subjected to a 100-percent 10-ampere current overload. Only the phase-A circuit breaker exceeded the time limit designated in the specifications, before it tripped. The Safety Board could not determine the reason for the failure of the phase-A circuit breaker to meet its specifications; however, all three circuit breakers showed evidence of damage due to an external heat source. [Page 42/37. Had the breakers for the three phases been mechanically interconnected, as per standard engineering practice, the phase-A breaker would have been forced to trip at the same time as the other two.]

[...]

Individual units are protected by individual trip-free circuit breakers. These, when tripped, cannot be electrically reset until cooled. Those units requiring three-phase supply will have individual breakers in each phase. [Page 49/44.]

[...]

Another possible source of ignition near the area where the fire was discovered was the flush motor wiring harness. The tripping of the three circuit breakers accompanied by the arcing sounds recorded by the CVR occurred at 1851:14. The three circuit breakers tripped almost simultaneously indicating that the circuitry of all three phases shorted at the same time. The only evidence of wiring damage was found where the flush motor wiring harness passed through the lightening hole in the partition between the amenities section and the toilet section of the lavatory. The damage noted in the wiring harness at this location could only have been the result of fire and heat, and the Safety Board concludes that the damage to the wiring which caused the three flush motor circuit breakers to trip was caused by heat and fire. [Page 60/55. Had the three breakers been mechanically interconnected, they would all have tripped simultaneously as soon as the first of the three phases shorted out, regardless of whether or not the other two had faulted.]

[...]

At 1851:27, the captain tried unsuccessfully to reset the circuit breakers. At 1859:58, he again tried unsuccessfully to reset the three circuit breakers. Air Canada flightcrews are taught to make one attempt to reset a tripped circuit breaker. They are taught that it may be necessary to allow a 3-minute cooling time before a circuit breaker will accept a reset and that circuit breakers protecting a single phase are trip free; therefore, a circuit cannot be completed by holding in an unlatched single-phase circuit breaker. Most important, they are taught that a tripped circuit breaker denotes that the circuit protected by the circuit breaker is no longer powered. The flightcrew and other Air Canada flight personnel stated that circuit breaker trips during flight are not an uncommon occurrence, and the procedure contained in Air Canada’s DC-9/AOM allows personnel to cope adequately with such occurrences.

In this case, the captain attempted to reset each of the tripped circuit breakers twice; the first attempt occurred almost immediately after they had tripped and was unsuccessful. He testified that he “thought at the time that the unit (flush motor) might be overheated so I just continued the routine of the flight... and after a certain time had passed... I attempted to reset the circuit breakers again to make sure. ... The circuit breakers would not move.” Although the captain was unable to detect any movement of the circuit breakers, the CVR showed that arcing sounds, which were not audible to either the captain or first officer, accompanied each attempt to reset each circuit breaker indicating that the circuit breaker had moved and momentary electrical contact had been made. However, once the contact was made, the protective circuitry caused the breaker to trip again. Since the fire was already well established, the attempts to reset the circuit breakers had no effect on the sequence of events. About 1902:40, 11 minutes 26 seconds after the initial trip of the three flush motor circuit breakers, the No. 2 flight attendant informed the flightcrew that there was a “fire.” [Pages 64-65/59-60.]

Why aren’t the DC-9’s three-phase electrical systems protected with proper three-phase breakers, instead of individual separate single-phase breakers for each phase?


1: No relation to the DC in the names of Douglas airliners, to the comic-book company, or to the seat of the U.S. government.

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    $\begingroup$ Nice touch with the list of unrelated DCs! $\endgroup$ – Cpt Reynolds Jan 29 at 22:20
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Your linked Google Images search leads to examples such as this:

enter image description here
Source: amazon.com

The manufacturer being Mechanical (the company's name). A little dive into the patents reveals this invention:

enter image description here
Source: Google Patents US3145281A

Also by Mechanical, for, you guessed it, a Multipole circuit breaker with trip devices located in the housing of a single pole. And the date was 1964, as shown on the image.

So the conclusion is that the invention was new when the DC-9 was being developed, and if Douglas Aircraft Company used existing proven technologies, say from the DC-8, that would explain why a multipole CB wasn't on that DC-9.

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A three-phase electric motor can in some cases continue to run on only two phases but at reduced power, depending on the nature of the load it is driving. Having separate breakers for each phase would allow this.

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