Good article. But nearly every medium to large commercial aircraft has a male version of the plug shown in the picture. Nearly every airport has 400Hz power supply systems that use this plug. It seems as universal as the two and three prong 120V plugs we use in our homes. The power supply configuration would have to be changed in all aircraft and at every single airport 400Hz power supply point to make the changes mentioned in the article. Since aircraft don't generally explode when attached to ground power at the gate and passengers don't get shocked while boarding there doesn't seem to be a lot of justification to make all those changes.

In my experience offloading commercial and military aircraft on the airport parking apron in Antigua, I observed that the FIRST action of the ground crew was to apply a large grounding clip from the APU (Auxiliary Power Unit) to the aircraft. This was done initially to dissipate the static electrical charge that the aircraft built up during the flight. (The static charge must be neutralized before refueling operations can begin, lest a static spark ignite the jet fuel, or a spark shock a person touching the aircraft and the ground.) I am not certain if the "static ground" also acts as an electrical ground for the APU, or if it is a high resistance ground. Your info states that the connector has pins A, B, C, and N. - Is pin "N" a neutral pin?

In answer to the, as yet unasked, question terrestrial based electricians may wonder about. 400Hz was chosen for aircraft applications due to magnetic considerations. 400Hz transformers and motors require smaller cores and thus are lighter weight. Commercial aircraft use of 400Hz power is not so much driven by commonality with military applications as the shared need for operational efficiency due to a lighter airframe.

I cannot resist.

This brings new meaning to the phrase "Hop on a plane." Is this the new safety mantra to avoid becoming part of a return path for current from the airframe to ground power?

Interesting topic. Have a question about the aircraft's AC system in regards to the neutral return. Does each load which requires a neutral have this neutral locally connected to the airframe for return? Or are their neutral conductors from a common neutral bus going to these loads?

Based on the replies to this, I will comment on another aspect of the original article.

David Rifkin

The neutral wire (same size as A, B, and C) is bonded to the generator case which is bonded to earth ground. The aircraft neutral buss is connected to the feed plug neutral. This neutral connection to the aircraft is NOT broken by any other means than the pluged connection.

Aircraft switches over to ground power via load contactors , first the ground power load concator which sends power to the aircraft, then after quality check passes, the aircraft load contactor energizes. The aircraft then monitors the power durring the entire power cycle and holds the generator ON via the smaller E & F 28 vdc signal. The neutral is NOT broken other than the matting plug pictured.

If the aircraft is grounded before the APU is connected the neutral will act as ground. The output from ground power units will also have their own protection from phase to the neutral (ground). More important is the bonding connection between grounded skybridge and the aircraft ground.

It seems that it would be a simple step to retrofit a Grounding conductor to the pendant cord, segregate the neutral from the frame to a Jack adjacent to the pendant jack and do a simple training (very simple) to have ground crews connect the ground P/J at the same time that the cord is connected. Why wouldn't the FAA require this alignment with the code in today's safety conscious environment?

I was a Crew Chief on a B-52. The first thing required, before connecting external power, refueling, etc. was a ground wire connected to the aircraft and then to the grounding grid.

Is this not common practice with all aircraft?

With all due respect Sir.

As an individual with an airframe and powerplant license, multi-decade experience with avaition, railroad and commercial electricity, please make sure this isn't a tempest in a teapot, an engineer in search of a cause. Because if this was an actual issue, we would be seeing aircraft exploding and people dropping like flies trying to board aircraft all over the world.

Every multiphase piece of equipment I have ever installed in an aircraft was completly self contained. That's why polyphase is used. No external neutral. This might be the issue, in that while it's called a "neutral", nothing in the airplane actually uses it, so in practice it becomes the bonding conductor.

Wiring, especially wiring carrying AC, to the sub equipment is usually shielded, with the shield grounded to the frame. The case of the equipment is bonded to the airframe or frame of the railroad car. You don't want current in the frame and shields, because then you have a big antenna, interfering with the navigation and communication equipment. Additionally each piece of equipment has it's own sub-breakers or fuse.

Another issue is that these peices of equipment, both the whole machine, and discreet appliances, recieves much more regular critical and intense inspection than building or house electrical system would ever recieve post approval. A "low level fault" would show up as something wrong, either with the system itself, or as interference with some other system.

In all of my experience, the metal frames, both the airframe itself and the individual components NEVER carried current. All equipment was bonded together, yes, but what have you seen inside the airplane to make you think that there is actually current in the airframe? Or are you looking at the plug and assuming that the inside of the airplane is working just like all the ground bound and stationary systems that you have designed?

The fuel tanks you mentioned as exploding? What reason was given for their explosion. Operator error? Did they explode while they were connected to the airplane? As far as I know it's always sop to ground the airframe to the fuel delivery device, either stationary or mobile. Was it actually the issue of current in the neutral that caused the explosion.

In conclusion, what I think we have here is something called a "legacy". When the MIL spec was written, it was sop. The industry worked to the spec. Now there are many, many things out there worked to that spec. The NEC moved on, rightfully so, but to change all of the things out there that are configured to the old spec, it is too impractical and too expensive to change.

Thank You

One thing that has not been mentioned. The NEC has no jurisdiction on aircraft.