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When an aircraft is being stored in a hangar between flights, are there any requirements to have the aircraft connected to an earth ground?

I believe that there is (although I'm looking for the reference), and if so, why and how should this be accomplished? Is attaching a cable to the metal structure of the hangar an accepted practice?

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  • $\begingroup$ Interesting question. In my time in the Marines we never grounded a single aircraft parked inside our hangars. $\endgroup$ – Juan Jimenez Jul 9 at 11:14
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NFPA (National Fire Protection Agency) 409 - Standard for Aircraft Hangars states in each chapter that deals with the 4 types of Hangars:

8.6.1* Grounding facilities shall be provided for removal and control of static electrical accumulations on aircraft while aircraft are stored or are undergoing servicing in a hangar.

8.6.2 Floor-grounding receptacles shall be provided and shall be either grounded through individual driven electrodes or electrically bonded together in a grid system and the entire system grounded to underground metal piping or driven electrodes. Where driven electrodes are used, they shall consist of 15.9 mm ( 5/8 in.) diameter or larger metal rods driven at least 1.5 m (5 ft) into the ground. Floor-grounding receptacles shall be designed to minimize the tripping hazard.

8.6.3* Grounding wires shall be bare and of a gauge that will be satisfactorily durable to withstand mechanical strains and usage.

So it is not a law per se, unless one has adopted a code that references NFPA 409, but at the very least it is a recognized standard for best practices that will be used in a court of law after an incident.

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In general any aircraft parked in hangar should be electrically grounded. The grounding should be done from basic aircraft structure to a low resistance ground.

Grounding of the aircraft is usually done to,

  • Protect aircraft and personnel against hazards from lightning discharge.
  • Provide current return paths
  • Protect personnel from shock hazard
  • Prevent accumulation of static charge

During de-fueling or refueling, the aircraft is grounded to the fuel truck.

In general, the aircraft are grounded to a point where the impedance is less than 10,000 ohms referenced to earth for static grounding. For power grounding, it should be less than 10 ohms to power systems neutral.

Aircraft are not grounded to the hangar structure as the external power supply, when connected, is grounded to that.

The best reference, if you ask me, would MIL- HDBK-274(AS)- Electrical Grounding for Aircraft Safety.

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    $\begingroup$ Lightning inside a hangar? Wouldn't the structure protect the aircraft from that? Also, I'm mainly looking to see if there is a requirement to do this, or if this is just a commonly practice. $\endgroup$ – Lnafziger Sep 2 '15 at 13:19
  • $\begingroup$ Actually, lightning would be a good reason to not ground the craft to the hangar itself. If the lightning hit the hangar, it would go direct to ground, if the craft is tethered to it (electrically) some power will flow into the aircraft and potentially cause damage. (Which sounds like a good question on its own, but most likely open to speculative answers only.) $\endgroup$ – FreeMan Sep 2 '15 at 13:42
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    $\begingroup$ @FreeMan I just had that conversation with someone yesterday! I would think that it wouldn't cause any damage in this case because the path to ground via the hangar would be much lower resistance than the path through the aircraft tires to ground and therefore wouldn't flow to the aircraft. That's just my "common sense" theory though, not based on anything substantial. :) $\endgroup$ – Lnafziger Sep 2 '15 at 16:06
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    $\begingroup$ A minor nitpick: When you connect an aircraft to a refueling truck (or a deicing truck) you're not grounding it - you are bonding the two structures electrically so they're at the same potential. (This is important, because if something connected to earth ground comes close enough to the two vehicles it could still generate a spark.) $\endgroup$ – voretaq7 Sep 2 '15 at 16:13
  • $\begingroup$ I have seen a lot of aircraft in a lot of hangars in several countries and never once seen one grounded or bonded (except perhaps incidentally if hooked up to a battery tender). $\endgroup$ – pericynthion Jul 9 at 8:26
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I know of no specific requirements for grounding an aircraft for "storage" within a hangar. However, there are mandatory requirements that address electrical system grounding, and static grounding. These are two separate issues. In the first, the National Electrical Code (NEC), Article 513.10 Special Equipment, (A), Aircraft Electrical Systems,(C), External Power Systems for Energizing Aircraft, (3), Cords; require that the Flexible cords used to power the aircraft "shall include an equipment grounding conductor". DC power system pendant cord plugs contain two contact pins, a + V and - V, the - V conductor is usually grounded at the power source; but this does not satisfy the Article 513 requirement above. In the case of 3-phase 400-HZ connectors, 6 active pins are provided in the standard connector; Pins for phases A, B, C, and the N neutral, as well as an "E" and "F" interlock circuit. The "N: Neutral pin must be bonded to building steel at the source. As noted above, the aircraft structure is used as a "return" path for both DC, and AC returns. Static grounding can be satisfied by the use of a static grounding strap. This is mandatory for US Military aircraft. I have no data on commercial or private manufacturer's requirements.

In the past, there have been horrific hangar fires, such as the Sabena Hangar fire in Belgium in May of 2006. Issues that need to be addressed are: compliance with NEC, effects or multiple voltage systems attached to parked aircraft. Before I retired, I addressed these issues in an article for EC&M Magazine; see at https://www.ecmweb.com/design/electrical-equipment-issues-parked-aircraft

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The Australian AC21.99 is a good example of a "reticulated" system; one with a central source, and transformer isolation at each service point. This is speaking to 400HZ service only. But I do not find a requirement in the US NEC, or FAA that talk about this. The old MIL-HDBK-274 had a lot of good information, especially in defining "power" grounds, and "static" grounds. I designed several Navy Hangar power systems, and before I retired, I inspected 83 or the US DoD hangars. My interest began when I did an investigation on a damaged F/A-18D, which was attributed to a faulty hangar power service station. The overvoltage damaged 7 of the Avionics WRAs that had to be replaced. Later, in a Texas hangar I found an improper 400HZ system bond which resulted in 31 volts (p-p) on the 400HZ neutral conductor. Since the aircraft generally use the structure as a neutral conductor and as a "ground" reference, it raises Cain with the electronics. Furthermore, the US Military does not have an inspection process in place for the ground power port, nor do they have a requirement to test power cart and service point cable heads for wear. When a pin becomes loose, the neutral may surge to the missing RMS phase voltage. This then puts 120V, 400HZ on the AC skin, damaging Avionics and shocking/burning maintenance people. Sorry, I can't speak to 28VDC systems. The only commercial reference I have to AC power requirements is the one from ATAA (Air Transport Association of America). Its copyrighted, 1980, but available. The title is "400Hz Fixed Power Systems Design Guidebook", author Ralph Morrison, McCormick-Morgan power systems engineers. Chapter 8 recommends system bonding in excess of NEC requirements due to the impedance effects of 400hz. Sorry for writing a book.

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There is a requirement to have the aircraft grounded to an approved grounding point in the hangar under certain conditions. This is for a number of reasons including the skin of the aircraft is the return path as we only use one wire in aircraft circuits and the return path is the aircraft skin itself. Also the grounding points must be approved. When the tarmac or hangar is built the grounding bars or points are tested to make sure they are within specs. The resistance must be low enough to ensure they are the path of least resistance. The first thing we do when an airplane enters the hangar is ground it. Every spot usually has its own grounding spot.

There are times that the airplane is not grounded, but typically for maintenance it is. Maintenance usually requires ground power to be applied to the airplane. Also when large aircraft and especially helicopters return from flight, they sometimes can have a large buildup of static electricity and this has caused some serious injuries in the past. This static electricity has to be dissipated and is done via a grounding wire to an approved grounding point.

AC21.99 Advisory Circular Electrical bonding of aircraft to earth has generally been aimed at protecting aircraft and personnel from the hazards associated with static electrical discharge. However, with utilisation of external power sources, electrical bonding to earth must also protect aircraft and personnel from the potential hazards associated with the electrical ground power supplies.

https://www.casa.gov.au/sites/g/files/net351/f/_assets/main/rules/1998casr/021/021c99s2c14.pdf

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    $\begingroup$ In our hangars, there are no grounding points like you describe, so it isn't in all hangars that have been built. You say that there is a requirement: Is it a FAR, federal law, or company policy requirement? That's mainly what I'm looking for: Is there an actual requirement for this, or just common practice? $\endgroup$ – Lnafziger Sep 2 '15 at 3:02
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    $\begingroup$ I've hangared Robinson R22 and Westland Wessex between flights and neither had statutory requirements, one CAA, one RAF, concerning fuel or earthing. If they are entering for servicing, then that's another matter. $\endgroup$ – Simon Sep 2 '15 at 6:47
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    $\begingroup$ I've been in a lot of hangars, from simple T style private hangars to large airline maintenance hangars. Not one of them had built in grounding points, and not one of the airplanes in them were grounded. $\endgroup$ – Ralgha Sep 2 '15 at 17:00
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    $\begingroup$ @ralgha I've found a lot of military references to this (procedures, etc.) but have a similar experience to yours in civil aircraft hangars. $\endgroup$ – Lnafziger Sep 2 '15 at 21:06
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    $\begingroup$ @Lnafziger I added the Advisory circular references for you. While it is from the Australian site we use it in Canada and the US. In the military we always ground the aircraft (fighters, helicopters, large transport) for safety reasons. I have seen lots of aircraft ungrounded but never during refuelling or maintenance with power applied. $\endgroup$ – Mark Sep 3 '15 at 20:28

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