Static Electricity Discharge Concerns In Fiberglass Aircraft

[Dynon101]

A member of our fiberglass aircraft type community had an incident with a GRT system where both EFIS attitude references failed when flying in IMC in light snow.

As he described the screens continued to have some kind of display and showed some sort of red "AHRS UNRELIABLE" indicator displayed instead of the "blue over brown" display because both AHRS units "tumbled" and the heading indicator spun around and the altitude and airspeed indicators showed "GPS CALCULATED". The autopilot contained to fly the airplane in a climb with wings level and it climbed out of the clouds. Even after climbing above the clouds in VMC both EFIS screens did not return to normal and he had to power them down and re-power and then both EFIS screens returned to normal operations. The flight continued without farther incident.

He worked with tech support and the logs determined that one AHRS system got confused and went off line...then came back...then the second unit did the same within a few seconds and while they were cycling the attitude display was normal...within a couple of seconds the logs determined that both units went off line again at the same time and that is when both EFIS screens displayed no attitude. Only after power off and on did the system return to normal.

He reported that the autopilot still was flying the airplane smoothly...naturally without an attitude display how would he know for sure however when the airplane climbed out of the clouds it was wings level and since the climb through the clouds was of short duration who will know for sure if the continued flight was due to a trimmed aircraft state that continued until the airplane exited the clouds.

He reported that the discussion with tech support indicated that the two AHRS units "tumbled" because of static electricity build up and then discharged rapidly with sufficient "noise" to confuse the AHRS. They said that some other fiberglass type airplane have this same issue because the static electricity builds up and it does not dissipate until the static is sufficient to rapidly discharge to the clouds as opposed to metal airplanes that dissipate the static over time and in smaller batches. The tech support stated that since there is not a copper mesh inside the fiberglass surfaces that a static wick on the surfaces would not help.

I do not know anything about the GRT system...nor do I really care...but I was wondering if there have been any such instances with the Dynon system?

Since retrofitting the aircraft surfaces with wire mesh and static discharge wicks is not an option I am trying to figure out if there is any way to mitigate the risk.

All I am now running is two SV-1000s and two stacked ADAHRS...no other mechanical spinning gyro backup attitude indicator.

I am considering installing a small attitude indicator and the most cost effective units are small digital screen units so would that not share the same concern?

There is some kind of units that will interact with the iPad however...again the same.

Both of my ADAHRS units are mounted on an aluminum tray so I was wondering if a static discharge wick bonded to the tray might be helpful to discharge the static from the tray and units however it is still surrounded by the fiberglass airplane.

So just wondering what the other plastic airplane drivers are doing out there.

THANKS for the help!!!!

 

[RVDan]

The phenomn is known as P-static. The static wicks need to be attached to the trailing edge of the flight surfaces, even if there is no mesh in the skin. Since in most cases the trailing edge is a flight control, make sure the hinge points are jumpered with ground braid straps. Put static wicks on the t/e of the wingtips. Mesh in the skin is mostly for lightning protection, which is also a problem that could kill everything.
I would be very concerned if P-static is shutting down the AHRS.

 

[thibault]

If the aircraft skins are truly fiberglass (eg. not carbon or anything else conductive), static wicks will not do anything. The P-static charge on the non-conductive skin does not "flow" in any conventional sense, it simply mills around without reason until the charge voltage is high enough to jump to something else. That could be a conductive part of the A/C or simply back to the atmosphere. The discharge is what upsets electronics or pops paint off the surface.

When I worked at Raytheon, I consulted with their premier ESD/EMI engineer on this topic and he advised me with the above info. If the airframe is a conductive composite, like carbon, then static wicks do perform well. One has to ensure electrical conductivity between all the surfaces involved and to the wicks themselves. They also work fine on metal A/C.

 

[RVDan]

Thibault, no offense to your engineer, but a commonly misunderstood phenomenon. P static is a problem when fields build up on low (non) conductive surfaces, resulting in voltages building up on the surface. The non- conductive surface, can be a layer of paint, or other materials like low conductance composites. This is not to say that bare metal doesn’t have P static issues, but they have less. P-static can create a corona field that will interfere with RF signals or discharge into the atmosphere, or in the case of a metal airplane with a skin that becomes isolated, arc from one skin to another. One common issue is where a static wick is placed on a fiberglass wing tip of a metal wing. Unless the static wick is electrically connected to the metal, it can’t drain the charge from the metal structure. In that case the wick must be on the metal. Radios and equipment that have poor electrical bonding to a common plane (ground) are more susceptible to voltages that build across the non-conductive structure. The OP was about AHRS that failed presumably due to P static. This is very concerning for a part of a critical system for IFR. Attaching static wicks to the AHRS trays won’t work. The tray does need to be bonded to the aircraft grounds with less than .0025milliohms. That is where I’d start. All modern avionics are tested to resist static with the .0025 milliohm bonding requirement.

Dry snow is the worst condition for P static, but any precipitation can do it.

 

[thibault]

RVDan, I don't disagree with your last post. Most of it agrees with mine. I was responding to your "The static wicks need to be attached to the trailing edge of the flight surfaces" from your prior post. Your latest doesn't really tell Dynon101 what he can do with his non-conductive airframe. I still believe wicks won't help him.