Date: 201505
Local time of day: 1201-1800
Location: ZHU.ARTCC, TX
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Turbulence
Light: Daylight
Aircraft: Citation I (C500)
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZHU
ATC / advisory: Center ZHU
Aircraft component: VHF
Component problem: Failed
Reporter organization: Corporate
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Flight Engineer; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Ambiguous

Narrative: This flight was to HYI (San Marcos; TX); IFR and occurred at approximately 30 minutes into this 45 minute flight to HYI. The time of this is approximate. We were level at FL180 deviating around storm cells in IMC. We encountered moderate to severe turbulence for about 15 seconds and lost all navigation and radios. Our only communication with Center was the transponder that was still working. Unable to get the navigation or radios back in operation we squawked 7600. We broke out into clear weather and decided to descend in VMC down to 3000 feet and look for a suitable airport. We landed at [an airport] and called Center by phone.After shutting down the engines and then turning the Battery Switch back on to trouble shoot this problem all navigation and radios came back on and operated normal.We did a preflight and departed VFR; called Center for VFR flight following to San Marcos (HYI).


Second narrative: On IFR flight plan to HYI (San Marcos; TX) first officer flying; we passed between two buildups. There was a smaller one between the buildups. I thought we would pass over it but we just clipped the top and hit two severe bumps about four seconds apart. Only two and then smooth ride. We then noticed both Garmins (530 & 430) were totally blank. Now we had no navigation and no communication. We recycled avionics and inverters to no avail. We discussed problem and squawked 7600 (lost communications). We were on border between FTW and HOU Center. Don't recall which we were talking to at the time of lost navigation/communication event. I realized we could not navigate accurately and decided we should get on ground before became a hazard to other traffic. I commanded the copilot who was flying to descend to 3;500 MSL so we could find a landing place as soon as possible. We were VMC after the turbulence. After about 10 or 15 minutes we spotted a paved; single runway airport and landed there.After parking and shutdown a man from the small FBO approached us with a phone [number] and name to call. I called; and I explained the situation and he seemed satisfied. Short conversation. We got back in; started up; powered up avionics and all started and worked well. We flew VFR with flight following to HYI. No problems.Next day I mentioned the situation to an avionics shop and learned there is a 5G switch in the Garmins. That was what shut us down. He said we could have totally powered down (batt and gens) and recycled to get Garmins working again. We could have done that in air. Learned something. Also; I will not ever try to hop over a buildup even if a small one. They are rough. They come up faster than you think. Even in smooth air.


Synopsis: Citation flight crew experiences moderate to severe turbulence at FL180 for 15 seconds that apparently causes both Garmins to shut down. This eliminates both Navigation and Communication and will not reset. The crew elects to remain VFR; squawk 7600; and divert to a suitable airport. When the aircraft is powered back up for trouble-shooting the Garmins work normally.