Date: 201705
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Rain; Thunderstorm; 10
Light: Daylight
Aircraft: King Air C90 E90
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Training
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Air/Ground Communication
Component problem: Design; Malfunctioning
Reporter organization: Personal
Reporter function: Instructor; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Confusion; Distraction; Communication Breakdown; Situational Awareness; Workload
Second reporter function: Trainee; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Aircraft Automation Overrode Flight Crew; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Aircraft; Procedure; Weather
Primary problem: Aircraft

Narrative: I was in the right seat serving in my capacity as an instrument and multiengine airplane flight instructor. We were in my student's King Air C90-A. My student was in the left seat. I was doing the radio work.Thunderstorms and rain showers were a factor all the way from our departure point to our destination. We made several adjustments to our route; relying on the on-board weather radar; XM weather; ADS-B weather; and our visual perceptions.As we approached [our destination]; we received and copied ATIS Information; a special observation. The airport was VFR; ceilings were 3;300 broken and 4;600 broken. There was lightning observed to the southwest. Aircraft were being vectored for the ILS approaches. Visibility was reported as 10 miles.On our cockpit weather depictions we could see some strong storms (later characterized by Approach Control in the follow-up telephone call as 'extreme precipitation') approaching the airport from the southwest; and since the ceilings were so high; on our initial contact with Approach Control we requested a visual approach (in order to get us on the ground as soon as possible). This external pressure to get on the ground will show up later in this report. Approach indicated we would have to see what worked out.As we were vectored northwest of the field; both pilots could clearly see the runway; with which we were aligned. We were at 5;000 feet. Again; just north of the field as we proceeded eastbound toward a right downwind; we could clearly see [the other] runways; still at an altitude of 5;000 feet.As we were turned south on the east side of the field; a right downwind; we entered clouds at 5;000 feet and lost contact with the ground. We were then instructed to descend to 4;000 feet and to turn left to 130 for traffic. We later were instructed to turn right to 270 southeast of the field. (Later adjusted to 280).Approach Control asked if we had the field in sight; and partially concerned that we had been vectored around for 8 - 10 minutes; all while extreme precipitation was fast approaching; and feeling a lot of external pressure to get my student safely on the ground; I reported the field in sight. The radios were already set to perform the ILS approach. We were now at 3;500 feet and had seen the airport more than once from 5;000 feet with good reported visibility. The field was clearly 1:30 just out my side of the aircraft. By the way; even though the ceiling was very ragged and uneven; and we had detoured around precipitation the entire trip; we were in VMC conditions at this point; and I knew which way the airport was; so I called 'my airplane' and initiated a right turn toward the field. I dodged further right to miss some light precipitation or virga and I inadvertently overshot my turn; putting us on a course approximately parallel to the final approach course; instead of on a course to intercept it. As I was beginning a turn back to the left toward the airport; the approach controller called with something like 'where are you going?' I admitted I had turned too far right and I think I stated I was correcting; but the controller canceled our visual approach clearance and turned us westbound.It was just after we had been turned westbound; toward the extreme precipitation southwest of the field that communications disaster struck. The Garmin-750 arbitrarily; and with no input from either pilot; toggled the active and standby communications frequencies.The action returned us to the previous frequency we had been using; Memphis Center; though unbeknown to us at the time.A little history of that GTN-750: when it was first installed; there were numerous instances of the standby com frequency becoming the active with absolutely no human intervention. A couple of times I actually witnessed the event as it occurred. The standby frequency numbers would highlight; then the two frequencies would swap places; the standby becoming the active.As I say; I am aware that this happened numerous times after the unit was first installed some time ago; but to my knowledge it has not happened once in over a year; and I assumed the problem had been addressed in one of the software updates Garmin provides.Apparently; we later learned; Approach Control desperately tried to reach us on their own frequency and on the emergency frequency; as the heading we were on was taking us right toward the extreme precipitation. Their efforts were to no avail; for we were not monitoring Guard; and were unaware that the com frequency had changed on its own. We did fly along for a couple of minutes westbound with no word from Approach; but I assumed they were vectoring us way west of the field for re-sequencing; and may have been irritated at the earlier turn overshoot.Finally; Center reached us and told us to contact approach. Good thinking; Approach!Once we were back in contact; we were turned to 020. We were told to descend first to 3;500 feet; [then] 2;600 feet and we were turned to 155 to proceed east for the ILS. Later headings of 020 and 340 led to the instruction to intercept the final approach course for a landing on 31.After we landed I was instructed to call the tower for a possible pilot deviation.Things I have learned:If there is a 2 person crew; it would be advisable for both pilots to identify the airport (or traffic) before accepting a visual approach or committing to maintaining visual separation.It would be prudent (as well as helpful to others) to monitor Guard. We would have been out of communication for only seconds instead of minutes had this been our habit.There is a VOR on the field and this aircraft was equipped with RMI. Instead of both radios being tuned to the ILS; it would have been better to have the RMI needle pointing to the field; for easier visualization.I will make a more determined effort to keep external pressures; such as the rapidly approaching weather we saw on radar; from negatively influencing my decision making.Finally; regarding the avionics malfunction; the old adage 'a problem that goes away by itself; comes back by itself' should be kept in mind.By the way; the owner's GTN-750; still under warranty; was sent in for repair at the earliest opportunity.

Callback: The reporter's GTN 750 has had numerous uncommanded frequency changes in the last several years. He reported witnessing the screen illuminating as it normally would after being touched and considered that possibly dust or other contaminant fell on it. The GTN 750 can communicate with the iPad ForeFlight App only for route changes; not frequency changes.

Second narrative: I was on a training flight. I was in the left seat; receiving training. In the right seat was a multi engine instructor. Both of us are rated for the aircraft. My copilot was handling the radio communications and giving me instruction; and I was doing the majority of the actual flying.I have learned a myriad of lessons from this incident. First; the visual approach on a busy day was a mistake number one. Being capable of doing an ILS approach; I should have opted for that. If the weather was just so terrible; I think that I should have considered landing at some other airport or have just stayed on the ground altogether.I have learned that just because another pilot has been flying longer; outranks you; and has more hours that it doesn't mean that pilot has better judgment or is doing the right thing. I need to question others early.I think not hearing from approach for so long should have been a red flag.


Synopsis: A BE9L flight crew reported the aircraft's GTN750 autonomously changed frequency after beginning a visual approach turn to base. The aircraft overshot final then entered IMC; but they could not communicate with Approach Control.