Citation X (C750)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Flight Crew Diverted; Flight Crew Took Evasive Action
Aircraft
We experienced a very loud noise at FL400. It sounded like an alarm at first or high pitch squealing sound. After a few minutes the passengers had said they detected an odor that they thought was electrical and hydraulic in nature. At that time we started to turn around and divert the aircraft. Weather was 2;600 Broken; 12 degrees C; 29.78; and winds of 360 at 17 knots. The loud sound in the aircraft did subside some in the descent and then returned; but not as loud. Also; the smell did change some as reported by the passengers. Once we were in the vicinity of [the airport] I actually did start to smell the odor as well. It reminded me of smells associated with both electrical and failed PACs. We requested the ILS XY; but were able to get a visual approach about 7 miles from the airport. The landing was approximately [30 minutes later]. We parked the aircraft; shut down; got passengers out and away from aircraft and then met with fire department and evaluated aircraft.
Took off and climbed to altitude. Approximately 10 minutes into cruise we heard a loud modulating alarm like noise coming from the cabin. So loud we could hear it despite wearing noise cancelling headsets. My initial thought was a medical alarm or similar from a device carried by a passenger. The modulating sound ended and was then replaced by a very loud squeal. I then guessed we had a bad pressure seal from the main door or emergency plug exit in the rear. I told the Pilot Not Flying (PNF) that I would fly the plane and he would turn to the passengers and see if they could tell where the sound was coming from. I checked the pressurization indications and found them to be normal. We both took out our oxygen masks and set them in our laps in case this was only the first step in a depressurization event. The PNF called back to our maintenance people to get input on the suspected seal failure. As he was speaking to maintenance to see if they had any alerts from the maintenance computer through datalink (they had not) the passengers communicated to us that they could smell an electrical burning followed by what they described as hydraulic fluid and or burning plastic smells. At this point both the PNF and I quickly agreed we were diverting. While he talked to the passengers I conveyed to ATC we had loud noises and smells coming from the back and needed to divert. Once the PNF put his headset back on and told me the smell was getting stronger we decided to divert to the nearest runway with at least 7000 feet. ATC gave us vectors and a descent and asked if [we were requesting priority handling] and we said yes. We still had no CAS or other abnormal indications in the cockpit as to what might be wrong. We descended at around 4000 feet/minute and kept the speed near VMO not knowing what we were dealing with. We went through all pages of aircraft systems looking for any indication of abnormal with particular emphasis to electrical and hydraulic. It then occurred to me it might be the Automatic Electronic Defibrillator (AED) we carried in the aft cabin. The alarm sound and electrical burning might be from a malfunctioning AED. Checking with the passengers they said the smell had stabilized and had not got worse and there was no evidence of smoke. We in the cockpit could not smell it. Before we went below 10;000 feet we discussed going more than 250 knots and decided there was no need since the smell had stabilized and had no evidence of fire and higher than normal speed might lead to mistakes on the approach by doing things too fast. We called the field in sight and landed without further incident. Deciding the aircraft was safe to taxi we cleared the runway and arrived at the FBO. As I secured the engines and shut the plane down the PNF calmly deplaned the passengers and led them to the FBO while I met the Fire Department. They inspected the aircraft and removed the AED and found it to be normal. As I went into the cabin it did smell of burnt plastic/melted wire casing and it seemed as if the cabin had a very slight haze to it. We emptied the baggage compartment and checked all Circuit Breakers (CBs) we could find with the help of the firefighters. We found no CBs popped. We then disconnected the ship's batteries at the direction of the firefighters. The firefighters checked the plane with their Forward-Looking Infrared (FLIR) device and confirmed the only heat hot enough to worry about was the brakes. I called home base and signed the paperwork for the fire department and got the passengers on their way in rental cars. Cessna dispatched a maintenance team and our flight department sent a team over also. It was discovered later that day that the In Flight Entertainment (IFE) audio box had 'melted down' and shorted out.
Flight Crew of CE-750 reported detecting smoke in cabin and diverted.