Date: 202105
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ1
ATC / advisory: Center ZZZ1
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Fatigue; Time Pressure; Workload
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I reviewed the maintenance log when I arrived at the aircraft for the first flight of the day. It had just been released from maintenance. It had previously been written up for a strong smell coming from the lavatory. The corrective action stated that the lavatory had been leaking into the cargo bay. It had been repaired the same day and I was the first to fly the aircraft. I included this information with my crew brief.During the descent to ZZZ1; sometime prior to FL180 because the descent checklist had yet to be completed; the FO (First Officer) and I heard what sounded like an extra loud flushing noise from the FWD LAV (Forward Lavatory); but then developed into a loud sound similar to an engine spooling down; immediately followed by a strange; slightly chemical odor. We both looked at each other and confirmed what we heard and smelled. I called the A F/A (Flight Attendant) to verify if anyone had just flushed the toilet. She said someone was in the lavatory; had just flushed; and it smelled strongly like excrement.The odor quickly subsided. We discussed the possible sources. The LAV was a different style from most of our aircraft so it could have just been a sound [different] than what we were used to. The LAV had just been repaired so the odor could have been related. Since there odor was gone and there were no other indications of abnormality we continued to ZZZ1 but I had the FO start reviewing synoptic pages for any clues and I started reviewing possible diversion airports. We made a list of airports closest to our position as we got closer to ZZZ1; ZZZ2; then ZZZ3; then ZZZ4.During descent after passing through FL180; I started to hear and feel the aircraft surging. It sounded like it was coming from the engines. I de-cluttered the EICAS to check for vibration. There was a notable difference in engine vibration; but they were still in the green and there were no other indications on the EICAS.At approximately 13;000 ft. the FO and I both noticed a strong acrid odor. I called the A F/A who confirmed the LAV was not in use. I then called for the Smoke; Fire; Fumes QRC (Quick Reference Checklist). We donned our oxygen masks and the FO started through the QRC. We were almost exactly on top of ZZZ3; which we had already decided as a suitable airport; so when the FO [read the steps related to ATC] of the QRH (Quick Reference Handbook) I told him to [request priority handling] and ask to land at ZZZ3 immediately.ATC assigned an ALT of 9;000 ft. As I started to descend; the aircraft began shaking violently. ATC continued to vector and assign lower altitudes. When the FO turned off the RECIRC fans per the QRH; I asked him to put the checklist down and set up the aircraft for landing. We were very close to the airport and descending quickly. I decided the priority was for the FO to get our landing numbers and set up the approach while coordinating with ATC and the FA's. I could not help because I suspected external damage to the control surfaces so I did not want to take my attention away from maintaining positive control of the aircraft.Somewhere around the 9;000 ft. mark or lower we received an EICAS PACK 1 FAIL. This was the first time the EICAS provided any kind of indication of an abnormality. We had experienced a loud noise; strong odors; violent shaking; and now a PACK 1 FAIL. It felt as if the aircraft had suffered major damage and safe completion of the flight was at serious risk. ATC had placed us on a downwind for the ILS but; fearing we may be losing more systems and controllability; I told the FO we were turning immediately to final. As we slowed to approach speeds the shaking reduced. The FO contacted the FA's to go over [procedures]. I did not order a brace and I did not intend to evacuate. The aircraft was stabilizing and there weren't any smoke or fire indications in the cabin or flight deck. I landed and stopped on the runway. I requested an inspection by the fire trucks to make sure the aircraft wasn't on fire and we were safe to taxi. I made an announcement for everyone to remain seated and that we had landed due to a maintenance issue; but that the aircraft was safe and we were taxiing to a gate.In our judgement; due to the order and severity of events; we thought the aircraft may be in serious jeopardy. An immediate landing was the safest option available. I have often conducted a very similar scenario with FO's on long flights out of the area and near ZZZ3; during cruise. The scenario is a fume event. The reason this is important is because the QRH for fumes is very long and involves a lot of system isolation for troubleshooting. It would take 20-30 minutes for us to review the QRH in a relaxed; stable environment. Only at the end of the QRH do you see just how much runway would be required if you're heavy and have taken a lot of performance penalties due to the QRH troubleshooting process. Because we were so close to the airport and descending quickly; I prioritized performance and navigation over completing the QRH. There simply was not enough time for the full checklist.Parked at the gate; I called Dispatch and explained what was going on. They weren't aware of the diversion. The FO had asked ATC to notify our company during descent. I was getting bounced around a lot between Dispatch and [Administration] with Maintenance talking to them in the background; but Maintenance immediately insisted that it was a simple air system failure and that the aircraft was safe. Both the FO and I conducted a walk-around. I said the aircraft was not safe to fly. I wouldn't have landed in ZZZ3 if I thought the aircraft was safe. This aircraft needs a full maintenance inspection based on the severity of conditions we had just experienced. [In the past] I've had a hydraulic line rupture at FL200; I've had a brake failure on landing; I've had a full system failure. This was like nothing I had experienced and felt far more severe at the time than just a simple air system failure. It was serious enough that I didn't know if we would make the runway. This aircraft had a recent history of fluids leaking into the cargo bay. How much unseen damage was caused by these fluids? Why wouldn't an inspection be required if the maintenance department is saying 'oh yeah the smell and shaking is from the system blowing out'? As I'm trying to convince Dispatch and Maintenance that an inspection needs to be conducted; this is already approaching about 2 hours after landing; the adrenaline started to fade and the fatigue settled in. The entire crew was exhausted to the point that even the FA's were fatigued. Having drilled this scenario; I believe I acted appropriately given the circumstances. QRH calls for system isolation to identify the source but with odor being cause of the [diversion] and no EICAS indications there is no way to verify the source without taking the O2 mask off; which is extremely dangerous. We were so close and working with so little time that getting on the ground immediately became the top priority.




Synopsis: Air Carrier Captain reported strong odors; a PACK 1 FAIL issue and vibrations during initial descent. The flight crew decided to divert to nearest suitable airport for an expedited descent and successful landing.