2025-01 · NASA ASRS report 2205470
Air carrier flight crew reported engine vibrations when entering unexpected icing conditions and diversion to a nearby suitable airport.
Our flight from ZZZ2 to ZZZ1 was restricted to FL250 due to a pack MEL. The flight route had us flying predominantly overwater to ZZZ1. While abeam the coast approximately 100 miles from the coastline; we entered a cloud layer with the outside air temp about -25 deg C. We immediately applied engine anti-ice heat to both engines. After several minutes at 280 kts. and getting bumped around quite a bit; I heard a very distinct change in outside noise that perpetuated. At first; I thought we ingested a bird into one of the engines and that was causing one of the engines to run rough; but the noise continued and began to get louder as now the sound was coming from both engines. I looked down at the secondary instrument display and noticed both engine vibe indicators were increasing. When I first glanced down; they were both in the mid 3s and steadily climbing. Correspondingly; engine oil temps were about 115 deg C and slowly climbing; engine oil pressure looked normal. I reached up to again verify that both engine anti-ice switches were on and they were; confirmed by the Thermal Anti-Ice (TAI) indication above both N1 gauges. I looked out the windscreen and noticed ice building up around the edges of the windscreen. This all happened very quickly and suddenly; as if we had just flown into a wall of wet sleet and slush; none of which showed up on our weather radar; which both of us had up on our nav display. I quickly came to the conclusion that we were in severe icing and it was affecting both engines. At this point I turned on wing anti-ice as well. I had been communicating with the First Officer (FO) all of this and asked him to run the Engine High Vibration QRH checklist. We followed all of the steps in the checklist to include autothrottle off and realized that we needed to descend immediately to both reduce the loads on the engines and get out of the severe icing condition. After a quick consultation about how to proceed; the FO [advised ATC]; and requested a lower altitude (in hindsight; I wished we would have made a MAYDAY call here and notified ATC of our intention to descend; but in the heat of the moment you go with what you think is best at the time). As we were waiting for the descent clearance; the left engine was above 5.0 on the vibe indicator; with reverse video on; and the right engine was at 4.9. ATC took more time than I liked to give us a descent clearance so I told the FO to [request priority handling] and tell them we need to descend immediately (again; we should have cleared ourselves with TCAS and descended; but in the moment that seemed like the best thing to do). Within a few seconds of that 2d call; ATC gave us a descent clearance and told us he can clear us to as low as we needed. During that 2d call to ATC the FO also requested direct ZZZ3 and the controller gave us a heading of 320. As we were descending; he cleared us down to 10;000 ft. During the descent I thought about going to ZZZ4 instead of ZZZ3 because it was closer but the amount of snow; ice; and sleet we were in ruled that out as we needed to exit icing conditions. We settled on heading to ZZZ3 as it was about 120 miles and better weather than ZZZ4. When we initiated the descent; I reduced power on both engines below 45% N1 as directed by the QRH and immediately the vibes decreased to the low 3s on the #1 engine and high 2s on the #2 engine. I was concerned for a dual engine flameout over water so I kept the power as close to 45% as I could.We continued our descent to 10;000 and headed to ZZZ3. The FO got the weather in ZZZ3 and it looked good enough - with light snow and overcast ceilings around 1200 ft. We continued our descent to min vectoring altitude which was 3400 ft; to get us to the warmest air possible. As we descended to warmer air; both engine vibe indications were getting lower and oil temps were coming down. Feeling better about the aircraft state; I passed the controls to the FO and began briefing dispatch; the flight attendants; and the passengers. During this time; Dispatch sent us an ACARS note saying that ZZZ was a better option than ZZZ3. With both engines recovered from the severe icing and the aircraft flying and responding well; we agreed to continue the flight to ZZZ. I re-briefed the flight attendants and passengers and informed Dispatch we were headed to ZZZ after we received a clearance from ATC. Initially; when we were headed to ZZZ3 with the engines and airplane in a precarious situation; I told ATC we would need Airport Rescue and Firefighting (ARFF) at ZZZ3. As we were headed to ZZZ; with the aircraft in a stable state; I told ATC we did not need ARFF services at ZZZ but they were still deployed along our runway when we landed there uneventfully; under max landing weight. Had we landed at ZZZ3 it would have been an overweight landing; but the additional flying to ZZZ brought us just under max landing weight.After arriving at the gate; I called Maintenance Control; made a logbook entry; spoke with ZZZ Maintenance; and spoke with the chief pilot. Before I had the chance to get out of my seat; the flight attendants were gone. I never had the chance to debrief with them. Upon entering the terminal; I realized that they had already started boarding our passengers on a new aircraft to ZZZ1. That meant that our flight attendants were already working their next flight. This upset me very much. Someone in the flight attendant's management chain had made the decision that boarding the continuation flight to ZZZ1 was more important than giving me the opportunity to debrief with my crew. Totally unacceptable. The human factors of what we went through should have been allowed to come to closure instead of pretending that we were ok to continue working the follow-on flight without a thorough debrief of our actions and events. During all of these events; the FO was outstanding; very proactive; and super proficient in all of his duties.Cause: Super cooled liquid droplets that were not depicted on weather radar; and the fact that the cabin was getting warm at the gate in ZZZ2 because the pack that cools the main cabin was inop. I was trying to get off the gate quickly to minimize passenger discomfort. Had we taken more time to analyze all of the data in WSI showing where the icing was at FL250; we may not have departed; or changed our route with more added gas. If a flight is restricted to FL250; both the Dispatcher and flight crew should study the route more to avoid flying into forecasted severe icing. I don't know if severe icing was forecast in the area we were flying to before the flight. I did look at weather prior to departing; to include weather radar and turbulence areas; but I did not dig into the icing charts. The radar depiction was green on the WSI radar depiction for our flight path.
While working flight XXXX ZZZ2-ZZZ1; we were dealing with a PACK Mel that limited us to 25000 ft. While enroute; and off the coast abeam ZZZ4; we felt and then noticed the engine vibrations go above 4 and into the 5s. We were in IMC and in icing conditions at the time; and the cowl anti-ice were on. The Captain; who was pilot flying (PF); directed me to get a lower altitude and run the QRH for Engine Vibes. I notified Center; and they gave us 15000 ft. Following the QRH; it directed the PF to reduce the power to less than 45%; which did get rid of the vibe indication. We sent a 7700 on ACARs; and assessed the weather of the nearest diversion airports. ZZZ4 had okay visibility; but the winds were more significant than ZZZ3 that did have snow and less vis; but it was still above mins and lighter winds. We decided to go to ZZZ3; officially [requested priority handling]; and asked for Airport Rescue and Firefighting (ARFF) to standby. We ended up descending to 5000 ft.; and by that time all engine indications stabilized. We had plenty of gas; and were planning on an overweight landing as well. Dispatch sent us an ACARs suggesting ZZZ; and we decided to do that since now that everything was stabilized and the weather was pretty much CAVOK.Captain did a good job of communicating with the flight attendants (FAs); and we did send multiple ACARs msgs to Dispatch; but our ACARs were going in and out. ATC asked if we still needed ARFF; we told them no; but they still were rolled. The Captain landed on XXC; we taxied to the gate; and had no further issues. Maintenance met us; and proper logbook entries were made. Having an MEL that limited us at FL250 where the highest likely hood of severe icing existed. Many other times with this MEL; I always have automatically associated it with convective weather systems; but that will be in mind as well. Also; though I found it quickly once I used search; I highly suggest the Vibrations QRH be just under 'Vibrations' and not 'Engine Vibrations'. It is already under the 'Engines' subsection; and the indicator is Vib; and no other system has an associated 'Vibrations' QRH. Lastly; having the opportunity to debrief as a crew whenever an emergency happens before we all get reassigned would be nice.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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