Engine knock/low altitude commercial flight

[pb26518]

A couple months ago I was on an American Airlines (Airbus A319) flight from Jackson Hole, WY to Chicago, IL. It was extremely cold that day (—10F when we took off) and snowing (plane had to be de-iced before takeoff). About 25min after takeoff there was a bit of a vibration/knock from one side of the plane. I barely noticed it but my fiancé immediately did (nervous flyer). Pretty soon after, we noticed we were definitely getting lower but nowhere near our destination (somewhere over South Dakota at this point). The captain came on the speaker and said “well you all probably heard that knock come from one of the engines…” and I don’t remember the rest of his message but I thought it was something about crossing a frontal boundary too quickly/cold front/engines needing time to catch up or something? He said it was environmental and that we were going to be flying lower where the air was warmer. He was communicating with dispatch but still planned to go to Chicago. He seemed rather nervous and unsure of himself stumbling over his words, which made me uneasy. The rest of the flight was relatively smooth (albeit a little nerve racking) and we landed in Chicago.

Has anyone heard of this type of issue before? I did some googling and could not find much. Is it normal to continue onto your destination with this sort of thing? After the initial message and no follow-up from the captain, I would have felt better about landing immediately, not knowing if I was being told the full story or not.

See below from flightaware
https://www.flightaware.com/live/flight/AAL772/history/20240117/2022Z/KJAC/KORD

Thanks,
Pat

 

[Jim K]

I'm not a jet pilot, but it sounds like it may have been a compressor stall. If the airflow through the engine is interrupted, the high pressure inside the engine can escape, kind of like a backfire in an old carbureted car. Possible they picked up some ice quickly passing through a front that disrupted airflow into the engine, or built up ice on the blades of the engine itself. As I understand it, it's pretty uncommon with modern engines, and it might have rattled them as they weren't expecting it and had possibly never experienced it before in real life.

If it was minor enough that you barely noticed, once the engine is running normally and the anti-ice system is working, there's no danger. Particularly if it was just one side. A 319 flies just fine on one engine. Sounds like they had a handle on it.

Here's a video about compressor stalls that you might find interesting:

 

[Jeff767]

Hard to tell what happened from what you wrote. Only thing I can think of is they secured the bleed air on one engine for some type of failure. That would put them in a single pack operation. Single pack flight can have altitude restrictions. Often as low as 25,000 feet depending on the aircraft type since you don’t have redundant aircraft pressurization.

 

[Larry in TN]

How loud/strong was this knock? A compressor stall is a loud bang.

If it was a mild vibration it could have been ice forming on the fan blades. Only one affected engine might suggest an issue with that engine's anti-ice system necessitating a descent out of the icing conditions.

If loud/strong, a compressor stall could be caused by a bleed-air valve failure, as Jeff said, and disabling that engine's bleed-air system.

Neither situation would require a divert as long as the vibration or stalls stopped.