Sudden "nose-down" event during a normal flight in normal weather conditions

Question

I had a flight scheduled from WAW to KIV on LOT airlines. The boarding pass told they changed the aircraft to a 737 MAX. When I saw that I've immediately remembered that whole MCAS story from 2018.

The flight went okay, just normal turbulence here and there when passing the clouds, nothing new.

However during the descent right at the border between Romania and Moldova we've slowed down and decreased the descent rate. After that we went in a heavy pitch down (the heaviest in my life, and I've flown more than a hundred times) almost lifting us up from our seats. That's when I've started to be really worried as I've thought this was the MAX model based on the boarding pass. After ≈30 seconds of this pitch down increasing the pilots started to pitch up hard, causing ≈1.8g of acceleration. After returning the plane onto horizontal flight - we've continued the flight without anything super abnormal.

I really thought that was the MAX aircraft and prayed that the pilots were briefed on how to handle it's malfunction and we wouldn't have gone into the second cycle.

However after landing I've checked that it was actually a 737-800 and now I'm reeeeeeally curios what caused that behavior. Any ideas?

Here's the flight radar link: https://fr24.com/data/flights/lo515#2e91e2d2

Answer 1

Well, the direct cause of such a sudden maneuver is almost certainly pilot action, rather than a system failure. Either a sudden change of autopilot settings (although the autopilot has rate limits, and outside of failure scenarios - which are incredibly rare, but possible - the maneuver should not be violent); or the pilot pushing and pulling on the control column.

Why would a pilot do that? Two possibilities that come to mind are ATC instructions ("expedite descent") or a TCAS RA. For the first case, it's not so unusual for ATC to ask an aircraft to descend quickly, to avoid spacing issues with other traffic. For the second case, automated systems onboard can detect collision threats and issue instructions to the pilots to climb or descend to avoid a collision. When this happens the pilots will maneuver strongly, in the way you described.

Unless an incident report is filed or you can somehow speak with the pilots, it will probably not be possible to figure out exactly what happened on your flight.