“Where was the forward momentum” is a great question.

To create lift, a wing is slightly longer over the top than on the bottom. In normal flight these two layers stay connected to the wings surface. The air on top takes slightly longer creating an area of low pressure. Bernoullis principle in its simplest terms is high pressure moves to low pressure. This is what makes a wing go up or create lift. When the airflow over the airfoil is disrupted by ice, an increased angle of attack is needed to maintain those areas of high and low pressure meaning the airplane nose must pitch up. Pitching up without adding more thrust will result in a slower airspeed. The ice also disrupted the laminar flow causing the smooth air to separate from the wing slightly making it less effective while increasing drag. In icing situations in propellered aircraft you are also losing thrust due to the propeller also being an airfoil. Rough ice reduces the effectiveness of that propellers ability to create a low pressure area in front of it because the laminar flow has been eliminated, and rough turbulent air is now passing on both sides of the airfoil greatly reducing its ability to pull you forward. With that being said it’s unlikely that prop ice contributed as those systems wouldn’t typically both fall at once. It has killed plenty of people in general aviation aircraft without those systems though and they’d usually die in a spin of the same manner.

A flat spin happens when you’ve lost all forward momentum after bleeding off all of your airspeed to the wings critical angle of attack or stall speed and your center of gravity is towards the back of or aft of the aircraft. Normal recovery by pushing forward on the yoke doesn’t work because you are too heavy in the tail. The horizontal stabilizer on the tail is another airfoil and surface that can build ice. It’s basically an upside down wing that creates down pressure. If it’s covered in ice it will get heavy and reduce or eliminate your ability to control pitch. Pair that with your props being accidentally feathered or full of ice and you’ve no longer got anything pulling you forward for thrust and no ability to push your nose over. The momentum left would look a lot like we see in the video. The airplane won’t do anything but fall at that point. The only thing that could be done was to get the propellers making thrust again. Without the tapes we don’t know what was said or tried. We also don’t know if they were hand flying or the autopilot was on. My guess is autopilot was maintaining altitude while they built ice to a point that their speed wasn’t enough to keep it flying. Continuously increasing pitch and losing airspeed due to increased weight. If they meant to drop flaps to increase lift and accidentally took that last remaining thrust away by feathering the props or a single prop an immediate loss of control could develop which is in my best guess what we see happening.

Hope this helps.

Oh, by the way, watch Top Gun! Maverick enters the same type of spin after losing thrust. Probably one of the greatest examples of a flat spin you’ll ever see besides this one.

That’s called a flat spin. When props are feathered there are turned so that the blades go to zero pitch meaning no thrust is being produced. Propellers are adjustable to provide max rpm and thrust at take off but then we can adjust them in cruise where the air is thinner to take a bigger “bite” of the air. This provides a big fuel savings. With ice building up on the propeller and wings neither are capable of creating thrust or lift because the laminar flow over them has been disrupted by the rough ice. With no thrust pulling you forward, a wing not creating lift and an airplane quickly getting heavier with ice, the airplane might as well be a brick. There are ways to recover from a stall. In most aircraft we push forward on the controls to regain that airspeed, add opposite rudder to stop the roll and recovery is not difficult. In an airliner with many passengers and bags and possibly a few thousand pounds of ice on the tail and wings the CG(center of gravity) could have been slightly aft making pushing forward on the controls ineffective. We’ll need to hear the tapes, but unfortunately stall spins are what kill a majority of pilots and passengers in aviation and is by far the most common type of accident. As mentioned in my previous post, accidentally moving those propeller pitch controls to the feathered low drag position when already at a slow speed would only accelerate the process of the stall and it would happen so fast they couldn’t be corrected in time to keep the airplane aloft. One more thing about stalls… typically when they happen one wing loses lift and stalls first causing that wing to drop and the other to quickly rise causing you to roll one direction or the other. That’s what starts the rotation or “spin”. Not only that but if the wrong control inputs are made it can tighten the spin increasing the rotation speed or even cause you to roll inverted. In todays world nothing would surprise me as far as a nefarious attack, but with my background and knowledge of airplanes and aviation I’ve seen this exact same scenario play out too many times. Most of the time it’s smaller general aviation aircraft so they don’t get as much attention. If you search stall spin crash on YouTube the results would take you years to watch though.

As airline pilots we are trained to a level that far exceeds many professions level of expertise before we are allowed to get behind the controls of an airplane carrying that many people, but we’re still human and still screw up. If the weather is adding complexity to the situation or a system such as the anti-icing equipment fails, those distractions can cascade into mistakes. Its usually takes a combination of things to happen for an airplane to crash, and I believe we’ll see that here as well after the investigation is complete.