CRJ-200 Captain reported a loss of aircraft control while flying an ILS approach on autopilot in visual conditions. The FO was the pilot flying when the aircraft suddenly pitched down; and became confused with the state of the aircraft due to the unstabilized approach. The Captain then took the controls; disconnected the autopilot; and continued the approach to a landing. During the landing; the Captain noted that another aircraft under tow was blocking the glideslope antenna.

Narrative

On Day 0 at approximately XA:10 from ZZZ to ZZZ1; while on approach to Runway XXC with the autopilot coupled and cleared for the visual approach; I was the pilot monitoring and the FO (First Officer) was pilot flying. Just after crossing ZZZZZ; FAF; approximately the Tower; local; Controller gave us clearance to land and warned us of crossing traffic. The pilot flying commanded gear down then flaps 30; 160 speed. All of a sudden the aircraft started to descend rapidly; the flight director commanded an approximate 7 - 10-degree nose down; with the glideslope indication showing we were approximately 1 1/2 - 2 dots high; the speed increased; [and] we got a sink rate warning. The pilot flying seemed to be in shock and not doing anything or disconnecting the autopilot so I immediately took the controls and disconnected the autopilot. I leveled off temporarily. Tower then issued a low altitude alert. I then began to intercept the glide path based on the PAPIs and configured the aircraft with the final flaps and landed. While landing; I pointed out the under tow 777 aircraft that was parked at Taxiway XX to the FO; was blocking the glideslope antenna.I should have been quicker with my reaction and taking the aircraft. Also having a discussion with the FO and training them to more vigilant with crossing traffic and the effect on the glideslope indications. After taking the aircraft; conducting a go-around would have been a smarter decision for a more stable approach instead of salvaging the landing close to the ground.