AvWeb has a chilling reprint from IFR Magazine on US airlines intentionally descending below the approach plate MDA …
“Flight inspection noted that a GPWS alert was received at the reported location if the aircraft continued to follow the published Vertical Descent Angle (VDA) below MDA. The airline (and several others) reported that it was their SOP to do so, pointing to the benefits of stabilized approaches and the use of a continuous descent angle.”
(In layman’s terms, often an airplane is supposed to descend from the clouds, level out, then fly at the FAA minimum descent altitude until the runway is in sight. Airlines admitted they were continuously descending almost into obstructions every day to avoid leveling out first – completely crazy.)
Descending below the MDA into terrain would void insurance policies and likely result in the airline company folding.
In the Polish state visit to Russia accident, the captain similarly made up his own approach procedure. He descended below MDA and used an on-board radar altimeter over uneven terrain at treetop height. The resulting crash killed 1/3 of their government and military leaders.