Figure: N807FT, from Aviation Safety Network
No doubt about it, the air traffic controller used non-standard phraseology and that was the first link in the chain that led to this mishap.
The pilot's Situational Awareness should have told him that was a ridiculous altitude for any part of the approach until just prior to landing. (They impacted 6.5 nm from the runway.)
The first officer was flying and had earlier complained about not having an approach plate, which the captain dismissed. The first officer was not assertive and it isn't clear if he had protested the altitude but there was evidently a lack of Crew Resource Management between the two pilots.
We as pilots can reduce the confusion level by forever banning the words "to" and "for" from our read backs. See the "Never Say To and For" technique in Clearance Read back.
The Boeing, named "Thomas Haywood", was less than half loaded with textiles, computer software and mail when it departed Singapore. Approaching Kuala Lumpur, the crew were cleared to route direct to the Kayell (KL) beacon for a runway 33 approach. While on the NDB approach, the crew were cleared to ". . . descend two four zero zero . . ." which was interpreted by the crew as ". . . to 400 . . .". The aircraft descended below minimum altitude and crashed into a hillside at 600 feet/180m msl just before reaching the Kayell NDB, where minimum descent height was 2400 feet. The Boeing hit treetops and started to break up until bursting into flames.
Non-standard phraseology was used by Kuala Lumpur ATC, causing the crew to misinterpret the instructions.