It would seem that the crew had miscalculated their true location because of flight within the jet stream, leading them to believe they were closer to Santiago than they really were.
The Mysterious Final Communication - STENDEC
An enduring mystery, however, involves the final morse code transmission sent my the radio operator. He radioed:
"ETA SANTIAGO 17.45 HRS STENDEC"
The Chilean Air Force Radio Operator in Santiago did not understand the last word and so asked it to be repeated. The radio operator on the Star Dust in response repeated "STENDEC" twice before contact was lost.
One theory is that the operator was experiencing hypoxia as a result of some problem on board the plane and, in a state of confusion, was intending to send the word DESCENT, which is formed from the same letters as STENDEC.
I find this explanation unrealistic, because the operator successfully transmitted these letters, in that order, a total of three times. That is not likely to happen as a result of an oxygen-deprived error.
It also needs to be acknowledged that the radio operator seems to have believed the message was comprehensible. Now, the fact that it was not understood, neither by the Chilean operator, nor by British investigators, leads me to believe it was an attempt to convey an expected message but using ad hoc abbreviations.
STENDEC - "Starting Engine Decrease"
For what it's worth, here's my wild speculation. The crew erroneously believed they were four minutes from the airport in Santiago. They did not know they were about to crash at full power into the side of a glacier. The message was routine. "Starting Engine Decrease."
The radio operator coined an abbreviation (indeed a whole phrase) he thought would be readily understood in context. He repeated it when asked for clarification, but their demise suddenly took them by surprise.