The helicopter, which crashed last September claiming the life of former Andhra Pradesh chief minister Y S Rajasekhara Reddy and four others, was "probably not airworthy", but this was not a contributing factor to the accident, an inquiry has found.
The probe panel, headed by Pawan Hans chief R K Tyagi, also established that there was "no evidence" to indicate any interference in the flight operations by the Voice over Internet Protocol on board.
The primary cause of the crash of the Bell-403 chopper on September 2 last year was the error of the crew, which after noticing a fault in the oil transmission pressure "was engrossed for vital six minutes before the impact in searching for the relevant checklist from the Flight Manual".
The report pointed out that the number one engine of the chopper had experienced two technical problems on earlier flights which required maintenance action before the helicopter was released for the next flight.
The maintenance engineers and the flight crew "ignored this advisory before operating the flight on September 2".
But both the engines operated normally and remained capable of producing the required power till the time of the crash, the enquiry committee found.
"Thus the helicopter was probably not airworthy when it was released for flight on September 2, even though it was not a contributory factor to the accident," the report said.
The former chief minister, his special secretary P Subramaniam, Chief Security Officer A S C Weseley and pilots Group Captain S K Bhatia and Captain M S Reddy, were killed when the Bell 430 crashed in the dense jungles of Nalamalla hills.
The report said, "The Committee in the present case could not find any evidence/indication wherein (the) VIP interfered/influenced the flight planning/progression".
It recommended conducting of suitable short-term courses for pilots structured to cater to the needs of VIP helicopter operations.
The committee also ruled out any bomb threat or fire on board the helicopter before the crash.
Regarding the weather on route, the enquiry panel said the crew was aware of the poor weather conditions, but "continued to proceed ahead in spite of inclement weather which was continuously aggravating and becoming more and more difficult to negotiate".
It said the co-pilot "also did not advise the pilot-in-command to return or divert to the nearest location" and blamed the PIC as well as the supervisory staff of the Andhra Pradesh Aviation Corporation Limited for "lack of knowledge or disregard of the rules".
During the final 14 seconds before the crash, the co-pilot repeatedly called out 'Go Around' thereby indicating that some problem like the close vicinity of the hill feature. "In spite of the co-pilot's 'Go Around' call, the PIC could not act apparently due to incapacitation," it said.
The helicopter came hurtling down and hit the hill at a very high speed because of apparently being caught in a down-draught of wind, the enquiry concluded.
Two factors, the fault with the oil transmission pressure and probable "spatial disorientation" of the crew as they were flying through the clouds, "may have led to loss of situational awareness", the report said.
It maintained that the "probable" cause of the accident was that it "occurred due to loss of control resulting in uncontrolled descent in the terrain at a very high rate of descent due to entry into severe down draught".
The crew flew the helicopter under the instrument flying condition even though their flight plan was cleared for visual flight rules, it said.
During the course of investigation, the probe team undertook technical analysis at the crash site for crucial evidence and also at the APACL hangar at Hyderabad.
The Cockpit Voice Recorder downloads, engine and electronic control unit were sent to the US for analysis and help sought from the helicopter and engine manufacturers, Bell Helicopters, Goodrich and Rolls Royce, among others.
