With the Congress seeking a probe into the chopper crash that killed Arunachal Pradesh chief minister Dorjee Khandu, various theories are doing rounds regarding the conditions that led to the crash.

While it is de rigeur to settle for either machine or human error as a reason for such crashes, the Khandu case seems to be far more complex. Sources say that the Pawan Hans chopper that the late chief minister was flying was a top-of-the-line Eurocopter and was just 10 months old in operation.

They added that the chopper was equipped with a Emergency Locator Transmitter (ELT) that works on two frequencies. Signals of this can be picked up by satellites or over flying aircraft. However, after the crash the ELT of Khandu's chopper failed to activate though it should have.

Registered in July last year the chopper was new. But it had just one engine which makes it unfit for hilly terrain, a directorate general of civil aviation official clarifies. New norms issued last year clearly says that a single-engine chopper can be deployed only on those routes or areas that allow the execution of a forced landing. It also says that single engine machines shall not be operated at night or in instrument meteorological conditions. Only when there is clear visibility, an exception is allowed, adds the official.

He maintained that the weather Khandu was flying in was unfavourable for a single engine craft to operate.

The official informed that the chopper -- AS 350 B3 of Eurocopter, nicknamed 'Ecureuil', which means squirrel in French -- had a single Turbomeca engine, bore serial number 4991, was registered as VT-PHT on July 7, 2010, and pressed into service in December last year.

It had been issued a certificate of airworthiness and had completed 306 hours of flying and 577 landings.

Requesting for anonymity, a Pawan Hans official has said that the pilot had been cleared for Special Visual Flight Rules (VFR) Operation. This means he could fly in even with  a visibility of 1,500 metres wheras normal VFR requires 5,000 m of visibility. VFR flying means flying in good visibility whereas the pilots ferrying Khandu had been cleared Instrument Flight Rules (IFR) which allows the pilots to fly in poor visibility (night/clouds) also. IFR clearance which allows pilots to fly safely in lower weather minimums mandates special training. The clearance also allows a pilot to leave or enter IFR airspace near an airport under ATC/radar surveillance if the pilot believes good visibility prevails beyond that airspace where the flight can continue in VFR conditions.

So, one of the questions that needs probing is whether the pilots of Khandu's chopper were trained for IFR flying in high altitudes?

The Pawan Hans officials say that the helicopter used by the deceased had been leased to the government of Arunachal Pradesh. The officials say that the a double engine was leased to the chief minister and he should not have opted for a single engine craft.

An aviation expert familiar with the terrain says that the CM’s choppers must have crashed after being struck by lightning. He says, “Flying in hilly terrain is very difficult and often air is thin when you go higher as lifting capacity of the craft becomes very less. In case of lightning, the pilot has no reaction time. In case of engine failure a craft slowly falls down and a pilot may have enough time to react.” He is also of the view that the helicopter might have made way into clouds with increasing exposure to thunderstorms and wind gusts.

He adds, “It becomes challenging for a pilot to handle a situation where there is double trouble. Visibility and the hilly terrain together create a disastrous cocktail.”

Khandu's death adds to the list of politicians killed in air crashes - Y S Rajasekhara Reddy, Sanjay Gandhi, Madhavrao Scindia, G M C Balayogi, S Mohan Kumaramangalam, O P Jindal, Surendra Singh, Dera Natung and C Sangma were all died in air crashes.