Mumbai, Oct 14 (IANS): Even as an official investigation into the Air India Express' Trichy take-off incident has commenced, the aviation fraternity has cited the pilots' decision as "dubious" and "dangerous" to carry on with the flight even after traffic controllers advised him otherwise.

The Air India Express' aircraft scheduled to fly between Trichy and Dubai early Friday struck the instrument landing system's (ILS) localiser antenna and then grazed the airport's perimeter wall before flying off.

The flight carried 130 passengers and six crew members. It was subsequently diverted and it landed in Mumbai at 5.40 a.m. -- four hours after it took off from Tiruchirappalli (Trichy). Its undercarriage suffered a deep gash.

Experts recall an incident on April 26, 1993, in which 55 people were killed on board an Indian Airlines plane in Aurangabad in Maharashtra when it crashed on take-off after apparently hitting a truck carrying cotton bales.

On Friday's incident, air safety expert Captain Mohan Ranganathan told IANS: "The senior pilot's decision to continue his flight onwards is dubious. The decision led to the loss of two hours of crucial on-board voice recording data (this instrument keeps such data only for two hours)."

"The in-cockpit conversation between the senior commander and the co-pilot at the time of the incident would have been stored in the voice recorder. However, as they landed in Mumbai after four hours of flying, the initial two-hour recording would have been lost," said Ranganathan.

Apart from hitting the ILS antenna and scraping the perimeter wall at Trichy airport, the pilots' biggest mistake was the decision to carry on with their flight.

"You can not risk flying over the sea with that type of damage to the aircraft's structure. He should have either listened to the ATC or have informed the company via ACARS (aircraft communications addressing and reporting system). The latter, he couldn't do as ACARS antenna was damaged at Trichy."

The ACARS is used to transmit and receive messages from ground stations.

A few retired and serving pilots told IANS that in their experience, such an incident would have surely come to the notice of the flight's commander and that his decision to fly on defies logic.

"At the 'VR' speed, when the aircraft is taking off at nearly 175 knots (300 km per hour), an incident like this would have been immediately noticed by the senior pilot," a senior commander currently operating a Boeing aircraft told IANS.

"The rotating angle of take-off might have been completely altered by the first impact between the aircraft and the antenna, which might have brought a nose-down altitude, thus hitting perimeter wall, which is about 300 metres from the runway's end, generally, and the aircraft should have attained a height of at least 35 to 50 feet above that point. This would not have escaped any pilot's notice."

"This is a clear case where the standard operating procedure has been violated. The SOP mandates the pilot to exhaust the fuel and land back safely. It does not imply to carry out the entire flight schedule. The pilot's decision defies logic."

However, industry insiders familiar with Boeing 737-800 aircraft pointed out that the damaged underbelly area was a pressurised and non-heated zone.

"Any penetration in the area could not have been detected by the aircraft's instruments, but a deep leak in the strcuture could have led to loss of pressurisation and could have been fatal especially over the sea," another pilot with an airline told IANS.

"Nonetheless, the damage would have increased the drag slightly. Now the question is: how much fuel consumption did this drag increase? Nowadays, when we are at a tight spot in terms of fuel usage, pilots notice even small changes in consumption trends."

Notwithstanding the lucky escape at the time of the impact, flying all the way to Dubai could have also led to a disaster. As the aircraft would have consumed more fuel, leaving the plane with less-than-anticipated fuel, thus forcing it to divert to any nearby airport.

In addition to SOP lapses, questions have also been raised over the load factor status of the aircraft. An overload condition would not have allowed the aircraft to gain sufficient speed or power.

Also in question is the aircraft engines generating sufficient power to touch the required speed to take off and gain the needed height in the given circumstances.

On the investigation front, an Air India statement on Friday said officers of the Directorate General of Civil Aviation (DGCA) have reached Trichy and so had Aircraft Accident Investigation Bureau officers.

Pending investigation, the airline has derostered both Captain D. Ganesh Babu and co-pilot and First Officer Captain Anurag.