Eugene Moosa, Tokyo – Aviation experts discount the theory that zero visibility in dense smog could have caused the crash of an Indonesian Garuda airliner, but they have not ruled out another theory: engine failure caused by smog intake.

The Garuda aircraft, an Airbus A300-B4, crashed into a hillside plantation 45 km from the west Indonesian city of Medan last Friday, killing all 234 people aboard.

The flight from Jakarta flew most of its way through a dense smog that has blanketed much of Indonesia and Malaysia in recent weeks due to forest and brush fires.

Many experts and pilots said it was unlikely that a loss of visibility could have caused the accident, because pilots are trained to fly "blind" on instruments and modern airliners are well-equipped for instrument flying.

But Japanese aviation experts say smog could have affected the airplane in a different manner.

"It is a well-known fact that jet and turboprop engines can fail when they fly through volcanic smoke," said Yasutomo Aoki, an aviation commentator and former editor of the magazine Aviation Journal.

"We must study the effect of engines flying through smog, especially on long flights. We are focusing on the residue of particles of smog on the engine blades," Aoki told Reuters.

Another expert, former Tokai University aeronautics professor Haruro Terao, said he, too, could not rule out the effects of smog on the airplane itself, rather than on the pilot.

"The massive smoke could have affected the atmosphere and it is not impossible that this had some effect on the airplane," Terao wrote in the daily Asahi Shimbun.

Aoki said sand-like grains of volcanic debris in the air have been known to cause engine failure.

The particles block small coolant holes on the surface of engine blades, which causes overheating. Blades can break off after heat expansion and these bits can damage the engine's interior.

But one Singapore-based airline pilot with 20 years of experience, who declined to be identified, said smoke particles from forest fires are too small to pose any problem to the engines and wings of modern airplanes.

Volcanic ash is the worst kind and if planes can survive that, the smoke is no big deal, he said.

In 1991, an expert told an international conference on the dangers of flying through volcanic clouds that many planes have risked disaster by doing so against the expressed advice of airplane manufacturers.

Captain Ernest Campbell, then the manager of airline support and flight training at Boeing Co, said his company replaced 10 engines on different planes that flew through ash from the plume of Mount Pinatubo in the previous year.

"This kind of trouble has also been seen among warplanes during the 1991 Gulf War (from the sand in the air)," Aoki said.

While the smog that has descended on Indonesia and Malaysia is different from volcanic smoke, no one has carefully analysed the minute particles in the fumes and their possible effects on airplane engines, Aoki said.

"This is the first crash of an airplane that flew for a long time through this dense fog. We are awaiting word about this possible factor after investigators study the engine blades and black boxes from the wreckage," he said.