A Philippine aircraft carrying soldiers crashed in July after an "unrecoverable stall", killing more than 50 people in one of the country's worst military air disasters, the armed forces said Thursday. 

The C-130 Hercules transport plane was loaded with nearly a hundred people, most of them fresh army graduates, when it overshot the runway while trying to land on the southern island of Jolo in Sulu province.

Most of the dead were soldiers being deployed to the island -- a haven for Islamist militants -- as part of a counter-insurgency effort. Dozens were injured. 

"Based on the investigating team's report, no single factor can be attributed to have exclusively caused the accident," the Armed Forces of the Philippines (AFP) said in a statement.

"The aircraft component, the environmental condition and aircrew response led to (an) unrecoverable stall in a critical phase of the aircraft operation," it said without providing details. 

Armed forces spokesman Colonel Ramon Zagala told AFP a component "failure" triggered a light that "caused the pilots to concentrate on that instead of recovering the stall or doing the turnaround". 

The plane stalled after losing "thrust and lift" at low altitude. 

"Usually you can recover a stall... if you have altitude, if you're way up, but in this case it doesn't have altitude," said Zagala, without specifying which part of the aircraft failed.

National Defence Secretary Delfin Lorenzana told a congressional committee hearing he had been informed the crash was caused by a "confluence of many events".

They included "defective instruments or systems, plus of course the reaction of the pilot was not also appropriate for the emergency," he said Wednesday. 

The military said previously the C-130 was in "very good condition" when it crashed in sunny weather.

The full results of the investigation have not been made public. It is not clear if the findings were also based on information recovered from the flight data recorders, which were sent to the United States for analysis.

Landed hard 

Witnesses and survivors told investigators shortly after the crash that the plane landed "hard" and then bounced twice before taking off again, Lieutenant General Corleto Vinluan, then chief of the Western Mindanao Command, told AFP days after the crash.

"Then at the right side of the airport it hit a tree -- that's the account of the injured," Vinluan said.

At least three civilians who were not on the flight were also killed as the plane ploughed through coconut trees and houses.

C-130s have been the workhorses of air forces around the world for decades, used to transport troops, supplies and vehicles.

The second-hand Hercules that crashed was acquired from the United States and delivered to the Philippines earlier this year.

The aim of the investigation was to "determine the cause of the accident rather than apportion blame", the statement said.

The armed forces also reported Thursday that a Black Hawk helicopter crashed in June after it "inadvertently entered a thunderstorm" and the pilot suffered "spatial disorientation or vertigo." All six on board died.