File photo of Piper Seneca plane carrying Robredo that crashed in Masbate. File photo of the Piper Seneca RP-C4431 plane that DILG Sec. Jesse Robredo boarded, which crashed Saturday afternoon in Masbate. A portion of the right wing was recovered Saturday night, with the numbers 431 visible. Courtesy of Aviatour Air

(Updated 5:20 p.m.) Divers searching for the plane of DILG Secretary Jesse Robredo are probing depths of up to 285 feet, and face a host of dangers from diving that deep. Transportation and Communications Secretary Mar Roxas, who has been posting updates on the fruitless search so far, says that divers are searching in waters from 131 to 285 feet, and are being advised not to stay underwater for more than 10 minutes.

By Sunday afternoon, divers had only recovered the plane's paper flight plan at about 150 feet. Roxas tweeted that the flight plan was found "alongside 'tracks' on the seabed along a north-south heading."

Roxas had earlier tweeted that "search area has depths of 40 to 87 meters. Human Scuba can only do 10 minutes at 40 meters (131 feet) before nitro narcosis." Roxas was referring to nitrogen narcosis, a condition in which divers experience an effect akin to tipsiness from alcohol. The diver's judgment is impaired, endangering himself and any dive companions.

In fact, according to experienced divers contacted by GMA News Online, divers can stay in water that deep for as long as 30 minutes with the right equipment and training.

"You cannot dive that deep with just air, you need mixed gases," says dive instructor Chen Mencias.

Technical diver Dave Dy, who is certified to explore shipwrecks, says ten minutes do not give a diver much time to search, and advises that the scuba divers involved in the search breathe mixed gases and bring multiple tanks to enable a longer time underwater and increase the chances of finding the plane fast. But deep-water diving takes specialized training.

Shortly after 4 p.m., Mar Roxas called out to scuba suppliers via Twitter that the dive search teams were running out of "trimix," a breathing gas used for deep diving. He said that the target depth for its use was 250-300 feet.

"It is possible to get out of deep water fast and safely using a blend of gases (in the scuba tank)," says Dy. "But the deeper one dives, the greater the chance of narcosis."

Anyone who dives that deep risks not only narcosis but bends or decompression sickness, which can cause paralysis or death, according to Dy. Technical divers are those trained to dive much deeper and far longer than recreational divers.

Interactive image of crash site below, location based on coordinates found on Pres. Aquino's Facebook page. Pilots of the Robredo plane aimed to make an emergency landing at Masbate Airport. They were less than a kilometer short.




Underwater search plan

The government is deploying Navy technical divers as well as underwater search techniques used last May when an Air Force trainer jet went missing off Mariveles, Bataan.

Jake Miranda, a technical diver familiar with both the Mariveles search and the current one in Masbate, says a side-scan sonar device is being employed to create images of large areas of the sea floor in the vicinity of the crash.

Once the plane's wreck is located, "then get the (geographical) coordinates, and bring down an ROV (Remotely Opreated Vehicle)," Miranda says via text, describing the plan. "When we have video, technical divers will go down."

Compression divers, or local fisherfolk who use improvised equipment to stay underwater for long periods of time, were reported to be assisting in the search on Saturday. "That's even  more dangerous," says Dy.

Most Filipino divers use feet as a unit of measurement when referring to depth.

DILG Secretary Jesse Robredo, pilot Jessup Bahinting, and Nepali co-pilot Kshitiz Chand

Aside from Robredo, the two others missing are veteran pilot and flight instructor Jessup Bahinting and his Nepali co-pilot Kshitiz Chand. Bahinting is also the chief executive officer of the Mactan-based aircraft rental company and flying school Aviatour Air, which owns the plane that crashed. According to Chand's Facebook page, he graduated from Aviatour Flight School in 2011 and identified himself as a commercial pilot.

Growing search and rescue effort