The mix of military aircraft at Marine Corps Base Hawaii at Kane'ohe Bay would change substantially beginning by 2012 under a plan released yesterday by the Navy.

All but three of the Navy's 27 aging propeller-driven P-3C Orion surveillance and reconnaissance aircraft would be replaced by 18 P-8A Poseidon multi-mission jet aircraft based on the Boeing 737-800, but with strengthened wings, weapons systems and added fuel tanks.

The final environmental impact statement released by the Navy yesterday also notes in the future aircraft complement 36 MV-22B Osprey tilt-rotor aircraft that will replace older CH-53D Sea Stallion helicopters.

Marine V-22 Ospreys — half-helicopter and half-plane — have seen a year's worth of duty ferrying troops and equipment across western Iraq after two decades of development and a deadly series of crashes in testing.

More recently, Air Force Special Operations Command has used the aircraft, which can take off like a helicopter and tilt its rotors forward to achieve fixed-wing speed, over western Africa.

A switch would be made from 10 SH-60R helicopters to nine MH-60R versions. The environmental report also said three C-20G Gulfstream jets would continue to be based at Kane'ohe Bay.

The move to Poseidon surveillance aircraft would result in fewer airplanes and personnel at the Marine Corps base, slightly more noise, and an investment of $147.5 million for infrastructure upgrades.

The Navy said in yesterday's environmental report that it wants to begin replacing the Orions in its fleet no later than 2012 and have the process completed by 2019.

The Navy still must issue a "record of decision" for the Poseidon plan.

Under a "preferred" basing plan, 84 Poseidons would replace 120 of the older Orions at the following bases: Kane'ohe Bay; Jacksonville, Fla.; and Whidbey Island, Wash. There also would be periodic squadron detachments to North Island, Calif.

The Poseidon is a military version of the Boeing 737 commercial jet. Kane'ohe Bay would receive three squadrons with six of the planes in each squadron.

Originally designed for hunting Soviet submarines, the P-3 Orion has been the Navy's frontline maritime patrol plane since the 1960s. The most capable Orion version is the P-3C, first delivered to the Navy in 1969.

After the 9/11 attacks, the Orion, with its long range, was pressed into overland service in Afghanistan, and Patrol Squadron 9, or VP-9, out of Kane'ohe Bay, conducted imaging, long-range targeting and firing of Stand-Off Land Attack missiles against Taliban and al-Qaida targets.

Hawai'i P-3s also served in Iraq, using sophisticated surveillance sensors on missions in support of ground troops.

However, nearly a quarter of the Navy's fleet of 161 P-3 Orions were grounded in December due to fatigue in wing sections. The service said some of those planes were based at Kane'ohe Bay.

In the mid-1980s, the Navy began studies for a replacement for the Orions, which had a service-life goal of 7,500 hours, but are now operating with 18,000 to 20,000 flight hours.

"The P-8A (Poseidon) offers advantages over the P-3C in that it will increase mission reliability while requiring a smaller force and less maintenance infrastructure," the Navy said.

According to the Navy, each of the three P-3C squadrons here has 346 personnel. Under the replacement plan, each of the three squadrons would have 166 personnel.

Hickam Air Force Base was considered but eliminated as a potential home base because ordnance handling and storage next to Honolulu International Airport would not meet Defense Department safety criteria, and because there is not enough land and infrastructure to support the new planes there.

Both the old and new planes can be armed with torpedoes, cruise missiles, bombs and mines.

The personnel total for the P-3Cs at Kane'ohe Bay, with support crews, is 1,641, according to the Navy. The environmental report predicts there will be 737 fewer people needed with the replacement planes.

For the Hawai'i basing, up to $147.5 million in new construction would be required for upgrades including a new hangar, but regional annual earnings would decrease by $93.5 million as a result of the personnel drop. Airfield operations would decrease by 10 percent.

The Navy said although noise levels are slightly higher for the Poseidon for takeoffs and landings, "touch-and-go" practice flights are "noticeably louder."

By comparison, the Air Force C-17 cargo planes that practice landings at Kane'ohe Bay are louder than the Poseidons will be, according to the Navy.

Marine Corps Base Hawai'i's workforce at Kane'ohe Bay includes 6,072 Marines, 2,765 sailors, and 2,340 civilians and contractors, according to Navy reports.