The US Navy has quietly overcome a grounding scare that programme officials now acknowledge brought the Lockheed Martin P-3C Orion fleet "to its knees" only 12 months ago.

With the P-3C's replacement aircraft - the Boeing P-8A Poseidon - still four years away from operational status, a series of inspections revealed structural damage that led to the grounding of all but 49 of 120 combat-coded Orions by September 2009.

The Naval Air Systems Command (NAVAIR) has since returned 33 of these aircraft to flying status within a 12-month period. Programme officials were forced to stand up a new supply chain to deliver critical aluminium extrusions in less than half the normal lead-time, says Capt Aaron Rondeau, the navy's P-3 integrated product team lead.

USN P-3C in Japan
 © US Navy

The goal now is to keep enough P-3Cs flying until the Boeing 737-based P-8A replaces the Orion completely by 2019.

Keeping the P-3C fleet flying even this long was never planned. Adapted from the Lockheed L-188 Electra regional airliner in the 1950s, the navy originally planned to start retiring the fleet almost 20 years ago, but cancelled the Lockheed P-7 replacement programme in 1990.

Concerns about the P-3C's structural health soon followed. Designed with a service life of 7,500 flight hours, the fleet today averages about 16,000h, says Bob Holmes, the navy's P-3C sustainment lead. Moreover, calculating the P-3C's fatigue life had been neglected, as the service's methodology previously focused on crack allowance versus fatigue life.

"When you start [a fatigue tracking policy] that late in the programme, it's very difficult to manage - unlike the air force which does it from the get-go," Rondeau says.

The navy formally launched a service life assessment programme for the Orion in 2000, which led to a full-scale fatigue test on a P-3C about two years later. "After full-scale fatigue tests came in it was pretty scary," Rondeau says.

The navy had hired Lockheed to perform the assessment based on a software algorithm developed in the 1980s.

"It turned out the old algorithm from the 1980s was underestimating the actual damage done to the airplane," says Mark Jarvis, Lockheed's director of P-3 design and production.

The new tools predicted that nearly the entire fleet faced catastrophic fatigue damage. Rather than grounding the fleet, NAVAIR engineers developed a special structural inspection kit to repair damaged areas. Starting in 2005, the navy also realised the underside of the wing was particularly damaged. After fixing the forward half of the underside of the wing, the navy turned to inspect the back half.

"When those results came in it was pretty devastating," Rondeau says. In December 2007 alone, the navy grounded 30 aircraft. Further inspections in September 2009 grounded another 10, with even more groundings occurring in between.

Source: Flight International