The sole manufacturer of engines for the Lockheed F-35 Lightning II discovered in May 2013 that it had used substandard titanium alloy that might have been illegally purchased from Russia.

Pratt & Whitney halted delivery of F135 engines that contained the suspect titanium and has subsequently sued the supplier, A&P Alloys, of West Bridgewater, Massachusetts.

Titanium is popular in aircraft manufacturing for its strength and light weight. In May 2013, P&W learned a majority of an order of 900 pieces of the metal was originally melted in Russia, according to the lawsuit filed on 29 August in the US District Court of Massachusetts.

The discovery last year of conflicting documentation of the metal’s origin resulted in a “quality hold” on delivery of the F135 engine, the company says. The titanium was used to manufacture some parts used in engines made by Pratt & Whitney Canada.

Critics of the $400 billion F-35 development programme, which has suffered repeated delays and cost overruns, have accused P&W of covering up the titanium issue for nearly four months. The revelation in late August comes amid ongoing flight restrictions following an engine fire in June that destroyed a US Air Force F-35 test aircraft and has hampered flight testing. An investigation found the fire was caused by rubbing of the third stage fan, an integrally bladed rotor (IBR). Engines deliveries have been on hold since the fire and P&W is set to test a fix this week, according to reports.

Russia is the world’s largest titanium producer, but US law prohibits the Defense Department from purchasing the metal from foreign sources, including Russia. The lawsuit alleges that A&P “knew or should have known that accurate information about the source of the titanium is critical,” but misrepresented the metal’s origin to P&W.

Documentation obtained by P&W “directly conflicts with the certifications A&P initially provided with the material and, therefore, calls into question the source and quality of all A&P material,” the lawsuit says.

A&P's false statements were not limited to titanium but included shipments of other materials and extend to other suppliers, which prompted P&W to issue an industry-wide alert for A&P materials.

The lawsuit against A&P and its sole owner, John Palie, asks for damages in an amount to be determined at trial on one count each of fraud, negligent misrepresentation, breach of contract and violation of Massachusetts state law.

The company impounded any products that were made with A&P materials “until the suspect parts were removed and replaced with alternately-sourced parts or were validated through mechanical testing.” It then eliminated A&P from its supply chain, the company says.

P&W says it has tested all the suspect parts, removed and replaced them where needed, and found new sources of titanium. Efforts to remedy the problem have cost P&W more than $1 million, the lawsuit says. The titanium in question was found to have deficiencies, but testing confirmed that the engine parts are not unsafe for flight.

“While the tested material may not meet every P&W material control standard, our engine designs have significant amounts of margin and, based on our engineering assessments, such material does not pose a risk to safety of flight,” Pratt & Whitney says in a statement.