HAMPTON, Va. -- The Air Force has awarded a contract worth about $19 million to Lockheed Martin Corp. to install a backup oxygen system in the F-22 Raptor.

The stealth fighter has had problems with pilots experiencing dizziness and blackouts from a lack of oxygen. The Air Force grounded its fleet last year while it tried to identify what was causing the hypoxia-like symptoms. No root cause was ever found and the jets were returned to the air several months later.

In May, Defense Secretary Leon Panetta ordered a stop to long-distance air patrol missions in Alaska until the planes had the automatic backup oxygen system installed. Other aircraft are performing those missions in the meantime.

An investigation found that a handle that engages the emergency oxygen system played a role in the death of Capt. Jeff Haney, 31, when his F-22 crashed in the Alaska wilderness in November 2010.

Retrofitting the fleet will start in December and finish in 2014. The contract was awarded on Tuesday.