Lockheed Martin, a global security and aerospace company, recently began a campaign on Twitter to defend their F-22s after pilots refused to fly them.

Complaints were made about oxygen deprivation in the cockpits. Investigations are being made by the Air Force into approximately 12 incidents involving hypoxia, or oxygen deficiency, according to CNN.

“For some reason, the onboard oxygen generating system and the environmental control system that feeds it may be inputting some contaminant,” Gen. Gregory Martin told CNN affiliate WAVY.

The problems began in 2008 when problems with the oxygen supply were first reported.

The jets were grounded for investigation but cleared to fly again in September 2011. They are limited to flying below 25,000 feet due to risk of blackouts above that altitude. Emergency oxygen deployment handles have been installed by the Air Force.

Lockheed Martin’s Twitter campaign began as 60 Minutes aired an episode interviewing pilots refusing to fly the fighter jets. Their tweets mention speeds that only the F-22s can claim as of yet.