SAN ANTONIO (REUTERS) - A former US Air Force training instructor was sentenced on Wednesday to six months in prison for his role in a military sex scandal that has led to changes in training practices.

Tech Sergeant Bobby Bass was also ordered reduced in rank one grade, to staff sergeant, and was fined US$3,000(S$3,700) at a court martial at Keesler Air Force Base in Mississippi, according to First Lieutenant Victoria Porto, a base spokeswoman.

Bass's offenses, which occurred at Lackland Air Force Base in Texas, included forcing a squadron of recruits to stand naked as he watched, according to Colonel Polly Kenny, a spokesman for the Air Force legal system. In all, he was convicted of 31 charges including wrongful sexual assault, cruelty, maltreatment, assault and dereliction of duty.

When the court martial began April 15, Bass was also charged with rape and faced the potential of life in prison. But the military court acquitted him of raping a male Airman.

Bass is the 17th Air Force military training instructor to face court martial in the past two years, in a growing scandal that has led some in Congress to call for sexual assault cases to be investigated and tried outside of the Uniform Code of Military Justice, which governs the armed services.

Kenny, an Air Force staff judge advocate, said Bass' offenses were uncovered as part of an effort by investigators to contact people who have gone through basic training since 2002 at Lackland Air Force Base to try to root out improper behavior by instructors.

She said Bass' offenses had occurred in 2009, before the pattern of abuse and assault came to light in 2011.

Previous cases have involved training instructors who engaged in improper sexual relationships with female trainees, flirted with trainees, exchanged sexual photographs on social networking sites, and dated trainees, all of which is forbidden under Air Force policy and military law. Some instructors have been accused of raping trainees.

The cases have also focused attention on the problem of sexual assault in the US military. Figures released by the Pentagon for 2010 show that 19,000 men and women had been sexually assaulted, but only 13.5 per cent of victims reported the incident.

"Survivors of rape and sexual assault encounter numerous obstacles in acquiring immediate and long-term care," said Nancy Parrish, who heads the support group Protect Our Defenders.

"They are often disbelieved, silenced, denied health care and discharged involuntarily."

Kenny said the fact that Bass and 16 others have faced trial in the scandal shows the military is committed to rooting out sexual assault.

The Air Force has also ordered that 25 per cent of military training instructors be women, and has started a "wingman"system, in which recruits and instructors never have an opportunity to be alone together.

Last fall, the Air Force swept installations around the world and removed material deemed to be offensive, such as pornography.

Colonel Deborah Liddick, who commands Basic Military Training at Lackland, said she now meets with every recruit to make sure they can report incidents of sexual abuse without fear of reprisal.

"I want them to know that this is a safe place, and they have a way to communicate with me, directly, if they need my help," she said.