The Marine Corps Military Occupational Specialty (MOS) 6173 is for Helicopter Crew Chiefs, CH-53. These crew chiefs are part of the flight crew and perform duties related to the maintenance and operation of CH-53 helicopters. The CH-53 is a heavy-lift transport helicopter.
To qualify for MOS 6173, a staff sergeant must:
Be qualified for independent duty prescreening checklist
Have a second or subsequent enlistment
Qualify for and obligate 48 months of service
Have 2 years or less time in grade
Have 3 years or more time on station at the time of orders
You can find more information in the official Department of the Navy Headquarters Marine Corps MOS Manual.