The U.S. Air Force continues to have a hard time persuading senior NCOs to serve a three year tour as a First Sergeant. This is not a promotion, not exactly. But the First Sergeant is the senior NCO in a unit, and is responsible for making the unit run smoothly. First Sergeants in the air force must be volunteers, and over the last decade there has been a chronic shortage of qualified volunteers.
First sergeants are the senior NCO (Non-Commissioned Officer) in the basic unit of the air force; the squadron. In addition to the familiar aircraft squadrons, there are also squadrons that do all sorts of other jobs (maintenance, administration, security and so on.) In effect, the air force squadron is roughly equivalent to the company in the army and marines. The first sergeant takes care of all administrative details and, in general, sees to it that things get done. The officers may command the squadron, and do things like fly aircraft, but the first sergeant is the guy (or gal) who makes sure things happen when they are supposed to. The first sergeant also works with the other sergeants in the squadron to prevent any of the junior enlisted personnel from getting into trouble, and also keeps an eye out for exceptional performers. When the squadron commander has to decide which enlisted personnel to promote, it is done after consultation with the first sergeant. To enlisted personnel, their first sergeant is something of a god. The first sergeant can intercede with officers if an airman gets in trouble, and will often crack down on wayward airmen before misbehavior catches the attention of an officer. To officers, the first sergeant is more than just a senior NCO. The first sergeant knows more about what is going on in the squadron than any of the squadron officers. Junior officers are told bluntly, before they report to their first squadron, that to cross the first sergeant is a very bad career move.
For the last few years, there's been a chronic shortage of about ten percent of the 1230 First Sergeant jobs that have to be filled. Because of the three year tour, the air force needs about 300 volunteers each year. Part of the problem is that the job has gotten a lot harder in the last decade. Unlike in the army and marines, air force first sergeants have to supervise a lot more people. The "company first sergeant" in the army and marines rarely has more than 300 personnel to look after. Air Force squadrons often have over 700 people in them. This is why some air force first sergeants are E-9 (a rank used in the army and marines for Sergeants Major, the senior NCO in battalions and larger units.) The rank of the first sergeant depends on how many airmen are in the unit (for 250 troops or less, it's an E-7, for 251-550 it's an E-8 and for larger units, it's an E-9.) Most army and marine first sergeants have only about 200 troops, or less, to worry about.
First sergeants have more problems to deal with these days. Since the end of the Cold War, more women have been joining the air force, and more junior enlisted personnel are getting married, and having families. In the Cold War days, most of the airmen were, well, men, and they lived in a barracks. The enlisted accommodations were actually more like a college dorm, with as few as two airmen to a room. But they were still all in one place and the first sergeant could go through the barracks building to keep an eye on his charges. Most of the NCOs were married and lived houses or apartments (both on the base or in civilian neighborhoods.) But the NCOs were older, usually in for a twenty year career and willing and able to help the first sergeant run the squadron. But now you often have up to twenty percent female airmen (there's no such thing as an "airwoman") and over half the enlisted personnel living outside the barracks. Not so easy to keep an eye on all those junior troops any more. The air force also discovered that supervising female airmen required some different management skills than those long used to supervise males.
And the first sergeant is the first responder when it comes to domestic problems among his troops. And all those young airmen (male and female) with spouses, and often young children, have lots more problems than the old, "single young males, living in the barracks" days. So added to the usual problems of getting the squadron to do its job efficiently, the first sergeant often has a dozen or more domestic crises to deal with (divorces, debt, sick children and so on.) While the air force has added many "family support" services, the first sergeant is still getting hit first with these problems. And no matter what any support effort outside the squadron does, the first sergeant is still dealing with side effects (poor performance, absence, bad attitude) inside the squadron. The job is basically a lot harder, and there just aren't as many volunteers as their used to be. While being a first sergeant gains one a lot of prestige and respect, the price to be paid is seen as too high.
Air Force first sergeants are usually referred to as the "first shirt," or just "the shirt." NCOs who are thinking of becoming first sergeants will often first serve as an unofficial assistant to the first sergeant, and fill in for the first shirt when he (or she) is away (on leave or for training.) This apprentice first sergeant is often called the "under shirt." First Sergeants add a diamond to the center of their rank badge (a bunch of chevrons worn on the arm), but there's usually no doubt who the first shirt is, in or out of uniform. First Sergeants receive a little extra pay for the job, and have to attend a three week training course before they are assigned to a squadron.
The air force is going to have to increase the bonus for first sergeant duty, and provide more support services, before the shortage goes away. Meanwhile, in those squadrons that are left without a first shirt, one the senior NCOs is assigned the job, which never works out as well as it does when you have a trained volunteer. In the past, the air force has ordered senior NCOs to take first sergeant jobs, and found that didn't work out too well. It was better to take the effort to get volunteers, and let a senior NCO, of the unit without a first shirt, fill in temporarily.