No likely any one on one combat. If you deploy to Afganistan, he will be with a Marine Squadron which is part of a Marine Air Group. You could be assigned perimeter security duty or guard duty on the flight line to protect the aircraft from direct attack. The likelihood of your engaging an enemy is relatively small, but not zero.
It is likely that you will have to deal with incoming rockets, RPGs (depending on how close his unit is to the perimeter), mortars, and possibly artillery. There should be bunkers in your area to jump into if necessary. Air bases are high target areas for an enemy force.
What I described above is what I did in the Vietnam days as an aviation MOS Marine. As I climbed up the ranks to Lieutenant Colonel, my responsibilities were expanded to consider the protection of my aviation assets. The infantry units assigned to keep insurgents away from the perimeter often must be used elsewhere. Hence, why Marine units must consider their own self-defense in case of a breakthrough.
Hopefully, you will never be in a position where you have to use your rifle, but if required, you will be trained to do so. Pay attention to everything on the rifle range and learn small unit tactics from your Marine Corps guide book and in practice at Marine Combat Training (never know when it will be needed).
Semper Fi,
Lieutenant Colonel, US Marine Corps-Retired (previous Marine Corps aviator and aircraft maintenance officer/aviation supply officer)
PS: My youngest brother is a National Guard Avionics Tech in Iraq. So far, they have only been hit with rockets and mortars three times in past 6 months. Afganistan is different, it is not as secure as Iraq is now. Based on the President's statement today, you have a high chance of being deployed to Afganistan sometime in your four year hitch.
EDIT: Amber is somewhat correct. Depends on your MOS, and time in the MOS whether you will be "allowed" to be a "crew chief" on a Marine chopper (a voluntary position). Usually, flight line personnel, guys who prepare a chopper for flight, are allowed to be crew chiefs. If you are in maintenance control, or assigned to the engine shop (engine mechanic), you will have a slim chance of being assigned as a crew chief. My engine mechs and other speciality trained Marines were in high demand due to the large number of flight hours flown - hence we could not afford to loose them to flight duties. Hence, not likely will see one to one combat with insurgents. The above still applies.
As she noted for fixed wing, not likely you would fly as a engine mech, but if you are a flight line Marine supporting KC-130s and were at least a Corporal, you could be assigned as a member of the aircrew (crew chief on a KC-130 is usually at least a Staff Sergeant).
EDIT: Devil Dawg - you are right. During Vietnam anyone who volunteered for flight skins could fly door gunner. I was in a CH-34 H&MS-17 squadron at DaNang (Cpl at the time, made Sgt after left country). I think the issue is what type of "mech" is the question focused on. Anyone in a squadron can volunteer for skins. The squadrons that I was associated with at MCAS Tustin allowed anyone in the helo squadrons to apply for fiight skins. However, the first focus of the MaintO was getting all aircraft OpReady to meet the flight schedule.
If you re-read my comment, my answer was related to one-on-one combat - Marine to insurgent. Not likely to happen with an aviation MOS (unless one is a door gunner - even then it is air-to-ground). I adjusted this for Amber's response based on who is defining "aviation mechanic". Every Marine in maintenance is a "mechanic" to include those assigned to flight line and those who are more specialized such as the engine shop, avionics shop, commnav, metal smith, and even the staff NCOs running Maintenance Control. Yeah, you have maint admin who maintain the aircraft log books - they are not normally mechs, but could be. In my previous MaintO job, a mech to me was an engine mech. Maybe it is the difference in interpretation between the Officer and the Enlisted guy. But I suspect we are really speaking the same lanuage, but understanding it differently. For me, it is the guy (s) in the engine shop who I never had enough who were fully trained and who removed and replaced the aircraft engines.
Many are on skins, but their first priority is getting the birds OpReady. If the MaintO could spare them, then they could be put on the flight schedule. In today's Marine Corps, there are not a lot of engine mechs in the squadrons with lots of experience, but there is a hugh workload. The MaintOs have to balance their aircraft repair requirements with the desire of their engine mechs to fly.
BTW: my response to Amber didn't focus on door gunners. It addressed crew chiefs. Most of the Marines selected for crew chief came off the flight line crew. Like you said, most have to be at least a Corporal or Sergeant (experience). A few sharp ones, as you mentioned, are LCpls. In today's world, the engine mechs are hard to come by, especially for the MV-22, 46s, and 53s. These birds are harder to repair and keep Op Ready than the UH-1 or AH-1. When I left active duty, the Marine Corps engine spares for the 46 was less than 50% in the Aviation Supply stocks. Not much better for the 53. The MV-22, as a new bird in the inventory, doesn't a full complement of spares either. What this means is, more inhouse engine maintenance time. This has not changed even though there is a war on. It is worse with the MV-22. The burden falls on the squadron MaintO and his engine shop (as well as the IMA MaintO) to do more inhouse repairs instead of sending the engines to the depot (usually the NARF) and who knows when they are shipped back! (for the MV-22, hydraulics is a challenge also).
As you know, because you were there once, the work load in a combat zone changes the equation, not for the good, but for the worse.
Back to your comment: If you consider a "aviation mech" to be a general flight line MOS, then you are 100% correct in your criticism. If that mech is a highly trained engine mech which most squadrons have few of, I stick by my comments. The ability to provide Op Ready combat aircraft is number 1 priority for the squadron's maintenance officer and the OPs O. Grounded aircraft due to supply or maint (NORS/M) isn't what the boss wants to hear!
If anything, I hope that we have agreed to mutually disagree.
Semper Fi to you. Who knows, we may have crossed paths sometime over the years. Besides, after thinking about it, this dialog is very beneficial to those who are thinking about a Marine Corps aviation MOS and reads our dialog. Gives them a feel for what aircraft maintenance may be like - fixed wing and rotary wing. I'm glad that you brought this issue up. I'm only in this forum to offer, what I hope is, helpful and useful comments to the newbies. Based on what I have seen in your answers, I know that you are here for the same reason. Challenge me anytime if it looks like I have strayed or provided too short an answer.