I'll try to give you quick rundown.
Exercise(weight lifting and/or cardio work) is definitely beneficial. You don't have to go all out or anything, but atleast 3 days a week. The way you exercise will also depend upon factors like your age, and what kind of health your in currently. As always, you should ask a doctor if it's fine to exercise, but since pilots have medical examinations I'm assuming your in somewhat good shape, but you only know. Exercise can reduce risk of heart disease, diabetes, increase sense of well being, reduce LDL cholesterol, increase muscle mass and bone density, etc....... Also, another misconception of cardio is thinking of treadmills, and stairs. Just find something fun to do to stay in shape like a sport (tennis, basketball), bike-riding, swimming, or many other activities.
As you probably know, diet and exercise go hand in hand. Eating "clean" foods is always best, but not always accessible to pilots. Buying fresh foods from the market that you cook yourself is great and enables you to see what's going into your meal. Eating healthier foods can also reduce problems like high cholesterol, blood pressure, heart disease, and others. Since pilots are away often, they normally eat out a lot. Soooo, if you have to eat out try to maintain a more healthy life-style by picking foods at a resteraunt with a lower content of saturated fat. For example, isntead of ordering cheese mashed potatoes loaded with cheese, sour cream, and butter, try ordering a baked potatoe(becomes less healthy if you obviously load butter and sour cream to that as well). Instead of a hamburger that contains much more saturated fat from ground beef, eat a grilled chicken burger instead. You don't have to have completely boring meals just be aware of what your having. I have a burger or steak sometimes because I feel like it, but it's more of a once in awhile type thing for me. That's also one of the things about eating healthier. Most people just stop eating what they love to be more healthy. If you like cookies, have one or two not 6. As long as the calories are the same over the days time, you shouldn't fluctuate too much. There are good types of fat like Olive oil which is mono-unsaturated fat. As long is that is eaten in in moderation it's good to include in the diet. You need your carbs (for many purposes like ENERGY, and glycogen replenishment, etc) and protein also. The RDA for protein is .8g per kilogram of bodyweight. You will most likely need more if your active and weight training. Protein is vital in body functions. It's not just to repair muscle's. Protein is used for growth and repair of tissue, transporting, immune, fluid balance, etc.......
If your looking for more muscle mass weight lifting is key. You have to stimulate the muscle in order for it to grow. If you want to lower bodyfat weight lifting can help maintain the muscle while your on a lower calorie diet. It's simple to understand how the either gain, maintain, or loose weight. If you consume more calories through your diet than your body needs for energy and normal body function you will gain weight because your body will store the unused calories as fat. If you eat less than your body needs you will loose weight. I'm sure you can guess by now why you maintain weight, right?
If your goal is to loose weight you need to create a calorie deficit of some sort, whether it's from diet alone, or exercise. I recommend both. More fat will be burned through implenting excersise in a weight loss program compared to just creating the deficit solely by diet, and this is considering the same calorie shortage is equal for both sides. When I loose weight I normally go for a pound a week. If I do more than that I loose more muscle. I know people who shoot for 2 pounds a week, and that works for them. You don't want to sacrifice much muscle if any at all. The more muscle mass a person has, the more calories their body will burn.
There is roughly 3500 in a pound of fat. Now, if you wanted to loose a pound of fat per week, you must be create the deficit reachy 3500 calories over the 7 days. Let's say you can maintain weight on 2500 calories a day. If you eat 500 calories below maintenance calories, which means you would be eating 2000 calories per day. Since you are eating 500 calories less than your maintenace over seven day's that means you had 3500 calories less over the weeks time (500cal X 7 days=3500 calories). The same goes for gaining weight, but you would have extra calories in the diet instead of a deficit.
You probably know this as well, but that's alright. If you were wondering where the calories came from, it's the macronutrients. Carbohydrates have 4 calories per gram, protein has 4 calories per gram as well, and fat has 9 calories per gram. Say you consume 30 grams of protein one meal, 50 grams of carbs, and 15 grams of fat. This meal would consist of 455 calories (30g Protein X 4cal/per g= 120 calories; 50g Carbs X 4cal/per g= 200 calories; 15g Fat X 9= 135 calories. 120+200+135=455 total calories)
Since your a pilot, staying healthy is important because of the obvious reasons above, and more, but also for your career. It should be more of a life long commitment. The people that look for a short term fix normally gain the weight back quickly, and more than than they had before. This is most likely because they eat everything they didn't eat while on their diet, and overindulge. This is basically the tip of the iceberg, but it should help you get started. Do all the research you can, and educate yourself on exercise/nutrition.
Good luck, and sorry if the things I described were breif, but you could write books and books about exercise/nurition and all the different aspects involved. That's why there are so many out there, even though some aren't that good at all.