When you first you get your private pilot certificate, you are allowed to fly only when visibility is good. The specific requirements depend a bit on what type of airspace you are in, but generally you are required to avoid clouds by a safe margin and can't fly in fog. You can fly at night, so long as you follow the same rules for clouds and fog.
Why are private pilots not allowed to fly through clouds? In case it's not obvious, there's no way to avoid other aircraft (or tall obstacles) if you can't see them. In addition, when you can't see the horizon it's easy to get disoriented while flying and you can quickly lose control of the plane. In essence, you need two things in order to safely fly through clouds:
- Special training on how to maintain control of the plane even in turbulent low-visibility conditions
- An eye in the sky selecting your route for you to de-conflict it with other aircraft and also keep you away from hazards like mountains, embedded (hidden) thunderstorms, etc.
That second item also requires special training. In order to obtain your instruments rating, you need roughly the same number of flight hours of training as you did to originally obtain your private pilot certificate. That is what I've been doing for most of this year.
In order to earn my instruments rating, I still need to complete the following:
- One cross country flight of at least 250 nautical miles in total length, with stops at two airports (plus the home airport I return to), one leg of the flight being greater than 100 nautical miles straight line distance, and a different instrument approach at each of the three airports
- End of course check (administered by a senior instructor at my school)
- Instrument written exam (computer based test administered by the FAA)
- Oral Exam and Practical Exam (with a designated pilot examiner)
I've met all of the other flight hour requirements at this point. In case you were wondering, an instrument approach is a procedure used to line yourself up with the runway and safely descend while visibility is zero. A large part of instrument training is learning how to use radio beacons and/or GPS to approach runways when visibility is poor.
So let's say I have my instrument rating... how do I fly through the clouds? In order to enter low-visibility conditions while flying, you need to meet the following criteria:
- Have an instruments rating
- Be "current," meaning you've practiced instrument techniques within the past 6 months in accordance with the minimums set by the Federal Aviation Administration
- File an instruments flight plan and obtain a clearance from Air Traffic Control
- Follow Air Traffic Control's instructions...you can't just fly wherever you want whenever you want if you are entering low-visibility conditions
So what has most of my training entailed? Basically, for most flights, we take off, I put on a view limiting device ("hood" or "foggles"), and my instructor acts as my safety pilot. This means he is looking around for other aircraft and tells me if we need to deviate. The view limiting device simulates that we are flying in clouds or fog. Occasionally, we actually do fly through the clouds, and my instructor advises me to take off my hood/foggles whenever we are actually in low-visibility conditions... no need for the view limiting device then!
When you fly on an airline flight, usually (if not always) the pilots are flying on an instrument flight plan. They indicate the destination in their flight plan, and Air Traffic Control routes them accordingly. As a private pilot, even with an instrument rating, I can choose whether or not to file an instruments flight plan when the weather is clear. However, when the weather is marginal (cloudy, but not too cloudy, or foggy, but not too foggy), most private pilots will be stuck on the ground, and I'll be flying through the clouds.