Instrument Meteorological Conditions

Instrument meteorological conditions (IMC) exist when an aircraft is in weather conditions that are not within the VFR visibility and cloud clearance requirements.

This means that you are in IMC if:
• You are inside of a cloud
• The visibility is too low
• You are too close to a cloud

When you are in IMC you need to be flying an instrument flight plan. Flying VFR into IMC is very dangerous and there are a multitude of accidents that occur this way. Don’t be a statistic! Stay away from the clouds.

In fact, I recommend having personal minimums for visibility and cloud clearance that are even more restrictive than the rules.

Visual Flight Rules

Visual Flight Rules, commonly called VFR, are a set of rules that apply to planes flying by visual reference. This is in opposition to IFR (Instrument Flight Rules), flown without visual references.

This set of rules allows a pilot to fly in most airspace without a flight plan, clearance, or ATC communication.

VFR flying requires constant vigilance for other traffic and reliance on oneself for navigation.

When you are flying VFR you must be in VMC.

Of course, while flying VFR you may choose to use air traffic control services to:

  • Get help in an emergency
  • Get directions
  • Get traffic advisories
  • Get clearance through airspace or to land

How do I know if I’m flying VFR?

Did you file and open an instrument flight plan? If not, then you are flying VFR. You must comply with all VFR regulations including visibility and cloud clearance minimums.

Visual Meteorological Conditions

Visual Meteorological Conditions, or VMC, are a set of weather conditions that allow for flying VFR.

Generally, VMC means good weather, but it also technically means that you are far enough between the clouds to allow for safe VFR flying.

For example, when flying IFR you might be popping in and out of clouds every few minutes.

If ATC notifies you about traffic then you must try to find the other plane. However, this only applies if you are in visual conditions at that moment. If you happen to be inside of a cloud you just respond with “IMC”.

In addition, it is also possible that you are near clouds and you can’t see the other plane because he is in a cloud. I will always take a quick look but if there is a chance that I can’t see the other plane because there is a large cloud right where he should be, I will still say “IMC”.

I am letting the controller know that the conditions where I am flying are not sufficient for VFR, even though I may be outside of a cloud at that time.

How do you know if you are in VMC?

I’ve devised a set of easy rules to help you remember the cloud clearance and visibility requirements for VMC.


Do you ever resent the FAA for making so many rules? This is an anti-authority attitude. It can be very hazardous because it can lead to poor decision making out of spite.

It’s ok to question the FAA about their many rules and feel free to even publicly denounce their many rules…..but only while on the ground. When you are in the air, you must fight against your anti-authority tendencies and remember this:

The rules are written in blood!

Most of the rules are there because somebody died doing something that was legal at the time. The rules don’t guarantee your safety but they do provide a framework of general safety limits.

Signs of an Anti-Authority Attitude

If you find yourself thinking some of the thoughts below (in bold) then you are experiencing anti-authority to some degree.

  • “Don’t tell me what to do.” The rules are usually telling you what to avoid, and while this may be inconvenient, there is a usually a pretty good reason. Follow the rule for now and find out why that rule exists after you land.
  • “This is a stupid rule.” It very well may be a stupid rule but professionalism and strict adherence to rules and procedures greatly enhance your survival chances.
  • “These rules don’t apply to me because I’m a better pilot than those who died.” Incorrect, you are a worse pilot than those who died in many ways. For example, you are letting yourself succumb to a hazardous anti-authority attitude. Those pilots who died before this rule existed were significantly more professional than you are being right now. Put down your pride and be safe.

Anti-authority goes hand-in-hand with invulnerability and is particularly dangerous because it leads to some of the most dangerous activities. Pilots who fly VFR into IMC or break up the plane doing unscheduled aerobatics usually suffer from both of these two delusions.

The rules and safety

Are the rules safe enough? No.

As pilots, we need to have personal minimums that are more restrictive than the rules. This is a personal decision and it will be different for everyone. For example, you only need 1 statute mile of visibility to fly in class G airspace, during the day, under 10,000 feet.

However, sticking to a higher minimum like 3 miles is probably a good idea.

Have you ever heard the phrase “8 hours bottle to throttle”? It means that you need to leave 8 hours time between drinking alcohol and flying. A better personal minimum that many use is “24 hours bottle to throttle”.

Be well aware of this attitude and decide in advance that you will be a professional pilot who follows the rules, even if you disagree with them.


Haze Layer

The air on our planet is full of particles of moisture. These are often visible as haze, usually in calm air or on humid days. Did you know that on many days, when the air is stable, you can get above the haze and the visibility goes way up?

Click on the picture below to view it full size. You can see the haze extending to the horizon and up to a clear “haze line”. Above this line the visibility is much higher. It will be easier to see other planes at this altitude.

Haze Layer – click to view full size

Calculating the top of the haze layer

The haze layer will generally be present in stable air where the temperature is above the dewpoint. At an altitude above the haze layer, the temperature will be below the dewpoint, so any moisture in the air will condense into droplets of mist of rain. Then gravity will pull these droplets down until they are back in the haze layer where they will turn back into water vapor (haze).

So the simplest way to find the top of the haze layer is to determine where the temperature and dewpoint will match.

The air is colder at altitude, but do you know how much colder? There is something called the adiabatic lapse rate, which is the rate at which the temperature drops as you climb.

It varies depending on the moisture of the air but as a rule of thumb just assume it is 2 degrees Celsius per 1000 feet of altitude. In dry air, it may be as high as 3 degrees Celsius.

So when you listen to the AWOS at sea level and it reports “temperature 12, dewpoint 6” then you can figure out the approximate top of the haze layer. That is a 6-degree difference, so we will need to climb 3000 feet.

This quick rule of thumb will let you make the haze layer calculation very quickly, just find the difference and divide it by 2 to get the altitude of the haze line.

for example:

Temperature: 10C, Dewpoint: 5C = Top of Haze Layer: 2500 feet

Temperature: 15C, Dewpoint 13C = Top of Haze Layer: 1000 feet

The next time you are flying start looking for the haze layer. It isn’t there every time, but it is there often enough (depending on where you fly).

Never Be 100% Sure

When I was a student pilot I had to fly a night cross-country flight with my instructor. I carefully planned the route and filled out my navlog. We took off in a Cessna 152 and proceeded to the destination, Carol County Airport in Maryland. About halfway through the flight, I was able to see the rotating beacon in the distance and I continued towards it.

I was 100% sure I had found my destination. I descended towards the airport and entered the pattern. As I landed I noticed that the runway number was wrong! This meant that the facility directory must not have been up to date. I taxied off the runway and then my instructor gave me the news. I had landed at York airport, in Pennsylvania. These two airports are 20 miles apart!

If I had not been 100% sure I would still have been evaluating the situation as I flew. That is why I will never tell you that I am more than 99% sure. This is a safeguard to ensure that I keep thinking and taking in new information to find the truth.

A huge 1% difference

My choice to be 100% sure meant that the new information (wrong runway number) was immediately treated as wrong! But if you take the 99% sure attitude, then you will treat new information as the truth and constantly reevaluate what you are seeing. If I had been 99% sure then I would not have put down my map and navlog. Instead, I would have seen rivers and cities and roads all in the wrong place and figured out where I really was.


An F-15 pilot was departing Elmendorf Air Force Base on a stormy night with a formation of 8 total F-15s. He realized he had an instrument problem and was moments from crashing. This led him to a calm feeling as he sat and awaited his fate.

Resignation means giving up. This hazardous attitude is one of defeat and pessimism. It can turn a dangerous situation into a deadly situation.

Signs of Resignation

It’s not too hard to spot resignation if you know what to look for. If you find yourself thinking some of the thoughts below (in bold) then you are experiencing resignation to some degree.

  • “I can’t fix this situation.” Wrong! You have the controls. You can absolutely change your situation at any time.
  • “This is just bad luck.” Wrong! In aviation, there is no luck, you make your own destiny. Take action.
  • “Someone else will make the decision.” No, you are the pilot in command, you are the one making the decision, even if you try to let someone else do it.

Don’t resign yourself to resignation

When you realize you are suffering from resignation, don’t resign yourself to your fate… something. You can always make things more positive for yourself by continuing to work at it. You are not helpless.

What happened to that F-15 pilot? He realized that he still had a chance to save himself, and he did! Read the full article about the F-15 incident here.

Class B Airspace

Class B airspace surrounds large, busy airports.

Its shape is generally an upside-down wedding cake but it can vary quite a bit. The main idea is that there are layers that get progressively wider and have a higher floor than inner layers.

For example, in St. Louis (pictured below) the inner ring extends from the surface up to 8000 feet MSL.

The next ring out extends from 2000 feet up to 8000, so there is room to fly under it. If you look closely you will see that the floor actually starts at 1700 in some places and 2500 in other places within this ring.

Going out further the next ring has a floor at 3000 and the larger ring has a 5000 foot floor (although it starts at 3500 feet in some sections).

There are also some extensions of this airspace that stick out of the normal area with a floor of 4500 feet. These are generally for the approach and departure paths into and out of Lambert International. They are shaped differently from the rest of the airspace but they still operate as class B.

Source: – St. Louis Class B

What is required to enter class B airspace?

VFR Aircraft must be “cleared into the bravo” before entering class B airspace.

When I get a clearance into bravo airspace I always read back “cleared into the bravo” in a slow, clear, enunciated way to make sure that the controller hears me correctly, and that it is picked up on the recording in case there is some claim of a misunderstanding later.

In addition, if you are within 30 miles of the center of class B airspace you need to have your transponder on and in altitude reporting mode. This is true even if you do not intend to actually enter the airspace.

What are the weather requirements in class B airspace?

In class B airspace you must remain clear of clouds. This is the least restrictive VFR requirement and allows you to get as close to the clouds as you want as long as you don’t fly into them. In class B airspace all aircraft are under positive ATC control and there is little risk of a plane appearing out of a cloud next to you. However, you should always be vigilant anyway because ATC is manned by humans who could possibly make a mistake.

The visibility must be 3 statute miles or greater.



Cruising Altitude

When flying anywhere you need to climb, cruise, and then descend. But you must comply with the rules and fly at certain predetermined altitudes.

The Rules: Neodd and Sweven

When you are flying above 3000 feet AGL you must fly at an even-numbered thousand feet if you are traveling west. That is what sweven means. If you are traveling directly South, or on any Westerly heading then fly on the even-numbered thousands.

Conversely, if you are traveling East or directly North, fly at an odd thousand (neodd).

For IFR flights to the East you will fly at 5000 or 7000 or 9000 feet, etc….

For VFR flights you must be 500 feet above these altitudes. So a VFR flight to the West would cruise at 4500 or 6500 or 8500 feet, etc….

These rules are for cruising altitude, meaning that if you are flying up above 3000 to practice an emergency descent then you can just climb to whatever altitude you want.

Why is this rule in place?

This rule is a bit of a compromise between safety and simplicity.

When two planes approach head-on their closure speed, the speed at which they are approaching each other, is very high. Even a relatively slow 152 will approach another 152 at around 180 knots TAS if they are head-on. For faster planes like an arrow traveling at 150 knots the closure speed is 300 knots!

It is safer to fly at these altitudes because VFR planes flying East will always be 1000 feet vertically separated from VFR planes flying West. Furthermore, they will be separated by 500 feet from all IFR traffic.

This is great, but there is a problem! What if planes approach nearly head-on but both traveling East? One of them could be traveling 010 degrees (which is East of North) and the other could be flying at 170 (which is East of South).

This is where the compromise comes in. The rule could split the compass into 4 segments but then it would be more complicated and difficult to remember.

Always stay vigilant looking for traffic that might be climbing/descending, not following the rules, or might be at a near head-on angle.

Are you on a Collision Course?

When an aircraft approaches you head-on, you are on a collision course. When you are following directly behind a slower plane, you are on a collision course. In these cases, it’s easy to tell that you are on this collision course and you can quickly decide to correct by turning to the right.

But what if another plane approaches at an angle? How can you quickly determine if you will collide or not?

Luckily there is an easy way to figure this out immediately!

Ask yourself,  is the other plane moving across my window? Look at the other plane and which part of your window it is in.

  • If it is not moving then you are on a collision course
  • If it is moving forwards then it will pass in front of you
  • If it is moving backward then it will pass behind you

Note: “Forwards” means that the plane appears to be traveling across your windscreen in its forward direction. “Backwards” means that the plane is moving across your windscreen towards its tail (although in reality it is obviously still going forwards).

You must decide in each situation how to handle it. A good rule of thumb I use is to give way and pass behind the other plane if it only requires a small course correction. This works in most cases and it tends to be safer because the pilot in the other plane may not see you.