by Bill Stack
Explain what VFR weather limitations are and how they affect flight decisions.
At the end of this lesson the student will generally understand how to include weather conditions in flight planning, how in-flight weather changes can affect a flight, and what to do when weather changes force the termination of a flight.
- Weather Minimums
AMSL = Above Mean Sea Level
AGL = Above Ground Level
ft = feet
IFR = Instrument Flight Rules
km = kilometres
m = meters
sm = statute miles
VFR = Visual Flight Rules
VMC = Visual Meteorological Conditions
Visual weather minimums are the lowest conditions under which visual flight is allowed by aviation regulations. These conditions are called "visual meteorological conditions" (VMC) and are officially defined as: "Meteorological conditions expressed in terms of visibility, distance from clouds, and ceilings equal to or better than specified minima". Aircraft may fly visually when conditions are at or better than these minimums and must fly on instruments or not at all when conditions are worse than these minimums.
Airspaces: Visual weather minimums apply in airspace classes B, C, D, E, and G. They do not apply in Class A airspace because VFR flights are not allowed. All flights in Class A airspace are conducted under instrument flight rules (IFR) per aviation regulations.
Types of Flight: Visual weather minimums apply to flights conducted under VFR. They do not apply to flights conducted under IFR. Most general-aviation flights are VFR whereas most commercial flights are IFR, per regulations and insurance requirements.
Visibility: For visual flight below 10,000ft AMSL, visibility must be at least 3sm (5km). When visibility is less than the required minimum, aircraft may not take off under visual flight rules (VFR). The pilot must either take off under IFR, delay until the required visibility exists, or not take off at all.
Once airborne, a VFR aircraft must change altitude, divert or land if visibility falls below the specified minimum. A VFR aircraft may not land visually at an airport where visibility is less than 3sm (5km) unless in emergency or due stress of weather.
Although 3sm (5km) seems like plenty of visibility, it isn't much for an aircraft flying at 150+ knots or approaching an airport at 100+ knots. There just isn't enough time for pilot and their aircraft to react to objects that suddenly appear through precipitation or fog.
For visual flight at or above 10,000ft AMSL, visibility must be at least 5sm (8km). The greater visibility requirement is because higher altitudes are associated with aircraft that have higher speeds.
Distance from Clouds: Clouds restrict visibility quite severely therefore visual flights not are allowed inside cloud. Because clouds continually change shape and size, visual flights are not allowed near clouds. Aircraft flying visually must stay out of clouds and at least 500ft below, 1,000ft above, and 2,000ft aside of clouds.
It's important to recognize that not even the world's most experienced and adept pilots know for certain when their aircraft is within these minima. No human being can judge 2,000ft, 1,000ft, or 500ft above, below, or near anything with any accuracy. Therefore, these minima are safety guidelines that should be honored to the pilot's best judgment and ability. A pilot interested in safe flight will apply a safety margin in excess of the minimum stated in the rules. Stay away from and out of clouds or bad weather whenever flying visually.
The following table shows typical VMC Minima.
- IFR Limitations In Weather
As an overview of weather effects on flight simulations, these articles are written in generalized terms, refer to other sources, and are therefore not intended to be complete or comprehensive explanations of the subject. They are for flight simulation only and are not intended for use in real-world aviation.
Bill Stack is an avid flight simmer, author of several books and magazine articles about flight simming, and president of TopSkills.