VATSIM Conference: Connexion 2017
29 & 30 July, London

By Bill Stack


Explain what IFR weather limits are and how they affect flight decisions.


At the end of this lesson the student will generally understand how to include weather conditions in IFR flight planning and what to do when changes in weather for changes in IFR flight activities.


Read these articles: 

  • Weather Minimums
  • VFR Limitations In Weather


There are no instrument weather minimums similar to visual weather minimums. There are, however, weather conditions that can make flight in instrument meteorological conditions (IMC) unwise and even dangerous. Pilot discretion is requisite. 

Aircraft Icing:

Icing is an accumulation of ice on an aircraft's surface that inhibits the aircraft's ability to be controlled and/or to remain airborne. Icing occurs most often in clouds and precipitation near the freezing level because it results from very cold water droplets striking subfreezing surfaces. Aircraft icing is not possible in warm air and is rare in severely cold air. 

Several types of icing occur, but none is simulated in today's home flight simulators. Realistic instrument flight should include consideration of aircraft icing even though the simulation is not possible.


Carburetor Icing:

Carburetor icing is an accumulation of ice inside the carburetor that reduces its ability to mix air with fuel. This accumulation can become heavy enough to block all air from entering the combustion chambers. The problem is corrected by engaging carburetor heat, and it can be prevented by engaging the heat whenever conditions that favor carburetor icing exist. Use of carburetor heat should be restricted to such conditions because carburetor heat diminishes the piston engines performance. Because jet engines lack carburetors, they do not experience this form of icing. The effects of carburetor icing are simulated in today's home flight simulators.

Snow and Ice: Snow storms and ice storms present aircraft icing hazards and low visibility aloft, as well as ground hazards at airports. Pilots are wise to avoid such weather and to use whatever anti-icing equipment is available to them if they cannot avoid these storms. They are also wise not to take off or land on runways that are not cleared of snow and ice. While snow and ice on runways are shown in today's home flight simulators, the effects of slick runways are not simulated. 

Low Visibility:

Airport visibility of a few hundred feet caused by clouds, fog, and heavy precipitation can be overcome by Category II and III pilots who are following Category II and III instrument approach procedures. These procedures are available only at certain runways, usually at busy commercial airports, and are shown on instrument approach charts. Their use requires advanced skills and certifications. Flight simmers are free to simulate these approaches whenever they wish.

Runway Visual Range:

Runway visual range (RVR) is a measurement of visibility down a runway. It is necessary for pilots taking off and landing because objects on the runway such as other aircraft or airport vehicles can appear when pilots and their fast-moving aircraft lack enough reaction time to avoid them. Approach charts usually specify safe RVRs for given runways, which pilots are wise to honor. Visibility reports from simulated ATC can be taken as RVR in lieu of actual RVR reports.


All pilots encounter turbulence whether flying visually or on instruments, but instrument pilots encounter it more often than visual pilots because they fly in foul weather and at high altitudes where turbulence is more common. Depending on severity, turbulence can be annoying, frightening, sickening, or dangerous to the aircraft and its occupants. For these reasons, pilots usually avoid turbulence by changing altitudes or staying clear of clouds and mountains where turbulence is more common. When flying prescribed instrument procedures, however, turbulence is difficult to avoid. Turbulence is simulated in home flight simulators.

Wind Shears:

Wind shears occur where winds of different directions and/or velocities meet. For example, when winds up to 10,000 feet are southwest at 20 knots and winds between 10,000 and 20,000 feet are northwest at 40 knots, a shear exists at 10,000 feet. Aircraft flying through shears experience jolts as the effects of winds on the aircraft change. Localized and more severe wind shears can exist around thunderstorms, which are explained below. Wind shears are simulated by today's home flight simulators.


Thunderstorms are sophisticated and severe weather systems that should be avoided by all aircraft because of potential danger from high winds, wind shears, severe turbulence, lightning, and heavy precipitation (including hail). The danger is not limited to inside the storm. Even air around thunderstorms is dangerously turbulent, and lightning strikes are always possible. Most thunderstorm hazards are simulated by today's home simulators, the most obvious exceptions being hail damage and lightning strikes.


Tornadoes are violently rotating columns of air in contact with ground and associated with thunderstorms. They are extremely destructive and are avoided by all sensible pilots. They are not simulated by today's home flight simulators, and there are no conditions under which any realistic flight simmer would want to simulate such dangerous flight, anyway.

Hurricanes and Typhoons:

Hurricanes and typhoons are huge cyclonic storms with heavy rains, strong winds, severe turbulence, thunderstorms, and tornadoes. They should be and are avoided by all sensible pilots. Those who penetrate them deliberately for weather data do so under strict procedures for their safety. These weather systems are not simulated in home flight simulators, but some of their components such as high wind, severe turbulence, heavy rain, and low visibility can be simulated.

Weather Alerts:

Real-world pilots can receive weather alerts from ATC, weather services, and/or other pilots. Severe weather reports sometimes carry recommendations against flying, which pilots are wise to follow. Weather reports from other pilots are called "Pireps". Many reports of conditions aloft are based on Pireps.


Flight simulation is more realistic when real-world weather conditions are simulated and applicable rules are followed. Conditions that cannot be simulated should be recognized as much as practical for the most realistic flight-simulation experiences.

The Author Bill Stack is an avid flight simmer, author of several books and magazine articles about flight simming, and president of TopSkills