CFI Brief: VFR vs. VMC

An instructor and student are preparing for an early afternoon flight to conduct maneuvers out in the practice area. The student receives a weather briefing and the instructor asks how the weather conditions are for the flight. The student responds that the weather looks to be VFR. OK, that’s great, but remember, the instructor’s specific question was in regards to how the weather conditions were for the flight. A more appropriate response would have been, weather is currently VMC. What’s the difference you might ask?

Visual flight rules (VFR) are just that, a set of rules adopted by the FAA to govern aircraft flight when the pilot has visual reference. These were the rules outlined in Monday’s post. So when the student initially answered the instructor he stated that the weather was such that they would be able to operate under these prescribed set of rules for the training flight.

On the other hand, visual meteorological conditions (VMC) are expressed in terms of visibility, distance from clouds, and ceiling meeting or exceeding the minimums specified by VFR. To operate under VFR you must have VMC. Stating that the weather was VMC followed up by the current conditions would have been a much better initial response from the student.

For example, the same student checks a METAR for a Class D airport:

201353Z 00000KT 4SM HZ CLR 16/03 A2996 RMK AO2 SLP140 T01610028

Per 14 CFR §91.155 and AIM ¶3-1-4, VFR weather minimums for class D airspace require 3 SM visibility, 500 feet below, 1,000 feet above, and 2,000 feet horizontal distance from clouds. According to this METAR, VMC exists at this airport because we have 4 SM visibility which is more than the prescribed 3 SM and the sky is clear of clouds. So in this particular situation we can operate under VFR because VMC is present.

Understanding your weather briefing relative to VMC rather than VFR will also help you apply your personal limitations. VMC is required to maintain VFR, but it may not be enough to meet your personal limitations established for the circumstances specific to today’s flight. Personal limitations change with experience so there is no set of numbers you can memorize to make your go / no-go decision: you must evaluate the weather relative to today’s personal limits—it’s a unique evaluation every flight.

In this particular example, yes, visual meteorological conditions do exist, but do those conditions meet your personal minimums as a student pilot? That’s a decision that you as the pilot in command will need to make. You will need to ask yourself: is 1 SM above the prescribed VFR minimums is satisfactory for me to complete today’s training flight? Only you can make that decision.

Below is a five question practice exam on VFR weather minimums. Answers and explanations are included below.

1. Normal VFR operations in Class D airspace with an operating control tower require the ceiling and visibility to be at least
A—1,000 feet and 1 mile.
B—1,000 feet and 3 miles.
C—2,500 feet and 3 miles.

2. What minimum visibility and clearance from clouds are required for VFR operations in Class G airspace at 700 feet AGL or below during daylight hours?
A—1 mile visibility and clear of clouds.
B—1 mile visibility, 500 feet below, 1,000 feet above, and 2,000 feet horizontal clearance from clouds.
C—3 miles visibility and clear of clouds.

3. The minimum distance from clouds required for VFR operations on an airway below 10,000 feet MSL is
A—remain clear of clouds.
B—500 feet below, 1,000 feet above, and 2,000 feet horizontally.
C—500 feet above, 1,000 feet below, and 2,000 feet horizontally.

4. A special VFR clearance authorizes the pilot of an aircraft to operate VFR while within Class D airspace when the visibility is
A—less than 1 mile and the ceiling is less than 1,000 feet.
B—at least 1 mile and the aircraft can remain clear of clouds.
C—at least 3 miles and the aircraft can remain clear of clouds.

5. Which VFR cruising altitude is appropriate when flying above 3,000 feet AGL on a magnetic course of 185°?
A—4,000 feet.
B—4,500 feet.
C—5,000 feet.

Click here for the VFR quiz answers.

Just because I know how much everyone loves standardized testing I will leave you with this bonus analogy question. First one to answer correctly in the comments section I will send an Oral Exam Guide of your choosing.

VFR : IFR :: VMC : (____)

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Regulations: VFR Minimums

This week we’ll expand on what we’ve discussed about regulations. Chapter 19 in the brand-new fourth edition of The Pilot’s Manual Volume 2: Ground School has a great overview of the FARs you’ll need to know. Beyond that, of course, is our FAR/AIM; the complete up-to-date set of aviation regulations. Today, we’ll feature an excerpt from the fourth edition of the PM-2C.

Basic VFR Weather Minimums
The basic weather minimums required for you to fly VFR are stated in terms of flight visibility and distance from clouds (horizontally and vertically). For VFR operations within Class B, C, D and E surface areas around airports with an operating control tower, you require:

  • cloud ceiling at least 1,000 feet AGL; and
  • ground visibility at least 3 statute miles (usually measured by ATC but, if not available, flight visibility at least 3 statute miles as estimated by the pilot).

This can seem very confusing and not just to the beginning pilot. Yet it really is pretty simple for most general aviation pilots, because below 10,000 feet, the following rules comply with all airspace ceiling and visibility requirements—maintain 3 SM visibility:

  • 500 feet below clouds;
  • 1,000 feet above clouds; and
  • at least 2,000 feet lateral separation from the clouds.

The requirements are slightly less restrictive in Class G airspace, with a less restrictive daytime visibility below 10,000 feet MSL (1 statute mile only) and, below 1,200 feet AGL by day a less-restrictive separation from clouds (clear of clouds, with no distance-from-cloud requirements). In Class B airspace aircraft are required to remain clear of clouds. In Class C, D, E and at night, Class G airspace, aircraft are required to maintain a minimum distance of 1,000 feet above, 500 feet below and 2,000 feet horizontal from clouds. Also, in Class G airspace, when the visibility is less than 3 statute miles but not less than 1 statute mile during night hours, an airplane may be operated clear of clouds if operated in an airport traffic pattern within one-half mile of the runway.

Special VFR Weather Minimums
A pilot operating below 10,000 feet MSL in or above the airspace designated on the surface for an airport may be issued an ATC clearance to operate under special VFR, which reduces the normal requirements down to:

  • flight visibility 1 statute mile (and ground visibility 1 statute mile for takeoff and landing); and
  • clear of clouds.

To take off or land at any airport in Class B, C, D and E airspace under special VFR, the ground visibility at the airport must be at least 1 statute mile. If ground visibility is not reported, then the flight visibility during takeoff or landing must be at least 1 statute mile. A noninstrument-rated pilot may be issued a special VFR clearance by day but, to operate under special VFR at night, you must be instrumentrated, instrument-current and flying in an IFR-equipped airplane.
ATC may issue a special VFR clearance

Airports in Class B, C or D airspace have a control tower from which you can request a special-VFR clearance. Airports in Class E airspace do not have a control tower, but your request for special VFR can be relayed via Flight Service Station to the ATC facility responsible for that Class E airspace (only ATC, and not a Flight Service Station, can issue an ATC clearance, although a Flight Service Station may relay it to you). Special VFR is prohibited at some airports (see 14 CFR, Part 91).

VFR Cruise Altitude or Flight Level
VFR cruise altitudes or flight levels, when more than 3,000 feet AGL, are:

  • on a magnetic course of magnetic north to magnetic 179: odds+500 feet—for example, 3,500 feet MSL, 15,500 feet MSL; and
  • on a magnetic course of magnetic 180 to magnetic 359: evens+500 feet—for example, 4,500 feet MSL, 16,500 feet MSL.

(You can memorize this as “West Evens, East Odds, plus 500 feet,” or “WEEO+500.”)
VFR cruise altitudes and flight levels above 3,000 feet AGL.

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CFI Brief: What a Drag!

Drag as it relates to aerodynamics in aviation is just one of those things that must be dealt with and overcome – literally  overcome. When we talk about drag in aviation it is usually discussed in relation to one of the four forces: lift, weight, thrust, and drag. It is the force that acts opposite to thrust, or opposite the direction of the aircraft flightpath. Aerodynamics for Aviators defines drag as the force that retards forward motion of an aircraft through the air. Drag as an aerodynamic parameter can be broken down into two basic types, parasite drag and induced drag.

Parasite Drag is the more complicated of the two and includes all the forces that work to slow an aircraft’s movement. It’s the difference between throwing a square object through the air compared to a circular object. Which of the two would generate a higher parasite drag? If you said the square object you would be correct. Simply put parasite drag is caused by the friction of air moving over the surface of the aircraft, or the displacement of air by the aircraft. There are many types of parasite drag; however, in aviation we typically discuss the three most common: form drag, interference drag, and skin friction drag. The Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B) does a great job at describing these.

  • Form drag is the portion of parasite drag generated by the aircraft due to its shape and airflow around it. Examples include the engine cowlings, antennas, and the aerodynamic shape of other components. When the air has to separate to move around a moving aircraft and its components, it eventually rejoins after passing the body. How quickly and smoothly it rejoins is representative of the resistance that it creates, which requires additional force to overcome. Form drag is the easiest to reduce when designing an aircraft. The solution is to streamline as many of the parts as possible.
  • Interference drag comes from the intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow. For example, the intersection of the wing and the fuselage at the wing root has significant interference drag. Air flowing around the fuselage collides with air flowing over the wing, merging into a current of air different from the two original currents. The most interference drag is observed when two surfaces meet at perpendicular angles. Fairings are used to reduce this tendency. If a jet fighter carries two identical wing tanks, the overall drag is greater than the sum of the individual tanks because both of these create and generate interference drag. Fairings and distance between lifting surfaces and external components (such as radar antennas hung from wings) reduce interference drag.
  • Skin friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft. Every surface, no matter how apparently smooth, has a rough, ragged surface when viewed under a microscope. The air molecules, which come in direct contact with the surface of the wing, are virtually motionless. Each layer of molecules above the surface moves slightly faster until the molecules are moving at the velocity of the air moving around the aircraft. This speed is called the free-stream velocity. The area between the wing and the free-stream velocity level is about as wide as a playing card and is called the boundary layer. At the top of the boundary layer, the molecules increase velocity and move at the same speed as the molecules outside the boundary layer. The actual speed at which the molecules move depends upon the shape of the wing, the viscosity (stickiness) of the air through which the wing or airfoil is moving, and its compressibility (how much it can be compacted).

The airflow outside of the boundary layer reacts to the shape of the edge of the boundary layer just as it would to the physical surface of an object. The boundary layer gives any object an “effective” shape that is usually slightly different from the physical shape. The boundary layer may also separate from the body, thus creating an effective shape much different from the physical shape of the object. This change in the physical shape of the boundary layer causes a dramatic decrease in lift and an increase in drag. When this happens, the airfoil has stalled.

In order to reduce the effect of skin friction drag, aircraft designers utilize flush mount rivets and remove any irregularities that may protrude above the wing surface. In addition, a smooth and glossy finish aids in transition of air across the surface of the wing. Since dirt on an aircraft disrupts the free flow of air and increases drag, keep the surfaces of an aircraft clean and waxed.

Induced Drag in a sense is a little more straight forward. It is a byproduct of lift directly proportional to the airspeed of the aircraft. As airspeed slows, a greater angle of attack is needed to create lift increasing the aircraft’s induced drag. When airspeed is increased, the wing is able to produce the same amount of lift at a shallower angle of attack decreasing the amount of induced drag. This type of drag is a result of wing circulation, high pressure below the wing moving to an area of lower pressure above the wing. You can learn more about wing circulation and induced drag in the second edition of Aerodynamics for Aviators.

The sum of parasite drag and induced drag is called total drag and is commonly expressed as shown in the figure below. Assuming constants like level flight, weight, configuration and altitude when plotted against velocity (airspeed), parasite drag increase while induced drag decreases with an increase in velocity.


What type of drag is associated with removing the wheel fairings from the landing gear of a Cessna 172? Let us know in the comments section.


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Aerodynamics: Forces in Climbs and Descents

We’re talking about aerodynamics again this week. Today, an excerpt from the Pilot’s Handbook of Aeronautical Knowledge on the forces in climbs and descents.

Forces in Climbs
For all practical purposes, the wing’s lift in a steady state normal climb is the same as it is in a steady level flight at the same airspeed. Although the aircraft’s flight path changed when the climb was established, the angle of attack (AOA) of the wing with respect to the inclined flight path reverts to practically the same values, as does the lift. There is an initial momentary change as shown in the figure below. During the transition from straight-and-level flight to a climb, a change in lift occurs when back elevator pressure is first applied. Raising the aircraft’s nose increases the AOA and momentarily increases the lift. Lift at this moment is now greater than weight and starts the aircraft climbing. After the flight path is stabilized on the upward incline, the AOA and lift again revert to about the level flight values.


If the climb is entered with no change in power setting, the airspeed gradually diminishes because the thrust required to maintain a given airspeed in level flight is insufficient to maintain the same airspeed in a climb. When the flight path is inclined upward, a component of the aircraft’s weight acts in the same direction as, and parallel to, the total drag of the aircraft, thereby increasing the total effective drag. Consequently, the total effective drag is greater than the power, and the airspeed decreases. The reduction in airspeed gradually results in a corresponding decrease in drag until the total drag (including the component of weight acting in the same direction) equals the thrust. Due to momentum, the change in airspeed is gradual, varying considerably with differences in aircraft size, weight, total drag, and other factors. Consequently, the total effective drag is greater than the thrust, and the airspeed decreases.


Generally, the forces of thrust and drag, and lift and weight, again become balanced when the airspeed stabilizes but at a value lower than in straight-and-level flight at the same power setting. Since the aircraft’s weight is acting not only downward but rearward with drag while in a climb, additional power is required to maintain the same airspeed as in level flight. The amount of power depends on the angle of climb. When the climb is established steep enough that there is insufficient power available, a slower speed results.

The thrust required for a stabilized climb equals drag plus a percentage of weight dependent on the angle of climb. For example, a 10° climb would require thrust to equal drag plus 17 percent of weight. To climb straight up would require thrust to equal all of weight and drag. Therefore, the angle of climb for climb performance is dependent on the amount of excess thrust available to overcome a portion of weight. Note that aircraft are able to sustain a climb due to excess thrust. When the excess thrust is gone, the aircraft is no longer able to climb. At this point, the aircraft has reached its “absolute ceiling.”

Forces in Descents
As in climbs, the forces that act on the aircraft go through definite changes when a descent is entered from straight-and-level flight. For the following example, the aircraft is descending at the same power as used in straight-and-level flight.

As forward pressure is applied to the control yoke to initiate the descent, the AOA is decreased momentarily. Initially, the momentum of the aircraft causes the aircraft to briefly continue along the same flight path. For this instant, the AOA decreases causing the total lift to decrease. With weight now being greater than lift, the aircraft begins to descend. At the same time, the flight path goes from level to a descending flight path. Do not confuse a reduction in lift with the inability to generate sufficient lift to maintain level flight. The flight path is being manipulated with available thrust in reserve and with the elevator.

To descend at the same airspeed as used in straight-and-level flight, the power must be reduced as the descent is entered. Entering the descent, the component of weight acting forward along the flight path increases as the angle of descent increases and, conversely, when leveling off, the component of weight acting along the flight path decreases as the angle of descent decreases.

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CFI Brief: Space Weather

Today I would like to discuss weather, specifically the all-important topics of Galactic Cosmic Radiation and Solar Eruptive Activity. Wait, say what? That’s what I thought when reviewing my newly printed edition of the FAA Advisory Circular (AC) Aviation Weather (AC 00-6B). I came across a new chapter 23, titled “Space Weather.” To say the least I was intrigued and a bit surprised—how often are pilots really operating 62 miles above the Earth? Now don’t worry, galactic cosmic radiation and solar eruptive activity is not actually a required knowledge element for any certificate rating, at least not yet. I’m sure down the road there will be a rocket ship add-on rating to your Commercial Pilot Certificate or something along those lines—Virgin Galactic isn’t going to fly itself (I’m only half-joking).


I know absolutely nothing about space weather, so I dove right in to learn something new. Right off the bat I noticed an interesting commonality, the sun is the dominant source of conditions described as space weather. Sound familiar? Well it should, because the same is said for weather here on planet earth. As the sun heats various parts of the atmosphere at different rates it causes masses of both warm and cold air. As these masses of air move they cause wind, further driving areas of higher and lower temperatures to move around. You can go pretty far in depth with an explanation as to what causes weather, but it all boils down to the simple fact you will hear throughout your aviation training: weather is caused by the uneven heating of the earth’s surface.

You may be asking yourself why space weather is included in this AC. Well to be honest in my opinion I don’t so much believe that the theory of space weather is important to your average general or commercial aviation pilot. I do however believe that the effects space weather can have on aircraft operations is important. The tail end of chapter 23 discusses four areas in relation to aircraft operations that on a daily basis can be affected by what is happening tens of thousands of miles away.

AC 00-6B 23.12
HF communications at low- to mid-latitudes are used by aircraft during transoceanic flights and routes where line-of-sight VHF communication is not an option. HF enables a skip mode to send a signal around the curvature of Earth. HF communications on the dayside can be adversely affected when a solar flare occurs and its photons rapidly alter the electron density of the lower altitudes of the ionosphere, causing fading, noise, or a total blackout. Usually these disruptions are short-lived (tens of minutes to a few hours), so the outage ends fairly quickly.

HF communications at high latitudes and polar regions are adversely affected for longer periods, sometimes days, due to some space weather events. The high latitude and polar ionosphere is a sink for charged particles, which alter the local ionization and provide steep local ionization gradients to deflect HF radio waves, as well as increase local absorption.

Satellite communication signals pass through the bulk of the ionosphere and are a popular means of communicating over a wide area. The frequencies normally used for satellite communications are high enough for the ionosphere to appear transparent. However, when the ionosphere is turbulent and nonhomogeneous, an effect called scintillation, a twinkling in both amplitude and phase, is imposed upon the transmitted signal. Scintillations can result in loss-of-lock and inability for the receiver to track a Doppler-shifted radio wave.

Navigation and Global Positioning System (GPS)
Space weather adversely affects GPS in three ways: it increases the error of the computed position, it causes a loss-of-lock for receivers, and it overwhelms the transmitted signal with solar radio noise.

Radiation Exposure to Flightcrews and Passengers
Solar radiation storms occurring under particular circumstances cause an increase in radiation dose to flightcrews and passengers. As high polar latitudes and high altitudes have the least shielding from the particles, the threat is the greatest for higher altitude polar flights. The increased dose is much less of an issue for low- and mid-latitude flights.

Radiation Effects on Avionics
The electronic components of aircraft avionic systems are susceptible to damage from the highly ionizing interactions of cosmic rays, solar particles, and the secondary particles generated in the atmosphere. As these components become increasingly smaller, and therefore more susceptible, the risk of damage also increases.


Very interesting stuff. These are all things that as you progress in your aviation career and maybe one day start flying international routes you will want to have in your bank of knowledge.


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Weather: Precipitation

For the first time since 1975, the FAA has updated Aviation Weather (AC 00-6B replacing AC 00-6A). A lot has changed since then in our understanding of meteorology and in the data available to pilots and how they can put it to use. The AC 00-6B is now available from ASA in print, PDF eBook, and eBundle. This is one of the essential manuals for everyone in aviation, and features hundreds of detailed, full-color figures.

Today, we’ll take a look at what the new edition of Aviation Weather says about precipitation. Be sure to check out our other posts on weather topics, too!

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are: drizzle, rain, snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets.

Precipitation formation requires three ingredients: water vapor, sufficient lift to condense the water vapor into clouds, and a growth process that allows cloud droplets to grow large and heavy enough to fall as precipitation. Significant precipitation usually requires clouds to be at least 4,000 feet thick. The heavier the precipitation, the thicker the clouds are likely to be. When arriving or departing from an airport reporting precipitation of light or greater intensity, expect clouds to be more than 4,000 feet thick.

All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice crystals are too small and light to fall to the ground as precipitation. Because of their microscopic size, the rate at which cloud droplets fall is incredibly slow. An average cloud droplet falling from a cloud base at 3,300 feet (1,000 meters) would require about 48 hours to reach the ground. It would never complete this journey because it would evaporate within minutes after falling below the cloud base. Two growth processes exist which allow cloud droplets (or ice crystals) to grow large enough to reach the ground as precipitation before they evaporate (or sublimate). One process is called the collision-coalescence, or warm rain process (see Figure 14-1). In this process, collisions occur between cloud droplets of varying size and different fall speeds, sticking together or coalescing to form larger drops. Finally, the drops become too large to be suspended in the air, and they fall to the ground as rain. This is thought to be the primary growth process in warm, tropical air masses where the freezing level is very high.
Figure 14-1. The collision-coalescence or warm rain process. Most cloud droplets are too small and light to fall to the ground as precipitation. However, the larger cloud droplets fall more rapidly and are able to sweep up the smaller ones in their path and grow.

The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation, it is easier for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface, it may snow; otherwise, the snowflakes may melt to rain. This is thought to be the primary growth process in mid- and high-latitudes.

The vertical distribution of temperature will often determine the type of precipitation that occurs at the surface. Snow occurs when the temperature remains below freezing throughout the entire depth of the atmosphere (see Figure 14-2).
Figure 14-2. Snow temperature environment.

Ice pellets (sleet) occur when there is a shallow layer aloft with above freezing temperatures and with a deep layer of below freezing air based at the surface. As snow falls into the shallow warm layer, the snowflakes partially melt. As the precipitation reenters air that is below freezing, it refreezes into ice pellets (see Figure 14-3).
Figure 14-3. Ice pellets temperature environment.

Freezing rain occurs when there is a deep layer aloft with above freezing temperatures and with a shallow layer of below freezing air at the surface. It can begin as either rain and/or snow, but becomes all rain in the warm layer. The rain falls back into below freezing air, but since the depth is shallow, the rain does not have time to freeze into ice pellets (see Figure 14-4). The drops freeze on contact with the ground or exposed objects.
Figure 14-4. Freezing rain temperature environment.

Rain occurs when there is a deep layer of above freezing air based at the surface (see Figure 14-5).
Figure 14-5. Rain temperature environment.

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CFI Brief: Runway Incursion, Disaster Averted.

Reading Monday’s post you learned that a pilot deviation (PD) is a pilot action that violates any Federal Aviation Regulation. The two broad categories of PDs are ground- or surface-based deviation and airborne deviation. A common example of a surface-based deviation would be a runway incursion, which just is today’s topic.

Runway Incursion: any occurrence in the airport runway environment involving an aircraft, vehicle, person, or object on the ground that creates a collision hazard or results in a loss of required separation with an aircraft taking off, intending to take off, landing, or intending to land.

I experienced my first runway incursion early on in my flight training; it was a rather bizarre situation. I was at a small non-towered airport just north of Daytona Beach, FL conducting my second of three full-stop landings to accomplish my first solo. The airfield consisted of two 5,000-foot runways and two closed runways that were no longer in service. During my first loop in the pattern, I noticed several motorcycles zooming around the tarmac on one of the closed runways. As a student pilot I thought that to be kind of odd but put it out of my mind as I was worried enough about getting the airplane back on the ground for the first time. The first loop in the pattern and landing went off without a hitch, the gear didn’t shear off nor did I pop a tire. I was now on short final for my second full-stop landing and out of the corner of my eye I see this dude on his Harley take a left off the taxiway right onto the runway in front of me. Yes, there is now a Harley Davidson cruising down the middle of the runway in which I am intending to land on in about 10 seconds. I momentarily froze thinking what the heck, then heard my instructor come over the radio via his hand-held yelling, “go-around, go-around!” Well yes, I would prefer not to kill anyone today so a go-around seems like a great idea! Full throttle, flaps up, and the go-around was initiated. The guy on his Harley still oblivious to the fact he almost got rear ended by a 17 year old in a 2,000 pound Cessna 172 with a giant spinning propeller. After the go-around, I completed my final two landings and once back on the ground my instructor and I had a pretty colorful debrief about the flight and the bozo on his cruiser.


This was probably one of the best things I could have experienced early on in my training, an actual runway incursion requiring a go-around. It gave me confidence and an awareness of the consequences of what could happen. Some of the best learning is through actual experience and learning from others’ mistakes. Now in my particular situation I was lucky not to have been the one who made the mistake, but in this FAA Safety simulation you will see a real life event of a pilot deviation resulting in a runway incursion. The PD occurred on January 5th 2007 in overcast conditions with visibility 0.5 miles in fog and snow.


After watching this simulation, did you note the lack of situational awareness the pilots displayed? You would think that operating in the conditions that were present that day the crew of this flight would have displayed a heightened sense of awareness to their surroundings. There were several cues in which the pilots should have picked up on denoting that they had taxied onto an active runway. The simulation points out a few specifics like: the change in pavement, signage, runway lighting, and runway markings. It was fortunate that this PD did not result in an accident but as you can see from the simulation it was very close. Remember to always keep your head outside the cockpit and be aware; if confused or unsure ask for additional instructions from the controller.

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Procedures and Airport Operations: Pilot Deviations

Today on the Learn to Blog, we’ll take a look again at safety in and around the airport. This post is excerpted from the new edition of the Pilot’s Handbook of Aeronautical Knowledge.

A pilot deviation (PD) is an action of a pilot that violates any Federal Aviation Regulation. While PDs should be avoided, the regulations do authorize deviations from a clearance in response to a traffic alert and collision avoidance system resolution advisory. You must notify ATC as soon as possible following a deviation.

Pilot deviations can occur in several different ways. Airborne deviations result when a pilot strays from an assigned heading or altitude or from an instrument procedure, or if the pilot penetrates controlled or restricted airspace without ATC clearance.

To prevent airborne deviations, follow these steps:

  • Plan each flight—you may have flown the flight many times before but conditions and situations can change rapidly, such as in the case of a pop-up temporary flight restriction (TFR). Take a few minutes prior to each flight to plan accordingly.
  • Talk and squawk—Proper communication with ATC has its benefits. Flight following often makes the controller’s job easier because they can better integrate VFR and IFR traffic.
  • Give yourself some room—GPS is usually more precise than ATC radar. Using your GPS to fly up to and along the line of the airspace you are trying to avoid could result in a pilot deviation because ATC radar may show you within the restricted airspace.

Ground deviations (also called surface deviations) include taxiing, taking off, or landing without clearance, deviating from an assigned taxi route, or failing to hold short of an assigned clearance limit. To prevent ground deviations, stay alert during ground operations. Pilot deviations can and frequently do occur on the ground. Many strategies and tactics pilots use to avoid airborne deviations also work on the ground.

Pilots should also remain vigilant about vehicle/pedestrian deviations (V/PDs). A vehicle or pedestrian deviation includes pedestrians, vehicles or other objects interfering with aircraft operations by entering or moving on the runway movement area without authorization from air traffic control.

In serious instances, any ground deviation (PD or VPD) can result in a runway incursion. Best practices in preventing ground deviations can be found in the following section under runway incursion avoidance.

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CFI Brief: Wind Shear

Wind shear is defined as a change in wind direction and/or speed over a very short distance in the atmosphere. This can occur at any level of the atmosphere and can be detected by the pilot as a sudden change in airspeed. As a pilot you can be certain that you will experience wind shear throughout your flying career. It is a common encounter and most often associated with thunderstorms, microburst, and frontal activity.  The majority of wind shear related accidents take place during the landing and takeoff phases of flight. In both phases of flight you are low and slow and will have less time to detect and recover from an encounter. Below are some of the more important aspects of wind shear in which you will be tested on during your FAA Private Pilot Knowledge Test.

  • Low-level (low-altitude) wind shear can be expected during strong temperature inversions, on all sides of a thunderstorm and directly below the cell. A pilot can expect a wind shear zone in a temperature inversion whenever the wind speed at 2,000 feet to 4,000 feet above the surface is at least 25 knots.
  • Low-level wind shear can also be found near frontal activity because winds can be significantly different in the two air masses which meet to form the front.
  • In warm front conditions, the most critical period is before the front passes. Warm front shear may exist below 5,000 feet for about 6 hours before surface passage of the front. The wind shear associated with a warm front is usually more extreme than that found in cold fronts.
  • The shear associated with cold fronts is usually found behind the front. If the front is moving at 30 knots or more, the shear zone will be 5,000 feet above the surface 3 hours after frontal passage.

Basically, there are two potentially hazardous shear situations—the loss of a tailwind or the loss of a headwind.

  1. A tailwind may shear to either a calm or headwind component. The airspeed initially increases, the aircraft pitches up, and altitude increases. Lower than normal power would be required initially, followed by a further decrease as the shear is encountered, and then an increase as the glide slope is regained. See Figure 1.

Figure 1: Tailwind shear to a Headwind.

Figure 1: Tailwind shear to a Headwind.

  1. A headwind may shear to a calm or tailwind component. Initially, the airspeed decreases, the aircraft pitches down, and altitude decreases. See Figure 2.

Figure 2: Headwind shear to a Tailwind.

Figure 2: Headwind shear to a Tailwind.

Some airports can report boundary winds as well as the wind at the tower. When a tower reports a boundary wind which is significantly different from the airport wind, there is a possibility of hazardous wind shear. Many modern commercial aircraft and jetliners have systems that have the ability to detect wind shear and notify the pilots.  The types of aircraft however in which you will be flying during training are not equipped with these types of systems, the responsibility lies on you the pilot to detect and understand how to recover from all types of wind shear encounters.

The three questions below are great examples of what you should expect a wind shear question to look like on the knowledge test. Can you get all three correct; the answers are all outlined in the text above so hopefully you read carefully.

1. A pilot can expect a wind-shear zone in a temperature inversion whenever the windspeed at 2,000 to 4,000 feet above the surface is at least
A—10 knots.
B—15 knots.
C—25 knots.

2. Where does wind shear occur?
A—Only at higher altitudes.
B—Only at lower altitudes.
C—At all altitudes, in all directions.

3. When may hazardous wind shear be expected?
A—When stable air crosses a mountain barrier where it tends to flow in layers forming lenticular clouds.
B—In areas of low-level temperature inversion, frontal zones, and clear air turbulence.
C—Following frontal passage when stratocumulus clouds form indicating mechanical mixing.

Check out the FAA Safety document on Wind Shear for some great additional knowledge on the topic.


Answers to questions: 1. C 2. C 3. B

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Navigation: The Effect of Wind

As you know by now, wind is a mass of air moving over the surface of the Earth in a definite direction. When the wind is blowing from the north at 25 knots, it simply means that air is moving southward at a rate of 25 NM in one hour. Under these conditions, any inert object free from contact with the Earth is carried 25 NM southward in one hour. This effect becomes apparent when clouds, dust, and toy balloons are observed being blown along by the wind. Likewise, an aircraft flying within the moving mass of air is similarly affected. Even though the aircraft does not flow freely with the wind, it moves through the air at the same time the air is moving over the ground. Today, we’ll discuss the effect wind has on navigation, with an excerpt from the new edition of the Pilot’s Handbook of Aeronautical Knowledge.

At the end of one hour of flight, the aircraft is in a position that results from a combination of the following two motions:

  • Movement of the air mass in reference to the ground; and
  • Forward movement of the aircraft through the airmass.

As shown in the figure below, an aircraft flying eastward at an airspeed of 120 knots in still air has a groundspeed (GS) exactly the same—120 knots. If the mass of air is moving eastward at 20 knots, the airspeed of the aircraft is not affected, but the progress of the aircraft over the ground is 120 plus 20 or a GS of 140 knots. On the other hand, if the mass of air is moving westward at 20 knots, the airspeed of the aircraft remains the same, but GS becomes 120 minus 20 or 100 knots.


Assuming no correction is made for wind effect, if an aircraft is heading eastward at 120 knots and the air mass moving southward at 20 knots, the aircraft at the end of 1 hour is almost 120 miles east of its point of departure because of its progress through the air. It is 20 miles south because of the motion of the air. Under these circumstances, the airspeed remains 120 knots, but the GS is determined by combining the movement of the aircraft with that of the air mass. GS can be measured as the distance from the point of departure to the position of the aircraft at the end of 1 hour. The GS can be computed by the time required to fly between two points a known distance apart. It also can be determined before flight by constructing a wind triangle.


The direction in which the aircraft is pointing as it flies is called heading. Its actual path over the ground, which is a combination of the motion of the aircraft and the motion of the air, is called track. The angle between the heading and the track is called drift angle. If the aircraft heading coincides with the TC and the wind is blowing from the left, the track does not coincide with the TC. The wind causes the aircraft to drift to the right, so the track falls to the right of the desired course or TC.


The following method is used by many pilots to determine compass heading: after the TC is measured, and wind correction applied resulting in a TH, the sequence TH ± variation (V) = magnetic heading (MH) ± deviation (D) = compass heading (CH) is followed to arrive at compass heading.


By determining the amount of drift, the pilot can counteract the effect of the wind and make the track of the aircraft coincide with the desired course. If the mass of air is moving across the course from the left, the aircraft drifts to the right, and a correction must be made by heading the aircraft sufficiently to the left to offset this drift. In other words, if the wind is from the left, the correction is made by pointing the aircraft to the left a certain number of degrees, therefore correcting for wind drift. This is the wind correction angle (WCA) and is expressed in terms of degrees right or left of the TC.


To summarize:

  • Course—intended path of an aircraft over the ground or the direction of a line drawn on a chart representing the intended aircraft path, expressed as the angle measured from a specific reference datum clockwise from 0° through 360° to the line.
  • Heading—direction in which the nose of the aircraft points during flight.
  • Track—actual path made over the ground in flight. (If proper correction has been made for the wind, track and course are identical.)
  • Drift angle—angle between heading and track.
  • WCA—correction applied to the course to establish a heading so that track coincides with course.
  • Airspeed—rate of the aircraft’s progress through the air.
  • GS—rate of the aircraft’s inflight progress over the ground.
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