CFI Brief: What causes weather?

The theory of weather is a complex yet intriguing topic to discuss. It’s often joked that by the time you complete training you’re an amateur meteorologist. Joke or not, that statement is fairly accurate. Weather can be one of the key factors in determining a go/no-go decision and to make an accurate assessment it is important for the pilot in command to understand the full spectrum of what causes weather.

Let’s start with the most basic and determine the overall source of where weather comes from. Below is an excerpt from ASA’s 2015 Private Pilot Test Prep.

The major source of all weather is the sun. Changes or variations of weather patterns are caused by the unequal heating of the Earth’s surface. Every physical process of weather is accompanied by or is a result of unequal heating of the Earth’s surface.

The heating of the Earth (and therefore the heating of the air surrounding the Earth) is unequal around the entire planet. Both north or south of the equator, one square foot of sunrays is not concentrated over one square foot of the surface, but over a larger area. This lower concentration of sunrays produces less radiation of heat over a given surface area; therefore, less atmospheric heating takes place in that area.

The unequal heating of the Earth’s atmosphere creates a large air-cell circulation pattern (wind) because the warmer air has a tendency to rise (low pressure) and the colder air has a tendency to settle or descend (high pressure) and replace the rising warmer air. This unequal heating, which causes pressure variations, will also cause variations in altimeter settings between weather reporting points.

Because the Earth rotates, this large, simple air-cell circulation pattern is greatly distorted by a phenomenon known as the Coriolis force. When the wind (which is created by high pressure trying to flow into low pressure) first begins to move at higher altitudes, the Coriolis force deflects it to the right (in the Northern Hemisphere) causing it to flow parallel to the isobars (lines of equal pressure). These deflections of the large-cell circulation pattern create general wind patterns as depicted in the figure below.

Prevailing wind systems.

Prevailing wind systems.

Let’s take a look at a few questions you might encounter on your FAA Private Pilot Knowledge Test. I will post the answers Monday morning in the comments section.

1. Every physical process of weather is accompanied by, or is the result of, a
A—movement of air.
B—pressure differential.
C—heat exchange.

2. What causes variations in altimeter settings between weather reporting points?
A—Unequal heating of the Earth’s surface.
B—Variation of terrain elevation.
C—Coriolis force.

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Weather: Americas Weather Regions, California

The United States of America has some diverse weather patterns. Thomas A. Horne‘s book Flying America’s Weather gives you a view of different weather regions around the U.S. from a pilots view. Here is an excerpt from the book discussing weather in the region of California:

For pilots or any other student of meteorology, the state of California represents one of the most interesting weather regions in the United States. It is a state with an abundance of different climatological areas in close proximity to each other, a condition made possible by a unique combination of widely disparate geographic features. This is especially true of southern California, where the pacific Ocean, coastal lowlands, mountain ranges, valleys, and a desert adjoin each other in a strip of land hardly more than 100 miles wide.

As might be expected of a state with 1,340 miles of coastline, California owes a great deal of its weather to the Pacific Ocean, and the air circulating above it. This oceanic circulation has some seasonal aspects central to the nature of California’s summers and winters–especially in southern California. Like the Atlantic Ocean, the Pacific is the site of a large, semipermanent high pressure system. And just as the Atlantic’s Bermuda high affects so much of the eastern United States’ weather, so the Pacific high influences the western coastal areas.

But the differences are great. The clockwise circulation around the “back side” of the Bermuda high sends warm, moist air from the Caribbean and Gulf of Mexico inland. In California, it’s the high’s “front side” that’s at work. Here, the clockwise flow sends air of more moderate temperature from the central pacific to Californian shores.

In the summer months, the Pacific high is situated more or less due west of central California, There, it exercises a blocking function, preventing low pressure areas from traveling to the southern regions of the state. The flow of air is nearly always from the west, both at the surface and aloft, and wind speeds are moderate–seldom exceeding 40 knots even at altitudes as high as 10,000 feet. In the fall and winter, the Pacific high drops to the south, and this permits low pressure and/or cold fronts to strike California as they are pushed through by the northernmost segment of the high’s flows.

In his book,Thomas A. Horne breaks the U.S. into 17 geographical weather regions and discusses weather aspects unique to that particular region. Want to know whats happening in your neck of the woods? Check out theFlying America’s Weather!

You can purchase Flying America’s Weather on our website at, where you can find more resources for student pilots.


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CFI Brief: Newton

It’s been a few days since I last posted; I believe we left off discussing Bernoulli’s principal as it relates to lift. Today, I want to discuss another important mathematician/physicist from around the same time, Sir Isaac Newton.

Sir Newton came up with what is known today as Newton’s Three Laws of Motion. The law in which we will focus on today is the third law: for every action there is an equal and opposite reaction. Along with Bernoulli, Newton’s Third Law is also a direct contributor to lift. Bob Gardner describes this in The Complete Private Pilot:

A second contributor to total lift is Newton’s Third Law: for every action there is an equal and opposite reaction. As the airfoil moves through the air it pushes the air downward and, in accordance with Newton’s Law, the air exerts an equal upward force. Because of differences in wing design and operating conditions, it is impossible to say what percentage of total lift can be attributed to Bernoulli or to Newton at any time.

Imagine as the wing passes through space the air strikes the underside of the wing and is deflected downward. Based on Newton’s Third Law the opposite reaction would be a force created in an upwards direction, better known to a pilot as lift. Remember when you were a kid and stuck your hand out the window like a wing? I’m sure you could feel the wind wanting to push your hand back and up, this is the same exact concept. Pretty cool stuff!

Can you think of any other areas where Newton’s Third Law might come into play? I’ll give you a clue, think about the four forces on the aircraft that we have already discussed.

How about thrust? An aircraft propeller’s job is to create thrust—it does this by pushing air backyards, the resultant force is the air pushing the propeller in the opposite direction (forwards). The same can be said for a jet engine.

Here’s another example, imagine yourself sitting in the cockpit of a Cessna 172 with the engine running. Looking out the window you will notice that the propeller is turning to your right. Obviously we all know that the prop is connected to the engine which in turn is bolted to the airframe (I hope you knew that). The opposing force from the propeller is felt on the airframe around the longitudinal axis, so the airframe wants to roll left as the prop turns right (see the figure below). This is known as torque reaction and manufactures take this into effect when designing aircraft. This is an area you will get more into when discussing P-Factor and torque.

Torque reaction.

Torque reaction.

Here are two questions that you are likely to see on your FAA Knowledge test that cover the information we have discussed. I will post the answers in the comments section of this post on Monday. Now enough of this Aerodynamics talk, let’s get into weather next week!

1. In what flight condition is torque effect the greatest in a single-engine airplane?
A—Low airspeed, high power, high angle of attack.
B—Low airspeed, low power, low angle of attack.
C—High airspeed, high power, high angle of attack.

2. Which statement relates to Bernoulli’s principle?
A—For every action there is an equal and opposite reaction.
B—An additional upward force is generated as the lower surface of the wing deflects air downward.
C—Air traveling faster over the curved upper surface of an airfoil causes lower pressure on the top surface.

Know the answer? Have a further question for the CFI? Leave a comment below, or send an email to, and be sure to check the comments on this post next week for the answers!

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Aerodynamics: Construction Part 3

This is the third part of our three part series about aircraft construction, which will be covering landing gear, propeller, engine, and lights. You can learn about the fuselage, wings, and empennage in Part 1, and the flight controls in Part 2. This excerpt comes from Bob Gardner‘s The Complete Private Pilot.

Landing Gear
The two main landing wheels and their supporting structure are designed to withstand landing loads and support the airplane on the ground. A third, smaller wheel mounted either forward (tricycle) or aft (conventional) is for ground steering control only. Nosewheels are usually close to or a part of the engine mount and are definitely not designed to absorb landing loads. (Your instructor will devote a lot of training time to making sure that you do not land on the nose wheel!)

The shiny cylinders on nose wheels and some main landing gear are called struts (the Katana’s nosewheel uses replaceable rubber “doughnuts”). They absorb the bumpiness of runways taxiways. The shiny kind are filled with air and oil, just like your car’s shock absorbers. When a strut is “flat” there is no cushioning effect and vibrations are transmitted to the entire airframe. You will see some airplanes which use a spring steel assembly on the main landing gear instead of a strut.

Almost all airplanes use disc brakes on the main landing gear, and you can see the discs if there are no wheel pants. Checking brake condition is considerably easier to do on airplanes than it is on cars. The nose wheel is usually not steerable with the rudder pedals and swivels freely, so steering is accomplished by tapping the brake lightly on the side toward the turn.

The propellers you see may be either fixed or variable in pitch, or blade angle. You will probably see some amphibians (airplanes that can land on either land or water) with pusher-type propellers, but most are mounted up front and pull the airplane through the air. The conical spinner is not only decorative but servers to direct air into the cooling air intakes.

Modern airplanes have four- or six-cylinder flat opposed engines: when you open the cowling, you see that the cylinders are on opposite sides of the engine, and that the flat profile allows maximum aerodynamic streaming of the cowling. As you walk along the ramp you may see an older airplane with a radial engine, its cylinders arranged in a “star” pattern. Most light sport aircraft use water-cooled Rotary engines.

The lighting system on a modern airplane consists of position lights on the wing tips (red on the left, green on the right) and a white light on the tail, and anticollision light system which may be either red or white (or both) and one or more landing lights. Many airplanes also have bright flashing strobe lights to increase the chances of being seen during both day and night flights.

You can purchase The Complete Private Pilot on our website at, which also contains even more resources for student pilots. On Thursday, the CFI will be back with more FAA Knowledge Exam questions.


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Aerodynamics: Construction Part 2

This is the second part of our three part series about aircraft construction, which will cover flight controls. You can learn about the fuselage, wings, and empennage here in Part 1. This excerpt comes from Bob Gardner‘s The Complete Private Pilot.

Flight Controls
Fore-and-aft movement of the control wheel or stick is transmitted by pushrods or cables and pulleys to these control surfaces, and left-right movement is controlled by the rudder, which is mounted at the rear of the vertical fin. The pilot depresses the rudder pedal in the desired direction of nose movement and a cable system moves the control surface. You will see V-Tails, T-tails, and straight tails, and maybe a home-built airplane with no horizontal surfaces mounted on the tail.

Flight Controls

The flight controls.

You won’t find many airplanes that do not have ailerons, which the moveable control surfaces at the outer trailing edge of the wings. Ailerons are used to bank the airplane. A control wheel or or stick at the pilot station is moved in the direction of the bank desired (left or right). The ailerons are deflected through a system of cables, pulleys, and bellcranks or pushrods. When no control forces is exerted, the ailerons are held flush with the wing surface by the airstream.

The hinged portions of the trailing edges of the wings near the fuselage are called flaps, and are normally used to steepen the glide angle without increasing airspeed. As you walk along the ramp you will see many different types of flaps, some that simply hinge down and others that extend down and backward. Older airplanes may not have any flaps at all.

Follow the Learn to Fly blog in the upcoming days to learn more about the Landing Gear, the Propeller and Engine, and the lights. You can purchase The Complete Private Pilot on our website at, which also contains even more resources for student pilots.

Check back on Monday for the final post in this series – Learn to Fly 9: Construction Part 3.

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