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CFI Brief: Looking at the Big Picture

This week you and I have been tasked with repositioning a Cessna 172 from the Savannah Airport (SAV) to Allendale County (88J) — airports neither of us has ever flown out of. To begin our planning we have obtained the local sectional chart to gain all available information from this important resource regarding our planned flight.

The first thing I like to do is look at the big picture to determine what types of airspace and other pertinent issues will affect the route of flight including terrain, aids to navigation, and aerial hazards to flight, such as parachute or glider activity. Notice that our departure point (SAV) is surrounded by a solid magenta line signifying it is within Class C airspace from the surface up to 4,100 feet, as noted by the 41/SFC  indication. Looking north, our arrival airport (88J) is surrounded by a thick shaded magenta line, which tells us Class E airspace begins at 700 feet AGL with Class G below. Other than our departure airport, there is no other significant airspace along our route that we will need to deal with.

Savannah Sectional

Savannah Sectional

Next, let’s look at local terrain features. The topography is shaded a light green color; this tells us it is a fairly flat area with no terrain over 2,000 feet MSL (remember we learned this in Monday’s post). Also, notice the maximum elevation figures along our route indicated as 17 (area 3) and 07 (area 1); flying above these altitudes means we will be clear of terrain or any vertical obstacles. The Savannah River that flows north to south will be a good visual aid to navigation while en route. The sectional depicts a VOR over both our departure and arrival points; there’s even a Victor airway (V37) that we may want to use to navigate to our destination. Fortunately there don’t appear to be any aerial hazards or areas where we need to exercise extreme caution along our route (which might be depicted by a glider or parachute icon).

Savannah Airport

Savannah Airport

Let’s take a closer look at the areas surrounding both our departure and arrival airports to see what additional information we can gather. To the left of the SAV airport we can see the airport data listed. In the order they are listed, we can see the airport has an operating control tower and can be reached on frequency 119.1 (CT 119.1*); the star indicates the tower operates part-time. Following tower frequency is a “C” notating that during hours the tower is closed this frequency should be used as a CTAF. Also on the first line, the ATIS frequency is shown as 123.75. The next line provides additional information about the airport: 50 indicates the airport elevation as 50 feet MSL, followed by *L indicating the airport has lighting limitations. The “93” tells us the longest runway is 9,300 feet, and lastly the UNICOM frequency is listed as 122.95.

Allendale Airport

Allendale Airport

Our arrival airport, 88J or Allendale County, is depicted as a solid magenta circle with tick marks and a star above. Magenta signifies no control tower in operation, the star tells us a rotating airport beacon is in operation sunset to sunrise, and the tick marks let us know that services like fuel are available during normal working hours. We can interpret the airport data to the left as saying the airport elevation is 162 feet MSL, there’s lighting restrictions, the longest runway is 5,000 feet, and the common traffic advisory frequency is 122.8.

At this point we have accomplished our initial objective of gathering all available information from the sectional chart. This has allowed us to gain an overall picture of our scheduled repositioning flight to SAV. From here, we would gather additional resources and information by using the Airport/Facility Directory, Weather Reports, Local NOTAMs, and finally, completing a flight plan.

Do you notice anything else from the sectional that would be good to note – anything that may affect our decision making, flight planning, or inflight operation? Let us know in the comments section below.

 

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Enroute Flight: Topography

A VFR Sectional Aeronautical Chart is a pictorial representation of a portion of the Earth’s surface upon which lines and symbols in a variety of colors represent features and/or details that can be seen on the Earth’s surface. Contour lines, shaded relief, color tints, obstruction symbols, and maximum elevation figures are all used to show topographical information. Explanations and examples may be found in the chart legend. Pilots should become familiar with all of the information provided in each Sectional Chart Legend. Today we’ll look at the FAA’s Aeronautical Chart User’s Guide introduction to land features (terrain) and obstructions. This publication is available from ASA in a print and eBook formats, and a pictorial guide to topographical features can be found on pages 28 to 36.

We use five different techniques to clearly show the shape of the earth and any obstructions: contour lines, shaded relief, color tints, obstruction symbols, and Maximum Elevation Figures (MEF).

  1. Contour lines join points of equal elevation. On Sectionals, basic contours are spaced at 500′ intervals. Intermediate contours are typically at 250′ intervals in moderately level or gently rolling areas. Auxiliary contours at 50′, 100′, 125′, or 150′ intervals occasionally show smaller relief features in areas of relatively low relief. The pattern of these lines and their spacing gives the pilot a visual concept of the terrain. Widely spaced contours represent gentle slopes, while closely spaced contours represent steep slopes.
  2. topography-1

  3. Shaded relief shows how terrain may appear from the air. Shadows are shown as if light is coming from the northwest, because studies show that our visual perception has been conditioned to this view.
  4. topography-2

  5. Different color tints show bands of elevation relative to sea level. These colors range from light green for the lower elevations, to dark brown for the higher elevations.
  6. topography-3-2

  7. Obstruction symbols show man made vertical features that could affect safe navigation. FAA’s Aeronautical Information Management maintains a database of over 1,200,000 obstacles in the U.S., Canada, Caribbean, Mexico, and U.S. Pacific Island Territories. Aeronautical Specialists evaluate each obstacle based on charting specifications before adding it to a visual chart. When a Specialist is not able to verify the position or elevation of an obstacle, it is marked UC, meaning it is “under construction” or being reported, but has not been verified.

    Sectional Charts and Terminal Area Charts (TACs) typically show manmade obstacles extending more than 200’ Above Ground Level (AGL), unless they appear in yellow city tint. Features considered to be hazardous obstacles to low-level flight are; smokestacks, tanks, factories, lookout towers, and antennas, etc. On World Aeronautical Charts (WACs) only those obstacles at 500’ AGL and higher are charted.
    topography-4
    Manmade features used by FAA Air Traffic Control as checkpoints use a graphic symbol shown in black with the required elevation data in blue. The elevation of the top of the obstacle above Mean Sea Level (MSL) and the height of the structure (AGL) is also indicated (when known or can be reliably determined by a Specialist). The AGL height is in parentheses below the MSL elevation. In extremely congested areas, the FAA typically omits the AGL values to avoid confusion.
    topography-5
    Whenever possible, the FAA depicts specific obstacles on charts. However, in
    high-density areas like city complexes, only the highest obstacle is represented on
    the chart using the group obstacle symbol to maximize legibility.
    topography-6
    Obstacles under construction are indicated by placing the letters UC next to the obstacle type.
    topography-7
    Obstacles with high-intensity strobe lighting systems may operate part-time or by proximity activation and are shown as follows:
    topography-8

  8. The Maximum Elevation Figure (MEF) represents the highest elevation within a quadrant, including terrain and other vertical obstacles (towers, trees, etc.). A quadrant on Sectionals is the area bounded by ticked lines dividing each 30 minutes of latitude and each 30 minutes of longitude. MEF figures are rounded up to the nearest 100’ value and the last two digits of the number are not shown.
    topography-9
    When a manmade obstacle is more than 200’ above the highest terrain within the quadrant:

    1. Determine the elevation of the top of the obstacle above MSL.
    2. Add the possible vertical error of the source material to the above figure (100′ or 1/2 contour interval when interval on source exceeds 200′.)
    3. Round the resultant figure up to the next higher hundred-foot level.
    4. topography-10

    When a natural terrain feature or natural vertical obstacle (e.g. a tree) is the highest feature within the quadrangle:

    1. Determine the elevation of the feature.
    2. Add the possible vertical error of the source to the above figure (100′ or 1/2 the contour interval when interval on source exceeds 200′).
    3. Add a 200′ allowance for uncharted or manmade obstacles. Chart specifications don’t require the portrayal of obstacles below minimum height.
    4. Round the figure up to the next higher hundred-foot level.
    5. topography-11

    We’ll be back with more on Thursday!

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CFI Brief: W&B Simple Calculations

As a pilot you should always expect change. Change can often occur prior to your flight even beginning. I’ve been in situations before where I have completed my entire cross country flight plan and weight and balance only to find out another friend wants to come along for the ride. Before doing another entire weight and balance problem you should know about a few quick tricks.

Weight Shift

Whenever weight is either added to or subtracted from a loaded airplane, both the gross weight and the center of gravity (CG) location will change. The solution to such a calculation is really a simplified loading problem. Instead of calculating a weight and moment for every section of the aircraft, it is only necessary to compute the original weight and moment—then, the effect of the change in weight.

For example, if an aircraft’s total weight was 8,600 pounds, and you shifted 100 pounds from station (or, arm) 100 to arm 150, a simple weight shift formula can be applied:

Weight Shift Formula

This is solved easily by cross-multiplying: 50 x 100 ÷ 8,600 = .06 inches. Therefore, the CG shifts .06 inches aft.

Weight Change

Use the following formula to find the change in CG, if weight has been added or subtracted. The amount of weight changed and the new total weight must be known, in addition to the distance between the original CG and the point where the weight is being added or subtracted.

Weight Change Formula

You can also use this formula to determine your new CG after fuel burn. Here is an example.

Problem:

Determine the new CG location after 1 hour 45 minutes of flight time, given the following:

Total weight: 4,037 lbs

CG location Station: 67.8

Fuel consumption: 14.7 GPH

Fuel CG Station: 68.0

Solution:

  1. Find the amount of weight change. The aircraft has consumed 14.7 GPH for 1 hour 45 minutes. The total fuel consumed is:
  2. 14.7 GPH x 1.75 hr = 25.7 gal

    Which weighs:

    25.7 gal x 6 lbs/gal = 154 lbs

  3. Determine the new total weight by subtracting the weight of the fuel consumed (154 lbs) from the total weight (4,037 lbs):
  4. 4,037 – 154 = 3,883 lbs new total weight

  5. Find the distance between the original CG (67.8) and the point weight removed (fuel CG = 68.0):
  6. 68.0 – 67.8 = 0.2 inches

  7. Place the three known values into the formula:
  8. 154 lbs. / 3,883 lbs. = Change in CG / 0.2 in

    Change in CG = 154 lbs. x 0.2 in / 3,883 lbs. = 0.01 in

  9. The CG was found to shift approximately 0.01 in. Since the weight was removed aft (68.0 in) of the CG (67.8 in), the CG shifted forward 0.01 in.
  10. 67.80 (original CG) – 0.01 (forward shift) = 67.79 (new CG)

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Aircraft Performance: Computing Weight and Balance

Back in February, we introduced the concept of weight and balance and its significance in preflight planning. This week, we’ll look at one of the methods of determining your loaded weight and CG. There are a variety of methods to do this, but this week we’ll illustrate the table method with an example from the Pilot’s Handbook of Aeronautical Knowledge.

With the table method, the CG position is determined by dividing the total moments by the total weight. As weights are shifted around within the airplane by moving baggage from forward to aft compartments, moving passengers, or even extending and retracting the landing gear, the CG will follow. To avoid confusion, moments are often divided by 100, 1,000, or 10,000.

The figure below is an example of a table and a weight and balance calculation based on that table. In this problem, the total weight of 2,799 pounds and moment of 2,278/100 are within the limits of the table.

PHAK_9-9
More on weight and balance this Thursday with our CFI.

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CFI Brief: Winds and Temperatures Aloft Forecast

Just the other day I was on a commercial airline flight on my way home from vacation. Prior to departure, the captain came on the overhead to advise the passengers that we would be arriving about 15 minutes late due to headwinds along the route. Like the captain did, it is an important part of your cross country flight planning to check the forecasted winds, known as the Winds and Temperatures Aloft Forecast (FB). Understanding what the winds are doing aloft will allow you to estimate several different factors, including your time en-route, approximate time of arrival, fuel burn, and compass headings.

The FB is issued 4 times daily for specified locations across the U.S and coastal waters as seen in the figure below.

Winds and Temperatures Aloft Forecast Network

Winds and Temperatures Aloft Forecast Network

It can get a little tricky understanding how to interpret the textual forecast. First, it’s important to understand that the winds are given in degrees true and that winds are specified in knots. The information is formatted like so DDffTT, Wind Direction (DD), Wind Speed (ff), and Temperature (TT). Using the forecast from below let’s look at DEN (Denver) at 9,000 feet MSL, 2321-04. The first two numbers 23 represent the direction of the wind in degree true, 230 (you will need to add on a 0). Next, the 21 represents the wind speed, in this case 21 knots. Finally, the temperate is shown as -04 or negative 4 degrees Celsius. You will either see a + or – displayed with the temperature to indicate positive or negative, above 24,000 feet no + or – sign is noted as all temperatures will be negative.

So far pretty simple, but it can get a bit trickier. Anytime the wind is in excess of 99 knots 50 will be added to the coded wind direction and 100 subtracted from the coded wind. A code indicating 7760-20 would decode as 270 degrees true at 160 knots and a temperature of negative 20 degree Celsius. Take the first two numbers 77 (our wind direction) and subtract 50 to get 27. Next take 60 (wind speed) and add on 100 to get 160. Lets try one more, 8242-15 decodes as 320 @ 142 knots and a temperature of negative 15. Subtract the 50 from 82 and then add on 100 to 42.

Now if you take another look at our example forecast below you will notice that no information is given for AMA at 3,000 feet MSL. This is due to the altitude at which the station is located. Wind forecasts are not issued for altitudes within 1,500 of the locations elevation and temperatures within 2,500 feet. Something else to take note of is anytime winds are light and variable or less than 5 knots it will be express as 9900. Try your hand at the three sample questions below, you can use figure 17 included here to answer each questions. The questions are similar in nature to what may appear on your Private Pilot Knowledge Exam.

Winds and Temperatures Aloft Forecast

Winds and Temperatures Aloft Forecast

1. (Refer to Figure 17.) What wind is forecast for STL at 9,000 feet?
A—230° true at 32 knots.
B—230° magnetic at 25 knots.
C—230° true at 25 knots.

2. (Refer to Figure 17.) What wind is forecast for STL at 12,000 feet?
A—230° true at 56 knots.
B—230° magnetic at 56 knots.
C—230° true at 39 knots.

3. (Refer to Figure 17.) What wind is forecast for STL at 12,000 feet?
A—230° true at 56 knots.
B—230° magnetic at 56 knots.
C—230° true at 39 knots.

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Weather Services: Terminal Aerodrome Forecast (TAF)

This week, we’re taking another look at aviation forecasts and focusing on the terminal aerodrome forecast (TAF), one of the printed forecasts that all pilots need to be familiar with. TAFs are designed to be useful in the preflight planning stage. Here’s how the FAA breaks down a TAF in the Pilot’s Handbook of Aeronautical Knowledge.

A sample TAF.

A sample TAF.

A TAF is a report established for the five statute mile radius around an airport. TAF reports are usually given for larger airports. Each TAF is valid for a 30-hour time period, and is updated four times a day at 0000Z, 0600Z, 1200Z, and 1800Z. The TAF utilizes the same descriptors and abbreviations as used in the METAR report. The TAF includes the following information in sequential order:

  1. Type of report—a TAF can be either a routine forecast (TAF) or an amended forecast (TAF AMD).
  2. ICAO station identifier—the station identifier is the same as that used in a METAR.
  3. Date and time of origin—time and date of TAF origination is given in the six-number code with the first two being the date, the last four being the time. Time is always given in UTC as denoted by the Z following the number group.
  4. Valid period date and time—the valid forecast time period is given by a six-digit number group. The first two numbers indicate the date, followed by the two-digit beginning time for the valid period, and the last two digits are the ending time.
  5. Forecast wind—the wind direction and speed forecast are given in a five-digit number group. The first three indicate the direction of the wind in reference to true north. The last two digits state the windspeed in knots as denoted by the letters “KT.” Like the METAR, winds greater than 99 knots are given in three digits.
  6. Forecast visibility—given in statute miles and may be in whole numbers or fractions. If the forecast is greater than six miles, it will be coded as “P6SM.”
  7. Forecast significant weather—weather phenomena are coded in the TAF reports in the same format as the METAR. If no significant weather is expected during the forecast time period, the denotation “NSW” is included in the “becoming” or “temporary” weather groups.
  8. Forecast sky condition—given in the same manner as the METAR. Only cumulonimbus (CB) clouds are forecast in this portion of the TAF report as opposed to CBs and towering cumulus in the METAR.
  9. Forecast change group—for any significant weather change forecast to occur during the TAF time period, the expected conditions and time period are included in this group. This information may be shown as from (FM), becoming (BECMG), and temporary (TEMPO).
  10. “FM” is used when a rapid and significant change, usually within an hour, is expected. “BECMG” is used when a gradual change in the weather is expected over a period of no more than 2 hours. “TEMPO” is used for temporary fluctuations of weather, expected to last less than one hour.
  11. Probability forecast—a given percentage that describes the probability of thunderstorms and precipitation occurring in the coming hours. This forecast is not used for the first 6 hours of the 24-hour forecast.

Example:
TAF
KPIR 111130Z 111212 15012KT P6SM BKN090
TEMPO 1214 5SM BR
FM1500 16015G25KT P6SM SCT040 BKN250
FM0000 14012KT P6SM BKN080 OVC150 PROB40 0004
3SM TSRA BKN030CB
FM0400 1408KT P6SM SCT040 OVC080
TEMPO 0408 3SM TSRA OVC030CB
BECMG 0810 32007KT=

Explanation:
Routine TAF for Pierre, South Dakota…on the 11th day of the month, at 1130Z…valid for 24 hours from 1200Z on the 11th to 1200Z on the 12th…wind from 150° at 12 knots…visibility greater than 6 sm…broken clouds at 9,000 feet…temporarily, between 1200Z and 1400Z, visibility 5 sm in mist…from 1500Z winds from 160° at 15 knots, gusting to 25 knots visibility greater than 6 sm…clouds scattered at 4,000 feet and broken at 25,000 feet…from 0000Z wind from 140° at 12 knots…visibility greater than 6 sm…clouds broken at 8,000 feet, overcast at 15,000 feet…between 0000Z and 0400Z, there is 40 percent probability of visibility 3 sm…thunderstorm with moderate rain showers…clouds broken at 3,000 feet with cumulonimbus clouds…from 0400Z…winds from 140° at 8 knots…visibility greater than 6 miles…clouds at 4,000 scattered and overcast at 8,000…temporarily between 0400Z and 0800Z…visibility 3 miles…thunderstorms with moderate rain showers…clouds overcast at 3,000 feet with cumulonimbus clouds…becoming between 0800Z and 1000Z…wind from 320° at 7 knots…end of report (=).

Look for more on deciphering TAFs from our CFI on Thursday. Other essential resources on weather services are the recently updated FAA text Aviation Weather Services (AC 00-45G.2) and Chapter 7 of the AIM, printed annually in ASA’s FAR/AIM.

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CFI Brief: Thunderstorms, BOOM!

For a thunderstorm to exist, you need two very basic elements: moisture and warm, rapidly rising air. This is why in the spring and summer months with the warmer conditions you tend to see an increase in thunderstorm activity. According to NOAA, at any given time across the world there are 1,800 thunderstorms occurring all of which are hazardous to flight. The average thunderstorm will last for a period of 30 minutes and spans approximately 15 miles.

A thunderstorm is a local storm produced by a cumulonimbus cloud. It is always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes hail. Three conditions are necessary for the formation of a thunderstorm: sufficient water vapor, an unstable lapse rate, and an initial upward boost (lifting action). The initial upward boost can be caused by heating from below, frontal lifting, or by mechanical lifting (wind blowing air upslope on a mountain).

There are three stages of a thunderstorm:

  1. The cumulus stage is characterized by continuous updrafts, and these updrafts create low-pressure areas.
  2. The mature stage is characterized by updrafts and downdrafts inside the cloud. Precipitation inside the cloud aids in the development of these downdrafts, and the start of rain from the base of the cloud signals the beginning of the mature stage. Thunderstorms reach their greatest intensity during the mature stage.
  3. The dissipating stage is characterized predominantly by downdrafts.

Life cycle of a thunderstorm.

Life cycle of a thunderstorm.


Thunderstorms that generally produce the most intense hazard to aircraft are called squall-line thunderstorms. These non-frontal, narrow bands of thunderstorms often develop ahead of a cold front. Embedded thunderstorms are those that are obscured by massive cloud layers and cannot be seen.

The majority of all FAA knowledge tests throughout your training will contain some type of question related to thunderstorms so it is important to understand this in-flight hazard. Here are a few questions that you may encounter on your private pilot knowledge test.

1. What feature is normally associated with the cumulus stage of a thunderstorm?
A—Roll cloud.
B—Continuous updraft.
C—Frequent lightning.

2. Which weather phenomenon signals the beginning of the mature stage of a thunderstorm?
A—The appearance of an anvil top.
B—Precipitation beginning to fall.
C—Maximum growth rate of the clouds.

3. What conditions are necessary for the formation of thunderstorms?
A—High humidity, lifting force, and unstable conditions.
B—High humidity, high temperature, and cumulus clouds.
C—Lifting force, moist air, and extensive cloud cover.

Leave your answers in the comments! We’ll be back Monday.

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Weather: Circulation and Wind

Differences in temperature create differences in pressure. These pressure differences drives a complex system of winds in a never ending attempt to reach equilibrium. Wind also transports water vapor and spreads fog, clouds, and precipitation. This week, we have a post from the classic FAA text Aviation Weather (AC 00-6A) to describe how the atmosphere circulates around our globe.

Convection
When two surfaces are heated unequally, they heat the overyling air unevenly. The warmer air expands and becomes lighter or less dense than the cool air. The more dense, cool air is drawn to the ground by its greater gravitational force lifting or forcing the warm air upward much as oil is forced to the top of water when the two are mixed. Figure 1 shows the convective process. The rising air spreads and cools, eventually descending to complete the convective circulation. As long as the uneven heating persists, convection maintains a continuous “convective current.”

The horizontal air flow in a convective current is “wind.” Convection of both large and small scales accounts for systems ranging from hemispheric circulations down to local eddies. This horizontal flow, wind, is sometimes called “advection.” However, the term “advection” more commonly applies to the transport of atmospheric properties by the wind, i.e., warm advection; cold advection; advection of water vapor, etc.

Figure 1. Convective current resulting from uneven heating of air by contrasting surface temperatures. Cool, heavy air forces warmer air aloft establishing a convective cell. Convection continues as long as uneven heating persists.

Figure 1. Convective current resulting from uneven heating of air by contrasting surface temperatures. Cool, heavy air forces warmer air aloft establishing a convective cell. Convection continues as long as uneven heating persists.


Pressure Gradient Force
Pressure differences must create a force in order to drive the wind. This force is the pressure gradient force. The force is from higher pressure to lower pressure. Because of uneven heating of the Earth, surface pressure is low in warm equatorial regions and high in cold polar regions. A pressure gradient develops from the poles to the Equator. If the Earth did not rotate, this pressure gradient force would be the only force acting on the wind. Circulation would be two giant hemispheric convective currents as shown in figure 2. Cold air would sink at the poles; wind would blow straight from the poles to the Equator; warm air at the Equator would be forced upward; and high level winds would blow directly toward the poles. However, the Earth does rotate; and because of its rotation, this simple circulation is greatly distorted.
Figure 2. Circulation as it would be on a nonrotating globe.

Figure 2. Circulation as it would be on a nonrotating globe.


Coriolis Force
A moving mass travels in a straight line until acted on by some outside force. However, if one views the moving mass relative to his platform appears to be deflected or curved. A similar apparent force deflects moving particles on the earth. Because the Earth is spherical, the deflective force is much more complex.

The Coriolis force affects the paths of aircraft; missiles; flying birds; ocean currents; and, most important to the study of weather, air currents. The force deflects air to the right in the Northern Hemisphere. Coriolis force is at a right angle to wind direction and directly proportional to wind speed. That is, as wind speed increases, Coriolis force increases. At a given latitude, double the wind speed and you double the Coriolis force.

Coriolis force varies with latitude from zero at the Equator to a maximum at the poles. It influences wind direction everywhere except immediately at the Equator; but the effects are more pronounced in middle and high latitudes.

The General Circulation
As air is forced aloft at the Equator and begins its high-level trek northward, the Coriolis force turns it to the right or to the east as shown in figure 3. Wind becomes westerly at about 30° latitude temporarily blocking further northward latitude temporarily blocking further northward movement. Similarly, as air over the poles begin its low-level journey southward toward the Equator, it likewise is deflected to the right and becomes an east wind, halting for a while its southerly progress–also shown in figure 3. As a result, air literally “piles up” at about 30° and 60° latitude in both hemispheres. The added weight of the air increases the pressure into semipermanent high pressure belts.

Figure 3.  In the Northern Hemisphere, Coriolis force turns high level southerly winds to westerlies at about 30° latitude, temporarily halting further northerly progress.

Figure 3. In the Northern Hemisphere, Coriolis force turns high level southerly winds to westerlies at about 30 latitude, temporarily halting further northerly progress.


The building of these high pressure belts creates a temporary impasse disrupting the simple convective transfer between the Equator and the poles. The restless atmosphere cannot live with this impasse in its effort to reach equilibrium. Something has to give. Huge masses of air begin overturning in middle latitudes to complete the exchange.

Large masses of cold air break through the northern barrier plunging southward toward the Tropics. Large midlatitude storms develop between cold outbreaks and carry warm air northward. The result is a midlatitude band of migratory storms with ever changing weather. Figure 4 is an attempt to standardize this chaotic circulation into an average general circulation.

Figure 4. General average circulation in the Northern Hemisphere.

Figure 4. General average circulation in the Northern Hemisphere.


Since pressure differences cause wind, seasonal pressure variations determine to a great extent the areas of these cold air outbreaks and midlatitude storms. But, seasonal temperature changes. We have learned that, at the surface, warm temperatures to a great extent determine low pressure and cold temperatures, high pressure. We have also learned that seasonal temperature changes over continents are much greater than over oceans.

During summer, warm continents tend to be areas of low pressure and the relatively cool oceans, high pressure. In winter, the reverse is true. As the air tries to blow outward from the high pressure, it is deflected to the right by the Coriolis force. Thus, the wind around a high blows clockwise. The high pressure with its associated wind system is an anticyclone.

The storms that develop between high pressure systems are characterized by low pressure. As winds try to blow inward toward the center of low pressure, they also are deflected to the right. Thus, the wind around a low is counterclockwise. The low pressure and its wind system is a cyclone. Figure 5 shows winds blowing parallel to isobars (contours on upper level charts). The winds are clockwise around highs and counterclockwise around lows.

Figure 5. Cyclonic and anticyclonic wind patterns.

Figure 5. Cyclonic and anticyclonic wind patterns.

Be here Thursday for a post of Thunderstorms from our very own CFI. Thanks for following the Learn to Fly Blog!

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CFI Brief: Pitot-Static Systems and Flight Instruments, Part III

Today we will finally wrap up our three part post on the pitot-static system and related instruments with a quick discussion on errors. If an error is noted on one of the three pitot-static instruments it is almost always caused by either a blockage in the pitot-tube or one of the static ports. As we’ve discussed previously part of you pre-flight preparation is to check the condition of both the pitot-tube and static port to verify each is free and clear of any visible debris or blockages like moisture, dirt, or insects. Blockages may not always be visible to the naked eye so it is important to also perform an instrument cockpit check. On the ground the ASI should indicate zero, VSI should indicate near the zero line, and the altimeter should indicate approximate field elevation when set to the current local altimeter. Remember a blocked pitot-tube will only affect the accuracy of the ASI while a blocked static port will affect all three instruments.

Because the pitot-tube has two openings (ram air inlet and drain hole) you can experience either a complete or partial blockage each affecting the ASI differently.

Ram air blocked, drain hole clear—Air already in the systems vents out the drain hole and the pressure becomes ambient (equal to the pressure outside). Because no difference in pressure is noted between ram and static the ASI will indicate zero.

Ram air and drain hole blocked—Pressure inside becomes trapped and no change will occur if airspeed is increased or decreased. However if the static port is still functioning and you change altitude then the ASI will indicate an increase of airspeed with a climb and decrease of airspeed with a descent (see figure 1). This is a result of the static air pressure changing against the trapped impact pressure which remains constant.

Figure 1. Blocked pitot system with clear static system.

Figure 1. Blocked pitot system with clear static system.

The static system will cause errors in all three instruments if a blockage occurs.

Static blocked pitot-tube clear—ASI will function, however, inaccurately. If a climb is made above the level of where the static port blockage occurred then the ASI will indicate an airspeed lower than actual. If a descent is made below the altitude where the blockage occurred then the ASI will indicate airspeed higher than actual.

Static blocked—Altimeter will freeze at the altitude where the blockage occurred. The VSI will indicate a continuous zero.

A quick reference chart for Pitot-Static Errors.

A quick reference chart for Pitot-Static Errors.

As we have already learned the majority of aircraft are equipped with an alternate static source located within the cockpit in the event of a system failure. It is important to remember pressure inside the cockpit is lower than pressure outside so there will be variations and some inaccuracy to the indications shown on the instruments. The pilots operating handbook will typically list corrections when using alternate static air.

This sums up our three part series on the pitot-static systems. If you should have any questions please feel free to leave them in the comments section below or send us an email.

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Procedures and Airport Operations: Airport Markings

Airport pavement markings and signs provide information useful to pilots during takeoff, landing, and taxiing. Uniformity in airport markings and signs from one airport to another enhances safety and improves efficiency. This introduction to markings and signs comes from the FAA’s Pilot’s Handbook of Aeronautical Knowledge. More on airport marking aids and signs, detailed with color images, can be found in Chapter 2 of the FAR/AIM, available from ASA in multiple formats.

Runway Markings
Runway markings vary depending on the type of operations conducted at the airport. Figure 1 shows a runway that is approved as a precision instrument approach runway and some other common runway markings. A basic VFR runway may only have centerline markings and runway numbers.

Selected airport markings and surface lighting.

Figure 1. Selected airport markings and surface lighting.

Since aircraft are affected by the wind during takeoffs and landings, runways are laid out according to the local prevailing winds. Runway numbers are in reference to magnetic north. Certain airports have two or even three runways laid out in the same direction. These are referred to as parallel runways and are distinguished by a letter added to the runway number (e.g., runway 36L (left), 36C (center), and 36R (right)).

Another feature of some runways is a displaced threshold. A threshold may be displaced because of an obstruction near the end of the runway. Although this portion of the runway is not to be used for landing, it may be available for taxiing, takeoff, or landing rollout. Some airports may have a blast pad/stopway area. The blast pad is an area where a propeller or jet blast can dissipate without creating a hazard. The stopway area is paved in order to provide space for an aircraft to decelerate and stop in the event of an aborted takeoff. These areas cannot be used for takeoff or landing.

Taxiway Markings
Aircraft use taxiways to transition from parking areas to the runway. Taxiways are identified by a continuous yellow centerline stripe and may include edge markings to define the edge of the taxiway. This is usually done when the taxiway edge does not correspond with the edge of the pavement. If an edge marking is a continuous line, the paved shoulder is not intended to be used by an aircraft. If it is a dashed marking, an aircraft may use that portion of the pavement. Where a taxiway approaches a runway, there may be a holding position marker. These consist of four yellow lines (two solid and two dashed). The solid lines are where the aircraft is to hold. At some towered airports, holding position markings may be found on a runway. They are used when there are intersecting runways, and ATC issues instructions such as “cleared to land—hold short of runway 30.”

Other Markings
Some other markings found on the airport include vehicle roadway markings, VOR receiver checkpoint markings, and non-movement area boundary markings.

Vehicle roadway markings are used when necessary to define a pathway for vehicle crossing areas that are also intended for aircraft. These markings usually consist of a solid white line to delineate each edge of the roadway and a dashed line to separate lanes within the edges of the roadway. In lieu of the solid lines, zipper markings may be used to delineate the edges of the vehicle roadway. [Figure 2]

Figure 2. Vehicle roadway markings.

Figure 2. Vehicle roadway markings.

A VOR receiver checkpoint marking consists of a painted circle with an arrow in the middle. The arrow is aligned in the direction of the checkpoint azimuth. This allows pilots to check aircraft instruments with navigational aid signals.

A non-movement area boundary marking delineates a movement area under ATC. These markings are yellow and located on the boundary between the movement and nonmovement area. They normally consist of two yellow lines (one solid and one dashed).

Airport Signs
There are six types of signs that may be found at airports. The more complex the layout of an airport, the more important the signs become to pilots. Figure 3 shows examples of signs, their purpose, and appropriate pilot action. The six types of signs are:

  • Mandatory instruction signs—red background with white inscription. These signs denote an entrance to a runway, critical area, or prohibited area.
  • Location signs—black with yellow inscription and a yellow border, no arrows. They are used to identify a taxiway or runway location, to identify the boundary of the runway, or identify an instrument landing system (ILS) critical area.
  • Direction signs—yellow background with black inscription. The inscription identifies the designation of the intersecting taxiway(s) leading out of an intersection.
  • Destination signs—yellow background with black inscription and also contain arrows. These signs provide information on locating things, such as runways, terminals, cargo areas, and civil aviation areas.
  • Information signs—yellow background with black inscription. These signs are used to provide the pilot with information on such things as areas that cannot be seen from the control tower, applicable radio frequencies, and noise abatement procedures. The airport operator determines the need, size, and location of these signs.
  • Runway distance remaining signs—black background with white numbers. The numbers indicate the distance of the remaining runway in thousands of feet.
Figure 3. Airport signs.

Figure 3. Airport signs.

Be here Thursday for Part III of our CFI’s series on pitot-static instruments!

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