Aerodynamics: Turns and Load Factors

We’re devoting this week to aerodynamics, specifically the load factors experienced in turns. There’s more to turning your airplane than smoothly coordinating your ailerons and rudder pressure. Understanding the role of lift and gravity in a turn will help you fly efficiently and within the limitations of your airplane. The following is excerpted from William Kershner’s textbook The Student Pilot’s Flight Manual.

Lift is considered to act perpendicularly to the wingspan. Consider an airplane in straight and level flight where lift equals weight. Assume the airplane’s weight to be 2,000 pounds.

In Figure 1A everything’s great—lift equals weight. If the plane is banked is 60° as in Figure 1B, things are not so rosy. The weight’s value or direction does not change. It’s still 2,000 pounds downward. The lift, however, is acting at a different angle. The vertical component of lift is only 1,000 pounds, since the cosine of 60° is 0.500 (60° is used for convenience here; you certainly won’t be doing a bank that steep in the earlier part of your training). With such an unbalance, the plane loses altitude. The answer, Figure 1C, is to increase the lift vector to 4,000 pounds so that the vertical component is 2,000 pounds. This is done by increasing the angle of attack. Back pressure is applied to the elevators to keep the nose up.

Figure 1. The vertical component of lift must be equal to weight in a constant-altitude turn.

Figure 1. The vertical component of lift must be equal to weight in a constant-altitude turn.

If in our example of the 60° bank, you roll out level but don’t get rid of the extra 2,000 pounds of lift you have, obviously the airplane will accelerate upward (the “up” and “down” forces will no longer be in balance). Maybe your idea of coordination is that demonstrated by a ballroom dancer, but it’s necessary in flying too. Think of control pressures rather than movement. The smoother your pressures, the better your control of the airplane.

The turn introduces a new idea. It is possible to bank so steeply that the wings cannot support the airplane. The lift must be increased so drastically that the critical angle of attack is exceeded, and the plane stalls. In the previous example at 60° of bank, it was found that our effective wing area was halved; therefore, each square foot of wing area had to support twice its normal load. This is called a load factor of 2. A plane in normal, straight and level flight has a load factor of 1, or 1 “g.” You have this same 1 g load on your body at the time. Mathematically speaking, the load factor in the turn is a function of the secant of the angle of bank. The secant varies from 1 at 0° to infinity at 90°; so maintain altitude indefinitely in a constant 90° bank, an infinite amount of lift is required—and this is not available.

Figure 2. The effects of bank angle on the amount of lift that is directly opposing weight.

Figure 2. The effects of bank angle on the amount of lift that is directly opposing weight.

Putting it simply: The stall speed goes up in the constant altitude turn; and the steeper the bank, the faster the small speed jumps up, as can be seen in Figures 3 and 4. The load factors just discussed are “positive” load factors and are attained by pulling the wheel back, causing you to be pressed down in the seat. A negative load factor is applied if the control wheel is pushed forward abruptly, and in this case you feel “light” and tend to leave the seat. Lightplanes are generally stressed to take a maximum positive load factor varying from 3.8 to 6, and a negative load factor of between 1.52 and 3, depending on the make and model. Both you and the plane are able to stand more positive g’s than negative. Positive or negative load factors can be imposed on the plane by sharp up or down gusts as well as by the pilot’s handling of the elevators.

Figure 3. The relationship of stall speed to angle of bank

Figure 3. The relationship of stall speed to angle of bank.


Figure 4. Stall speed increases with bank.

Figure 4. Stall speed increases with bank.

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CFI Brief: Magnetos

So I’ve been told my airplane engine has two magnetos, but what the heck is a magneto? Well in a reciprocating airplane engine like that of Lycoming IO-360 found in many Cessna 172 aircraft the magnetos are a source of high-voltage electrical energy. This electrical energy is used to produce the spark to ignite the fuel-air mixture inside the cylinders of a reciprocating engine. They are called magnetos because they use a permanent magnet to generate the electrical current sent to the spark plugs.

Airplane Magneto

Airplane Magneto–About the size of a Coke can

Once the starter is engaged and the crank shaft begins turning, the magnetos will activate and start producing the electrical energy needed to create a spark in the cylinders. It is important to understand that the magnetos operate completely independent of the aircraft’s electrical system. This is done for safety; in the event of a complete electrical failure, the engine will not shut down.

Key point: as the crankshaft turns so do the magnets within the magnetos creating the aforementioned energy. So for the engine to initially start, some source of outside energy needs turn the crankshaft. This is most commonly done by engaging the starter within the engine which does require an initial amount of electrical energy that comes from the batteries. However, you may fly an older aircraft that does not contain a starter, as one of the aircraft engine components, in a case like this an individual would physical turn the crankshaft by hand propping the airplane (caution: hand propping is extremely dangerous, always consult the aircraft’s operating handbook and follow proper hand propping procedures).

You learned in Monday’s post that each magneto operates independently of one another and contains a 5-position ignition switch: OFF, R, L, BOTH, and START. When OFF is selected you have in turn grounded both magnetos preventing them from creating the necessary spark for engine ignition. If selecting R or L (Right or Left magneto) you are grounding only one magneto, the one which is not selected. For example, if the L is selected the right magneto is grounded. The system will operate on both magnetos when BOTH is selected. By moving the ignition switch to the START indication you will engage the aircraft engine starter and un-ground both magnetos. As in most cases, you will have to hold the switch in this position while engaging the starter, releasing the switch it will snap back into the BOTH position, as it is designed to do allowing the engine to run on the both magnetos.

5 Position Ignition Indicator Switch

5-Position Ignition Indicator Switch

So what happens if you accidentally turn the magnetos to the OFF position in-flight? Well, the engine will stop as no spark will be provided to the cylinders to create combustion. Even though the engine is stopped the propeller will likely still be windmilling due to aerodynamic forces. Because that prop is still spinning so is the crankshaft, so simply turning the ignition switch back to BOTH should allow your engine to restart without problem. Always consult the pilot operating handbook for all in-flight restart procedures as these can and will vary between aircraft. And please try not to accidentally turn your magnetos off.

Questions about the magnetos or ignition systems? Let us know in the comments section and we will do our best to answer your questions.

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Aircraft Systems: Ignition

You don’t have to be a mechanic to be a safe pilot, but a knowledge of how your engine works and what the engine instruments are telling you will make it easier to give your engine tender loving care and get long, reliable service from it. Today we’ll cover your airplane’s ignition system, with a post taken from The Complete Private Pilot by Bob Gardner.

There are many similarities between an automobile engine and an airplane engine. Both are internal combustion engines, both use spark plugs, and both use some type of fuel metering system related to throttle position. An aircraft is a four-cycle engine: Figure 1 illustrates the four cycles. The fuelair mixture is drawn into the cylinder as the piston moves downward on the intake stroke; as the piston moves upward with the valves closed, the mixture is compressed during the compression stroke. The burning of the fuel-air mixture after ignition drives the piston downward during the power stroke, and as the piston rises again with the exhaust valve open the exhaust stroke ends the four stroke cycle. Because your aircraft engine has four or more cylinders, each igniting at a different time, there is always one piston on a power stroke, and the process is continuous.

Figure 1. Four strokes of an aircraft engine.

Figure 1. Four strokes of an aircraft engine.

Your gas-powered airplane engine uses a magneto as the source of ignition. Magneto may not be a familiar term to you, but your gas lawnmower, chain saw, or outboard motor all use magnetos. A magneto is a self-contained source of electrical impulses, using the physical motion of a coil and a fixed magnetic field to develop ignition voltage. To start the engine, you provide that physical motion by pulling a cord on your lawnmower, chain saw, or outboard. The starter motor does the job in the airplane, rotating the engine until the magneto-developed spark ignites the mixture. You have probably seen older airplanes without electrical systems (and newer airplanes with starter problems) being started manually—rotating the propeller by hand (“propping”) causes the magneto to generate a voltage which goes to the spark plug to ignite the fuel/air mixture. Hand-propping an airplane is a hazardous undertaking which requires an experienced and knowledgeable person both in the cockpit and at the propeller. Once an airplane engine is started, the magnetos provide continuous ignition on their own—the airplane’s electrical system and the starter motor have done their jobs. The master switch plays no further role in engine operation.

Each cylinder in your airplane engine has two spark plugs, each fired by a different magneto (see Figure 2). This has two advantages: better combustion efficiency, and safety. The engine will run on either magneto if one should develop a problem. Magnetos are totally independent of the aircraft electrical system.

Figure 2. Spark plugs and magnetos.

Figure 2. Spark plugs and magnetos.

When you turn the ignition off, with a key or with switches, you are connecting the electrical output of the magneto to the metal block of the engine where it is shorted to electrical ground and cannot fire the spark plugs. This “shorting out” is done through a wire called a P-lead, and if a P-lead is broken its associated magneto can fire the spark plugs even with the ignition in the OFF position. For this reason, you should treat all propellers with respect—moving the propeller might cause a magneto to start the engine unexpectedly if a P-lead has broken. During the preflight check of the airplane and its systems you will run the engine on each magneto separately. The ignition switch is marked OFF-LEFT-RIGHT-BOTH, if there is a start button, and OFF-L-R-BOTH-START if there is not (the START position is spring-loaded to return to the BOTH position when finger pressure is removed). In the OFF position, the P-leads of both magnetos are grounded; in the LEFT position, the right magneto is grounded and you are checking the operation of the left magneto. In the RIGHT position, then, the P-lead of the left magneto is grounded, and in the BOTH position, both magnetos are capable of delivering a spark.

As you cut the ignition sources in half you will lose some power, reflected as a drop in revolutions per minute (rpm). If no drop occurs when one magneto is shut off, that magneto probably has a broken P-lead, and the flight should be delayed until a mechanic checks it. Some authorities recommend checking for a broken P-lead just before shutting the engine down after a flight, by turning the ignition switch to its “OFF” position momentarily while at idle power; if the engine continues to run, there is probably a broken P-lead.

You should check your engine’s magnetos each time you are in the runup area preparing for takeoff. Magnetos can develop faults that are not readily detectable in cruising flight but which might rob the engine of the power required for takeoff.

We’ll have more on aircraft engines from our CFI on Thursday.

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CFI Brief: Effective Communication

According to FAA research and safety reports there are three distinct types of communication errors:

An absence of a pilot readback. The pilot merely acknowledges the clearance that he/she in actuality misunderstood.

A Readback/hearback error. When a pilot reads back a clearance incorrectly and the controller fails to catch the error.

Hearback Type II Errors. This is when the pilot correctly reads back a clearance but the controller fails to notice that the clearance issued was not the intended one.

I point these errors out to show that effective communication relies heavily upon two people. If you plan on becoming an instructor, one day you will learn that the process of communication is composed of three elements: source, symbols, and receiver. Think of the source as the person (pilot or controller) transmitting the information and the receiver (pilot or controller) as just that the person listening and receiving that information. Symbols are those words or even signs used to transmit the information between the two. When a symbol is misunderstood confusion can occur.

In Monday’s post we learned that there is established phraseology and accepted techniques to be used while communicating on the radio. This is in place to alleviate the confusion that can occur between the pilot and controller through the process of transmitting symbols.

Standard procedural words and phrases.

Standard procedural words and phrases.

These question below you may likely encounter on the FAA Private Pilot Knowledge Exam and will test your knowledge on correct phraseology and technique used on the radios.

1. The correct method of stating 10,500 feet MSL to ATC is

2. The correct method of stating 4,500 feet MSL to ATC is

3. If instructed by ground control to taxi to Runway 9, the pilot may proceed
A—via taxiways and across runways to, but not onto, Runway 9.
B—to the next intersecting runway where further clearance is required.
C—via taxiways and across runways to Runway 9, where an immediate takeoff may be made.

4. When flying HAWK N666CB, the proper phraseology for initial contact with McAlester AFSS is

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Communication Procedures: Phraseology, Techniques, and Procedures

Effective communication is absolutely critical to your safety and the safety of those in the air around you and on the ground. There’s a well established phraseology and accepted techniques in aviation, so mastering this will be key in your flight training. Take a look at the introduction to radio communications, excerpted from Bob Gardner’s communication textbook Say Again, Please, as well as our CFI’s post on how the FAA expects you to understand radio phraseology. Today’s post comes from Bob Gardner’s The Complete Private Pilot and from the 2015 FAR/AIM.

Always use the phonetic alphabet when identifying your aircraft and to spell out groups of letter or unusual words under difficult communications conditions. Do not make up your own phonetic equivalents; this alphabet was developed internationally to be understandable by non-English speaking pilots and ground personnel.

AIM Table 4-2-2. Phonetic Alphabet/Morse Code

AIM Table 4-2-2. Phonetic Alphabet/Morse Code

Misunderstandings about altitude assignments can be hazardous. Check the Aeronautical Information Manual for more officially accepted techniques. However, as you monitor aviation frequencies you will hear pilots use a decimal system: TWO POINT FIVE for 2,500. The FAA has never commented on this practice and it is commonly accepted—don’t try it in another country.

500 . . . . . . . FIVE HUNDRED
10,000 . . . . . TEN THOUSAND

Pilots flying above 18,000 feet use flight levels when referring to altitude:


Bearings, courses, and radials are always spoken in three digits:


Address ground controllers as “SEATTLE GROUND CONTROL,” control towers as “O’HARE TOWER,” radar facilities as “MIAMI APPROACH,” or “ATLANTA CENTER.” When calling a flight service station, use radio: “PORTLAND RADIO, MOONEY FOUR VICTOR WHISKEY.”

When making the initial contact with a controller, use your full callsign, without the initial november: “BOISE GROUND, PIPER THREE SIX NINER ECHO ROMEO AT THE RAMP WITH INFORMATION GOLF, TAXI FOR TAKEOFF.”

Use of the last three digits is acceptable for subsequent transmissions: “ROGER, TAXI TWO EIGHT RIGHT, NINER ECHO ROMEO.”

Many uncontrolled airports share UNICOM frequencies, and if you do not identify the airport at which you are operating, your transmissions may serve to confuse other pilots monitoring the frequency. Don’t say, “STINSON FOUR SIX WHISKEY DOWNWIND FOR RUNWAY SIX,” say, “ARLINGTON TRAFFIC, STINSON FOUR SIX WHISKEY DOWNWIND FOR RUNWAY SIX, ARLINGTON.” That way, pilots at other airports sharing Arlington’s UNICOM frequency will not be nervously looking over their shoulders.

The most valuable word in radio communication is “unable.” It should be used whenever you are asked to do something you don’t want to do or are prohibited from doing, like flying to close to a cloud. An air traffic controller who hears “unable” will come up with an alternative plan. You are the pilot-in-command and the only person in position to determine the safety of a proposed action. The second most important word is “immediately.” If a controller tells you to turn left immediately, or to climb immediately or to do anything else immediately, do not reach for the microphone—do it. The other side of the coin is when you need to cut corners to get on the ground in a hurry—a sick passenger, for example. When you make your initial call, tell the controller that you need to land immediately.

Online Resources

Note: None of these sites are official and they should be used only for planning and orientation. Aerial views are not current.

You can find the frequencies to be used at any airport at

An excellent resource for radio communication procedures is “Say it Right,” produced by the Air Safety Foundation. It can be found at

More on communication procedures from our CFI on Thursday.

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CFI Brief: VOR Service Volumes

VORs are anything but standard. When operating under the guidance of radio navigational aids like a VOR it’s important to understand the restrictions and standard service volumes (SSV) associated with each aid of intended use. VOR standard service volumes are designated in three classes: Terminal (T), Low Altitude (L), and High Altitude (H). Your aeronautical charts will at minimum show position, frequency, and Morse code identifier of each VOR; you can find additional information like the SSV in your AF/D.

Terminal VOR (TVOR) has a range of 25NM from 1,000 feet AGL to 12,000 feet AGL.

Fig 1-1-03

Low Altitude VOR (LVOR) has a range of 40NM from 1,000 feet AGL to 18,000 feet AGL.

Fig 1-1-02

High Altitude VOR (HVOR) is a little trickier and has several different service volumes based on altitude. The first of these starts at 1,000 feet AGL up to 14,500 feet AGL with a range of 40NM. As we increase in altitude so does range, from 14,500 feet AGL up to 18,000 feet—a range of 100NM. Then again, range will increase to 130NM from 18,000 feet up to 45,000 feet. Once we get over 45,000 feet the range will decrease back down to 100NM up to 60,000.

Fig 1-1-01

A VOR allows for high quality line of sight reception so even though you may be within the SSV of a particular VOR you are not guaranteed reception. This would be caused by local terrain features surrounding the ground station. Furthermore, it’s important to check NOTAMs prior to using any VOR for navigational purposes as often happens a VOR may be placed out of service. You can see from the NOTAM below how the Santa Monica VOR is shown to be out of service from February 10, 2015 at 1600 until March 14, 2015 at 2359.

!SMO 02/013 SMO NAV VOR/DME OUT OF SERVICE 1502101600-1503142359

More commonly, a VOR may have further range and boundary restrictions imposed. This can occur for a number of reasons, something as simple as a crane being erected near the ground station causing blockage of a select range of radials.

For further information on VORs you can refer to ASA’s publication of the Aeronautical Information Manual. Or check out the Navigation section of the Learn to Fly Blog for more posts on VORs.

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Navigation: VHF Omnidirectional Range (VOR)

We’re devoting this week’s posts to the VOR, a radio navigation system used worldwide by private and commercial pilots. This introduction comes from The Student Pilot’s Flight Manual, by William Kershner.

The most useful of the enroute radio navigation aids, other than GPS, is the VHF omnirange, or VOR as it is sometimes called. The VOR frequency band is from 108.00 to 117.95 MHz and uses the principle of electronically measuring an angle. The VOR puts out two signals. One is all-directional (or omnidirectional) and the other is a rotating signal. The all-directional signal contracts and expands 30 times a second, and the rotating signal rotates clockwise at 30 revolutions per second. The rotating signal has a positive and a negative side.

The all-directional or reference signal is timed to transmit at the same instant the rotating beam passes magnetic north. These rotating beams and the reference signal result in radial measurements.

Your omni receiver picks up the all-directional signal. Some time later it picks up the maximum point of the positive rotating signal. The receiver electronically measures the time difference, and it is indicated in degrees as your magnetic bearing in relation to the station (Figure 1). For instance, assume it took a minute instead of 1/30 of a second for the rotating signal to make one revolution. You receive the all-directional signal and 20 seconds later you receive the rotating signal. This means that your position is 20/60 or 1/3 of the way around. (One-third of 360° is 120° and you are on the 120 radial.) The VOR receiver does this in a quicker, more accurate way.

Figure 1. Principle of the VOR.

Figure 1. Principle of the VOR.

The aircraft VOR receiver presentation is composed of four main parts: (1) a dial to select the frequency of the station you want to use; (2) an azimuth or omni bearing selector (OBS) calibrated from 0 to 360; (3) a course deviation indicator (CDI), a vertical needle that moves left or right; and (4) a TO-FROM indicator. Figure 2 is one type of VOR receiver.

Figure 2. A VOR receiver. (Courtesy of Narco Avionics)

Figure 2. A VOR receiver. (Courtesy of Narco Avionics)

Suppose you want to fly to a certain VOR, say, 30 miles away. First you would tune the frequency and identify the station. You should have some idea where you are in relation to the station, but if not, turn the azimuth or direction selector until the deviation indicator or needle is centered, and the TO-FROM indicator says TO. Read the OBS. This is your course TO the VOR. If you turn on that magnetic course and keep the needle centered, you’ll fly right over the VOR. If the TOFROM says TO and you are going to the station, fly the needle. If the needle moves to the left, the selected bearing is to the left and you will turn the plane in that direction and fly until the needle is centered again. You will have to correct for wind to stay on the selected bearing (Figure 3).

Figure 3. Using a VOR receiver to track to the station.

Figure 3. Using a VOR receiver to track to the station.

While your bearing to the station is 300°, you are on the 120 (one-two-zero) radial. (The radials are like spokes from the VOR.) (See Figure 3A.) The radials are numbered from 0 through 359, so if the station asked where you were, you’d say, “I’m inbound to the station on the 120 (one-two-zero) radial.” For example, if there’s a westerly wind, the plane may drift from the selected bearing as shown in Figure 3B, and the LEFT-RIGHT needle would look like Figure 4.

Figure 4. A heading indicator and two types of VOR indicators. The selected radial is to the left.

Figure 4. A heading indicator and two types of VOR indicators. The selected radial is to the left.

The angle of correction or the “cut” you’ll take will depend on the amount that you’ve drifted from course. Usually 30° would be the maximum even at some distance from the station. It may take some time for the needle to center again if you’re far out. After the needle returns to center, turn back toward the original heading, but this time include an estimated wind correction on your compass or heading indicator. Watch the needle and make further corrections as needed.

When you cross over the station, the TO-FROM indicator will oscillate, then fall to FROM. The receiver now says that you are on a bearing of 300° FROM the station. Always make sure your OBS is set close to your compass heading. This way the needle always points toward the selected radial. If you turned around and headed back to the station after passing it on a course of 300° and did not reset the bearing selector to coincide with your heading, the needle would work in reverse. Always set your OBS to the course to be followed; then the needle senses correctly.

The sensing is incorrect if you correct toward the needle, and the needle moves farther away from the center as you fly.

But, back to the station passage: The TO-FROM says FROM and your plane is on a course of 300° FROM the VOR, so the needle is correct. Continue to fly the needle as you did before the VOR was reached.

Most omni needles are set up so that a full deflection from center is 10° or more. If the needle is deflected halfway, you can figure that you are about 5° from your selected bearing.

VORs are identified by Morse code and/or by the automatic recorded voice identification (“Airville VOR”).

The accuracy of the VOR ground facility is generally ±1°, but some stations in mountainous terrain may have errors greater than this for some radials or may be unusable below certain altitudes; this is duly noted in the Airport/Facility Directory.

We’ll have more on the VOR from our very own CFI this Thursday.

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CFI Brief: Time Zones

Coordinated Universal Time, Universal Time Coordinated, Greenwich Mean Time, Zulu Time—I am sure you have heard these terms at some point in your flight training, but what is all of it? To keep it simple these are essentially one in the same: time corrected for seasonal variations in the earth’s rotation about the sun. As you may know, moving east or west across the lines of longitude we travel through various time zone changes. For example, 10 AM in Seattle, WA is 1 PM in New York City (3 hours ahead). This can get extremely confusing to pilots traveling across multiple time zones, so that’s why we use Coordinated Universal Time or UTC for short. All aeronautical communications across the world are expressed in UTC.

Standard Time Zones in the United States — Click to enlarge!

Standard Time Zones in the United States — Click to enlarge!

If we were to convert 10 AM in Seattle to UTC time we would get 1800 UTC. Let’s take a look at how we did that.

Step 1 is to convert our local time to the 24 hour clock.

10:00 AM Local = 1000 Local

Step 2 using the chart below we can determine our time zone conversion of + 8.

1000 + 8 = 1800 UTC

For daylight savings time you would subtract 1, so be careful you understand what time of the year it is.

U.S. Time Zones in relation to UTC — Click to enlarge!

U.S. Time Zones in relation to UTC — Click to enlarge!

You may not always have this chart available but you can always find time zone conversions listed in the A/FD under each airport.

Let’s work through a realistic scenario (at least my idea of one). A few friends and I plan on traveling to San Diego, CA from Dallas, TX. We have an afternoon golf tee time at 3 PM local pacific standard time. We have determined the flight will take us 4 hours. If we want to arrive in San Diego at 2 PM local time, when should we leave?

Step 1. Convert 2 PM to the 24 hour clock:

02:00 PM + 12 = 1400 Local

Step 2. Convert 1400 Local to UTC (refer to the chart above):

1400 + 8 = 2200 UTC

Step 3. Since we have already determined that the flight will take us 4 hours, simply subtract the travel time from the UTC time:

2200 – 0400 = 1800 UTC

Step 4. To arrive on time, we must leave Dallas by 1800 UTC. Convert this to local time by referring back to our chart. Dallas is in Central Standard Time, so we would subtract 6 hours:

1800 – 0600 = 1200 (on the 24 hour clock) or 12:00 PM local central standard time

Perfect. We would need to be wheels up in Dallas at 12 PM local. If you think about it, this makes sense too: there is a two hour time gap between the time we leave local (12 PM) and the time we arrive local (2 PM), including the two hour time zone change between Central Standard Time and Pacific Standard Time and we get 4 hours.

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Enroute Flight: Latitude and Longitude

Understanding the imaginary grid we’ve laid out around and across our planet is key in flight planning and ultimately your safety. Today, we’ll review some of the basics with help from the FAA textbook Pilot’s Handbook of Aeronautical Knowledge.

The equator is an imaginary circle equidistant from the poles of the Earth. Circles parallel to the equator (lines running east and west) are parallels of latitude. They are used to measure degrees of latitude north (N) or south (S) of the equator. The angular distance from the equator to the pole is one-fourth of a circle or 90°. The 48 conterminous states of the United States are located between 25° and 49° N latitude. The arrows in the figure below labeled “Latitude” point to lines of latitude.

Meridians and parallels--the basis of measuring time, distance, and direction.

Meridians and parallels–the basis of measuring time, distance, and direction.

Meridians of longitude are drawn from the North Pole to the South Pole and are at right angles to the Equator. The “Prime Meridian” which passes through Greenwich, England, is used as the zero line from which measurements are made in degrees east (E) and west (W) to 180°. The 48 conterminous states of the United States are between 67° and 125° W longitude. The arrows in the figure above labeled “Longitude” point to lines of longitude.

Any specific geographical point can be located by reference to its longitude and latitude. Washington, D.C., for example, is approximately 39° N latitude, 77° W longitude. Chicago is approximately 42° N latitude, 88° W longitude.

The meridians are also useful for designating time zones. A day is defined as the time required for the Earth to make one complete rotation of 360°. Since the day is divided into 24 hours, the Earth revolves at the rate of 15° an hour. Noon is the time when the sun is directly above a meridian; to the west of that meridian is morning, to the east is afternoon. The standard practice is to establish a time zone for each 15° of longitude. This makes a difference of exactly 1 hour between each zone.

By using the meridians, direction from one point to another can be measured in degrees, in a clockwise direction from true north. To indicate a course to be followed in flight, draw a line on the chart from the point of departure to the destination and measure the angle which this line forms with a meridian. Direction is expressed in degrees.

Because meridians converge toward the poles, course measurement should be taken at a meridian near the midpoint of the course rather than at the point of departure. The course measured on the chart is known as the true course (TC). This is the direction measured by reference to a meridian or true north. It is the direction of intended flight as measured in degrees clockwise from true north.

As shown in the figure below, the direction from A to B would be a true course of 065°, whereas the return trip (called the reciprocal) would be a true course of 245°.

Courses are determined by reference to meridians on aeronautical charts.

Courses are determined by reference to meridians on aeronautical charts.

The true heading (TH) is the direction in which the nose of the aircraft points during a flight when measured in degrees clockwise from true north. Usually, it is necessary to head the aircraft in a direction slightly different from the true course to offset the effect of wind. Consequently, numerical value of the true heading may not correspond with that of the true course. For the purpose of this discussion, assume a no-wind condition exists under which heading and course would coincide. Thus, for a true course of 065°, the true heading would be 065°. To use the compass accurately, however, corrections must be made for a magnetic variation and compass deviation.

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CFI Brief: W&B Terms

Part of your preflight duties as a pilot will be to determine the weight and balance of the aircraft. Trying to takeoff and fly with an aircraft over max gross weight or out of balance can cause drastic consequences on the airplane’s ability to fly. Below are some of the terms that will be important to understand when conducting weight and balance computations.

Empty Weight—The weight of the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the aircraft. Empty weight includes optional and special equipment, fixed ballast, full reservoirs of hydraulic fluid, engine lubricating oil, and the unusable fuel, but does not include occupants, baggage, or cargo.

Useful Load—The difference between the maximum allowable weight of the aircraft and its empty weight. The useful load of an aircraft includes the weight of the fuel and oil, the crew, passengers, all of their baggage, and any cargo carried.

Takeoff Weight—The weight of an aircraft just before brake release. It is the ramp weight less the weight of the fuel burned during start and taxi.

Landing Weight—The maximum weight an aircraft is allowed to have for landing. Landings put far more stress into an aircraft structure than takeoffs, and therefore large aircraft that fly for long distances are allowed to have a greater weight for takeoff than for landing.

Datum—An imaginary vertical reference plane or line chosen by the aircraft manufacturer from which all arms used for weight and balance computation are measured.

Arm—The horizontal distance in inches between a reference datum line and the center of gravity of an object. If the object is behind the datum, the arm is positive, and if it is ahead of the datum, the arm is negative.

Moment—The product of the weight of an object multiplied by its arm expressed in pound-inches (lbs-in). A formula that is used to find moment is usually: Weight x Arm = Moment.

Datum Reference

Moment Index—A moment divided by a constant, such as 10, 100, 1,000, or an even larger number. The use of a moment index allows weight and balance computations to be made using smaller numbers, decreasing the chance for errors.

Center of Gravity—The point at which an aircraft will balance, expressed in inches from datum. The center of gravity is found by dividing the total moment by the total weight: Total Moment / Total Weight = CG (inches aft of datum).

Standard Weight—Values used when specific weights are not available.

  • General Aviation Crew and Passenger. 170lbs each.
  • AvGas–6 lbs/U.S. gallon
  • Turbine Engine Fuel–6.7 lbs/U.S. gallon
  • Lubricating Oil–7.5 lbs/U.S. gallon
  • Water–8.35 lbs/U.S. gallon

A question for you: if an aircraft is loaded 90 pounds over maximum certificated gross weight and fuel (AvGas) is drained to bring the aircraft weight within limits, how much fuel should be drained?

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