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Aircraft Systems: Fuel Injection Systems

Today we’re featuring an excerpt from The Pilot’s Manual: Ground School (PM-2C).

Many sophisticated engines have fuel directly metered into the induction manifold and then into the cylinders without using a carburetor. This is known as fuel injection.

A venturi system is still used to create the pressure differential. This is coupled to a fuel control unit (FCU), from which metered fuel is piped to the fuel manifold unit (fuel distributor). From here, a separate fuel line carries fuel to the discharge nozzle in each cylinder head, or into the inlet port prior to the inlet valve. The mixture control in the fuel injection system controls the idle cut-off.

With fuel injection, each individual cylinder is provided with a correct mixture by its own separate fuel line. (This is unlike the carburetor system, which supplies the same fuel/air mixture to all cylinders. This requires a slightly richer-than-ideal mixture to ensure that the leanest-running cylinder does not run too lean.)

The advantages of fuel injection include:

  • freedom from fuel ice (no suitable place for it to form);
  • more uniform delivery of the fuel/air mixture to each cylinder;
  • improved control of fuel/air ratio;
  • fewer maintenance problems;
  • instant acceleration of the engine after idling with no tendency for it to stall; and
  • increased engine efficiency.

Starting an already hot engine that has a fuel injection system may be difficult because of vapor locking in the fuel lines. Electric boost pumps that pressurize the fuel lines can help alleviate this problem. Having very fine fuel lines, fuel injection engines are more susceptible to any contamination in the fuel such as dirt or water. Correct fuel management is imperative! Know the fuel system of your particular airplane. Surplus fuel provided by a fuel injection system will pass through a return line which may be routed to only one of the fuel tanks. If the pilot does not remain aware of where the surplus fuel is being returned to, it may result in uneven fuel loading in the tanks or fuel being vented overboard (thus reducing flight fuel available).

Typical fuel injection system.

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Aircraft Systems: Propeller Principles

The propeller, the unit which must absorb the power output of the engine, has passed through many stages of development. Today we’ll feature an excerpt introducing the general concepts of a propeller from our recently released book Aircraft Systems for Pilots.

Propeller Principles
The aircraft propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing. As a result of their construction, the propeller blades produce forces that create thrust to pull or push the airplane through the air.

The power needed to rotate the propeller blades is furnished by the engine. The propeller is mounted on a shaft. which may be an extension of the crankshaft on low-horsepower engines; on high-horsepower engines, it is mounted on a propeller shaft which is geared to the engine crankshaft. In either case. the engine rotates the airfoils of the blades through the air at high speeds, and the propeller transforms the rotary motion (power) of the engine into thrust.

The engine supplies brake horsepower through a rotating shaft. and the propeller converts it into thrust horsepower. In this conversion, some power is wasted. For maximum efficiency, the propeller must be designed to keep this waste as small as possible. Since the efficiency of any machine is the ratio of the useful power output to the power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. The usual symbol for propeller efficiency is the Greek letter η (eta). Propeller efficiency varies from 50% to 87%, depending on how much the propeller “slips.”

THP = BHP × Propeller Efficiency

Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch (see figure 1). Geometric pitch is the distance a propeller should advance in one revolution; effective pitch is the distance it actually advances. Thus, geometric or theoretical pitch is based on no slippage, but actual, or effective pitch, recognizes propeller slippage in the air.

Figure 1. Effective and geometric pitch.

The typical propeller blade can be described as a twisted airfoil of irregular planform. Two views of a propeller blade are shown in figure 2. For purposes of analysis, a blade can be divided into segments, which are located by station numbers in inches from the center of the blade hub. The cross sections of each 6-in. blade segment are shown as airfoils in the right-hand side of figure 2. Also identified in figure 2 are the blade shank and the blade butt. The blade shank is the thick, rounded portion of the propeller blade near the hub, which is designed to give strength to the blade. The blade butt, also called the blade base or root, is the end of the blade that fits in the propeller hub. The blade tip is that part of the propeller blade farthest from the hub, generally defined as the last 6 in. of the blade.

Figure 2. Propeller blade design.

A cross section of a typical propeller blade is shown in figure 3. This section or blade element is an airfoil comparable to a cross section of an aircraft wing. The blade back is the cambered or curved side of the blade, similar to the upper surface of an aircraft wing. The blade face is the flat side of the propeller blade (“facing” the pilot, if the propeller is up front in the tractor position). The chord line is an imaginary line drawn through the blade from the leading edge to the trailing edge. The leading edge is the thick edge of the blade that meets the air as the propeller rotates.

Figure 3. Cross section of a propeller blade.

Blade angle, usually measured in degrees, is the angle between the chord line of the blade and the plane of rotation (figure 4). The chord of the propeller blade is determined in about the same manner as the chord of an airfoil. In fact, a propeller blade can be considered as being made up of an infinite number of thin blade elements, each of which is a miniature airfoil section whose chord is the width of the propeller blade at that section. Because most propellers have a flat blade face, pitch is easily measured by finding the angle between a line drawn along the face of the propeller blade and a line scribed by the plane of rotation. Pitch is not the same as blade angle, but, because pitch is largely determined by blade angle, the two terms are often used interchangeably. An increase or decrease in one is usually associated with an increase or decrease in the other.

Figure 4. Propeller efficiency varies with airspeed while constant speed propellers maintain high efficiency over a wide range of airspeeds.

Forces Acting On The Propeller
A rotating propeller is acted upon by centrifugal, twisting, and bending forces. The principal forces acting on a rotating propeller are illustrated in figure 5.

Figure 5. Forces acting on a rotating propeller.

Centrifugal force (A of figure 5) is a physical force that tends to throw the rotating propeller blades away from the hub. Torque bending force (B of figure 5), in the form of air resistance, tends to bend the propeller blades opposite to the direction of rotation. Thrust bending force (C of figure 5) is the thrust load that tends to bend propeller blades forward as the aircraft is pulled through the air. Aerodynamic twisting force (D of figure 5) creates a rotational force (twisting moment) about the center of pressure, causing the blade to tend to pitch to a lower blade angle (streamline).

At high angles of attack this twisting moment is reduced as the center of lift moves forward. At very high blade angles of attack, the blades may exhibit a weak tendency to pitch toward a greater blade angle.

Centrifugal twisting force also twists the blade to flat pitch (unless the blade is counterweighted so its center of mass is behind the center of rotation). This is a strong force at normal propeller speeds. Imagine a string tied to your finger and to a small weight (see figure 6). If the weight is spinning about your finger, the weight will align itself directly in line with the point on your finger where it is tied (reference the plane scribed by the spinning weight). This same force causes the center of mass of the propeller to align itself with the center of rotation on the spin-plane, causing a strong pitch tendency toward minimum blade pitch angle.

Figure 6. Propeller forces.

A propeller must be capable of withstanding severe stresses, which are greater near the hub, caused by centrifugal force and thrust. The stresses increase in proportion to the RPM. The blade face is also subjected to tension from the centrifugal force and additional tension from the bending. For these reasons, nicks or scratches on the blade may cause very serious consequences.

A propeller must also be rigid enough to prevent fluttering, a type of vibration in which the ends of the blade twist back and forth at high frequency around an axis perpendicular to the engine crankshaft. Fluttering is accompanied by a distinctive noise often mistaken for exhaust noise. The constant vibration tends to weaken the blade and eventually causes failure.

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IFR: Precision Instrument Runway Markings

Today, we’re sharing an excerpt from The Pilot’s Manual Volume Three: Instrument Flying. This post is a follow-up to last month’s IFR: The Instrument Landing System (ILS).

To assist pilots transitioning to a visual landing at the conclusion of a precision instrument approach, precision instrument runways have specific markings.

A displaced threshold on an instrument runway is indicated by arrows in the middle of the runway leading to the displaced threshold mark. The runway edge lights to the displaced threshold appear red to an airplane on approach, and to an airplane taxiing to the displaced threshold from the absolute end of the runway. They appear white when taxiing back from the displaced threshold toward the absolute end of the runway. The green runway end lights seen on approach to a runway with a displaced threshold are found off the edge of the runway.

The runway surface with arrows to the displaced threshold is available for taxiing, takeoff and landing roll-out, but not for landing. The initial part of this runway is a non-touchdown area. If chevrons rather than arrows are used to mark the displaced threshold, then the surface is not available for any use, other than aborted takeoff from the other direction.

Displaced threshold markings with preceding blast pad or stopway.

Displaced threshold markings with preceding blast pad or stopway.

A precision instrument runway will contain a designation, centerline, threshold, aiming point, touchdown zone, and side strips as seen in the figure below. Runway threshold strips can be configured in two ways. Four solid strips on either side of the centerline or configured as such that the number of strips correlates to the width of the runway (see table). The runway aiming point markers are large rectangular marks on each side of the runway centerline usually placed 1,000 feet after the threshold and serve as a visual aiming point for the pilot. Touchdown zone markers identify the touchdown zone for landing operations, providing coded distance information in 500 foot intervals and shown as either one, two, or three vertical stripes on either side of the centerline.

Runway width based on number of runway threshold strips.

Runway width based on number of runway threshold strips.

Markings on a precision instrument runway.

Markings on a precision instrument runway.

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Aircraft Performance: Runway Surface and Gradient

Today’s post comes from the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B), which is now available as an eBook from ASA, iTunes, and Kindle.

Runway conditions affect takeoff and landing performance. Typically, performance chart information assumes paved, level, smooth, and dry runway surfaces. Since no two runways are alike, the runway surface differs from one runway to another, as does the runway gradient or slope.

Takeoff distance chart

Takeoff distance chart

Runway surfaces vary widely from one airport to another. The runway surface encountered may be concrete, asphalt, gravel, dirt, or grass. The runway surface for a specific airport is noted in the Chart Supplement U.S. (formerly Airport/Facility Directory). Any surface that is not hard and smooth increases the ground roll during takeoff. This is due to the inability of the tires to roll smoothly along the runway. Tires can sink into soft, grassy, or muddy runways. Potholes or other ruts in the pavement can be the cause of poor tire movement along the runway. Obstructions such as mud, snow, or standing water reduce the airplane’s acceleration down the runway. Although muddy and wet surface conditions can reduce friction between the runway and the tires, they can also act as obstructions and reduce the landing distance. Braking effectiveness is another consideration when dealing with various runway types. The condition of the surface affects the braking ability of the aircraft.

The amount of power that is applied to the brakes without skidding the tires is referred to as braking effectiveness. Ensure that runways are adequate in length for takeoff acceleration and landing deceleration when less than ideal surface conditions are being reported.

The gradient or slope of the runway is the amount of change in runway height over the length of the runway. The gradient is expressed as a percentage, such as a 3 percent gradient. This means that for every 100 feet of runway length, the runway height changes by 3 feet. A positive gradient indicates the runway height increases, and a negative gradient indicates the runway decreases in height. An upsloping runway impedes acceleration and results in a longer ground run during takeoff. However, landing on an upsloping runway typically reduces the landing roll. A downsloping runway aids in acceleration on takeoff resulting in shorter takeoff distances. The opposite is true when landing, as landing on a downsloping runway increases landing distances. Runway slope information is contained in the Chart Supplement U.S. (formerly Airport/ Facility Directory).

Chart Supplement U.S. information

Chart Supplement U.S. information

Water on the Runway and Dynamic Hydroplaning
Water on the runways reduces the friction between the tires and the ground and can reduce braking effectiveness. The ability to brake can be completely lost when the tires are hydroplaning because a layer of water separates the tires from the runway surface. This is also true of braking effectiveness when runways are covered in ice.

When the runway is wet, the pilot may be confronted with dynamic hydroplaning. Dynamic hydroplaning is a condition in which the aircraft tires ride on a thin sheet of water rather than on the runway’s surface. Because hydroplaning wheels are not touching the runway, braking and directional control are almost nil. To help minimize dynamic hydroplaning, some runways are grooved to help drain off water; most runways are not.

Tire pressure is a factor in dynamic hydroplaning. Using the simple formula in the figure below, a pilot can calculate the minimum speed, in knots, at which hydroplaning begins. In plain language, the minimum hydroplaning speed is determined by multiplying the square root of the main gear tire pressure in psi by nine. For example, if the main gear tire pressure is at 36 psi, the aircraft would begin hydroplaning at 54 knots.

Tire pressure

Tire pressure

Landing at higher than recommended touchdown speeds exposes the aircraft to a greater potential for hydroplaning. And once hydroplaning starts, it can continue well below the minimum initial hydroplaning speed.

On wet runways, directional control can be maximized by landing into the wind. Abrupt control inputs should be avoided. When the runway is wet, anticipate braking problems well before landing and be prepared for hydroplaning. Opt for a suitable runway most aligned with the wind. Mechanical braking may be ineffective, so aerodynamic braking should be used to its fullest advantage.

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Navigation: Automatic Dependent Surveillance-Broadcast

Today, we’re featuring an excerpt from The Pilot’s Manual: Instrument Flying (PM-3D).

Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance technology being deployed throughout the entire National Airspace System. ADS-B enables improved surveillance services, both air-to-air and air-to-ground, especially in areas where radar is ineffective due to terrain or where radar is impractical or cost prohibitive. Eventually, ADS-B will replace most ground-based surveillance radars.

The basic principle of ADS-B is that each aircraft broadcasts a radio transmission approximately once per second, which contains the aircraft’s position, velocity, identification, and other information. This capability is referred to as “ADS-B out.” This transmission is received by the ground-based transceivers (GBTs) and by other appropriately-equipped aircraft. (The capability to receive ADS-B information is referred to as “ADS-B in.”) The ADS-B ground station processes the information and uses it to provide surveillance services. The composite traffic information is uplinked as the product, “Traffic Information Service-Broadcast (TIS-B).” In order to detect each other, no ground infrastructure is necessary for ADS-B equipped aircraft (i.e., those with both ADS-B out and in).

ADS-B ground based transceiver (GBT) antenna.

ADS-B ground based transceiver (GBT) antenna.

There are two completely different methods for aircraft to transmit and receive the ADS-B information. Aircraft that primarily operate in high-altitude airspace send and receive the information using an enhanced Mode S transponder. Aircraft that primarily operate in the low-altitude airspace send and receive the information using the Universal Access Transceiver (UAT). The GBT receives information from both sources and rebroadcasts (ADS-R) it so that all aircraft have all the information.

ADS-B, TIS-B, and FIS-B: broadcast services architecture.

ADS-B, TIS-B, and FIS-B: broadcast services architecture.

Another feature related to ADS-B is called Flight Information Services-Broadcast (FIS-B). FIS-B provides current weather products via an uplink from the GBT antennas to the UAT on the airplane. There is no fee for this weather service. More information on FIS-B is given in the “Datalink Weather Systems” section ahead on page 208.

Before FIS-B and datalink weather capability, pilots had to contact Flight Watch or a flight service station to have someone describe the weather radar picture for them. Now that picture is available in the cockpit. For example, NOTAMs and Temporary Flight Restrictions (TFRs) can be graphically presented over the top of electronic charts, so pilots know what areas to avoid.

The ADS-B system also has downlink capabilities that may someday transmit actual weather data to the ground. In the future, airplanes could send down actual (not forecast) wind direction, velocity, freezing levels, turbulence, and more, to be analyzed in real time. The information could be constantly sent to the ground to build an enhanced, comprehensive, and current weather picture. This information is called an electronic PIREP.

Mandatory ADS-B Out Requirement
The term “ADS-B out” refers to the broadcast of ADS-B transmissions from aircraft, without the installation of complementary receiving equipment to process and display ADS-B data on cockpit displays to pilots. This complementary processing is called “ADS-B in.” ADS-B out is needed for cockpit displays to be able to directly observe traffic. ADS-B out can be deployed earlier than ADS-B in, since ATC surveillance (air-ground) can operate without ADS-B in.

Effective January 1, 2020, ADS-B out capability will be required for aircraft in the following airspace areas:

  • Class A, B, and C airspace;
  • all airspace above 10,000 ft MSL over the 48 contiguous states and the District of Columbia (excluding the airspace above 10,000 feet, but within 2,500 feet of the ground);
  • within the 30 NM veil of airports listed in 14 CFR §91.225; and
  • Class E airspace over the Gulf of Mexico from the coastline of the United States out to 12 nautical miles, at and above 3,000 feet MSL.
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Introducing the New CX-3 Flight Computer

Today on the Learn to Fly Blog, we’d like to share some information on ASA’s next generation CX-3 Flight Computer, available this November. The CX-3 is an excellent companion in the cockpit, on the tarmac, or the ground school classroom, whether you need to make a rate of descent calculation or plan a flight. You can even use it when you take your FAA Knowledge exam. Using the latest microchip and display technologies, the CX-3 features make it the most versatile and useful aviation calculator available.

CX-3_HiRes

May be used during FAA and Canadian Knowledge Exams. The CX-3 complies with FAA Order 8080.6 and Advisory Circular (AC) 60-11, “Test Aids and Materials that May be Used by Airman Knowledge Testing Applicants”; therefore you may bring the CX-3 with you to the testing centers for all pilot, mechanic, and dispatcher FAA exams.

Numerous aviation functions. You can calculate everything from true airspeed and Mach number, fuel burn, holding patterns, to headwind/crosswind components, center of gravity (CG), and everything in between. The menu structure provides easy entry, review, and editing within each function. Multiple problems can be solved within one function.

User-friendly. The color LCD screen displays a menu of functions and the inputs and outputs of a selected function, for easy-to-read menus and data displays. The inputs and outputs of each function are separated on the display screen so it’s clear which numbers were entered and which were calculated, along with their corresponding units of measurement. The menu organization reflects how a flight is normally planned and executed. The result is a natural flow from one function to the next with a minimum of keystrokes. To plan a flight, simply work from the menus in sequential order as you fill in your flight plan form.

1_LTFBCX3

Non-volatile memory. All settings including aircraft profile, weight and balance data, trip plan data, values entered by the user, and calculations performed by the device will be retained until the batteries are removed or the user performs a memory reset. Aircraft profiles for multiple aircraft can be created and saved, and imported from or exported to a computer via a micro-usb port.

Ergonomic design. The CX-3 features a simple keyboard and slim design. The non-slip cover will protect your computer inside the flight bag and it fits on the backside of the unit for easy storage while in use.

Unit conversions. The CX-3 has 12 unit conversions: Distance, Speed, Duration, Temperature, Pressure, Volume, Rate, Weight, Rate of Climb/Descent, Angle of Climb/Descent, Torque, and Angle. These 12 conversion categories contain 38 different conversion units for over 100 functions. Unit conversions can be performed during any step in a calculation.

3_LTFBCX3

Timers and clocks. The CX-3 has two timers: a stopwatch that counts up and a countdown timer. The stopwatch can be used to keep track of elapsed time or to determine the time required to fly a known distance. The countdown timer can be used as a reminder to switch fuel tanks, or to determine the missed approach point on a non-precision instrument approach. An internal clock continues running even when the flight computer is turned off. UTC and local time can be displayed, and the time can be set with UTC, destination or local time.

Interactive functions. The CX-3 is designed so the functions can be used together. You can perform “chain” calculations where the answer to a preceding problem is automatically entered in subsequent problems. Standard mathematical calculations and conversions can be performed within each aviation function.

Up to date. Check often for new CX-3 updates online at www.asa2fly.com/CX3. Firmware updates and user-data backups are made easy with a micro-usb port to connect the CX-3 to computer.

4_LTFBCX3

The CX-3 will begin shipping in November. Check in with your local FBO, favorite online retailer, or ASA for availability. On Thursday, our CFI will share some sample calculations and tips on using the CX-3.

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Aircraft Systems: Engine Cooling Systems

Today’s post is excerpted from Pilot’s Handbook of Aeronautical Knowledge.

The burning fuel within the cylinders produces intense heat, most of which is expelled through the exhaust system. Much of the remaining heat, however, must be removed, or at least dissipated, to prevent the engine from overheating. Otherwise, the extremely high engine temperatures can lead to loss of power, excessive oil consumption, detonation, and serious engine damage.

While the oil system is vital to the internal cooling of the engine, an additional method of cooling is necessary for the engine’s external surface. Most small aircraft are air cooled, although some are liquid cooled.

Air cooling is accomplished by air flowing into the engine compartment through openings in front of the engine cowling. Baffles route this air over fins attached to the engine cylinders, and other parts of the engine, where the air absorbs the engine heat. Expulsion of the hot air takes place through one or more openings in the lower, aft portion of the engine cowling.

7-19

Outside the air aids in cooling the engine.

The outside air enters the engine compartment through an inlet behind the propeller hub. Baffles direct it to the hottest parts of the engine, primarily the cylinders, which have fins that increase the area exposed to the airflow.

The air cooling system is less effective during ground operations, takeoffs, go-arounds, and other periods of highpower, low-airspeed operation. Conversely, high-speed descents provide excess air and can shock cool the engine, subjecting it to abrupt temperature fluctuations.

Operating the engine at higher than its designed temperature can cause loss of power, excessive oil consumption, and detonation. It will also lead to serious permanent damage, such as scoring the cylinder walls, damaging the pistons and rings, and burning and warping the valves. Monitoring the flight deck engine temperature instruments aids in avoiding high operating temperature.

Under normal operating conditions in aircraft not equipped with cowl flaps, the engine temperature can be controlled by changing the airspeed or the power output of the engine. High engine temperatures can be decreased by increasing the airspeed and/or reducing the power.

The oil temperature gauge gives an indirect and delayed indication of rising engine temperature, but can be used for determining engine temperature if this is the only means available.

Most aircraft are equipped with a cylinder-head temperature gauge that indicates a direct and immediate cylinder temperature change. This instrument is calibrated in degrees Celsius or Fahrenheit and is usually color coded with a green arc to indicate the normal operating range. A red line on the instrument indicates maximum allowable cylinder head temperature.

To avoid excessive cylinder head temperatures, increase airspeed, enrich the fuel-air mixture, and/or reduce power. Any of these procedures help to reduce the engine temperature. On aircraft equipped with cowl flaps, use the cowl flap positions to control the temperature. Cowl flaps are hinged covers that fit over the opening through which the hot air is expelled. If the engine temperature is low, the cowl flaps can be closed, thereby restricting the flow of expelled hot air and increasing engine temperature. If the engine temperature is high, the cowl flaps can be opened to permit a greater flow of air through the system, thereby decreasing the engine temperature.

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IFR: The Instrument Landing System (ILS)

Today, we’re featuring an excerpt from The Pilot’s Manual Volume Three: Instrument Flying. In A Pilot’s Accident Review, author John Lowery recommends that “after about 100 hours of flying with a new private certificate it’s important to the new pilot’s safety and longevity to begin training for an instrument rating.” If you’re a private pilot curious about the IFR rating, a great place to start is our CFI’s “An Introduction to the IFR Rating” as well as other IFR category posts we’ve shared here on the L2FB.

The instrument landing system is known as the ILS. It enables a suitably equipped airplane to make a precision approach to a particular runway. A precision approach is one in which electronic glide slope guidance, as well as tracking guidance, is given. Each ILS is known by the airport and runway it serves, for example, the Lafayette ILS Rwy 10, in Indiana.

The instrument landing system has four main elements:

  1. the localizer, which provides course guidance along the extended centerline of the runway (guidance in azimuth left or right of the extended centerline);
  2. the glide slope, which provides vertical guidance toward the runway touchdown point, usually at a slope of approximately 3° to the horizontal, or 1:20 (vertical guidance above or below the glide slope);
  3. marker beacons, which provide accurate range fixes along the approach path (usually an outer marker and a middle marker) are provided; and
  4. approach lights, VASI (visual approach slope indicator), and other lights (touchdown zone lighting, runway lights, etc.) to assist in transitioning from instrument to visual flight.

There may be supplementary NAVAIDs available, including:

  • a compass locator (NDB); and
  • DME.
The instrument landing system.

The instrument landing system. (Click to view full size.)

The outer marker may be replaced as a range marker on some ILS’s by a compass locator, a DME distance, or an ASR or PAR radar position from ATC. The middle marker, where more accuracy is required, may be replaced as a range marker on some ILS’s by a compass locator or PAR radar position from ATC (but not by a DME distance or ASR radar position). These range markers provide you with an accurate distance fix along the localizer.

A co-located compass locator and outer marker will appear on the approach chart as “LOM.” A co-located compass locator and middle marker will appear on the approach chart as “LMM.”

The ideal flight path on an ILS approach, where the localizer plane and the glide slope plane intersect, is referred to as the glide path. The word glide is really a misnomer carried over from earlier days, since modern airplanes make powered approaches down the glide path, rather than glide approaches. However, the term glide path is still used.

Since ILS approaches will often be made in conditions of poor visibility or at night, there is always associated visual information that can be used once the pilot becomes “visual” (has the runway environment in sight). This may include approach lights leading toward the runway, runway lights, touchdown lights, and centerline lights. Lighting is indispensable for night operations, but it can also be invaluable during daylight hours in conditions of restricted visibility.

There may also be a VASI situated near the touchdown zone to provide visual slope guidance during the latter stages of the approach. This, and other visual information, will assist you in maintaining a stable descent path toward the runway, where you can complete the landing.

The ILS is selected in the cockpit on the NAV/COM radio. Its cockpit display is usually the same instrument as for the VOR except that, in addition to the vertical localizer needle (CDI) that moves left and right for course guidance, there is a second needle or indicators that come into view. It is horizontal, and is able to move up and down to represent the position of the glide slope relative to the airplane. Some ILS indicators have needles that are hinged and move like wipers, others have needles that move rectilinearly. The airplane may be thought of as the center dot, and the intersection of the needles as the relative position of the glide path.

ILS cockpit displays.

ILS cockpit displays.

We’ll have more to share on the ILS, and much more on IFR, in future Monday posts.

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Weather: Measurement of Atmospheric Pressure

Today’s post is an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (8083-25B).

Atmospheric pressure historically was measured in inches of mercury (“Hg) by a mercurial barometer. The barometer measures the height of a column of mercury inside a glass tube. A section of the mercury is exposed to the pressure of the atmosphere, which exerts a force on the mercury. An increase in pressure forces the mercury to rise inside the tube. When the pressure drops, mercury drains out of the tube decreasing the height of the column. This type of barometer is typically used in a laboratory or weather observation station, is not easily transported, and difficult to read.

Although mercurial barometers are no longer used in the U. S., they are still a good historical reference for where the altimeter setting came from (inches of mercury).

Although mercurial barometers are no longer used in the U. S., they are still a good historical reference for where the altimeter setting came from (inches of mercury).

An aneroid barometer is the standard instrument used to measure pressure; it is easier to read and transport. The aneroid barometer contains a closed vessel called an aneroid cell that contracts or expands with changes in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The pressure sensing part of an aircraft altimeter is essentially an aneroid barometer. It is important to note that due to the linkage mechanism of an aneroid barometer, it is not as accurate as a mercurial barometer.

Aneroid barometer.

Aneroid barometer.

To provide a common reference, the International Standard Atmosphere (ISA) has been established. These standard conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure is defined as 29.92 “Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 “Hg equal to approximately 34 mb. Standard sea level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers, and hurricane data are reported using mb.

Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, each station converts its barometric pressure by adding approximately 1 “Hg for every 1,000 feet of elevation. For example, a station at 5,000 feet above sea level, with a reading of 24.92 “Hg, reports a sea level pressure reading of 29.92″Hg. Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.

Station pressure is converted to and reported in sea level pressure.

Station pressure is converted to and reported in sea level pressure.

By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe storms.

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Human Factors: Vision, Scanning, and Judgement

Eyes provide the brain with a visual image of the environment. Each eye acts as a natural and very sophisticated digital camera. Its basic function is to collect light rays reflected from an object, using the lens to focus these rays into an image on a screen (the retina), and then converting this image into electrical signals that are sent via the optic nerve to the brain. This is how you see. The brain matches the image to previously stored data so you recognize (perceive) the object. The connection of the optic nerve to the brain is so close and integral, and the importance of the messages sent to the brain is so dominant, that the eyes can almost be considered an extension of the brain. Today we’ll talk more about vision with an excerpt from our textbook The Pilot’s Manual Volume 2: Ground School (PM-2C).
12-7
Scanning by Day
The central (foveal) region of the retina provides the best vision, and in full color but only during reasonable daylight. Objects are best seen by day if you can focus their image on the foveal region, and you do this by looking directly at them. The most effective method of scanning for other aircraft for collision avoidance during daylight hours is to use a series of short, regularly spaced eye movements to search each 10° sector of the sky. Systematically focusing on different segments of the sky for short intervals is a better technique than continuously sweeping the sky. This is sometimes called the saccade/fixation cycle, where the saccade or movement takes about one-third of a second.

Methodical scan.

Methodical scan.

Relative Movement
If there is no apparent relative motion between you and another aircraft, you may be on a collision course, especially if the other aircraft appears to be getting bigger and bigger in the windshield. Due to the lack of movement across your windshield, an aircraft on a collision course with you will be more difficult to spot than one that is not on a collision course.

Any relative movement of an object against its background usually makes it easier to notice in your peripheral vision. The image of the other aircraft may not increase in size much at first, but, shortly before impact, it would rapidly increase in size. The time available for you to avoid a collision may be quite brief, depending upon when you see the other aircraft and the rate of closure.

Constant relative position = collision course.

Constant relative position = collision course.

If you are flying at 100 knots and it is flying at 500 knots in the opposite direction, the rate of closure is 600 knots, i.e. ten nautical miles per minute. If you spot the other aircraft at a distance of one nautical mile, you only have 1/10 of a minute (six seconds) to potential impact. If you are a vigilant pilot and spot it at 3 nautical miles you have eighteen seconds in which to act.

In hazy or low-visibility conditions, your ability to see other aircraft and objects with edges that might be blurred will be diminished and, if you can see them, they may appear to be further away than their actual distance. You might be closer than you think.

Empty-Field Myopia
When trying to search for other aircraft in an empty sky, the natural tendency of a resting eye is to focus at about six feet. Consequently, distant aircraft may not be noticed. To avoid this empty-field myopia, you should focus on any available distant object, such as a cloud or a landmark, to lengthen your focus. If the sky is empty of clouds or other objects, then focus briefly on a relatively distant part of the airplane like a wing tip as a means of lengthening your focus. Having spotted an airplane in an otherwise empty sky, be aware that it could be closer to you than it appears to be, because you have no other object with which to compare its size.

Specks
A small, dark image formed on the retina could be a distant aircraft, or it could be a speck of dirt or dust, or an insect spot, on the windshield. Specks, dust particles, a scratch, or an insect on the windshield might be mistaken for a distant airplane. Simply moving your head will allow you to discriminate between marks on the windshield and distant objects.

Specks?

Specks?

Scanning by Night
The central (foveal) region of the retina containing mainly cones is not as effective at night, causing an area of reduced visual sensitivity in your central vision. Peripheral vision, provided by the rods in the outer band of the retina, is more effective albeit color blind. An object at night is more readily visible when you are looking to the side of it by ten or twenty degrees, rather than directly at it. Color is not perceived by the rods, and so your night vision will be in shades of gray. Objects will not be as sharply defined (focused) as in daytime foveal vision.

The most effective way to use your eyes during night flight is to scan small sectors of sky more slowly than in daylight to permit off-center viewing of objects in your peripheral vision, and to deliberately focus your perception (mind) a few degrees from your visual center of attention (that is, look at a point but look for objects around it). Since you may not be able to see the aircraft shape at night, you will have to determine its direction of travel making use of its visible lighting:

  • the flashing red beacon;
  • the red navigation light on the left wing tip;
  • the green navigation light on the right wing tip; and
  • a steady white light on the tail.
Position lights.

Position lights.

Visual Judgment on Approach
The eyes and brain use many clues and stored images of known objects to help in judging distance, size and height. The relative size and relative clarity of objects give clues to their relative distances: a bigger object is assumed to be nearer than a smaller one and a more clearly defined object nearer than a blurry one. When the object is near, binocular vision (the slightly different images of a nearby object relative to its background seen by each eye) assists in depth perception.

Texture also assists in depth perception: the more visible the texture, the closer the object appears to be. On final approach as you near the aim point, the surface texture will appear to flow outward in all directions from the point on which you are focused. This is one means by which you can visually maintain the flight path to the aim point: adjust the attitude and heading so that the point from which the texture appears to be moving outward remains the desired aim point.

Aim point.

Aim point.

Texture is also used for the estimation of height; for instance, as you approach flare height for a landing, the actual texture of the runway or the grass passing by the cockpit becomes increasingly noticeable. Relative motion also aids in depth perception. Near objects generally appear to pass by faster than more distant objects. This helps a visual pilot estimate height above the runway before and during the flare: the closer the airplane is to the runway, the faster the runway surface and the surrounding environment appears to pass by.

Depth perception can be difficult in hazy or misty conditions, where edges are blurred, colors are muted, and light rays may be refracted unusually. This gives the impression of greater distance, an impression reinforced by the fact that we often have to look at distant objects through a smoggy or hazy atmosphere. This illusion is referred to as environmental perspective. In hazy conditions, the object might be closer than it seems; in very clear conditions, the object might be further away than it seems. On hazy days, you might touch down earlier than expected; on very clear nights, you might flare a little too soon.

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