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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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CFI Brief: How does a Propeller Work?

The propeller is a rotating airfoil which produces thrust by creating a positive dynamic pressure, usually on the engine side. Some exceptions include the Piaggio Avanti, shown below which uses propellers mounted in what’s often referred to as the pusher configuration.

When a propeller rotates, the tips travel at a greater speed than the hub. To compensate for the greater speed at the tips, the blades are twisted slightly. The propeller blade angles decrease from the hub to the tips with the greatest angle of incidence, or highest pitch, at the hub and the smallest at the tip. This produces a relatively uniform angle of attack (uniform lift) along the blade’s length in cruise flight.

No propeller is 100% efficient. There is always some loss of power when converting engine output into thrust. This loss is primarily due to propeller slippage. A propeller’s efficiency is the ratio of thrust horsepower (propeller output) to brake horsepower (engine output). A fixed propeller will have a peak (best) efficiency at only one combination of airspeed and RPM.

A constant-speed (controllable-pitch) propeller allows the pilot to select the most efficient propeller blade angle for each phase of flight. In this system, the throttle controls the power output as registered on the manifold pressure gauge, and the propeller control regulates the engine RPM (propeller RPM). The pitch angle of the blades is changed by governor regulated oil pressure which keeps engine speed at a constant selected RPM. A constant-speed propeller allows the pilot to select a small propeller blade angle (flat pitch) and high RPM to develop maximum power and thrust for takeoff.

To reduce the engine output to climb power after takeoff, a pilot should decrease the manifold pressure. The RPM is decreased by increasing the propeller blade angle. When the throttle is advanced (increased) during cruise, the propeller pitch angle will automatically increase to allow engine RPM to remain the same. A pilot should avoid a high manifold pressure setting with low RPM on engines equipped with a constant-speed propeller to prevent placing undue stress on engine components. To avoid high manifold pressure combined with low RPM, the manifold pressure should be reduced before reducing RPM when decreasing power settings, and the RPM increased before increasing the manifold pressure when increasing power settings.

Let’s take a look at these three sample knowledge test questions and see if we can answer them given the information from Monday and todays posts.

1. Which statement best describes the operating principle of a constant-speed propeller?
A—As throttle setting is changed by the pilot, the prop governor causes pitch angle of the propeller blades to remain unchanged.
B—A high blade angle, or increased pitch, reduces the propeller drag and allows more engine power for takeoffs.
C—The propeller control regulates the engine RPM and in turn the propeller RPM.

2. Propeller efficiency is the
A—ratio of thrust horsepower to brake horsepower.
B—actual distance a propeller advances in one revolution.
C—ratio of geometric pitch to effective pitch.

3. A fixed-pitch propeller is designed for best efficiency only at a given combination of
A—altitude and RPM.
B—airspeed and RPM.
C—airspeed and altitude.

Answers in the comments section.

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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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CFI Brief: FAA Safety Briefing, January 2018

The first edition of the FAA Safety Briefing for 2018 is now available and includes some great articles. One in particular that I found to be very informative is “Simple?” written by Susan Parsons. This is a great article that discusses getting back to the basics of piloting in an otherwise complex environment. To read this, and other articles, download the latest edition by selecting the below image.

Also recently published is Advisory Circular 61-65G, which replaces the -65F. This advisory circular (AC) provides guidance for pilot applicants, pilots, flight instructors, ground instructors, and examiners on the certification standards, knowledge test procedures, and other requirements in Title 14 of the Code of Federal Regulations (14 CFR) Part 61. This AC is also commonly known as your go to reference for sample endorsements for use by authorized instructors when endorsing logbooks, or other means found acceptable to the Administrator for airmen applying for a knowledge or practical test, or when certifying accomplishment of requirements for pilot operating privileges.

ASA has noted all these changes throughout this AC and updated our fill-in PDF Endorsement Labels accordingly. To download this free product check out the link below:

ASA Endorsement Labels

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CFI Brief: CX-3 Flight Computer – Indicated Airspeed

Some questions on the FAA Knowledge Exam will require you to determine approximate indicated airspeed. These types of problems are best solved with the use of a CX-3 Flight Computer. Today, we will work through a sample knowledge test question requiring us to run several different calculations on the CX-3 to determine indicated airspeed from the given information. Problems like the one below are a great way to get use to using your new go to tool in the flight bag.

On a cross-country flight, point A is crossed at 1500 hours and the plan is to reach point B at 1530 hours. Use the following information to determine the indicated airspeed required to reach point B on schedule.

Distance between A and B 70 NM
Forecast wind 310° at 15 kts
Pressure altitude 8,000 ft
Ambient temperature -10 °C
True course 270°

The required indicated airspeed would be approximately

A. 126 knots.
B. 137 knots.
C. 152 knots.

Step 1 is to determine the Ground Speed it will take to cover 70 NM in 30 minutes (1500 – 1530 hours), this information is given in the question.

From the FLT menu select the Ground Speed Function.

Input a Distance (Dist) of 70 NM and Duration (Dur) of 0.50 HR to get a Ground Speed (GS) of 140 KTS.

Step 2 we need to find the True Airspeed using our equated ground speed, forecast wind, and true course.

From the FLT menu select Wind Correction.

Input a Ground Speed (GS) of 140 KTS, True Course (TCrs) of 270°, Wind Speed (WSpd) of 15 KTS, and Wind Direction (WDir) of 310°. The CX-3 will show a TAS of 151.8 KTS.

Step 3 we can now determine the required indicated airspeed using the true airspeed determined in step 2 and the pressure altitude and ambient temperature given in the question. Note that the Indicate Airspeed is shown in the CX-3 as Calibrated Airspeed (CAS).

From the FLT menu select Airspeed.

Input a True Airspeed (TAS) of 151.8 KTS, Outside Air Temperature (OAT) of -10°C, and Pressure Altitude (PAlt) of 8,000 FT. The CX-3 will show a CAS of 137.15.

The correct answer to the above question is B: 137 knots.

For additional information on using your CX-3 Flight Computer check out the complete Users Guide at www.asa2fly.com/CX3.

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CFI Brief: Pressure Altitude Conversions

Pressure altitude is the height above the standard datum plane (SDP). The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 “Hg SDP, the altitude indicated is the pressure altitude—the altitude in the standard atmosphere corresponding to the sensed pressure.

The SDP is a theoretical level at which the pressure of the atmosphere is 29.92 “Hg and the weight of air is 14.7 psi. As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining aircraft performance, as well as for assigning flight levels to aircraft operating at above 18,000 feet.

The pressure altitude can be determined by any of the three following methods:

  1. By setting the barometric scale of the altimeter to 29.92 “Hg and reading the indicated altitude,
  2. By applying a correction factor to the indicated altitude according to the reported “altimeter setting,” see figure below.
  3. By using a CX-3 Flight Computer.

Let’s try a sample problem using the above chart.

1. Determine the pressure altitude at an airport that is 1,386 feet MSL with an altimeter setting of 29.55.
A—1,631 feet MSL.
B—1,731 feet MSL.
C—1,778 feet MSL.

Looking at the above chart you will see that 29.65 is not actually shown so we will need to interpolate between 29.50 and 29.60. Subtract 298 from 392 to get 94 and then divide by 2 since 29.55 is directly in the middle. You get 47 which you can now add to 298 to come up with 345 altitude correction. Add 345 onto you airport elevation to find the pressure altitude, 1,386 + 345 = 1,731 PAlt.

The correct answer is B, 1,731 feet MSL.

Here is another problem which can be solved by using your CX-3 Flight Computer.

2. Determine the pressure altitude at an airport that is 3,563 feet MSL with an altimeter setting of 29.96.
A—3,527 feet MSL.
B—3,556 feet MSL.
C—3,639 feet MSL.

Using you CX-3 open the FLT menu and select Altitude from the list. Enter your IAlt (indicated altitude) or airport elevation of 3,563 feet. Scroll down to the Baro field and enter your altimeter setting of 29.96. The CX-3 will then give you a PAlt of 3,527 feet MSL. The correct answer is A.

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CFI Brief: Altimeter Pressure Errors

High to low look out below, low to high clear the sky! If you have never heard that saying before you are probably pretty confused right now. Let me help ease that confusion and explain that today we are discussing altimeter errors when flying in areas of changing atmospheric pressures. The discussion will revolve around two specific Private Pilot Knowledge Test questions that I get calls about regularly. These questions outline common areas that trip students up, resulting in mass confusion.

1. If a flight is made from an area of low pressure into an area of high pressure without the altimeter setting being adjusted, the altimeter will indicate
A. the actual altitude above sea level.
B. higher than the actual altitude above sea level.
C. lower than the actual altitude above sea level.

2. If a flight is made from an area of high pressure into an area of lower pressure without the altimeter setting being adjusted, the altimeter will indicate
A. lower than the actual altitude above sea level.
B. higher than the actual altitude above sea level.
C. the actual altitude above sea level.

If you answered B an A, respectively, then you are also having some of the same confusion about this topic that many other students experience. Let’s start with the knowledge required to answer these questions.

It is easy to maintain a consistent height above ground if the barometric pressure and temperature remain constant, but this is rarely the case. The pressure and temperature can change between takeoff and landing on a local flight and even more drastically when flying cross country between areas of varying pressure and temperature. If these changes are not taken into consideration, flight becomes dangerous.

For example, when flying from an area of high pressure to an area of low pressure without adjusting the altimeter, a constant indicated altitude will remain but the aircraft’s actual altitude above ground level will be lower than indicated. Conversely, when flying from an area of low pressure to an area of high pressure the aircraft’s actual altitude above ground level will be higher than the indicated altitude on the altimeter. The image below is a good visual depiction of flying from an area of high pressure to an area of low pressure and the resulting altitude of the aircraft above the ground level if the altimeter is not adjusted. 

The reason so many students answer these questions incorrectly is not so much because they don’t understand the principle knowledge, but rather because they are not understanding what the question is asking. Both questions are asking what the ALTIMETER will indicate, not what the aircraft’s actual altitude is relative to the ground.

The correct answer to question 1 is C: the altimeter will indicate lower than the actual altitude above sea level. When flying from a low pressure area into a high pressure area the aircraft’s altitude will slowly increase while the altimeter remains constant; therefore, the ALTIMETER is indicating a LOWER altitude then what the aircraft is actually flying.

Vice versa, the correct answer to question 2 is B: the altimeter will indicate higher than the actual altitude above sea level. When flying from a high pressure area into a low pressure area, the aircraft’s altitude will slowly decrease while the altimeter remains constant therefore the ALTIMETER is indicating a HIGHER altitude then what the aircraft is actually flying.

Tricky questions yes, but also a very important piece of knowledge to fully understand. You should now be able to correctly answer these on your knowledge test as well as remember to adjust the altimeter when flying between different air pressure systems.

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Editor’s Picks for the 2017 Holiday Season

2017 was a big year for us at ASA. We’ve been busy, releasing new editions of all three Pilot’s Manual textbooks (Flight School, Ground School, and Instrument Flying), William Kershner’s The Student Pilot’s Flight Manual, Bob Gardner’s The Complete Private Pilot, our Oral Exam Guide titles (Private, Instrument, CommercialMulti-Engine, CFI, and Aircraft Dispatcher), Dale Crane’s Dictionary of Aeronautical Terms and Aviation Mechanic Handbook, multiple FAA Handbooks, and of course our Test Prep and FAR series titles. Today, I’d like to recommend three brand-new products from ASA, sure to suit the aviator on your holiday shopping list.

Small Unmanned Aircraft Systems Guide
by Brent Terwilliger, David Ison, John Robbins, Dennis Vincenzi

This is the perfect gift for a remote pilot, whether they’re just a hobbyist or looking into monetizing their drone flying. Written by a team of UAS-industry experts, this book covers the history of drones, design philosophies, technology, and safety practices, as well as resources to help you make well-informed decisions regarding purchase and use and determine a path forward through the complex legal, business, operational, and support considerations.

The Flight Instructor’s Survival Guide
by Arlynn McMahon

For the CFI in your life, Arylnn McMahon shares forty-four stories that demonstrate the fundamentals of instructon (FOI) principles in flight and offer practical strategies for dealing with both common and unexpected situations—all with wisdom, grace, and humor. Through artful storytelling, McMahon shows how a successful instructor is sometimes a psychologist, other times a detective, and always a gatekeeper enforcing rules and cultivating the behaviors required to be a responsible aviation citizen.

CX-3 Flight Computer
by Aviation Supplies & Academics, Inc.

Years in the making, the new CX-3 Flight Computer is the ultimate gift to the pilot on your list. Check out our rundown of all of it’s features in last month’s blog post. The CX-3 Flight Computer makes flight planning simple by taking confusion out of the equation. Fast, versatile and easy to use, the CX-3 delivers accurate results quickly and efficiently. It can even be used on all FAA and Canadian pilot, mechanic, and dispatcher knowledge exams. Whether used for flight planning, ground school, or knowlledge testing, the menu organization reflects the order in which a flight is normally planned and executed, resulting in a natural flow from one function to the next with a minimum of keystrokes.


For more holiday gift ideas, check out our 2017 Holiday Gift Guide. And stay tuned, we’re working hard to deliver a lot of new books in 2018 which we will be able to tell you about soon!

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CFI Brief: Airport Rotating Beacon

Have you ever wondered how pilots are able to determine the location of an airport at night or in reduced visibility? Well the answer is actually very simple. At night, the location of an airport can be determined by the presence of an airport rotating beacon light like the one seen in the image below. An airport beacon will assist you as a pilot in identifying the location and type of airport by the color combination the beacon is emitting.

The colors and color combinations that denote the type of airports are:

*Note: Green alone or amber alone is used only in connection with a white-and-green or white-and-amber beacon display, respectively.

A civil-lighted land airport beacon will show alternating white and green flashes. A military airfield will be identified by dual-peaked (two quick) white flashes between green flashes.

In Class B, C, D, or E airspace, operation of the airport beacon during the hours of daylight often indicates the ceiling is less than 1,000 feet and/or the visibility is less than 3 miles. However, pilots should not rely solely on the operation of the airport beacon to indicate if weather conditions are IFR or VFR.

The beacon has a vertical light distribution to make it most effective from 1–10° above the horizon, although it can be seen well above or below this spread.

Here is another nifty little tidbit you will learn once you start night flight training. Radio control of lighting is available at some airports, providing airborne control of lights by keying the aircraft’s microphone. The control system is responsive to 7, 5, or 3 microphone clicks. Keying the microphone 7 times within 5 seconds will turn the lighting to its highest intensity; 5 times in 5 seconds will set the lights to medium intensity; low intensity is set by keying 3 times in 5 seconds. Many airports, particularly airports without an operating control tower, will not keep runway lights on constantly throughout the night so it becomes the pilots responsibility to turn the runway lights on for landing or takeoff. Once the lights are keyed on they will typically remain on for 15 minutes. A quick glance in the Chart Supplement U.S. will identify an airport with pilot controlled lighting.

 

 

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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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