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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.

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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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CFI Brief: The Instrument Approach Procedure Chart

On Monday, we learned about the Instrument Landing System and it’s components. Today, I would like to further our discussion and talk about Instrument Approach Procedure Charts. These charts are what depict to pilots how to fly a particular approach into an airport. Many instrument approaches will require the use of an ILS or it’s Localizer component.

With use of the depicted information on an IAP chart a pilot will be assured of terrain and obstruction clearance and runway or airport alignment during approach for landing.

The IAP chart may be divided into four distinct areas: the Plan View, showing the route to the airport; the Profile View, showing altitude and descent information; the Minimums Section, showing approach categories, minimum altitudes, and visibility requirements; and the Airport diagram, showing runway alignments, runway lights, and approach lighting systems.

  1. The Plan View is that portion of the IAP chart depicted at “A” in the figure below. Atop the IAP chart is the procedure identifications which will depict the A/C equipment necessary to execute the approach, the runway alignment, the name of the airport, the city and state of airport location (See Figure Area #1). An ILS approach, for example, requires the aircraft to have an operable localizer, glide slope, and marker beacon receiver. An LOC/DME approach would require the aircraft to be equipped with both a localizer receiver and distance measuring equipment (DME). If the approach is aligned within 30° of the centerline, the runway number listed at the top of the approach chart means straight-in landing minimums are published for that runway. If the approach course is not within 30° of the runway centerline, an alphabetic code will be assigned to tie IAP identification (for example, NDB-A, VOR-C), indicating that only circle-to-land minimums are published. This would not preclude a pilot from landing straight-in, however, if the pilot has the runway in sight in sufficient time to make a normal approach for landing, and has been cleared to land.

The IAP plan view will list in either upper corner, the approach control, tower, and other communications frequencies a pilot will need. Some listings may include a direction (for example, North 120.2, South 120.8).

The IAP plan view may contain a Minimum Sector Altitude (MSA) diagram. The diagram shows the altitude that would provide obstacle clearance of at least 1,000 feet in the defined sector while within 25 NM of the primary omnidirectional NAVAID; usually a VOR or NDB (See Figure Area #2).

An IAP may include a procedural track around a DME arc to intercept a radial. An arc-to-radial altitude restriction applies while established on that segment of the IAP.

  1. The Profile View is that portion of the IAP chart depicted at “B” in the Figure. The profile view shows a side view of the procedures. This view includes the minimum altitude and maximum distance for the procedure turn, altitudes over prescribed fixes, distances between fixes, and the missed approach procedure.
  2. The Minimums Section is that portion of the IAP chart depicted at “C” in the Figure. The categories listed on instrument approach charts are based on aircraft speed. The speed is 1.3 times VS0 at maximum certificated gross landing weight.
  3. The Aerodrome Data is that portion of the IAP chart which includes an airport diagram, and depicts runway alignments, runway lights, approach lights, and other important information, such as the touchdown zone elevation (TDZE) and airport elevation (See figure area “D”).

TP-I-08-02

Take a look a the IAP Chart Figure below and see if you can determine the following. Answers will be posted in the comments section.

  1. What is the minimum equipment required for this approach?
  2. What are the noted minimum safe altitudes (MSA)?
  3. What is the decision altitude (DA) if conducting a straight in approach?

instrument_179

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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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CFI Brief: October 2017 Test Roll

The FAA October test cycle resulted in very few changes or updates to the FAA Airman Knowledge Tests. The FAA Aviation Exam Board continues to work to align questions within the context of a specific Area of Operation/Task as outlined in the various Airman Certification Standards publications. The goal of this boarding process is to ensure all test questions correlate to a knowledge, risk management or skill element. The FAA makes their intentions clear by the Frequently Asked Questions and What’s New documents which are posted each test cycle. Below is a list of the most recent changes affecting all knowledge test question banks. The next test cycle is expected February 2018.

  • References to the Airport/Facility Directory (A/FD) have been changed to this publication’s new name, “Chart Supplement.”
  • U.S. format Flight Plans – New questions based on the new U.S. flight plan will be developed and implemented by June 2018.
  • Student Pilot/Medical Certificate – New questions based on the Student Pilot Certificate rule that took effect on 1 April 2016 are expected by October 16, 2017.
  • Rote memorization questions such as the following have been removed (e.g., Validity period for unscheduled products such as SIGMETS).
  • Operationally irrelevant questions have been removed (e.g., Meaning of brackets near station model on a WX depiction chart).
  • The following topics have been removed from FAA Knowledge Tests (effective June 12, 2017):
    • 4-panel prog charts
    • Weather depiction chart
    • Area forecasts
    • Aerobatic flight

Recent changes affecting the Private Pilot Airplane Knowledge Test:

  • Aircraft performance and weather questions that involve multiple interpolations across multiple charts do not include multiple interpolations across multiple charts.

Recent changes affecting the Instrument Rating Airplane Knowledge Test:

  • The following subjects have been removed:
    • Airport Surveillance Radar (ASR) approaches
    • Composite Flight Plans
    • Designation of instruments as “primary” or “secondary” for aircraft control
    • Inner Marker, Middle Marker
    • Specific number of degrees on glide path
    • Time and distance questions involving multiple interpolation
    • BARO VNAV (IRA ONLY)
    • Back Course Approaches (IRA ONLY)
    • LDA & SDF (IRA ONLY)
    • Aircraft performance and weather questions that involve multiple interpolations across multiple charts

These changes have been noted by ASA and updates for Prepware Software, Prepware Online, and Test Prep books will be available shortly. If you would like to be notified when these updates have become available be sure to follow the link below and sign-up for notifications.

http://www.asa2fly.com/testupdate

UPDATES from ASA

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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).
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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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CFI Brief: Pilot Deviations, Stay Alert!

Yesterday, the FAA Safety Team distributed a newly published Fly Safe Fact Sheet, Avoiding Pilot Deviations (PDs). Now listen, if you’ve read this blog over the years you know we have discussed this topic before. However, it’s worth discussing on the regular since PDs can lead to serious consequences in the form of accidents or enforcement violations.

If you are not already familiar with what a pilot deviation is, it is defined as an action of a pilot that violates any Federal Aviation Regulation. While PDs should be avoided, the regulations do authorize deviations from a clearance in response to a traffic alert and collision avoidance system resolution advisory. Meaning, if a possible collision with another aircraft or vehicle is imminent it is OK to deviate. You must however notify ATC as soon as possible following a deviation.

Piot deviations are broken down into two separate categories, airborne and ground. Airborne deviations result when a pilot strays from an assigned heading or altitude or from an instrument procedure, or if the pilot penetrates controlled or restricted airspace without ATC clearance. Ground deviations (also called surface deviations) include taxiing, taking off, or landing without clearance, deviating from an assigned taxi route, or failing to hold short of an assigned clearance limit.

Ways to Avoid Pilot Deviations:

Plan each flight —you may have flown the flight many times before but conditions and situations can change rapidly, such as in the case of a pop-up temporary flight restriction (TFR). Take a few minutes prior to each flight to plan accordingly.

Talk and squawk —Proper communication with ATC has its benefits. Flight following often makes the controller’s job easier because they can better integrate VFR and IFR traffic.

Give yourself some room —GPS is usually more precise than ATC radar. Using your GPS to fly up to and along the line of the airspace you are trying to avoid could result in a pilot deviation because ATC radar may show you within the restricted airspace.

Stay Alert – This is often overlooked during ground operations. It’s important that whether you are in the air or on the ground you maintain focus and alertness at all times. Keep your head out of the cockpit and on a swivel.

Click the below image to access the FAA Fact Sheet and see the full text on the 4 steps to avoid pilot deviations.

RunwaySafety_24x18_21A

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Weather: Precipitation

We’re seeing rain for the first time in over two months in the Seattle area right now, so how about a refresher on precipitation today on the Learn to Fly Blog? Today’s post is excerpted from Aviation Weather (AC 00-6B).

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are: drizzle, rain, snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets.

Precipitation formation requires three ingredients: water vapor, sufficient lift to condense the water vapor into clouds, and a growth process that allows cloud droplets to grow large and heavy enough to fall as precipitation. Significant precipitation usually requires clouds to be at least 4,000 feet thick. The heavier the precipitation, the thicker the clouds are likely to be. When arriving or departing from an airport reporting precipitation of light or greater intensity, expect clouds to be more than 4,000 feet thick.

All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice crystals are too small and light to fall to the ground as precipitation. Because of their microscopic size, the rate at which cloud droplets fall is incredibly slow. An average cloud droplet falling from a cloud base at 3,300 feet (1,000 meters) would require about 48 hours to reach the ground. It would never complete this journey because it would evaporate within minutes after falling below the cloud base. Two growth processes exist which allow cloud droplets (or ice crystals) to grow large enough to reach the ground as precipitation before they evaporate (or sublimate). One process is called the collision-coalescence, or warm rain process (see Figure 14-1). In this process, collisions occur between cloud droplets of varying size and different fall speeds, sticking together or coalescing to form larger drops. Finally, the drops become too large to be suspended in the air, and they fall to the ground as rain. This is thought to be the primary growth process in warm, tropical air masses where the freezing level is very high.
00-6b_14-1
Figure 14-1. The collision-coalescence or warm rain process. Most cloud droplets are too small and light to fall to the ground as precipitation. However, the larger cloud droplets fall more rapidly and are able to sweep up the smaller ones in their path and grow.

The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation, it is easier for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface, it may snow; otherwise, the snowflakes may melt to rain. This is thought to be the primary growth process in mid- and high-latitudes.

The vertical distribution of temperature will often determine the type of precipitation that occurs at the surface. Snow occurs when the temperature remains below freezing throughout the entire depth of the atmosphere (see Figure 14-2).
00-6b_14-2
Figure 14-2. Snow temperature environment.

Ice pellets (sleet) occur when there is a shallow layer aloft with above freezing temperatures and with a deep layer of below freezing air based at the surface. As snow falls into the shallow warm layer, the snowflakes partially melt. As the precipitation reenters air that is below freezing, it refreezes into ice pellets (see Figure 14-3).
00-6b_14-3
Figure 14-3. Ice pellets temperature environment.

Freezing rain occurs when there is a deep layer aloft with above freezing temperatures and with a shallow layer of below freezing air at the surface. It can begin as either rain and/or snow, but becomes all rain in the warm layer. The rain falls back into below freezing air, but since the depth is shallow, the rain does not have time to freeze into ice pellets (see Figure 14-4). The drops freeze on contact with the ground or exposed objects.
00-6b_14-4
Figure 14-4. Freezing rain temperature environment.

Rain occurs when there is a deep layer of above freezing air based at the surface (see Figure 14-5).
00-6b_14-5
Figure 14-5. Rain temperature environment.

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Aerodynamics: Descent and Gliding Flight

Our CFI is out enjoying the Reno Air Races this week, so today we’ll share a follow up to Monday’s post with another excerpt from Aerodynamics for Aviators.

Descending a light propeller-driven general aviation aircraft is a fairly simple task. Reduce power to a point where there is more power required than power available, and the basic principle of weight takes over. Under normal flight conditions, descending flight is initiated by the pilot creating a decrement of power (more power required than available). Once the aircraft begins descending, the weight vector can be broken up into two parts, just like with the climb. One component acts perpendicular to the flightpath (down), the other acts forward and parallel to the flightpath, helping accelerate the aircraft.

Gliding flight can be self-induced by bringing the power back to idle, but in most piston aircraft, descents are not conducted at idle power, thus they are called a powered descent. This is because of shock cooling and the possible damage it could cause to the engine. A true gliding descent would be used if the engine fails. Gliding flight can be broken down into two parts, minimum sink and maximum range.

The minimum sink glide is used to prolong the time aloft in the event the engine or engines fail. This is a speed that is not published, but could be useful if you are over your current landing site and wish to stay aloft a little longer. Most light single-engine airplanes will be at (or close to) minimum sink with full aft trim. This is slower than best glide speed. A pilot who elects to use this method should accelerate to best glide once a normal pattern altitude is reached. This will provide a larger margin above stall and the aircraft will have more positive maneuverability. It should be noted that the best glide speed should be used unless the pilot has training and experience flying at the minimum sink glide speed.

The maximum glide range occurs at the speed for maximum range: L/DMAX. This is generally a published speed and is used when the engine stops or fails in flight. Some Airplane Flying Manuals (AFMs) contain glide ratio charts. There are some concerns with these charts:

  1. They do not account for wind.
  2. They are usually calculated in a minimum drag configuration (gear and flaps up).
  3. They are usually calculated with controllable-pitch propellers in the full decrease position (high AOA).
  4. They are usually calculated at maximum gross weight.

Wind is a factor in glide distance and angle. A headwind will decrease glide distance, and the angle of descent will increase (steepen). A tailwind will increase the glide distance and flatten the angle of descent. You experience the effects of both a headwind and a tailwind when you do a power-off approach. On downwind the aircraft has a flatter descent and a higher groundspeed. Turning base to final, the angle of descent steepens and the groundspeed slows.

Weight is also a factor in glide distance if L/DMAX is not maintained. Without an AOA indicator, the only way to maintain a specific AOA at L/DMAX is to vary the airspeed. As weight increases, the airspeed would need to be increased to maintain L/DMAX.

Altitude also affects the airplane’s gliding distance. To understand this we need to step back and look at the effects of altitude on true airspeed. As the aircraft climbs, TAS increases about 2% per 1,000 feet. An aircraft gliding at higher altitudes will have a higher TAS, this means that it will be moving down the slope at a faster rate. This is of particular importance when operating an aircraft at high density altitudes.

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Aerodynamics: Cruise Flight

Cruise flight centers on two basic principles: how far we can fly, and for how long. How far we can fly is defined as the aircraft’s range. How long we can fly is defined as endurance. Today’s post is an excerpt from our textbook Aerodynamics for Aviators.

When flying, we generally consider range in two ways:

  1. Maximizing the distance we fly for a given fuel load.
  2. Traveling a specified distance while burning minimum fuel.

Endurance
It’s important to understand that range and endurance are not the same. Range relates to distance, endurance relates to time. The formula for endurance is:

endurance = hours ÷ fuel

Hours is simply flight time expressed in whatever units you want: hours, minutes or seconds. Fuel can be expressed in gallons or pounds. A pilot who wants to achieve maximum endurance would slow the aircraft to the minimum power required speed. Figure 5-24 shows the minimum power point being the lowest point in the drag curve.

5-24

Figure 5-24. Maximum endurance.

If the aircraft were to slow even further, to point A, drag would increase rapidly, more power would be required, and the engine would burn more fuel. If the aircraft were to accelerate above point B, drag also increases, which increases fuel burn. As you can see, flying at maximum endurance speed is not practical in the real world; you may save fuel but it would take forever to get to the destination. This speed is also not practical for operations such as holding because it is generally close to stall. From a practical standpoint, endurance comes from the selection of a cruise power setting of 55%, 65%, or 75% endurance charts. The point of this type of flying is generally to minimize or eliminate fuel stops (very time consuming) along the route, or to minimize fuel burn for cost purposes—not necessarily to stay aloft for hours on end.

Range
Range can be broken down into two parts: specific range and total range. An easy way to understand the difference is to use a car trip scenario. If I have a car that has a 20 gallon fuel tank and gets 30 miles per gallon, I can travel 600 miles on one tank of gas. The specific range in this example is 30 miles per gallon, and the total range is 600 miles. In an airplane, specific range is how many nautical miles you can travel on one gallon or pound of fuel. The total range is how far the airplane can fly with the remaining fuel load on board the aircraft. The definition for specific range is:

specific range = NM ÷ gallons of fuel (Note: pounds can be inserted for gallons.)

Specific range is affected by three things: (1) aircraft weight, (2) altitude, and (3) configuration. The maximum range of the aircraft can be found at L/DMAX. Unlike endurance, which is found on the drag curve where minimum power is required, maximum range is found where the ratio of speed to power required is the greatest. This is located on the graph by drawing a tangent line from the origin to the power required curve (Figure 5-24). Another way to think about this is that as you move from the origin point along the tangent line toward L/DMAX you increase airspeed at a greater rate than fuel burn (think of the ratio). At L/DMAX, the ratio of fuel to airspeed should be 1. At any speed above L/DMAX, the fuel burn ratio increases at a greater rate than the airspeed. Therefore, L/DMAX is the point where the speed-to-power ratio required is the greatest.

Another aspect of range that we need to examine is the effect of weight on range. Because L/DMAX occurs at a specific angle of attack, and most general aviation airplanes do not have AOA indicators, the airspeed has to be varied as weight changes to maintain a constant AOA. Figure 5-25 illustrates this: as weight increases, the speed must be increased to maintain the AOA. This is because as weight is increased, the AOA must be increased to produce more lift; the only way to lower the AOA is to increase speed. As weight decreases, the speed must decrease. The reasoning is that as the aircraft becomes lighter, the AOA is lowered to compensate for less weight; the only way to increase AOA is to reduce speed (Figure 5-25).

5-25

Figure 5-25. The effect of weight on range.

The effect of altitude on range can be seen in Figure 5-26. Flights operating at high altitude require a higher TAS, which will require more power.

5-26

Figure 5-26. The effect of altitude on range.

Another aspect of cruise flight relating to range and endurance, one that is often not talked about in textbooks, is cruise performance. From a practical standpoint, the pilot will not fly the aircraft at maximum endurance or range—it is just too slow. In reality, pilots often operate propeller-driven airplanes at 55%, 65%, or 75% best power or endurance.

In order to calculate how to get to your destination as fast as possible, find the highest true airspeed for your aircraft. Most fixed-gear single-engine aircraft that cruise in the 110–130 knot range will have their highest TAS in the 6,000 to 7,000-foot range. This is a good place to start; however, the wind, terrain, and the need for fuel stops will dictate the altitude and speed at which the aircraft ultimately flies.

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