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Aerodynamics: Shock Waves

Today we’re taking a look at a concept related to high speed flight, shock waves, with an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B). During flight, a wing produces lift by accelerating the airflow over the upper surface. This accelerated air can, and does, reach sonic speeds even though the aircraft itself may be flying subsonic. At some extreme angles of attack (AOA), in some aircraft, the speed of the air over the top surface of the wing may be double the aircraft’s speed. It is therefore entirely possible to have both supersonic and subsonic airflow on an aircraft at the same time. When flow velocities reach sonic speeds at some location on an aircraft (such as the area of maximum camber on the wing), further acceleration results in the onset of compressibility effects, such as shock wave formation, drag increase, buffeting, stability, and control difficulties. Subsonic flow principles are invalid at all speeds above this point.

When an airplane flies at subsonic speeds, the air ahead is “warned” of the airplane’s coming by a pressure change transmitted ahead of the airplane at the speed of sound. Because of this warning, the air begins to move aside before the airplane arrives and is prepared to let it pass easily. When the airplane’s speed reaches the speed of sound, the pressure change can no longer warn the air ahead because the airplane is keeping up with its own pressure waves. Rather, the air particles pile up in front of the airplane causing a sharp decrease in the flow velocity directly in front of the airplane with a corresponding increase in air pressure and density.

As the airplane’s speed increases beyond the speed of sound, the pressure and density of the compressed air ahead of it increase, the area of compression extending some distance ahead of the airplane. At some point in the airstream, the air particles are completely undisturbed, having had no advanced warning of the airplane’s approach, and in the next instant the same air particles are forced to undergo sudden and drastic changes in temperature, pressure, density, and velocity. The boundary between the undisturbed air and the region of compressed air is called a shock or “compression” wave. This same type of wave is formed whenever a supersonic airstream is slowed to subsonic without a change in direction, such as when the airstream is accelerated to sonic speed over the cambered portion of a wing, and then decelerated to subsonic speed as the area of maximum camber is passed. A shock wave forms as a boundary between the supersonic and subsonic ranges.

Whenever a shock wave forms perpendicular to the airflow, it is termed a “normal” shock wave, and the flow immediately behind the wave is subsonic. A supersonic airstream passing through a normal shock wave experiences these changes:

  • The airstream is slowed to subsonic.
  • The airflow immediately behind the shock wave does not change direction.
  • The static pressure and density of the airstream behind the wave is greatly increased.
  • The energy of the airstream (indicated by total pressure—dynamic plus static) is greatly reduced.

Shock wave formation causes an increase in drag. One of the principal effects of a shock wave is the formation of a dense high pressure region immediately behind the wave. The instability of the high pressure region, and the fact that part of the velocity energy of the airstream is converted to heat as it flows through the wave, is a contributing factor in the drag increase, but the drag resulting from airflow separation is much greater. If the shock wave is strong, the boundary layer may not have sufficient kinetic energy to withstand airflow separation. The drag incurred in the transonic region due to shock wave formation and airflow separation is known as “wave drag.” When speed exceeds the critical Mach number by about 10 percent, wave drag increases sharply. A considerable increase in thrust (power) is required to increase flight speed beyond this point into the supersonic range where, depending on the airfoil shape and the AOA, the boundary layer may reattach.

Normal shock waves form on the wing’s upper surface and form an additional area of supersonic flow and a normal shock wave on the lower surface. As flight speed approaches the speed of sound, the areas of supersonic flow enlarge and the shock waves move nearer the trailing edge.

Shock waves

Associated with “drag rise” are buffet (known as Mach buffet), trim, and stability changes and a decrease in control force effectiveness. The loss of lift due to airflow separation results in a loss of downwash and a change in the position of the center pressure on the wing. Airflow separation produces a turbulent wake behind the wing, which causes the tail surfaces to buffet (vibrate). The nose-up and nose-down pitch control provided by the horizontal tail is dependent on the downwash behind the wing. Thus, an increase in downwash decreases the horizontal tail’s pitch control effectiveness since it effectively increases the AOA that the tail surface is seeing. Movement of the wing center of pressure (CP) affects the wing pitching moment. If the CP moves aft, a diving moment referred to as “Mach tuck” or “tuck under” is produced, and if it moves forward, a nose-up moment is produced. This is the primary reason for the development of the T-tail configuration on many turbine-powered aircraft, which places the horizontal stabilizer as far as practical from the turbulence of the wings.

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Aircraft Systems: Electrical System

Today, we’re featuring an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (FAA-8083-25B).

Most aircraft are equipped with either a 14- or a 28-volt direct current (DC) electrical system. A basic aircraft electrical system consists of the following components:

  • Alternator/generator
  • Battery
  • Master/battery switch
  • Alternator/generator switch
  • Bus bar, fuses, and circuit breakers
  • Voltage regulator
  • Ammeter/loadmeter
  • Associated electrical wiring

Engine-driven alternators or generators supply electric current to the electrical system. They also maintain a sufficient electrical charge in the battery. Electrical energy stored in a battery provides a source of electrical power for starting the engine and a limited supply of electrical power for use in the event the alternator or generator fails. Most DC generators do not produce a sufficient amount of electrical current at low engine rpm to operate the entire electrical system. During operations at low engine rpm, the electrical needs must be drawn from the battery, which can quickly be depleted.

Alternators have several advantages over generators. Alternators produce sufficient current to operate the entire electrical system, even at slower engine speeds, by producing alternating current (AC), which is converted to DC. The electrical output of an alternator is more constant throughout a wide range of engine speeds.

Some aircraft have receptacles to which an external ground power unit (GPU) may be connected to provide electrical energy for starting. These are very useful, especially during cold weather starting. Follow the manufacturer’s recommendations for engine starting using a GPU. The electrical system is turned on or off with a master switch. Turning the master switch to the ON position provides electrical energy to all the electrical equipment circuits except the ignition system. Equipment that commonly uses the electrical system for its source of energy includes:

  • Position lights
  • Anticollision lights
  • Landing lights
  • Taxi lights
  • Interior cabin lights
  • Instrument lights
  • Radio equipment
  • Turn indicator
  • Fuel gauges
  • Electric fuel pump
  • Stall warning system
  • Pitot heat
  • Starting motor

Many aircraft are equipped with a battery switch that controls the electrical power to the aircraft in a manner similar to the master switch. In addition, an alternator switch is installed that permits the pilot to exclude the alternator from the electrical system in the event of alternator failure.

On this master switch, the left half is for the alternator and the right half is for the battery.

With the alternator half of the switch in the OFF position, the entire electrical load is placed on the battery. All nonessential electrical equipment should be turned off to conserve battery power.

A bus bar is used as a terminal in the aircraft electrical system to connect the main electrical system to the equipment using electricity as a source of power. This simplifies the wiring system and provides a common point from which voltage can be distributed throughout the system.

Electrical system schematic.

Fuses or circuit breakers are used in the electrical system to protect the circuits and equipment from electrical overload. Spare fuses of the proper amperage limit should be carried in the aircraft to replace defective or blown fuses. Circuit breakers have the same function as a fuse but can be manually reset, rather than replaced, if an overload condition occurs in the electrical system. Placards at the fuse or circuit breaker panel identify the circuit by name and show the amperage limit. An ammeter is used to monitor the performance of the aircraft electrical system. The ammeter shows if the alternator/generator is producing an adequate supply of electrical power. It also indicates whether or not the battery is receiving an electrical charge.

Ammeters are designed with the zero point in the center of the face and a negative or positive indication on either side. When the pointer of the ammeter is on the plus side, it shows the charging rate of the battery. A minus indication means more current is being drawn from the battery than is being replaced. A full-scale minus deflection indicates a malfunction of the alternator/generator. A full-scale positive deflection indicates a malfunction of the regulator. In either case, consult the AFM/POH for appropriate action to be taken.

Ammeter and loadmeter

Not all aircraft are equipped with an ammeter. Some have a warning light that, when lighted, indicates a discharge in the system as a generator/alternator malfunction. Refer to the AFM/POH for appropriate action to be taken.

Another electrical monitoring indicator is a loadmeter. This type of gauge has a scale beginning with zero and shows the load being placed on the alternator/generator. The loadmeter reflects the total percentage of the load placed on the generating capacity of the electrical system by the electrical accessories and battery. When all electrical components are turned off, it reflects only the amount of charging current demanded by the battery.

A voltage regulator controls the rate of charge to the battery by stabilizing the generator or alternator electrical output. The generator/alternator voltage output should be higher than the battery voltage. For example, a 12-volt battery would be fed by a generator/alternator system of approximately 14 volts. The difference in voltage keeps the battery charged.

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sUAS: Preflight Inspections

Today we’re pleased to feature an excerpt from our latest remote pilot textbook, The Complete Remote Pilot, by Bob Gardner and David Ison. Built on the foundation of Bob Gardner’s popular The Complete Private Pilot series, this textbook is tailored for anyone interested in pursuing and obtaining a Remote Pilot Certificate, which is required in order to operate drones for commercial use. The Complete Remote Pilot is designed to not only prepare you for the exam but to teach you about how UAS fly, their components and systems, and the aeronautical knowledge required to fly these systems in the same airspace as large commercial jets. This book covers specifics on the language of drones, regulations, airspace and navigation, airport and off-airport operations, radio communication procedures, weather, aerodynamics and aircraft performance, emergency procedures, human factors, maintenance, and preflight inspection procedures.

REGULATORY REQUIREMENTS
Since an sUAS potentially operates in proximity to and within the same airspace as manned aircraft, a high level of care is required to ensure that the aircraft is safe to operate and will not do anything to jeopardize the ability of the remote PIC to maintain positive control of the system while in use. The FAA explicitly spells this out in 14 CFR §107.15, stating that prior to each flight, “the remote pilot in command must check the small unmanned aircraft system to determine whether it is in a condition for safe operation.” Further, if at any time it is determined that this condition is compromised, the operation must cease immediately. Unmanned aircraft pilots should mimic their manned counterparts who are very familiar with the preflight inspection process, which is (or should be) a very thorough evaluation of the aircraft before taking flight.

While manned aircraft manufacturers typically provide a comprehensive checklist to use for preflight inspections, not all sUAS manufacturers do so. It may be necessary to create your own checklist. However, you may not need to start from scratch—or worse, learn the “hard” way; instead, look online to find out what work has been done on your individual system. Some great resources are available on various websites, especially for systems with minimal documentation provided by the manufacturer. In short, if the aircraft comes with a checklist or procedure for ensuring safe operation, use it. Feel free to add to it if you find some additional things that you feel need to be checked prior to use. If no such guidance is provided, create your own. So what should you include? Let’s take a look.

PREFLIGHT INSPECTION CONSIDERATIONS
While each sUAS will vary, here are some key areas to consider for careful inspection before flight or on a regular basis. Before every flight, it’s important to do a thorough visual examination of the aircraft. Are there any loose parts? Is anything hanging off that should not be? Does anything look abnormal? Is there any damage to the structure? Next, you should take a look at the propellers. Before putting them on the aircraft, flex the blades slightly to confirm their integrity. While you are doing this, look over the blades and run your finger along their edges and surfaces. If there are nicks or cracks, you should replace the propeller. Once they are installed on the aircraft, propellers should be secure (and locked if applicable), but don’t overtighten them, as that can damage the threads or connections.

While you are in the vicinity of the motors, check them for proper rotation and security. Do they spin freely? Is anything sticking? Are any motors too loose? The best way to know for sure is by comparing one motor to another (or if the sUAS only has one motor, comparing it to how it appeared the last time). If anything is abnormal, it is advisable to remove and replace the item in question. (More experienced users may want to do some bench tests prior to replacing or flying.)

Next, check all peripheral items, such as the camera gimbal or other payloads. Are they properly connected and secured? How about the camera or other sensors? These typically are expensive pieces of equipment; you do not want to accidentally damage or lose them because you were in a hurry to fly.

You will also want to inspect the battery prior to installation. Note: be sure you know the correct power-on procedures for your sUAS. Some will be powered once the battery is installed, so if that is the case, you will want to be ready for that change in status. Typically, the controller is turned on first and then the sUAS is turned on, to reduce the chances of something unexpected happening (and if it does, by following this procedure, you should have control).

Check the battery connection pins/slots to verify that they are not damaged or dirty. Check the body of the battery. Is there any “puffiness” or bulging of the outer coating or sides? If the answer is yes, your battery is failing. Never use a puffy or bulging battery. Is the battery warm or hot? If so, let it cool before using. Do you know the charging status? If not, it is best to know this before flight for quality assurance and performance tracking. Once you are happy with the battery’s status, install it (or position it for installation).

The ground control station and any other equipment also should be inspected. Are the antennae properly installed and attached? Are there any missing or loose parts? What is the battery status of the components? Will the capacity be enough to complete the mission with an additional time cushion?

Is your crew briefed and ready to go? If yes, power on the sUAS (after completing necessary checklists) to check lights and other markings. Do a control check (if possible) and gimbal check. Does everything move freely without abnormal resistance or noises? Next, you will want to do an idle check with motors on (again, only when ready and post-checklists). Check for any unusual noises or vibrations. If any are detected, shut down and investigate further before departing. The final check prior to initiating the mission is an airborne control check. For example, if you’re using a quadcopter, lift the aircraft off to a hover just above the ground. Input left/right, forward/back, yaw, and power inputs to ensure the system reacts as expected. Additionally, confirm that there are no weird noises or “ticks” experienced during this operational check.

POSTFLIGHT INSPECTION CONSIDERATIONS
Upon completing a flight, the sUAS should be inspected again. Essentially you will want to check the same items as in the preflight inspection, noting any changes in aircraft status. Batteries will generally be warm because of discharging, but they should not be hot. Use caution if the battery condition changes between the pre- and post-flight inspections (e.g., it begins to bulge). Note any issues in your maintenance log (explained later) and replace any parts that are damaged or at the end of their service life.

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Weather: Clouds

Today we’ll review one of the fundamental concepts in aviation weather, understanding clouds. This post feature’s an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (8083-25B).

Clouds are visible indicators and are often indicative of future weather. For clouds to form, there must be adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled. When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes of deposition (also referred to as sublimation) and condensation, moisture condenses or sublimates onto miniscule particles of matter like dust, salt, and smoke known as condensation nuclei. The nuclei are important because they provide a means for the moisture to change from one state to another.

Cloud type is determined by its height, shape, and characteristics. They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development.

Basic cloud types

Low clouds are those that form near the Earth’s surface and extend up to about 6,500 feet AGL. They are made primarily of water droplets but can include supercooled water droplets that induce hazardous aircraft icing. Typical low clouds are stratus, stratocumulus, and nimbostratus. Fog is also classified as a type of low cloud formation. Clouds in this family create low ceilings, hamper visibility, and can change rapidly. Because of this, they influence flight planning and can make visual flight rules (VFR) flight impossible.

Middle clouds form around 6,500 feet AGL and extend up to 20,000 feet AGL. They are composed of water, ice crystals, and supercooled water droplets. Typical middle-level clouds include altostratus and altocumulus. These types of clouds may be encountered on cross-country flights at higher altitudes. Altostratus clouds can produce turbulence and may contain moderate icing. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may contain light turbulence and icing.

High clouds form above 20,000 feet AGL and usually form only in stable air. They are made up of ice crystals and pose no real threat of turbulence or aircraft icing. Typical high level clouds are cirrus, cirrostratus, and cirrocumulus.

Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds. The bases of these clouds form in the low to middle cloud base region but can extend into high altitude cloud levels. Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent. These types of clouds often develop into cumulonimbus clouds or thunderstorms. Cumulonimbus clouds contain large amounts of moisture and unstable air and usually produce hazardous weather phenomena, such as lightning, hail, tornadoes, gusty winds, and wind shear. These extensive vertical clouds can be obscured by other cloud formations and are not always visible from the ground or while in flight. When this happens, these clouds are said to be embedded, hence the term, embedded thunderstorms.

To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or in groups and is known as either an air mass or orographic thunderstorm. Heating of the air near the Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.

Since rising air currents cause cumulonimbus clouds, they are extremely turbulent and pose a significant hazard to flight safety. For example, if an aircraft enters a thunderstorm, the aircraft could experience updrafts and downdrafts that exceed 3,000 fpm. In addition, thunderstorms can produce large hailstones, damaging lightning, tornadoes, and large quantities of water, all of which are potentially hazardous to aircraft.

Cloud classification can be further broken down into specific cloud types according to the outward appearance and cloud composition. Knowing these terms can help a pilot identify visible clouds.

The following is a list of cloud classifications:

  • Cumulus—heaped or piled clouds
  • Stratus—formed in layers
  • Cirrus—ringlets, fibrous clouds, also high level clouds above 20,000 feet
  • Castellanus—common base with separate vertical development, castle-like
  • Lenticularus—lens-shaped, formed over mountains in strong winds
  • Nimbus—rain-bearing clouds
  • Fracto—ragged or broken
  • Alto—middle level clouds existing at 5,000 to 20,000 feet
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Ground Reference Maneuvers: Turns Around a Point

Today we’re featuring a follow-up to our recent post on the rectangular course with an excerpt from the Airplane Flying Handbook (FAA-H-8083-3B).

Turns around a point are a logical extension of both the rectangular course and S-turns across a road. The maneuver is a 360° constant radius turn around a single ground-based reference point. The principles are the same in any turning ground reference maneuver—higher groundspeeds require steeper banks and slower ground speeds require shallower banks. The objectives of turns around a point are as follows:

  • Maintaining a specific relationship between the airplane and the ground.
  • Dividing attention between the flightpath, groundbased references, manipulating of the flight controls, and scanning for outside hazards and instrument indications.
  • Adjusting the bank angle during turns to correct for groundspeed changes in order to maintain a constant radius turn; steeper bank angles for higher ground speeds, shallow bank angles for slower groundspeeds.
  • Improving competency in managing the quickly changing bank angles.
  • Establishing and adjusting the wind correction angle in order to maintain the track over the ground.
  • Developing the ability to compensate for drift in quickly changing orientations.
  • Developing further awareness that the radius of a turn is correlated to the bank angle.

Turns around a point.

To perform a turn around a point, the pilot must complete at least one 360° turn; however, to properly assess wind direction, velocity, bank required, and other factors related to turns in wind, the pilot should complete two or more turns. As in other ground reference maneuvers, when wind is present, the pilot must a constantly adjust the airplane’s bank and wind correction angle to maintain a constant radius turn around a point. In contrast to the ground reference maneuvers discussed previously in which turns were approximately limited to either 90° or 180°, turns around a point are consecutive 360° turns where, throughout the maneuver, the pilot must constantly adjust the bank angle and the resulting rate of turn in proportion to the groundspeed as the airplane sequences through the various wind directions. The pilot should make these adjustments by applying coordinated aileron and rudder pressure throughout the turn.

When performing a turn around a point, the pilot should select a prominent, ground-based reference that is easily distinguishable yet small enough to present a precise reference. The pilot should enter the maneuver downwind, where the groundspeed is at its fastest, at the appropriate radius of turn and distance from the selected ground-based reference point. In a high-wing airplane, the lowered wing may block the view of the ground reference point, especially in airplanes with side-by-side seating during a left turn (assuming that the pilot is flying from the left seat). To prevent this, the pilot may need to change the maneuvering altitude or the desired turn radius. The pilot should ensure that the reference point is visible at all times throughout the maneuver, even with the wing lowered in a bank.

Upon entering the maneuver, depending on the wind’s speed, it may be necessary to roll into the initial bank at a rapid rate so that the steepest bank is set quickly to prevent the airplane from drifting outside of the desired turn radius. This is best accomplished by repeated practice and assessing the required roll in rate. Thereafter, the pilot should gradually decrease the angle of bank until the airplane is headed directly upwind. As the upwind becomes a crosswind and then a downwind, the pilot should gradually steepen the bank to the steepest angle upon reaching the initial point of entry.

During the downwind half of the turn, the pilot should progressively adjust the airplane’s heading toward the inside of the turn. During the upwind half, the pilot should progressively adjust the airplane’s heading toward the outside of the turn. Recall from the previous discussion on wind correction angle that the airplane’s heading should be ahead of its position over the ground during the downwind half of the turn behind its position during the upwind half. Remember that the goal is to make a constant radius turn over the ground and, because the airplane is flying through a moving air mass, the pilot must constantly adjust the bank angle to achieve this goal.

The following are the most common errors in the performance of turns around a point:

  • Failure to adequately clear the area above, below, and on either side of the airplane for safety hazards, initially and throughout the maneuver.
  • Failure to establish a constant, level altitude prior to entering the maneuver.
  • Failure to maintain altitude during the maneuver.
  • Failure to properly assess wind direction.
  • Failure to properly execute constant radius turns.
  • Failure to manipulate the flight controls in a smooth and continuous manner.
  • Failure to establish the appropriate wind correction angle.
  • Failure to apply coordinated aileron and rudder pressure, resulting in slips or skids.
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Procedures and Airport Operations: Traffic Patterns

Today we’re featuring an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B).

At airports without an operating control tower, a segmented circle visual indicator system, if installed, is designed to provide traffic pattern information. Usually located in a position affording maximum visibility to pilots in the air and on the ground and providing a centralized location for other elements of the system, the segmented circle consists of the following components: wind direction indicators, landing direction indicators, landing strip indicators, and traffic pattern indicators.

Segmented circle

A tetrahedron is installed to indicate the direction of landings and takeoffs when conditions at the airport warrant its use. It may be located at the center of a segmented circle and may be lighted for night operations. The small end of the tetrahedron points in the direction of landing. Pilots are cautioned against using a tetrahedron for any purpose other than as an indicator of landing direction. At airports with control towers, the tetrahedron should only be referenced when the control tower is not in operation. Tower instructions supersede tetrahedron indications.

Landing strip indicators are installed in pairs and are used to show the alignment of landing strips. Traffic pattern indicators are arranged in pairs in conjunction with landing strip indicators and used to indicate the direction of turns when there is a variation from the normal left traffic pattern. (If there is no segmented circle installed at the airport, traffic pattern indicators may be installed on or near the end of the runway.)

At most airports and military air bases, traffic pattern altitudes for propeller-driven aircraft generally extend from 600 feet to as high as 1,500 feet above ground level (AGL). Pilots can obtain the traffic pattern altitude for an airport from the Chart Supplement U.S. Also, traffic pattern altitudes for military turbojet aircraft sometimes extend up to 2,500 feet AGL. Therefore, pilots of en route aircraft should be constantly on alert for other aircraft in traffic patterns and avoid these areas whenever possible. When operating at an airport, traffic pattern altitudes should be maintained unless otherwise required by the applicable distance from cloud criteria according to 14 CFR §91.155. Additional information on airport traffic pattern operations can be found in Chapter 4, “Air Traffic Control,” of the AIM. Pilots can find traffic pattern information and restrictions, such as noise abatement in the Chart Supplement U.S.

Example: Key to Traffic Pattern Operations—Single Runway

  1. Enter pattern in level flight, abeam the midpoint of the runway, at pattern altitude. (1,000′ AGL is recommended pattern altitude unless otherwise established.)
  2. Maintain pattern altitude until abeam approach end of the landing runway on downwind leg.
  3. Complete turn to final at least ¼ mile from the runway.
  4. After takeoff or go-around, continue straight ahead until beyond departure end of runway.
  5. If remaining in the traffic pattern, commence turn to crosswind leg beyond the departure end of the runway within 300 feet of pattern altitude.
  6. If departing the traffic pattern, continue straight out, or exit with a 45° turn (to the left when in a left-hand traffic pattern; to the right when in a right-hand traffic pattern) beyond the departure end of the runway, after reaching pattern altitude.

Traffic pattern operations—single runway.

Example: Key to Traffic Pattern Operations—Parallel Runways

  1. Enter pattern in level flight, abeam the midpoint of the runway, at pattern altitude. (1,000′ AGL is recommended pattern altitude unless otherwise established.)
  2. Maintain pattern altitude until abeam approach end of the landing runway on downwind leg.
  3. Complete turn to final at least ¼ mile from the runway.
  4. Do not overshoot final or continue on a track that penetrates the final approach of the parallel runway
  5. After takeoff or go-around, continue straight ahead until beyond departure end of runway.
  6. If remaining in the traffic pattern, commence turn to crosswind leg beyond the departure end of the runway within 300 feet of pattern altitude.
  7. If departing the traffic pattern, continue straight out, or exit with a 45° turn (to the left when in a left-hand traffic pattern; to the right when in a right-hand traffic pattern) beyond the departure end of the runway, after reaching pattern altitude.
  8. Do not continue on a track that penetrates the departure path of the parallel runway.

Traffic pattern operation—parallel runways.

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Helicopters: Hovering

Today, we’ll introduce one of the aerodynamic fundamentals of helicopter flight, hovering, with an excerpt from the Helicopter Flying Handbook (FAA-H-8083-21A).

Hovering is the most challenging part of flying a helicopter. This is because a helicopter generates its own gusty air while in a hover, which acts against the fuselage and flight control surfaces. The end result is constant control inputs and corrections by the pilot to keep the helicopter where it is required to be. Despite the complexity of the task, the control inputs in a hover are simple. The cyclic is used to eliminate drift in the horizontal plane, controlling forward, backward, right and left movement or travel. The throttle, if not governor controlled, is used to control revolutions per minute (rpm). The collective is used to maintain altitude. The pedals are used to control nose direction or heading. It is the interaction of these controls that makes hovering difficult, since an adjustment in any one control requires an adjustment of the other two, creating a cycle of constant correction. During hovering flight, a helicopter maintains a constant position over a selected point, usually a few feet above the ground. The ability of the helicopter to hover comes from the both the lift component, which is the force developed by the main rotor(s) to overcome gravity and aircraft weight, and the thrust component, which acts horizontally to accelerate or decelerate the helicopter in the desired direction. Pilots direct the thrust of the rotor system by using the cyclic to change the tip-path plane as compared to the visible horizon to induce travel or compensate for the wind and hold a position. At a hover in a no-wind condition, all opposing forces (lift, thrust, drag, and weight) are in balance; they are equal and opposite. Therefore, lift and weight are equal, resulting in the helicopter remaining at a stationary hover.

To maintain a hover at a constant altitude, the lift must equal the weight of the helicopter. Thrust must equal any wind and tail rotor thrust to maintain position. The power must be sufficient to turn the rotors and overcome the various drags and frictions involved.

While hovering, the amount of main rotor thrust can be changed to maintain the desired hovering altitude. This is done by changing the angle of incidence (by moving the collective) of the rotor blades and hence the angle of attack (AOA) of the main rotor blades. Changing the AOA changes the drag on the rotor blades, and the power delivered by the engine must change as well to keep the rotor speed constant.

The weight that must be supported is the total weight of the helicopter and its occupants. If the amount of lift is greater than the actual weight, the helicopter accelerates upwards until the lift force equals the weight gain altitude; if thrust is less than weight, the helicopter accelerates downward. When operating near the ground, the effects of the proximity to the surface change this response.

The drag of a hovering helicopter is mainly induced drag incurred while the blades are producing lift. There is, however, some profile drag on the blades as they rotate through the air and a small amount of parasite drag from the non-lift-producing surfaces of the helicopter, such as the rotor hub, cowlings, and landing gear. Throughout the rest of this discussion, the term “drag” includes induced, profile and parasite drag.

An important consequence of producing thrust is torque. Remember Newton’s Third Law: for every action there is an equal and opposite reaction. Therefore, as the engine turns the main rotor system in a counterclockwise direction, the helicopter fuselage wants to turn clockwise. The amount of torque is directly related to the amount of engine power being used to turn the main rotor system. Remember, as power changes, torque changes.

To counteract this torque-induced turning tendency, an antitorque rotor or tail rotor is incorporated into most helicopter designs. A pilot can vary the amount of thrust produced by the tail rotor in relation to the amount of torque produced by the engine. As the engine supplies more power to the main rotor, the tail rotor must produce more thrust to overcome the increased torque effect. This control change is accomplished through the use of antitorque pedals.

A tail rotor is designed to produce thrust in a direction opposite torque. The thrust produced by the tail rotor is sufficient to move the helicopter laterally.

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Aircraft Systems: Pressurized Aircraft

Aircraft are flown at high altitudes for two reasons. First, an aircraft flown at high altitude consumes less fuel for a given airspeed than it does for the same speed at a lower altitude because the aircraft is more efficient at a high altitude. Second, bad weather and turbulence may be avoided by flying in relatively smooth air above the storms. Many modern aircraft are being designed to operate at high altitudes, taking advantage of that environment. In order to fly at higher altitudes, the aircraft must be pressurized or suitable supplemental oxygen must be provided for each occupant. It is important for pilots who fly these aircraft to be familiar with the basic operating principles, which we’ll talk about today with an excerpt from the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B).

In a typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit capable of containing air under a pressure higher than outside atmospheric pressure. On aircraft powered by turbine engines, bleed air from the engine compressor section is used to pressurize the cabin. Superchargers may be used on older model turbine-powered aircraft to pump air into the sealed fuselage. Piston-powered aircraft may use air supplied from each engine turbocharger through a sonic venturi (flow limiter). Air is released from the fuselage by a device called an outflow valve. By regulating the air exit, the outflow valve allows for a constant inflow of air to the pressurized area.

High performance airplane pressurization system.

A cabin pressurization system typically maintains a cabin pressure altitude of approximately 8,000 feet at the maximum designed cruising altitude of an aircraft. This prevents rapid changes of cabin altitude that may be uncomfortable or cause injury to passengers and crew. In addition, the pressurization system permits a reasonably fast exchange of air from the inside to the outside of the cabin. This is necessary to eliminate odors and to remove stale air.

Standard atmospheric pressure chart.

Pressurization of the aircraft cabin is necessary in order to protect occupants against hypoxia. Within a pressurized cabin, occupants can be transported comfortably and safely for long periods of time, particularly if the cabin altitude is maintained at 8,000 feet or below, where the use of oxygen equipment is not required. The flight crew in this type of aircraft must be aware of the danger of accidental loss of cabin pressure and be prepared to deal with such an emergency whenever it occurs.

The following terms will aid in understanding the operating principles of pressurization and air conditioning systems:

  • Aircraft altitude—the actual height above sea level at which the aircraft is flying
  • Ambient temperature—the temperature in the area immediately surrounding the aircraft
  • Ambient pressure—the pressure in the area immediately surrounding the aircraft
  • Cabin altitude—cabin pressure in terms of equivalent altitude above sea level
  • Differential pressure—the difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air-conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure.

The cabin pressure control system provides cabin pressure regulation, pressure relief, vacuum relief, and the means for selecting the desired cabin altitude in the isobaric and differential range. In addition, dumping of the cabin pressure is a function of the pressure control system. A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish these functions.

The cabin pressure regulator controls cabin pressure to a selected value in the isobaric range and limits cabin pressure to a preset differential value in the differential range. When an aircraft reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed, a further increase in aircraft altitude will result in a corresponding increase in cabin altitude. Differential control is used to prevent the maximum differential pressure, for which the fuselage was designed, from being exceeded. This differential pressure is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors.

The cabin air pressure safety valve is a combination pressure relief, vacuum relief, and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure. The vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure. The flight deck control switch actuates the dump valve. When this switch is positioned to ram, a solenoid valve opens, causing the valve to dump cabin air into the atmosphere.

The degree of pressurization and the operating altitude of the aircraft are limited by several critical design factors. Primarily, the fuselage is designed to withstand a particular maximum cabin differential pressure.

Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gauge indicates the difference between inside and outside pressure. This gauge should be monitored to assure that the cabin does not exceed the maximum allowable differential pressure. A cabin altimeter is also provided as a check on the performance of the system. In some cases, these two instruments are combined into one. A third instrument indicates the cabin rate of climb or descent. A cabin rate-of-climb instrument and a cabin altimeter are illustrated in the figure below.

Cabin pressurization instruments. (Click to expand.)

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Procedures and Airport Operations: Short-Field Approach and Landing

Short-field approaches and landings require the use of procedures for approaches and landings at fields with a relatively short landing area or where an approach is made over obstacles that limit the available landing area.  Short-field operations require the pilot fly the airplane at one of its crucial performance capabilities while close to the ground in order to safely land within confined areas. This low-speed type of power-on approach is closely related to the performance of flight at minimum controllable airspeeds. Today’s post is an excerpt from the Airplane Flying Handbook (FAA-8083-3B).

Landing over an obstacle.

Landing on a short-field.

To land within a short-field or a confined area, the pilot must have precise, positive control of the rate of descent and airspeed to produce an approach that clears any obstacles, result in little or no floating during the round out, and permit the airplane to be stopped in the shortest possible distance.

The procedures for landing in a short-field or for landing approaches over obstacles as recommended in the AFM/ POH should be used. A stabilized approach is essential. These procedures generally involve the use of full flaps and the final approach started from an altitude of at least 500 feet higher than the touchdown area. A wider than normal pattern is normally used so that the airplane can be properly configured and trimmed. In the absence of the manufacturer’s recommended approach speed, a speed of not more than 1.3 VSO is used. For example, in an airplane that stalls at 60 knots with power off, and flaps and landing gear extended, an approach speed no higher than 78 knots is used. In gusty air, no more than one-half the gust factor is added. An excessive amount of airspeed could result in a touchdown too far from the runway threshold or an after landing roll that exceeds the available landing area. After the landing gear and full flaps have been extended, simultaneously adjust the power and the pitch attitude to establish and maintain the proper descent angle and airspeed. A coordinated combination of both pitch and power adjustments is required. When this is done properly, very little change in the airplane’s pitch attitude and power setting is necessary to make corrections in the angle of descent and airspeed.

Stabilized approach.

Unstabilized approach.

The short-field approach and landing is in reality an accuracy approach to a spot landing. The procedures previously outlined in the section on the stabilized approach concept are used. If it appears that the obstacle clearance is excessive and touchdown occurs well beyond the desired spot leaving insufficient room to stop, power is reduced while lowering the pitch attitude to steepen the descent path and increase the rate of descent. If it appears that the descent angle does not ensure safe clearance of obstacles, power is increased while simultaneously raising the pitch attitude to shallow the descent path and decrease the rate of descent. Care must be taken to avoid an excessively low airspeed. If the speed is allowed to become too slow, an increase in pitch and application of full power may only result in a further rate of descent. This occurs when the AOA is so great and creating so much drag that the maximum available power is insufficient to overcome it. This is generally referred to as operating in the region of reversed command or operating on the back side of the power curve. When there is doubt regarding the outcome of the approach, make a go around and try again or divert to a more suitable landing area.

Because the final approach over obstacles is made at a relatively steep approach angle and close to the airplane’s stalling speed, the initiation of the round out or flare must be judged accurately to avoid flying into the ground or stalling prematurely and sinking rapidly. A lack of floating during the flare with sufficient control to touch down properly is verification that the approach speed was correct.

Touchdown should occur at the minimum controllable airspeed with the airplane in approximately the pitch attitude that results in a power-off stall when the throttle is closed. Care must be exercised to avoid closing the throttle too rapidly, as closing the throttle may result in an immediate increase in the rate of descent and a hard landing.

Upon touchdown, the airplane is held in this positive pitch attitude as long as the elevators remain effective. This provides aerodynamic braking to assist in deceleration. Immediately upon touchdown and closing the throttle, appropriate braking is applied to minimize the after-landing roll. The airplane is normally stopped within the shortest possible distance consistent with safety and controllability. If the proper approach speed has been maintained, resulting in minimum float during the round out and the touchdown made at minimum control speed, minimum braking is required.

Common errors in the performance of short-field approaches and landings are:

  • Failure to allow enough room on final to set up the approach, necessitating an overly steep approach and high sink rate
  • Unstable approach
  • Undue delay in initiating glide path corrections
  • Too low an airspeed on final resulting in inability to flare properly and landing hard
  • Too high an airspeed resulting in floating on round out
  • Prematurely reducing power to idle on round out resulting in hard landing
  • Touchdown with excessive airspeed
  • Excessive and/or unnecessary braking after touchdown
  • Failure to maintain directional control
  • Failure to recognize and abort a poor approach that cannot be completed safely
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