RIVETED JOINTS
By MANISH KUMARRIVETED JOINTS
Aircraft raw materials come in different but limited sizes due to manufacturing limitations as well as economical distribution. The designer has to choose materials which are available, can be transported to the manufacturing facility (even the homebuilder's basement or garage), can be cut to required sizes with the minimum tools, and can be handled without causing too many rejects due to mishandling ... and still end up with an aircraft of appreciable size, adequate strength and good looks. Aircraft can't just be made out of one big sheet of material and "wrapped together." Rather, various parts have to be formed out of different types of material and joined together. Each of those parts carries a load and the fastener that brings these parts together has to carry the load from one part to the other. If we have, for example, 1,000 lbs. to be carried over from one skin to another, we can choose various ways of achieving this (see figure 1).
The designer of an aircraft chooses the solutions best adapted to the materials used - a continuous joint with wood and composites, a single bolt or heavy (thick) fittings with steel; or riveted joints on relatively light gauge materials and/or when the joints are long (to avoid the weight penalty of many steel bolts).
For over 50 years, riveted aluminum structures have been very successful, and are found to varying degrees on virtually all aircraft (whether the complete airframe or just an instrument panel). They do not fail under static or repeated loads and they do not corrode if the rivets are well chosen and properly set.
How to set the rivets correctly can be learned quite easily and should be explained by the designer when he sells drawings or kits to build an aluminum aircraft. The choice of rivets is very simple: only 2017 alloy rivets are commercially readily available (these are the "AD" rivets mentioned in earlier columns). They have good corrosion resistance and are compatible with 2024 and 6061 materials.
Now, let's look at why they are also a good structural fastener. (See figure 2). First the hole is drilled slightly oversized (via the use of number drills) so that the rivet can easily be introduced after deburring (see Figure 2, item E).
Note that the drawing also indicates correct rivet size depending on the total metal thickness, called the "grip'. Then the rivet is squeezed (compression is achieved by a rivet 'gun" and a "bucking bar". The pneumatic gun hammers on one side while the bucking bar, which is simply a heavy chunk of steel, provides the reaction on the other.)
When the rivet shank is compressed, its diameter grows until the hole is completely filled. (See Figure 2, item F). When we further compress the rivet it can only grow further outside the hole and thus the formed head is shaped (see Figure 2, item G), which also gives a correct formed head dimension. Note that a visual inspection of the rivet will immediately tell you if the rivet is good or if it has to be drilled out and replaced.
Such easy inspection is obviously not possible on a bonded or glued joint, which can cause such joints to be less reliable.
Next, let's look at what makes the set rivet (AD rivet) a good fastener.
- First, AD rivets are manufactured with adequate quality control which guarantees you the correct alloy (when you mix bonding cement or resins. you are responsible!)
- The rivet fills the hole completely so that no relative motion is possible.
- The original as well as the formed head both rest both very well an the parts having been compressed into place. This makes for a snug and sealed joint which will prevent any water from creeping under the heads and corroding underneath.
Also very important is the fact that the heads squeeze assembled parts tightly together and when the loads are applied (see arrows on Figure 2), part of the load is transmitted from one sheet to the other by friction. It just happens in aircraft (this is not the case with racing cars) that the part of the load transmitted by friction corresponds to the high frequency engine loads which would otherwise fatigue the rivet (or require an overdesign of the rivet joint which is done in racing cars where the engine vibration loads are much larger with respect to the static loads). As mentioned, solid riveting when correctly done is an excellent fastener - both reliable and durable. But it also has some drawbacks:
- You need special equipment (you'll need to buy an air compressor, rivet gun(s), rivet snaps and bucking bars);
- You need some expertise and prior practice (you'll need a good teacher for this - errors can be costly in more ways than one);
- It is noisy (your family and neighbors may object to your setting rivets in your basement or garage after 10 p.m. or on Sunday morning . . . and that is just when you have the time for it);
- You need access to both sides of the parts to be assembled (and this is obviously not always easy or possible: How will you get the bucking bar inside an aileron of a small aircraft?). You'll often need a helper to "buck" the rivet on the other side, or have long skinny arms and/or a full assortment of bucking bars.
So another solution has been devised - blind rivets, which have none of the above-mentioned disadvantages. Blind rivets, often incorrectly referred to as "pop" rivets, have been used on aircraft since the production of the DC-3 (the tubular 'Chobert' rivets). In the next article we will discuss the good and also the questionable qualities of blind rivets in more detail.
In the first part of this article we examined the advantages (i.e. reliability and durability) of solid "bucked" rivets as well as their disadvantages (i.e. need for expensive equipment, required skill level, noisy operation, and accessibility). Blind rivets have been developed to overcome the disadvantages of solid rivets, and some of the blind rivets now available have retained virtually all the advantages of solid rivets. Let's look at blind rivets in some detail.
First of all, let us understand that a "pop" rivet is a blind rivet, but a blind rivet is not necessarily a pop rivet. ("Pop" rivet is a brand name manufactured by USM - United Shoe Machine - and obviously a "shoe" rivet is not ideal for aircraft use.)
As a typical example, we will use Textron's Avdel Avex rivet (see Note at end of article).
When setting a blind rivet we first drill a slightly oversize hole so that the rivet can easily be inserted (see item H, Figure 3).
A special hand rivet puller (hand rivet gun from a local hardware store - at $15 to $50, depending on quality) is used to pull on the rivet stem and the reaction is applied to the rivet head. The stem has a special head which compresses the rivet tube and makes it grow and fill the hole (see item 1, Figure 3) pulling further. The rivet can only grow outside the parts until the rivet and stem head create a good formed head resting well on the part and squeezing the parts together. At that stage, the stem breaks in tension at the notch. (The set rivet is shown in item L, Figure 3.)
When we examine this blind rivet and compare it to the solid rivet discussed in Part 1 of this discussion, we find some of the same advantages:
The rivet is manufactured under adequate quality control, which guarantees you the quality. (Again, see note at the end of this article.)
The rivet fills the hole completely preventing any relative motion.
Original and formed heads seal on and compress the parts together (no corrosion, the engine vibration loads do not fatigue the rivet because they are transmitted by friction.)
There is one prime disadvantage to blind rivets. The rivet, being tubular, has a section that is obviously smaller than that of a solid rivet. This means one blind rivet is not as strong as one solid rivet of the same diameter. The designer needs more blind rivets, a larger diameter rivet or a stronger material.
Many designers seem to like the "monel" (stainless steel type) rivets which are stronger, but they may forget that there is a corrosion problem involved with stainless steel. As mentioned in an earlier article, as the aluminum corrodes away, the aircraft owner has no choice but to replace the rivet with a larger diameter rivet later on. Or, if using stainless steel rivets, the builder has the messy burden of dipping every single rivet in zinc-chromate (ZnCr) primer before setting it in the hole ... and this is all beside the fact that there is no "good" stainless steel blind rivet readily available on the market!
Going to larger rivet diameters is a limited choice as the large blind rivets are so hard to set by hand that a very expensive and cumbersome tool is required. In my opinion, this defeats the purpose of these rivets in the first place.
Consequently, then, if the decision is made to go with blind rivets as opposed to solid rivets, the builder/designer is left with little choice other than increasing the numbers of rivets. A good rule to be used is that the number of blind rivets needs to be increased roughly in the proportion of 5 blind rivets for 3 solid rivets. In actual fact, this is not a consideration either on light airplanes as most rivets are used on the skins, which need a relatively small rivet pitch (spacing between rivets) anyway in order to prevent waviness in the skin panel. So, the designer is stuck, solid or blind rivets, not with the strength, but with choosing a pitch which provides a nice finish (for aerodynamic and aesthetic reasons).
We have given the example of the Avex blind rivet because this is the only reasonably priced "good" blind rivet readily available (see note). Cost of the Avex rivets is approximately 8 cents per rivet, which compares to 30 cents to $1.00 for a Cherry blind rivet (and, remember, you need 4,000 to 8,000 rivets per aircraft). One other very determinant factor for selecting the Avex rivets is that they are "grip" insensitive. The standard Avex rivets will join from grip 0 to grip 1/4" (6 mm) with the same rivet. (This compares to four different lengths for the Cherry type). This is a very important factor to prevent errors and must bear heavily on the designer's decision to make construction as easy and reliable as possible for the builder.
There is one other objection to blind rivets. The rivet is okay for corrosion, but what about the stem? The stem is steel and phosphated, which is the correct protection, but, obviously, where the stem breaks there is no protection. Will this rust? Any galvanic corrosion protection (such as phosphating steel or zinc chromating aluminum) has a reach of about 1/8" (3 mm) beyond the protected area. With Avex rivets the broken part is only 1/16" at the most, and extensive experience has confirmed that this is not a problem. (Zenith CH 200 / 300 aircraft assembled with Avex rivets still look like new after more than two decades, with outside storage.)
In this article we do not give any specific shear strength, just some relative values. It is the responsibility of every designer to obtain the values he or she feels can be consistently achieved by the builders (and this takes into account many things, such as basic design philosophy, materials to be jointed, working conditions, etc.)
Nevertheless, I feel impelled to warn some experimental designers that the shear values given by the blind rivet manufacturers in catalogues are to be looked at with some common sense as well. The manufacturer is not a liar, but he does present his product in the best possible way. For example, when they make tests they use very thin sheets so that the stem is long enough to fill the rivet (see figure 4), which is the reason why the individual shear strength is higher than an aircraft solid rivet (the steel stem participates). But on our aircraft, this is relatively seldom the case. As a rule of thumb, a reliable shear value should be 1/2 the catalog specification. But again, the designer should make tests. (Just as an example, when I do blind rivet tests, I knock the stem out before the test, just to be on the safe side!)
Figure 4 shows a bad blind rivet (a standard hardware "pop" rivet). Note that the rivet does not fill the hole and that there is note a nice, formed head (just the tube is opened); the stem will fall out after some vibrations.
Use the right rivets and you will be very pleased with the results!
Bombardier Learjet 40 XR
By MANISH KUMAR|
Bombardier Learjet 40 XR
The Bombardier Learjet 40 XR business jet is, without
question, the premium aircraft in today’s light jet
category, outperforming its competitors on almost every
criterion. Its cruise speed is faster than any other
aircraft in its class, shortening travel time and
bringing destinations closer. With enhanced engines, it
is faster off the ground and in the air than his category
leader predecessor, the Bombardier Learjet 40.
Learjet 40 XR Interior As with the Bombardier Learjet 40, the Learjet 40 XR cabin is twenty percent larger than that of any other light business jet. Learjet 40 XR is the tallest in its class, reflecting the aircraft's premium status. With superior style and functionality throughout, every amenity maximizes passenger comfort and productivity.
Capitalizing on its 363 cubic feet (10.28 m3) volume, the aircraft's large cabin offers more seating possibilities than you would expect in a light jet. Its flat floor throughout and "track and swivel" seating enables passengers to move comfortably throughout the aircraft.
Standard floor plan
Learjet 40 XR Performance Outpacing the competition with its enhanced engine performance, the Bombardier Learjet 40 XR business jet rockets to 51,000 feet (15,545 m) soaring above traffic and bad weather. Fully fueled it flies 7 passengers to their destinations faster than any competing jet.
Learjet 40 XR Technology With its premium ergonomic environment, “dark cockpit” methodology and advanced avionics, the Bombardier Learjet 40 XR offers pilots more systems safety features than any competitive jet in its class. Certification
The Bombardier Learjet 40 XR business jet complies with
exceptionally rigorous standards. In fact, it is
certified to a higher level than any of its direct
competitors—higher even than many modern airliners. This
ringing endorsement by regulatory bodies testifies to
both the technological benchmark the aircraft represents
and the safety and reliability its operators can
expect.
• 16 G Dynamic Seat Certification, which more accurately
simulates dynamic events during an accident, improves
seat attachment and restraint system loading, and
stiffens requirements related to head impact and leg
compression injuries. Systems
A principal advantage of the Bombardier Learjet 40 XR
aircraft over other light business jets is its heritage.
Capitalizing on advanced technology successfully
developed for the high-performance Learjet 45 program -
and leveraging Bombardier's unmatched engineering and
integration expertise - all Learjet 40 XR systems assure
proven reliability and uncompromising quality. The
results are performance excellence, unequalled ease of
maintenance and serviceability and total operator
confidence. No other aircraft in the category comes even
close.
• Hydraulically powered anti-skid system for the shortest
landing distance in its class
Powerplant Imagine the power Imagine: It’s midday in Jackson Hole, Wyoming, when a team of six executives receives an urgent call to return to New York. The temperature is 82° F (28° C), the altitude is 6,444 feet (1,964 m) above Sea Level, the runway is just 6,300 feet (1,920 m) long – and delay is not an option. Only one light jet can rise to such a challenge. The Bombardier Learjet 40 XR aircraft, with its enhanced engines, takes off with a full complement of passengers and fuel. It powers effortlessly above the Teton Range’s highest peak, climbs directly to 45,000 feet (13,716 m) in under 30 minutes, and flies on to Teterboro, New Jersey – a distance that is over 900 nautical miles (1,667 km) farther than even the performance-driven Learjet 40 business jet could achieve. While a Maximum Takeoff Weight increase allows the Learjet 40 XR jet to fill both the cabin and the fuel tanks, it is the increased thrust made available by its TFE-731-20-BR engines that make the aircraft’s stellar hot and high performance possible. Imagine what that extra power can do for you under normal conditions on everyday missions. Honeywell TFE731-20-BR-1B Engines The Bombardier Learjet 40 XR executive aircraft is powered by two aft fuselage-mounted TFE731-20-BR-1B turbofan engines from Honeywell Engines & Systems. An upgrade of the TFE731-20-AR-1B engine, developed in partnership with Bombardier for the clean-sheet designed Bombardier Learjet 45 business jet, the TFE731-20-BR-1B brings proven maturity along with a host of high-end features that add value beyond anything else in the light jet category. Avionics
The Bombardier Learjet 40 XR business jet is equipped
with a Honeywell Primus 1000 avionics system, including
an Electronic Flight Instrument System (EFIS), equipped
with an EICAS, for unparalleled new levels of systems
management. The EICAS integrates the multitude of
electromechanical instruments that previously were
required to display critical engine parameters, offering
significant safety and operational benefits. The EFIS,
which employs four large displays, presents vital
information to the crew in an uncluttered format,
simplifying cockpit scan and thereby reducing pilot
workload and fatigue.
• EFIS with four 8 x 7 in (20.3 x 17.8 cm) displays
Primary and Multi-Function Flight
Displays
Options An aircraft is personalized through the many finishing touches that make it your own. Be your tastes conservative or unrestrained, your technology needs modest or cutting-edge, Bombardier Learjet aircraft can be outfitted to meet your preferences with a full complement of interior, exterior, equipment and avionics options.
Extended range Flying at its long range cruise speed of Mach 0.75 (800 km/h), the Learjet 40 XR aircraft now offers a non-stop range of 1,991 NM (3,687 km)* – over 15 percent more distance covered. The extended range increases travel flexibility to include key city pairs such as London-Cairo 1,942 NM (3,596 km), Singapore-Taipei 1,835 NM (3,398 km), Hong Kong-Jakarta 1,864 NM (3,452 km), New York-Aspen 1,519 NM (2,813 km), and Dubai-Istanbul 1,715 NM (3,176 km)*. *under certain conditions: 4 passengers, long range cruise speed , zero wind, NBAA (100 NM) IFR fuel reserves, 85 % Boeing annual winds, ISA.
Exterior and Interior Options Modestly detailed or proudly emblazoned with your corporate colors, the ramp appeal and interior splendor of Bombardier Learjet aircraft is but enhanced with your options - inside and out. A selection of our available options: Exterior Options Split Base Coat -- Required for paint schemes that have two different colors for the top and bottom of the aircraft. Interior Options
Universal
Aero-M SATCOM -- Includes a cockpit handset, cabin
handset at seat #2 and fax/data port. Airshow Network -- In addition to Airshow 400 features, Airshow Network includes up-to-the-minute financial news and stock quotes from Bloomberg, news briefs from CNN, business updates from the Wall Street Journal and current sports scores from ESPN SportsTicker. The flight crew can also access and receive airport, regional and national weather updates from the Wall Street Journal. The display format includes one of the following map packages: North America / South America / Europe / Africa/Middle East / North Pacific / South Pacific... Note: Requires the selection of a video monitor and airborne telephone system. Airshow Network is not compatible with option 23-15-0000 Universal Aero-M SATCOM.
10.4-inch (26
cm) Aft Tracking Video Monitor -- A
high-resolution, flat-panel LCD screen, surface-mounted
on the forward side of the aft left-hand partition. The
system tracks inboard over the aisle during flight for
improved viewing.
Equipment Options Equipment options are designed to facilitate your every mission requirement, be they for added safety or ease of operation. A selection of our available options: Concorde Batteries – 28 Ampere-hour (Exchange) -- Includes two Concorde lead acid batteries. Provides the same power as a standard battery.. Note: Exchange for standard NiCad main batteries. Concorde Batteries – 38 Ampere-hour (Exchange) -- Includes two Concorde lead acid 24 volt, 38 ampere-hour batteries in exchange for the standard SAFT NiCad 24 volt, 27 ampere-hour batteries... Note: Meets JAA compliance requirements.
VARTA
Batteries (Exchange) 24-32-0000 Tail Illumination Package -- Includes two lights, installed on the lower surface of the horizontal stabilizer, which illuminate both sides of the vertical stabilizer for enhanced night runway visibility. The NAV light switch is modified to add a third (NAV/LOGO) position. Exterior Lighting Package -- Provides flood lighting for the single-point refueling adapter and external baggage door areas. Lighted Control Wheel Chart Holders -- Installed at each control wheel. Provide illumination of the approach plates... Note: Required for aircraft operating under JAR-OPS 1 regulations. Pulsating Recognition Lights -- Alternately pulses outboard wing root recognition lights for enhanced aircraft visual identification. Aircraft Locking Package -- In addition to the standard configuration, which has locks on the entry door, baggage bay door and tailcone door, the optional Aircraft Locking Package provides supplemental locks for both nose avionics doors and the single-point refueling and filler door. All locks are keyed alike. R134A Air-Conditioning System (Vapor Cycle System) -- The Keith Products Vapor Cycle System provides ground air-conditioning and an auxiliary heater for ground operations. The system features separate forward and aft evaporators that independently condition the cockpit and cabin air. The air-conditioning can be powered by either a Ground Power Unit (GPU) or by both generators operating. The heater can only be powered by a GPU. The cabin and cockpit fans can be operated for air circulation without limitation. Increased Oxygen Capacity, 40 Cubic Feet (Exchange) -- Increases the aircraft's oxygen system capacity from 23 to 40 cubic feet (651 l to 1,133 l).
Avionics Options A complete selection of optional avionics equipment is available for Bombardier Learjet aircraft. Whether to meet current and upcoming FAA and EASA legislation, or for the latest airborne telephone systems, Bombardier can outfit your jet accordingly. A selection of our available options: Second Universal UNS-1E FMS -- Provides second, pedestal-mounted Universal Avionics UNS-1E FMS. Note: The annual revision service is not included. Single HF Communication with SELCAL -- The Honeywell KHF-950 provides 280,000 HF frequencies at 2 to 29.999 MHz with 99 pilot-programmable preset channels. The system includes a KCU-951 control and an inverted “V” long wire antenna. The Coltech SELCAL provides selective calling for VHF 1, VHF 2 and HF. Dual HF Communication with SELCAL -- The dual Honeywell KHF-950 provides 280,000 HF frequencies at 2 to 29.999 MHz with 99 pilot-programmable preset channels. The system includes two KCU-951 controls and a single inverted “V” long wire antenna. Both HF radios can be in operation in the receive mode, but only one radio can be in transmit mode. The Coltech SELCAL provides selective calling for VHF 1, VHF 2, HF 1 and HF 2. Note: Required for certain operations for aircraft operating under JAR-OPS 1 regulations. Honeywell Airborne Flight Information System (AFIS) -- AFIS interfaces with the FMS to provide the crew with the following services from the Global Data Center: Domestic and International Flight Planning and Filing, Text Weather Services, Dispatching Services, Air Traffic Services and Message Services. The built-in VHF comm communicates with the Global Data Center through the ARINC, SITA and AVICOM networks. Coverage areas include North and Central America, Europe, Japan and parts of South America, Africa, Asia, Australia and the South Pacific. The system includes a Data Management Unit, Data Transfer Unit, Configuration Unit and antenna. Note: The AFIS cannot be selected in combination with option 31-30-0000 Digital Flight Data Recorder. Primus 880 Radar (Exchange) -- In addition to Primus 660 features, the Primus 880 radar includes Doppler Turbulence Detection, which is displayed on the weather radar map, and Altitude Compensation Tilt, which automatically adjusts radar tilt for changes in altitude. Dual Angle of Attack Indicators -- Round gauges display the relative angle of attack on pilot and co-pilot instrument panels. Dual VHF Communication with 8.33 kHz Channel Spacing (Exchange) -- VHF comm radios with 8.33 kHz channel spacing and a frequency range of 118.0 to 136.975 kHz. This option includes an 8.33 kHz capable radio management unit and a CD-850 clearance delivery unit. This radio is required in most European countries for flight above 24,500 feet (7,468 m). Second Automatic Direction Finder (ADF) -- Adds a second ADF with the same capabilities as the standard unit. Note: This option cannot be selected in combination with Aero-M or Aero-I SATCOM phones options 23-15-0000 and 23-15-0001. Cockpit Voice Recorder (Exchange) -- A solid-state Cockpit Voice Recorder that records voice and other cockpit sounds and retains them for a minimum of two hours. The unit complies with the requirements of ED-56A and TSO-C123a. Note: Required for aircraft operating under JAR-OPS 1 regulations. UniLink -- UniLink is a two-way data link which allows the crew to connect with a service provider (currently Universal Weather or Global Data Center) for any number of conveniences, such as pre-departure and oceanic clearances, flight plans, weather (including graphics), digital ATIS, Terminal Weather Information for Pilots (TWIP) and messaging. Weather maps are displayed on the UNS-1E FMS CDU. The built-in VHF comm communicates with the service provider through the ARINC, SITA and AVICOM networks. Coverage areas include North and Central America, Europe, Japan and parts of South America, Africa, Asia, Australia and the South Pacific. Note: An airborne telephone system is required to upload weather maps. Universal Weather is currently the only service provider of graphic weather maps (at the time of printing). Digital Flight Data Recorder -- A 128-word solid-state recorder that stores a minimum of 25 hours of data and complies with the requirements of FAA, TSO-C124a and EUROCAE ED-55... Note: Required for aircraft operating under JAR-OPS 1 regulations. The Digital Flight Data Recorder cannot be selected in combination with option 23-23-0001 Airborne Flight Information System (AFIS). Lightning Detection System (LDS) -- The LSZ-860 Lightning Sensor System detects electromagnetic discharges resulting from lightning activity 360 degrees around the aircraft and displays the strikes on the EFIS display with the Weather Radar. It detects lightning activity out to 200 nautical miles (370 km). Note: Installation of this option reduces storage space in the tailcone baggage compartment area.
Warranty Along with the premium performance capabilities and comfort of the Bombardier Learjet 40 XR comes strong warranty coverage, to help you better forecast cost of ownership(1). Virtually all “Alert and Recommended Service Bulletins” issued by Learjet during the basic aircraft warranty period are covered for parts and labor.
• 5 years on airframe primary metal structure
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Turbulence: The Basics
By MANISH KUMARSurprising as it is to most people, an airplane moves through air that is itself moving. Moving air flows in ways that are quite similar to moving water, only we cannot see the air currents.
Air currents vary, and flying rapidly from one current to another is what leads to the feeling of turbulence. Some people call this “hitting an air pocket,” but this description is a misnomer. The jolt comes not from falling into a “hole” in the air but from crossing a barrier between different currents.
There are several causes of turbulence:|
• |
Convective currents result from the sun heating the ground, causing air to rise. As the air rises, it cools and forms clouds—those pretty, white, fluffy cumulus clouds that look so nice and soft on the outside and are boiling with activity inside. Hence pilots are always looking for smooth air above the clouds where the convection stops. After sunset the air is generally much smoother because of a lack of convective activity. But other forms of turbulence can occur at any time, even at night: |
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• |
Obstructions to wind flow cause all kinds of eddies and currents. On a city street you may have seen papers blowing around in circles between buildings; in an airplane you will notice this kind of turbulence when flying over mountains; e.g., especially on the eastern side of the Rockies in the US. |
|
• |
Wind shear occurs at the boundary between winds that differ in speed or direction, or both. Common near temperature inversions and at the border between weather fronts, this kind of turbulence is most likely encountered in a large aircraft at high altitudes near the jet stream. |
Turbulence: The Dangers
Although turbulence is generally not dangerous, it would be a lie to say that it is never dangerous. So let’s sort out the dangerous aspects of it.
Commercial aircraft prefer to fly “straight and level” (at a constant heading and altitude) because it is convenient. First of all, air traffic controllers have to keep aircraft from flying into each other, and having pilots fly at predictable paths during each segment of a flight makes a controller’s job possible. Second, people tend to get airsick when an airplane moves erratically, so straight and level flight makes things more comfortable for the passengers and crew. And third, because the shortest distance between two points is a straight line, straight and level flight makes for economy. (Actually, since the earth’s surface is curved, aircraft routes are always curves, not straight lines—but that doesn’t matter to this discussion.)
When an aircraft flies through turbulent air, though, it will tend to rock its wings and dip and bob, all because the air in which it is flying is moving every which way. There is nothing dangerous about this because there is no physical law that says an airplane has to fly in a straight line at a constant altitude and at a constant airspeed. If you watch an aerobatic airshow sometime you will realize that airplanes can fly in all sorts of positions—even backwards, for a short time—and still be safe. So, even though level flight may be preferred, if an airplane enters turbulent air, its erratic flight poses no real safety issue to the airplane itself. Usually.
So here is the first real danger of turbulence:
Structural Failure
According to FAA regulations, all aircraft are designed and built to withstand far more stress than occurs in normal flight, including ordinary turbulence. But the turbulent air in severe thunderstorms can be so powerful that it can literally rip an airplane to pieces.
Now, I’m
talking here about the danger of flying right into the middle of
the biggest and meanest thunderclouds there are. No competent
pilot would ever do that deliberately.
It is true that small, general aviation airplanes have often gotten destroyed in thunderstorms, all because the pilot was flying in the clouds and, not having on-board radar to distinguish a thunderstorm from the surrounding clouds, inadvertently flew right into a big thundercloud.
But commercial aviation has a far happier history. Dispatchers who plan the flights will route flights away from thunderstorms. Sometimes they will even cancel flights because of thunderstorms. Pilots of commercial aircraft also have on-board radar to spot and avoid thunderstorms, and they will often request a course change to avoid bad weather. So, if your flight is delayed or cancelled because of weather, be grateful, not angry.
The second real danger of turbulence:
Passenger Injury
When an airplane flies into downward-moving air, the airplane will drop with the air. But anything not securely attached to the airplane itself—such as passengers who are not strapped to their seats—can get thrown around the cabin.
You should be aware that turbulence can be forecast by aviation weather services, so pilots are likely to be aware of it in advance and will try to avoid it. This explains why the “Fasten Seat Belts” sign comes on well before the bumps start. Occasionally, though, turbulence (such as “clear air turbulence” which doesn’t have any clouds around it to give a visual warning) can be unexpected.
|
|
Many passengers who get injured because of turbulence are
those who, unlike more experienced travelers, do not keep
their seat belts loosely fastened at all times. |
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The
third real danger of turbulence:
Altitude Loss Near the Ground.
Sudden altitude changes can also be a consequence of flying in turbulent air. When you are thousands of feet above the ground, a few hundred feet of altitude loss doesn’t count for much. But if the airplane is only a few hundred feet above the ground, as when it is in the process of taking off or landing, then a few hundred feet of altitude loss can make all the difference in the world.
Severe
turbulence near the ground is usually the result of one of two
things:
|
1. |
Wake turbulence occurs when an aircraft leaves a trail of disturbed air behind it simply as a result of its flying through the air. This turbulence is greatest when it is flying slowly during take-off or landing. The turbulence poses no danger to the aircraft itself, but any other aircraft following too closely behind it can fly into the turbulent air and lose control. For this reason, air traffic controllers maintain strict limits of spacing between aircraft, both on arrival and departure. This concern for safety can cause traffic delays, but they are well worth the safety advantage. |
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2. |
Thunderstorms, as I mentioned above, can cause big problems for airplanes flying near them, especially because the storms can produce strong, unexpected downdrafts. And there have been planes that crashed while landing all because the plane dropped onto the ground before it could recover from a sudden downdraft. Thankfully, these accidents of the past have only made pilots more aware of the problem of sudden wind shifts during take-off and landing. In addition, many airports now have special detectors to warn pilots of unusual wind behavior in the vicinity of the airport. |
The
fourth real danger of turbulence:
Wind Changes Near the Ground.
Turbulent air aloft is not a problem in regard to an aircraft’s airspeed, because no matter how fast the wind is “blowing,” and no matter whether the aircraft is flying “with” or “against” the wind, all that matters aerodynamically is that the aircraft be moving sufficiently fast relative to the air around it to generate the lift necessary to keep flying.
For example, if an aircraft’s airspeed is 300 knots that means it is moving through the air mass around it at 300 knots. If that same mass of air is also moving (relative to the ground) at 300 knots opposite to the direction the aircraft is flying, the aircraft’s airspeed is still 300 knots. Even though we might think that the aircraft is flying “against” the wind, it is really flying quite safely within a moving air mass.
In the above example, although the aircraft has an airspeed of 300 knots, its groundspeed is 0 knots. Technically, it is hovering over the ground because the air is moving it backwards (relative to the ground) at the same rate as the aircraft is flying forwards (relative to the ground). Of course, my example of wind blowing at 300 knots is highly exaggerated, even in the jet stream, and I use the example just to make the point about groundspeed easier to comprehend. More realistically, though, if you watch gliding birds such as hawks and seagulls, you can occasionally see them hover over one spot on the ground just by pointing themselves into the wind and matching their airspeed with the speed of the wind.
On the other hand, if the aircraft flies in an air mass moving in the same direction as the aircraft, the aircraft’s groundspeed will be increased, sometimes dramatically. In fact, air travel across the US from the west coast to the east coast can be greatly facilitated by flying in the west-to-east jet stream. (Flying east-to-west, of course, airplanes avoid the jet stream as best as possible—otherwise, they might end up hovering above the ground, and that’s not an effective way to travel anywhere. )
OK. So understanding this much about airspeed and groundspeed, you can now grasp the safety problem in regard to wind changes when the aircraft is near the ground. If the aircraft is just about to land and suddenly the wind changes to a tail wind, the plane can actually get “blown” right off the end of the runway. In fact, several aircraft accidents have happened like this. In trying to land in the vicinity of a thunderstorm, pilots under pressure to land, rather than divert to another airport, have been surprised by strong, erratic winds and, on touchdown, have landed too far down the runway, lost control, and slid right off the runway.
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Please note that sudden, unpredictable wind changes near the ground usually happen because of a thunderstorm in the vicinity of the airport. Wind changes can happen because of a larger weather system such as a front, but such wind changes are usually a matter of changes in wind speed, not radical changes in wind direction. Therefore, strong winds in themselves do not usually cause problems for commercial aviation.
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ON BOARD MAINTENANCE SYSTEM (OBMS)
By MANISH KUMARPurpose of the OBMS (from A320 Manual) REMEMBER ALWAYS REFER TO THE AMM
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The purpose of the onboard maintenance is to provide maintenance personnel with an aid to fault diagnosis further to a complaint of the crew.
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To accomplish this goal:
(1) Each system includes a Built-In-Test Equipment (BITE) used for detection and isolation of faulty equipment. Furthermore the system is able to initiate tests for the purpose of confirming a fault condition or checking that proper system operation is restored after corrective action.
Remark:
To simplify the task of maintenance personnel:
- the faces of the computers and the maintenance test functions have been standardized
- the maintenance messages are displayed in clear English language and always concern the faulty component or, in some cases, the faulty system.
(2) A Centralized Fault Display Interface Unit (CFDIU) acquires and processes (completes, correlates, memorizes and presents) the data transmitted by the BITEs and the warnings which have originated the crew complaint.
(3) The result of fault diagnosis is displayed to the maintenance operator through the Multipurpose Control and Display Units (MCDU) and the Printer which constitute the user interface.
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Levels of Maintenance
The failure information delivered by the Centralized Fault Display System (CFDS) corresponds to several levels of maintenance.
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(1) Line maintenance
This maintenance is characterized by rapid intervention of maintenance personnel in a short time period; it is limited to the isolation and replacement of a faulty equipment.
This action consists of the identification and/or confirmation of fault condition(s), the isolation of the fault and the replacement of the faulty unit (i.e the Line Replaceable Unit (LRU)).
A test is carried out before and after the removal/installation procedure to check the correct operation of the system.
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(2) Hangar or main base maintenance
This maintenance is characterized by intervention of maintenance personnel in a longer time period and generally concerns actions that cannot be performed at line maintenance level, either because the procedures are too lengthy or because more skilled personnel are required.
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(3) Workshop maintenance
These maintenance actions are performed at regular intervals (check
A, 2A, B...). Intervention of maintenance personnel is then scheduled according to aircraft utilization and concerns the items of equipment for which some mechanical parts are not monitored and/or tested. These failures are called hidden failures.
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B. System BITE Functions
(1) General
- Each electrical or electronic system of the aircraft includes a fault detection, isolating and storing device called a BITE
- A system consists of a set of removable components called LRUs that are specific to the system and that are said to be internal to the system. In many cases, a system uses data from other systems that can be considered as _sensors_ and that are said to be LRUs external to the system.
Example : In an autopilot, the control unit, the computers which determine the laws and the power systems are internal LRUs.
The air data and inertial reference units, the radio altimeter, are external LRUs.
- The BITE system reacts to any fault affecting operation, whether INTERNAL or EXTERNAL to the system.
- Each LRU is a component internal to a given system and no other.
Example : A pressure sensor used for several systems is taken into account as an internal component by one of these systems only.
- All systems including a BITE system are connected to the CFDIU.
- The systems are connected to the CFDIU by means of the system bus.
Exceptions : the FMGS via the FAC 1 and the EFCS via the FCDC use specific b uses.
- The responsibility for fault detection and isolation lies with the system. The CFDS does not perform any processing and it does not modify the diagnosis made by the system.
- For complex systems, one of the computers plays the role of system
BITE, in other words it collects the maintenance data from the peripheral computers and it ensures the interface between the computers and the CFDS.
NOTE : A system BITE is supposed to analyze the data received with a view to establishing a consolidated diagnosis.
STUDY MANAGEMENT
By MANISH KUMARSmart students know how to use their time efficiently to accomplish more in shorter time.
But how? Here are the top-7 ways of better time management for students:
1. Use the weekly planner: Organize your study on a weekly basis. Use the printable template to print out copies and add them to a 3-ring binder. Plan your weekly schedule every Sunday evening and adjust it daily.
2. Book the firm activities first: Block off class time, work time, and any other pre-determined weekly activities. Also schedule the essentials such as sleep, eating, and relaxation (fun and exercise).
3. Schedule short study sessions before and after lectures: Set aside 20 minutes each to preview the textbook before the lecture and to quickly check your notes after the lecture. This will save hours of study time later on.
4. Avoid marathon study sessions: Break down your study sessions into one-hour segments with 50 minutes of study and 10-minute breaks. Alternate subjects if you have scheduled consecutive hours.
5. Use a timer: A countdown timer is a great tool to help you concentrate for intensive study and to trim down your non-productive hours.
6. Arrange peak hours for core study: In time, you will learn your best study hours of the day. Use them for core study activities that require high concentration, such as reviewing lecture notes and reading the textbook.
7. Allow flexibility: Leave room for the unexpected and plan for the unplanned. Set aside open time on your planner for this
Exam Technique for EASA Part 66 multi-choice questions
By MANISH KUMAR1. Read the questions carefully. Don't rush and make sure that you've understood the question before you select one of the answers. This should go without saying but it still catches people out!
2. Think it through. Not all questions are straightforward and some are downright confusing! If you can't understand the question try to get inside the examiner's head and ask yourself what the question is really about and what knowledge the question is actually trying to test.
3. Have you used all the information given in the question? You might just have overlooked a vital piece of information. Once again, this comes down to checking that you have read the question carefully before attempting to arrive at an answer.
4. A few questions don't actually give you all the information that you need. There might be something missing from the question that you might have to assume? Questions of this type are not good practice but they DO exist. If you think that
something is missing from the question you might need to decide on what could be reasonably assumed (for example, that temperature does not change or that the supply voltage remains constant).
5. Don't make up your mind too quickly. If one of the answers looks obviously correct take another look – the examiner will often provide you with an answer that might look more inviting than the others but is actually incorrect.
6. Never guess an answer. Always try to reason out the correct answer. If this doesn't work, try to eliminate one of the answers so it becomes a choice of two rather than three potential answers. In many cases you should be able to select one answer
that is patently wrong.
7. A few questions may have more than one correct answer - it's just that one of the answers is “more correct” than the others! If you think that more than one answer
could be right you need to ask yourself which of the answers is the one that the examiner wants to see. For example, does one of the answers convey more
meaningful information than the other (potentially correct) answer(s)? If so, this is the one to go for! (Once again this is rather bad practice and the examiner should
ideally provide one answer that is uniquely correct and two others that are patently wrong).
8. Don't give up! However hard you find the questions remember that people do pass these exams and only one or two questions correctly answered can make all the difference between a pass and a fail.
Good luck with your exams!
Aircraft Tire Selection and Maintenance
By MANISH KUMAR|
Aircraft Tire Selection and Maintenance |
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Maintenance |
An aircraft tire is a sophisticated, computer-designed, multi-component product consisting of three major materials: steel, rubber and fabric. Taking this down a level, there are multiple types of nylon and rubber compounds in tire construction, each with its own special properties designed to complete the task assigned. The only thing they have in common with auto tires is that they are round.
Tires are available in tubeless and radial construction for the heavy iron, but by and large light twins and single-engine, piston-prop aircraft have a choice limited to tube-type, bias-ply tire brands and subsets within brands. Your choice is nominally an economical model, a mid-price version, a high-end model or a retread. Retreads are particularly popular with flying schools.
Aircraft tires are approved under the FAA's Technical Standard Order system (TSO). All TSO-C62b-qualified tires with a speed rating of 160 mph or less -- as well as TSO-C62c-qualified tires -- do not require re-qualification to TSO-C62d unless the tire is changed.
Tire Selection
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Tires are marked with
lots of information you need when you're ready to replace
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When selecting a tire, check your POH for the requirements for your aircraft. It will include both a size, such as 6.00-6, and a ply rating, and sometimes a brand and model recommendation. The ply rating determines the load-carrying capability of the tire. Stick to the POH on both size and ply, because you can end up with unexpected results if you elect to experiment with unapproved tires or ply ratings. If there is an STC for specialty tires, such as flotation types as may be used on a bush plane, that's fine. You, of course, will pay an airspeed penalty in that case.
Note that the term "ply" is used to identify
the maximum-rated, static-load capability and corresponding
inflation pressure applicable to specific operational
requirements. The ply rating is an indication of tire strength
and does not specify the actual number of carcass plies within
that tire.
The Embraer ERJ-170/175/190/195
By MANISH KUMARThe Embraer ERJ-170/175/190/195
Country of origin Brazil
Powerplants
ERJ-170 - Two 62.3kN (14,000lb) General Electric CF34-8E turbofans.
ERJ-190 - Two 82.3kN (18,500lb) CF34-10Es.
Performance
ERJ-170 - Max cruising speed Mach 0.82 or 890km/h (481kt). Standard range with max passengers at long range cruising speed 3334km (1800nm), LR range at same conditions 3889km (2100nm).
Pilot Fatigue
By MANISH KUMARFatigue and flight operations
Fatigue is a threat to aviation safety because of the impairments in alertness and performance it creates. "Fatigue" is defined as "a non-pathologic state resulting in a decreased ability to maintain function or workload due to mental or physical stress." The term used to describe a range of experiences from sleepy, or tired, to exhausted. There are two major physiological phenomena that have been demonstrated to create fatigue: sleep loss and circadian rhythm disruption. Fatigue is a normal response to many conditions common to flight operations because of sleep loss, shift work, and long duty cycles. It has significant physiological and performance consequences because it is essential that all flight crew members remain alert and contribute to flight safety by their actions, observations and communications. The only effective treatment for fatigue is adequate sleep (1).
In a National Transportation Safety Board (NTSB) safety study of US major carrier accidents involving flight crew from 1978 to 1990, one finding directly addressed the concern about fatigue. It stated: "Half the captains for whom data were available had been awake for more than 12 hours prior to their accidents. Half the first officers had been awake for more than 11 hours. Crews comprising captains and first officers whose time since awake was above the median for their crew position made more errors overall, and significantly more procedural and tactical decision errors (2)."
Jet-lag & Transmeridian flight
By MANISH KUMARJet lag is caused by travelling at great speeds over many time zones. This unbalances the "circadian rhythms," or biological lock, which is set by the pineal gland (a tiny organ in the brain). Eye cells send light and darkness messages to this gland, which releases melatonin (a sleep-inducing hormone) in response to darkness. Thus, abrupt changes in time zones can upset melatonin production, which ultimately unbalances the body's sleep-wake cycle. These biological functions, combined with travel-related physical and emotional stress, cause jet lag.
Common symptoms of jet lag include headaches, irritability, upset stomach, sleeplessness, gastric discomfort, chills and inability to concentrate. Symptoms may be worse if you are travelling west to east (away from the sun), because light helps to preserve the body's equilibrium. Travel from east to west (to an earlier time zone) results in fewer jet lag symptoms, and travelling northward or southward does not affect the body's circadian rhythms at all.
