Winglets The most noticeable feature to appear on new 737s are the winglets. These are wing tip extensions which reduce lift induced drag and provide some extra lift. They have been credited to Dr Louis Gratzer formerly Chief of Aerodynamics at Boeing and now with Aviation Partners Boeing (APB). They were first flown on a 737-800 in June 1998 as a testbed for use on the BBJ. They are now available as a standard production line option for all NGs with the exception of the -600 series, for which Boeing is “continuing to assess the applicability”. They are also available as a retrofit from APB. They are 8ft 2in tall and about 4 feet wide at the base, narrowing to approximately two feet at the tip and add almost 5 feet to the total wingspan. The winglet for the Classic is slightly shorter at 7ft tall. Over half of all 737NGs have had winglets retrofitted.

All of the information, photographs & schematics from this website and much more is now available in a 370 page, 8.5" x 11" book available here. Updated 29 Jan 11

Winglets are also available for Classics. The first winglet equipped 737-300 flew in Nov 2002 and gained its FAA supplemental type certificate (STC) on 30 May 2003. Winglet equipped Classics are known as Special Performance (SP). Winglets have the potential to give the following benefits: Improved climb gradient. This will enable a higher RTOW from climb limited airports (hot, high or noise abatement) or obstacle limited runways. Reduced climb thrust. A winglet equipped aircraft can typically take a 3% derate over the non-winglet equivalent aircraft. This can extend engine life and reduce maintenance costs. Environmentally friendly. The derate, if taken, will reduce the noise footprint by 6.5% and NOx emissions by 5%. This could give savings on airport noise quotas or fines. Reduced cruise thrust. Cruise fuel flow is reduced by up to 6% giving savings in fuel costs and increasing range. Improved cruise performance. Winglets can allow aircraft to reach higher levels sooner. Air Berlin notes, “Previously, we'd step-climb from 35,000 to 41,000 feet. With Blended Winglets, we can now climb direct to 41,000 feet where traffic congestion is much less and we can take advantage of direct routings and shortcuts which we could not otherwise consider.” Good looks. Winglets bring a modern look and feel to aircraft, and improve customers' perceptions of the airline. If winglets are so good, you may wonder why all 737s don’t have them. In fact 85% of all new 737s are now built with winglets, particularly the 800 and 900 series and of course all BBJs. It comes down to cost versus benefits. Winglets cost about $725,000USD and take about 1 week to install which costs an extra $25-80,000USD. Once fitted, they add 170-235kg (375-518lbs) to the weight of the aircraft, depending upon whether they were installed at production or a retrofit. The fuel cost of carrying this extra weight will take some flying time each sector to recover, although this is offset by the need to carry less fuel because of the increased range. In simple terms, if your average sector length is short (less than one hour) you wont get much the benefit from winglets - unless you need any of the other benefits such as reduced noise or you regularly operate from obstacle limited runways. There is a small difference in rotation rate for aircraft with winglets installed and, as a result, the crew needs to be cautious of pitch rate. There is approximately a ½ unit take-off trim change between non-winglet and winglet aircraft so the green band is slightly different for winglet aircraft. Finally, the dry “maximum demonstrated” crosswind limit is slightly reduced with winglets to 34kts. According to APB this is because“the FAA will only let us document the max winds experienced during flight test... so if we had been able to find more crosswind, then the 33kts might have been more. There appears to be no weather cocking effect due to winglets.”

Aerodymaics
(Discuss the basics of aerodynamics, and what makes an airplane fly)


Contents:
1) What is Aerodynamics
2) What is Lift
2.1) Bernoulli's Theory of Lift (incorrect theory)
2.2) Real Theories on Lift (correct theory)
3) What is Drag
4) What is Thrust
5) What are high Angles of Attack (AOA)
6) What is a Spin & Stall


Section 1 :: What is Aerodynamics?
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is often used synonymously with fluid and gas dynamics, with the difference being fluid dynamics is applied to liquids, and gas dynamics to gases. Understanding the motion of air (aka "flow field") around an object enables the calculation of forces and moments acting on the object. Typical properties calculated for a flow field include: velocity, pressure, density, and temperature. By defining a control volute around the flow field, equations for the conservation of mass, momentum, and energy can be defined and used to solve for the properties.

 
 


Section 2 :: What is Lift?
Lift is a force, which is perpendicular to the flow field direction. It is opposite of the drag force (explained later), which isparallel to the flow field. Lift is generated in accordance with the fundamental principles of physics, such as: Newton's Laws of Motion, Bernoulli's Principle, Conservation of Mass, and Balance of Momentum (similar to Newton's 2nd law).

* Bernoulli's Theory of Lift:
This theory ignores friction over surfaces, which is easier to understand, but is not completely accurate.

In this diagram, "streamline" is the air which goes over the wing (aka airfoil from a side-view). As air flows over the top surface of the airfoil, which has a longer distance than the lower surface, it has a lower pressure, thus a faster velocity or speed. Thus, airflow on the lower surface has a higher pressure, and slower velocity.
Because airflow over the top surface is faster, it produces greater force than the bottom, thus creating our "Lift" force perpendicular to the airflow.
(NOTE: Circles = time intervals of airflow over the airfoil. Notice how the times are all the same.)


* Real Theories on Lift:
The above theory in a way is correct, but it does not explain correctly how airflow over the wing produces lift. No single theory can explain how a wing produces lift, it is a combination of the four main theories mentioned above, and many other more complex theories and formulas.
(No formulas will be given here either, so if you wish to know those, please GOOGLE or read up on aviation books on lift and aerodynamics.)


If you look at the above image, you will noticed various colored lines. These lines show where on the airfoil air is flowing over it, and at what times (ms). Unlike Bernoulli's simplified version, you will notice air does not meet at the end of the airfoil at the same time.


Above is a similar images (with various Angles of Attack), but it also shows the various pressure bubbles formed over the airfoil. When the Angle of Attack is at 0-degrees, you'll notice the pressure bubbles are roughly equal; with the top blue bubble being larger and further back. When the Angle of Attack reaches 10-degrees, the top blue pressure is larger and further back than the smaller red bubble.
It is actually the size and location of these pressure bubbles which causes lift on the wing, rather than the top airfoil having a longer travel distance than the bottom airfoil. The Angle of Attack and shape of the airfoil does affect airflow speed over the airfoil, which leads to various shapes and designs.
If the Angle of Attack is too high, and the airfoil is not designed properly, the pressure bubble on the top wing will not form correctly, thus produce almost zero lift force.



Section 3 :: What is Drag?
There are various types of drag, and the most common type of drag everyone talks about is Lift Induced Drag. This type of drag is caused by the lift force and angle of the wing. The greater the wing angle is with respect to the airflow and airplane direction, the greater the induced drag. This type of drag is greatest at low airspeeds. Since there is little force or thrustfrom the engines to push the aircraft forward (which provides greater airflow), higher Angles of Attack of the wing are required to produce sufficient lift. This higher Angle of Attack is what causes greater induced drag.
http://upload.wikimedia.org/wikipedi...drag_r.svg.png

A second type of drag force is Parasitic Drag (aka Profile Drag). This form of drag is caused by a combination of three other drag forces: Form drag, Skin friction, and Interference drag.

Skin friction is basically friction between the air and wing or airplane surfaces (smoother surface = less skin friction).

Form drag is drag caused by the shape of the wing airfoil and/or airplane design. Smoother and more flowing designs result in lower form drag.



Interference drag is caused by odd shapes on the body of the aircraft, such as engines on a commercial airliner.

The last type of drag is called Wave Drag. This type of drag only takes affect when an airplanes is within the transonicairspeed range. This is a specific range of speeds (Mach 0.8 to 1.2) which is close to the speed of sound. This form of drag is caused by minor shock-waves formed by breaking the sound-barrier on various parts of the plane due to its shape. The greatest resistance is at exactly Mach 1.0, and is least at Mach 0.8 and 1.2.
Speeds under Mach 0.8 are considered Subsonic, speeds over Mach 1.2 are Supersonic, and speeds over Mach 5.0 are considered Hypersonic.



Section 4 :: What is Thrust? 
Thrust is a force, which is described quantitatively by Newton's 2nd and 3rd Laws of Physics. When an engine accelerates an airplane in one direction, the acceleration will cause a proportional but opposite force on the body.
If there is insufficient thrust from the engines to force air over the wings and other bodies of the plane to cause lift, the plane will not fly.
In the case of a flat winged plane (like foamy planes), if there is sufficient thrust, a plane will fly. This also applies to paper airplanes.
http://upload.wikimedia.org/wikipedi...forces.svg.png




Section 5 :: What are High Angles of Attack (AOA)?
Angle of Attack basically means the angle of the wing or airfoil with respect to the direction of the airflow and airplane. If the wing is not designed properly, very low or high Angles of Attack could cause bad airflow over the wings (not forming good pressure bubbles), thus insufficient lift on the wings. When this happens, a stall occurs on the wing and its control surfaces.



Section 6 :: What is a Spin & Stall?
Stalling occurs when insufficient airflow or pressure over the wings and control surfaces causes the airplane to lose control. This can be caused by insufficient thrust from the engines, poorly designed wing airfoils, bad control surface locations, or simply the aircraft reaching its maximum Angle of Attack. This last reason is very important for aircraft pilots to know, so they can avoid such stalling conditions.

A common and deadly type of stall is called a "Flat Spin". When this occurs, the plane has almost no airflow over the wing or tail control surfaces. Since there is no air on the surfaces to cause lift to turn the plane's direction, it keeps on spinning and falls towards the Earth. Sometimes, because the plane is falling and spinning so fast, the stress on the hull actually rips the plane apart before it hits the ground.



(Diagram of where wing stalls START from...)
 
 
 



-- If you find any errors, please let me know, and I will fix them.

Hydraulic Leak in Aircraft System

Washout

Lets learn about washout. What is it and how is it achieved and why do we use it? The easiest way to think about washout is to consider it a "built in twist" into the structure of a wing. A controlled warp if you will. Normally we would try to build things as straight as possible. Especially when it comes to wings. In aerobatic models that is most certainly the case, as we would like the model to behave the same whether it is upright or inverted. Aerobatic models are aerobatic by the very fact that they are designed to be inherently unstable or right on the "hairy edge". Hence, any means of establishing inherent stability is generally not used so it is not uncommon to see aerobatic models not use washout. Scale models on the other hand have a great desire to want to be stable so that the pilot in command stays calmly in command. One of the most popular means of building in some stability is to incorporate washout into the construction of the wings outer panels. Some designers apply washout to only the last few ribs on the outboard end of each wing panel. The better approach is to incorporate a varying amount of washout from root to tip of the "outer" panel. For example: Aircraft that have wings that are made up of multiple segments should have wash out only on the outer panels of the wing. Some examples would be a Corsair, Typhoon, Stuka, Hurricane or Hellcat etc. Aircraft with "complete" wing panels such as a Mustang, Spitfire, ME-109, FW-190 etc. should have washout incorporated in the entire panel from the aircraft centerline right to the tip rib.

"What is washout really, and how does it work?" you may ask.

Washout works like this: If a wing panel is twisted so that the trailing edge of the tip rib is slightly higher then the trailing edge of the root rib this will in effect reduce the angle of attack of the tip ribs. If this twist is gradual from the root of the panel to the tip each rib in this panel will progressively have a reduced angle of attack. This means that in flight as the airflow approaches the tip of the wing the tip ribs will stall later then the ribs closer to the root. This is good, as this will prevent "tip stall" at slower speeds when the angle of attack is generally higher then normal flight. This will help prevent the plane from snap rolling when you least want it. Like on landing approaches and slow fly-bys.

Washout should be "built in" to the wing structure during construction. This will insure that each wing panel has the same degree of washout and that it will be permanent. Some modellers will complete the wing and then attempt to add washout by "stressing" the skin to achieve washout. These techniques sometimes require the wing to be soaked with water or ammonia in the case of sheeted wings. Then twisted and held in place until everything dries. If the wing is an open frame work structure heat is applied to the covering and the wing is twisted to the shape desired. Both of these methods are not a "sure thing". Soaked wings never twist the same for each panel. Shrink fabric covered panels can change twist as the temperature at the flying field changes, especially if you live in one of the hot areas of the country like Florida or Arizona. The best method to insure permanent and accurate washout is to build it in right from the start. How do we do this? Pretty simple really. First we must start by doing a little drawing.

On each rib draw a centerline that runs from the center of the leading edge to the point of the trailing edge, (see fig. 1). This is the line that the true incidence angles can be measured from. Draw this line on every rib. Next we lay down the main spar for the required panel right on the plans. Place the root rib of the panel on the main spar and hold in place with a few pins or weights. Do not glue yet. Next, place the tip rib on the spar and also hold in place. Again do not glue. Taking an "extra" piece of spar material to act as a shim, slide this under the trailing edges of the root and tip ribs. Now here is the tricky part. At the root rib we want to measure from the rib's centerline to the tabletop. Measure at both the leading edge and the trailing edge. With the rib resting on the table at the main spar and resting on the "shim" near the trailing edge we want these two measurements to be the same. Slide the "shim" in or out to achieve this condition. This will establish the previously drawn centerline to be parallel to the tabletop, and the root rib will have zero incidence relative to the tabletop, (see fig. 2).  Mark on the plans under the root rib where this shim needs to be to achieve this. Next, move out to the tip rib and repeat the measurement steps. Naturally these numbers will be different than those at the root rib, as the tip rib is generally smaller then the root ribs. But, what we want here is to have this rib at a negative incidence angle relative to the root rib. Negative incidence means that the leading edge will be lower then the trailing edge. Or in other words, the trailing edge is higher then the leading edge. How much higher will depend on the incidence angle we need here. This negative incidence angle at the tip rib will be our washout angle. Generally we would like at least 1 degree, but sometimes up to 2 ½ degrees, depending on the type of plane, but never more than the incidence angle that the root rib should have when the wing is installed in the plane.

There are two easy ways to measure this washout angle. The easiest is to use an incidence meter. The other is to do a little trigonometry, where the "delta" between the leading edge measurement and the trailing edge measurement will determine the washout angle, (see fig. 3). Move the trailing edge shim in or out to achieve the proper negative angle. Again, mark this location on the plans. Once your shim is in place you can load the rest of the ribs and you'll get an automatic built in twist to the wing panel from the root rib to the tip rib.  Once the top spar is added you can progress with all the other wing structure stuff. Duplicate this procedure for the other panel and you'll be assured that both panels are identical and permanent in holding the washout angle. You'll find that once you get going, this procedure is relatively simple and can be applied to many wing configurations. The beauty of this technique is that it does not require any fancy jigs or alignment tools and will produce consistent and accurate wing panels.

If you haven't used washout before, and have had a few birds "drop out of the sky" in slow flight, give "washout" a try on your next project. You'll find the plane more manageable on landings, and you'll keep your plane longer……. Using washout will help prevent you from getting "washed out"…….

 

Good luck and happy flying.

RIVETED 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.

1. 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!)
2. The rivet fills the hole completely so that no relative motion is possible.
3. 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:

1. You need special equipment (you'll need to buy an air compressor, rivet gun(s), rivet snaps and bucking bars);
2. You need some expertise and prior practice (you'll need a good teacher for this - errors can be costly in more ways than one);
3. 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);
4. 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.

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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!

Wisconsin Aviation’s Jon Weigand adjusts standby compass compensator during a compass swing. Setting up aircraft on compass rose to perform a compass swing. Aircraft positioned on compass rose for compass swing.

Purpose 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.

ARTIFICIAL STABILITY  &  FLY-BY-WIRE CONTROL

Probably the single most important system in an aircraft, aside from those natural prerequisites for high performance flight, an airframe and powerplant, is the control system. Aircraft control systems have undergone considerable development in the last seventy years and the end of that path of development is by no means in clear sight. The primary function of any control system is to convey instructions, in an aircraft, from the pilot to the vehicle. Aside from several not so conventional instances, such as the Harrier in hover or the Space Shuttle in orbit, this is achieved aerodynamically, by means of control surfaces. In a modern aircraft this becomes a very complex design problem, the system must enable the pilot to retain full control in a speed range starting around 100 kts and extending to Mach 2 plus, including the transonic region with all of its characteristic properties; a modern fighter must also be capable of flight at large (positive and negative) angles of attack. Aside from the aerodynamic complexities involved, there are also the aspects of lifetime, reliability, maintainability and ability to withstand damage. The lifetime and reliability of a system are factors which go hand-in-hand. Fatigue and, to a lesser extent, wear are the basic causes of the majority of failures experienced. A control system's ability to withstand combat damage is one of the crucial considerations in current air warfare, as the Americans learned in Viet-Nam, the hard way. The problem, aside from the issue of surviving SAM/AAM hits, which tend to inflict heavier damage, can be the case of the after effects of insidious small arms fire. SAM/radar sites can be spoofed or busted by Wild Weasels, just as flak emplacements can be, however it's improbable that anyone will ever find an absolute means of eliminating the omnipresent infantry-man with his AK47 (or M16, for that). The obvious solution to this problem is to provide the aircraft with redundant backup systems, which is exactly what's being done. On the other hand, the more systems, the more maintenance and in effect, the lesser the reliability, as the probability of components failing is n-times the probability of one component failing. Weight is another factor to bear in mind, the effect of multiplying systems is obvious here. Cost likewise. The conclusions which can be drawn are: The system must satisfy all of the aircraft's performance requirements.  The system must be capable of absorbing damage.  The system must have a high degree of reliability.  The system must be maintainable.  Cost and weight must be kept at a minimum. Aircraft, such as the F-14, satisfy 1.,2.,4. however they lag in 3. and definitely fail in 5.. The basic cause is that they employ conventional mechanical/hydraulic systems, which, in spite of their conceptual simplicity, become enormously complex in these instances. The only current system which satisfies 1. to 5. adequately is fly-by-wire control. Fly-by-wire Control Systems. As the name implies, fly-by-wire employs electrical signals to transmit information from the cockpit to the control actuators. Control elements, e.g. control stick, rudder pedals, are fitted with mechanical / electrical transducers - either force (F-16) or position sensing devices, which generate electrical signals corresponding to the given command. Here is where we must make the distinction between analogue and digital systems. Analogue systems operate with electrical analogues to real quantities. An example would be a device transmitting a quantity from 0 to 100% with an electrical voltage output of 0 to 10 Volts. A value of 15% would generate an output of 1.5 Volts. The number of ways in which analog information can be encoded is virtually unlimited. Information can be coded into voltage, frequency, phase or combinations of these, it can also be compressed prior to encoding, enabling more of it to be transmitted at once. Digital systems operate in binary. The binary number system (as compared to the decimal system we use) has only two values, 1 and 0. 2 forms a unit analogous to 10, 4 to 100, 8 to 1000, hence we can express a number such as six (6) as 110. 1,2,3... corresponds to 1, 10, 11.... Any number can be converted into binary, a digital device can then generate an output with only two states, on and off, corresponding to 1 and 0. All digital computers employ binary. Analog and digital systems both have advantages and disadvantages. Analog systems are, generally, simpler and less demanding in component parameters, such as speed. On the other hand, they are more susceptible to induced noise and interference, as the information content is carried within fine variations of some signal parameter. A digital system need only discriminate between on and off, the information being carried by sequences of binary numbers. Digital systems may be easily reconfigured by changes in software, whereas an analog system, hardwired, would require rebuilding. When analog systems fail, they often merely degrade in performance, a failure of a similar type could completely disable a digital device. The signals generated by the control elements are then used to control the control surface actuators. However, the raw output of a cockpit control element is hardly enough for that. It is modified by a stability augmentation computer. The computer compares the aircraft's actual motion, as sensed by gyros and accelerometers, and corrects it to a control law, improving the aircraft's handling. This type of system is used by the Tornado GR.1/F.2, which employs triplex control output sensing and computing, and quadruple electric control of the hydraulic control surface actuators. Further safety is provided by a mechanical backup for pitch/roll control. A similar approach was used in the YC-15 AMST STOL transport, which uses blown flaps to enhance short field performance. Variations in engine thrust can cause additional moments in roll and pitch, which would complicate the handling of such an aircraft. Aircraft configured for direct force control featuring variable incidence wings, beavertail elevator and control surfaces beneath nose. DSF - direct side force, VLF - vectored lift force. (Carlo Kopp) These tiny robot-like units are actually intricate components - subminiature gyroscopes used to guide aircraft, ships, missiles and torpedoes. The finished GR-G5 rate gyros, built by Northrop, are only two inches long and weigh less than five ounces. The GR-G5 is the most widely used rate gyroscope in the world, and is used in a quadruple-redundant system in the F-16 fly-by-wire flight control system. (Northrop) In either instance the fly-by-wire system, aside from improving reliability and its associated factors, is used to modify the response of the aircraft. This is, by no means, full utilization of the potential offered by electronic flight control.

On major hanager inspection you find corrosion in an unpresurised panel ,

Pls deatails your action to return the aircraft to servise?

Safety

If an aircraft is on a major hangar inspection, it must be made in a safe condition for maintenance i.a.w. the appropriate AMM and company procedures.  The appropriate safety pins and guards must be fitted, relevant CB pulled, and all warning placards and signs made clearly visible to all personnel.  Every one working on or around the aircraft must be fully trained and qualified to do so and be fully aware of all health and safety policies.  Personnel must clear appropriate personal protective equipment (PPE) and there must be suitable staging and access platforms used.

Assessment

The SRM inspection should be carried out by an appropriate licensed engineer i.a.w. the AMM, Structural Repair Manual (SRM) or Drawing.  It is important to asses the type, extent and severity of the corrosion before starting to remove it.  In some cases Non-Destructive Testing (NDT) may be required if the corrosion is not visible.  If corrosion is removed, the material thickness must be measured thereafter, to make sure the thickness is still within limits.

Your chances of passing the CAA-approved exams can be greatly enhanced by the use of “exam technique”. The following points should help you to improve your performance:
1. 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!