Aircraft Workshop Principles and Practice grading system

Unit Name                     : Aircraft Workshop Principles and Practice.                      

Unit No.                         : 76

Unit Level                      : 3                                                                                 

Staff                               : TBA

Grading Tracking Sheet: 2007/08

                                                                                                            

Group:

Student’s Name	Assignment 1 P1, P2	Assignment 2 P3, P4, P6, P7, P8, P9, P10	Assignment 3 P5, M1, M2	Assignment 4 M3, D1, D2		Overall grade Obtainable
						Pass	Merit	Dist’
						10	3	2
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-
					-

Verbal and written communication

Verbal and written communication

Verbal communication (i.e. speaking and listening) is widely used in everyday situations, including:

• Informal discussions either on the telephone or face to face.

• Formal presentations to groups of persons who all require the same information.

Where a group of persons all require the same information, a formal presentation must be used. On no account should information be `passed down the line' from person to person because errors are bound to creep in. There is a story that during World War I the message `send reinforcements, we are going to advance' arrived at headquarters, by word of mouth, as ^`send three and four pence (old money), we are going to a dance'. We will let you decide on the truth behind this story, but we feel it makes the point.

In any event, it is important to remember that the spoken word is easily forgotten and oral communication should be reinforced by:

• Notes taken at the time.
• Tape recording the conversation.
• A written summary. For example, the published `proceedings' of formal lectures and presentations. Another example is a `press release' that is provided to journalists and reporters in order to ensure: the factual accuracy of information intended for the public.

Oral communication must be presented in a manner appropriate to the audience. It must be brief and to the point. The key facts must be emphasized so that they can be easily remembered. The presentation must be interesting so that the attention of the audience does not wander.

When communicating by the spoken word, it is as equally important to be a good listener as it is to be a good speaker. This applies to conversations between two or three people as well as to formal presentations.

Activity 2.39

Use presentation software to prepare a 5-minute presentation to the rest of the class (using appropriate visual aids) on any one of the following topics:

• How to choose a digital camera.
• How to connect to the Internet.
• What to look for when purchasing a second-hand car.

You should prepare a set of brief printed notes summarizing the key points for your audience. Also include printed copies of any screens or overhead projector transparencies that you use. At the end of your talk you should invite questions from your audience and provide appropriate answers.

Activity 2.40

Conduct a brief interview (lasting no longer than 15 minutes) with another student and take notes to summarize the outcome. Do not forget to allow time for questions at the end of the interview. Your interview should be based around the following questions:

• Why did you decide to take a course in Engineering?
• Why did you choose the BTEC National Diploma course?
• What made you choose this school/college?
• What subjects/topics have you enjoyed the most?
• What subjects/topics have you enjoyed the least?
• What plans have you got for the future?
• Where would you hope to be and what would you hope to be doing in 10 years' time?

You should add further questions to clarify the above. Do not forget to thank your interviewee! Present your findings in the form of hand-written interview notes.

Written communication

This is a more reliable method of communication since it usually provides a permanent written record of the key information. The same information is available for all those who require it.

Anyone who has ever marked an English comprehension test will know that the same written passage can mean very different things to different people. Therefore, care must be taken in preparing written information. To avoid confusion, the normal conventions of grammar and punctuation must be used. Words must be correctly spelt. Use a dictionary if you are uncertain. If you are using a word-processing package use the spell checker. However, take care, many software packages originate in the USA and the spell checker may reflect this.

Never use jargon terms and acronyms unless you are sure that those reading the message are as equally familiar with them as is the writer.

An engineer often has to write notes, memoranda and reports. He/she often has to maintain logbooks and complete service sheets. An engineer may also have to communicate with other engineers, suppliers and customers by letter. Being able to express yourself clearly and concisely is of great importance.

Activity 2.41

Prepare a brief article for the local press (using not more than 1000 words) on any one of the following topics:

• A sporting event that you took part in.
• A recent school or college activity.
• A newly available product or technology.

Include contact or other details for further information. Present your work in word-processed form and include relevant photographs, diagrams or sketches.

Non-return valves and shuttle valves

Non-return valves and shuttle valves

The non-return valve (NRV), or check valve as it is sometimes known, is a special type of directional control valve. It allows the fluid to flow in one direction only and it blocks the flow in the reverse direction. These valves may be operated directly or by a pilot circuit. Some examples are shown in  

• Figure 2.88a shows a valve that opens (is free) when the inlet pressure is higher than the outlet pressure (back pressure).
• Figure 2.88b shows a spring-loaded valve that only opens when the inlet pressure can overcome the combined effects of the outlet pressure and the force exerted by the spring.
• Figure 2.88c shows a pilot controlled NRV. It opens only if the inlet pressure is greater than the outlet pressure. However, these pressures can be augmented by the pilot circuit pressure.

(i)      The pilot pressure is applied to the inlet side of the NRV. We now have the    combined pressures of the main (primary) circuit and the pilot circuit acting against the outlet pressure. This enables the valve to open at a lower main            circuit pressure than would normally be possible.

(ii)     The pilot pressure is applied to the outlet side of the NRV This assists the      outlet or back pressure in holding the valve closed. Therefore, it requires a             greater main circuit pressure to open the valve. By adjusting the pilot pressure in these two examples we can control the circumstances under   which the NRV opens.

Fluid power schematic diagrams

These diagrams cover both pneumatic and hydraulic circuits. The symbols that we shall use do not illustrate the physical make-up, construction or shape of the components. Neither are the symbols to scale or orientated in any particular position. They are only intended to show the `function' of the component they portray, the connections and the fluid flow path.

Complete symbols are made up from one or more basic symbols and from one or more functional symbols. Examples of some basic symbols are shown in   and some functional symbols are shown in  

Energy converters

Let us now see how we can combine some of these basic and functional symbols to produce a complete symbol representing a component. For example, let us start with a motor. The complete symbol is shown in

The large circle indicates that we have an energy conversion unit such as a motor or pump. Notice that the fluid flow is into the device and that it is pneumatic. The direction of the arrowhead indicates the direction of flow. The fact that the arrowhead is clear (open) indicates that the fluid is air. Therefore, the device must be a motor. If it were a pump the fluid flow would be out of the circle. The single line at the bottom of the circle is the outlet (exhaust) from the motor and the double line is the mechanical output from the motor.

Now let us analyse the symbol shown in Figure 2.75.

• The circle tells us that it is an energy conversion unit.
• The arrowheads show that the flow is from the unit so it must be a pump.
•  The arrowheads are solid so it must be a hydraulic pump.
• . The arrowheads point in opposite directions so the pump can deliver the hydraulic fluid in either direction depending upon its direction of rotation.

• The arrow slanting across the pump is the variability symbol, so the pump has variable displacement.
• The double lines indicate the mechanical input to the pump from some engine or motor.

Summing up, we have a variable displacement, hydraulic pump that is bi-directional.

Test your knowledge 2.25

Draw the symbol for:

(a) a unidirectional, fixed displacement pneumatic pump (compressor)

(b) a fixed capacity hydraulic motor.

Directional control valves

The function of a directional control valve is to open or close flow lines in a system. Control valve symbols are always drawn in square boxes or groups of square boxes to form a rectangle. This is how you recognize them. Each box indicates a discrete position for the control valve. Flow paths through a valve are known as `ways'. Thus, a 4-way valve has four flow paths through the valve. This will be the same as the number of connections. We can, therefore, use a number code to describe the function of a valve. Figure 2.76 shows a 4/2 directional control valve (DVC). This valve has four flow paths, ports or connections and two positions. The two boxes indicate the two positions. The appropriate box is shunted from side to side so that, in your imagination, the internal flow paths line up with the connections. Connections are shown by the lines that extend `outside' the perimeters of the boxes.

Test your knowledge 2.26

A valve symbol is shown in  

(a) State the numerical code that describes the valve.

(b) Describe the flow path drawn.

 (c) Sketch and describe the flow path when the valve is in its alternative position.

As drawn, the fluid can flow into port 1 and out of port 2. Fluid can also flow into port 3 and out of port 4. In the second position, the fluid flows into port 3 and out of port 1. Fluid can also flow into port 4 and out of port 2.

Valve control methods

Before we look at other examples of directional control valves, let us see how we can control the positions of a valve. There are five basic methods of control, these are:

• Manual control of the valve position.
• Mechanical control of the valve position.
• Electromagnetic control of the valve position.
• Pressure control of the valve positions (direct and indirect).
• Combined control methods.

The methods of control are shown in   With simple electrical or pressure control, it is possible only to move the valve to one, two or three discrete positions. The valve spool may be located in such positions by a spring loaded detent.

Combinations of the above control methods are possible. For example, a single solenoid with spring return for a two-position valve. Let us now look at some further DCVs.

HYDRAULIC INTRODUCTION AND PLUMBING LINES

2.1 INTRODUCTION  

The term "hydraulic plumbing" refers to process of forming, installing as well as associated repairing/maintenance of the hose, tubing, fittings, and connectors used in the aircraft hydraulic system.
Occasionally it may be necessary to repair or replace damaged aircraft hydraulic plumbing lines. Very often the repair can be made sim ply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabri-cated. Replacement tubing should be of the same size and material as the original line. All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subjected. If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object. All tubing failures should be carefully studied and the cause of the failure determined.
This week will also highlight plumbing connectors.

2.2 PLUMBING LINES :
Aircraft plumbing lines usually are made of metal tubing and fittings or of flexible hose. Metal tubing is widely used in aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines. Flexible hose is generally used with moving parts or where the hose is subject to considerable vibration.
Generally, aluminum alloy or corrosion-resistant steel tubing have replaced copper tubing. The high fatigue factor of copper tubing is the chief reason for its replacement. It becomes hard and brittle from vibration and finally breaks, however it may be restored to its soft annealed state by heating it red hot and quenching it in cold water. Cooling in air will result in a degree of softness but not equal to that obtained with the cold water quench. This annealing process must be accomplished if copper tubing is removed for any reason. Inspection of copper tubing for cracks, hardness, brittleness and general condition should be accomplished at regular intervals to preclude failure. The work ability, resistance to corrosion, and lightweight of aluminum alloy are major factors in its adoption for aircraft plumbing.

In some special high-pressure(3,000 p.s.i.) hydraulic installations, corrosion resistant steel tubing, either annealed or 1/4 -hard, is used. Corrosion resistant steel tubing does not have to be annealed for flaring or forming; in fact, the flared section is somewhat strengthened by the cold working and strain hardening during the flaring process. Its higher tensile strength permits the use of tubing with thinner walls; consequently the final installation weight is not much greater than that of the thicker-wall aluminum alloy tubing.