Aircraft Marshaling

When aircraft are moving under their own power the view from the cockpit may be restricted or the pilot may find extreme difficulty in judging the clearances between other aircraft or obstructions especially when trying to park in a parking area. To do this in a safe way a method of aircraft marshaling is utilized.

• Aircraft Marshaller should wear a distinctive bright high visibility surcoat or vest to be easily identified.
• When an aircraft lands, the marshaller concerned must first identify himself to the pilot by extending his arms above his head, with palms or bats facing forwards.
• Once identified, the marshaller should then stand where he can be clearly seen by the pilot and all of the marshaller signals are clearly observed.
• During start-up the marshaller should always stand in a set position, which is usually, slightly ahead of the aircraft between the left wingtip and the cockpit.
• At night marshaling signals are given using wand torches.

Pilot`s singnals to the marshaller on ground  are as follows.

Raised arm with clenched fist.	Brakes 'On'
Raised arm with fingers extended.	Brakes ‘Off’
Arms extended palms facing outwards thumbs extended, move hands inwards to cross in front of face.	Insert chocks
Arms extended palms facing inwards thumbs extended, move hands outwards from in front of face.	Remove chocks
Raise the number of fingers on one hand indicating the number of the engine to be started.  For this purpose the aircraft engines are numbered as follows: No 1 engine should be the port outer engine. No 2 engine should be the port inner engine. No 3 engine should be starboard inner engine. No 4 engine should be the starboard outer engine.	Ready to start engine

Aircraft Auxiliary Power Unit (APU)

The auxiliary power unit or APU as it is commonly known, is a small gas turbine engine, fitted to aircraft and can provide

•     Electric power from shaft driven generators.
•     Pneumatic duct pressure for air conditioning and engine starting purposes.
•     Hydraulic Pressure (In Some aircraft).

  

An Auxiliary Power Unit (APU) is an automatic engine, which normally runs at a governed speed of 100%. Some APUs have an idle facility that allows the engine to run at 85% when no loads are applied. As it is an automatic engine the fuel system must control the engine throughout the start and running phases of operation. The engine will be shut down if a critical control function is lost or a serious malfunction such as low oil pressure occurs.

APU’s are mainly used on the ground when their main engines are not running and ground carts (electrical and pneumatic) are not available.   On most modern aircraft the APU will also be used in the air to provide air-conditioning during take off and landing phases, or to back up the main engines in case of a generator or air system failure. 

Although the APU is usually rated to run at the max cruise altitude of the aircraft it is fitted to, its ability to take load diminishes with altitude. As the major load on any APU is the air load it can be usual that the APU’s ability to provide sufficient air for the aircraft is limited to 15-20,000 ft. 

Above this height the APU will only provide electrical power, this may also be limited to less than the max cruise height. Most APU’s give shaft priority which means that if air and electric generators are on the generators are given priority. Most Aircraft use constant frequency generators, and their APU’s which run at a constant 100% do not therefore require a constant speed drive unit to maintain a constant output. If the air loads become to high the APU will reach its max EGT and the control system will back off the fuel to prevent damage, this would bring the APU generator off frequency and take the generator ‘off line’. Instead the air load is reduced to maintain a constant APU speed.

FLIGHT-LINE SAFETY (Line Maintenance)

Many sources of accidents on the flight line are involved with propellers and rotor blades. They are difficult to see when they are turning, and personnel sometimes become distracted and forget about the danger. The main difference between these, and other flight-line accidents, is that they are almost always fatal.

Most blades have high-visibility markings, to ensure that they can be seen when they are turning. These markings vary from a yellow blade tip marking, to black and white alternate stripes along the full blade length.

To reduce the risk of propeller and rotor blade strikes, it is best to follow strict rules as to the correct way to approach and leave the vicinity of an aircraft or helicopter whilst it is under power. For example (and allowing for the fact that there are specific rules laid down for each aircraft), installing and removing chocks should normally be done from the wing-tip direction. Boarding and leaving a helicopter should always be done from the side.

When dealing with running jet engines there are similar dangers. These come not only from the noise risk, which can result in deafness, but also from the risk of intake suction, which has resulted in ramp personnel being sucked into the engine and being killed. At the rear of the aircraft, there is the risk of jet blast, which, at maximum thrust is quite capable of overturning a vehicle if it passes too close behind the aircraft. Piston-powered aircraft (depending on their size) will have similar danger areas.

Aircraft DC generator construction

Aircraft dc generators have for the most part been replaced by alternators on modern aircraft.  However, there are still several dc generators currently in operation in older aircraft.  

The Yoke: This is a cylinder of cast iron which supports the pole pieces of the electromagnetic field.

The Armature: This is driven by the aircraft engine and holds the windings in which the output voltage of the machine is induced.

The Commutator: The voltage induced in the armature is AC.  The commutator changes the AC voltage into DC voltage.

The Quill Drive: This is a weak point which is designed to shear and protect the engine, if the generator seizes.

The Brushes: These are made of carbon and collect the DC voltage from the moving armature and commutator.

The Suppressor: This reduces radio interference, which may be caused by sparking between the brushed and commutator.

Aircraft Electronics- Diodes

Rectifier Diodes

A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor device which allows current to flow in one direction but not in the other. 

When conducting, the diode is said to be 'forward biased'. Under these conditions the diode offers little resistance to current flow. 

When opposing current flow, the diode is said to be 'reverse biased'. Under reverse biased conditions the diode has a high resistance.

Symbols & Identification

The various symbols used for diodes 

Whether the triangles are filled or unfilled depends only on the drawing office preference.  Where it is considered necessary, it is possible to show that one of the electrodes is connected to the case of the device by adding a dot to the symbol, but this is not often used.  In every symbol, the arrow indicates the direction of conventional current flow.

The base of the triangle is the end where conventional current enters the diode, this end is called the anode.  The end through which current leaves the diode is the cathode.  In some cases the arrow symbol is marked on the diode, where it is not, the cathode is identified by a band or distinctive shape as shown Two identification codes are used for diodes.  In the American system the code always starts with 1N and is followed by a serial number, i.e. 1N4001.  In the continental system, the first letter gives the semiconductor material; A for germanium; B for silicon, and the second letter identifies the use; A - signal diode; Y - rectifier diode and Z for zener diode.  To complicate the situation some manufacturers have their own codes.

Operating Characteristics

Most semiconductor diodes are made from silicon or germanium, these two materials have different operating characteristics, although the principle of operation and circuit symbols are both the same.

Biasing

A diode is said to be 'biased' when a voltage is applied between the terminals such that the diode operates as required.

An external voltage applied so that the anode is positive and the cathode negative is called 'forward bias'.  There are many ways of achieving this, for example:

·        Connect the anode to +3V and the cathode to 0V.

·        Connect the anode to +1V and the cathode to -1V.

·        Connect the anode to -50V and the cathode to -52V.

So far as the diode is concerned, it is the voltage of the anode with respect to the cathode which determines the bias.

If the voltage is applied so that the anode is negative with respect to the cathode, the diode is ‘reverse biased’, again, there are many ways of achieving this.

The forward voltage required to make the diode conduct depends on the material it is made from.  Germanium diodes require a voltage of approximately 0.1 to 0.2 volts and silicon diodes 0.6 to 0.7 volts.

Forward Voltage Drop

Ideally a diode should have zero resistance when conducting and should cause no voltage drop, unfortunately this does not happen.  Germanium diodes create a voltage drop of approximately 0.6V and silicon diodes a drop of approximately 1.1V.  This needs to be taken into account when doing circuit calculations.

Reverse Leakage Current

When a diode is reverse biased, it should ideally have infinite resistance and no current should flow.  Unfortunately when a diode is reverse biased, a small current called 'reverse leakage current' flows, generally this is too small to be of significance, however, it should be noted that the value of this current increases with an increase in diode temperature.  The reverse current of silicon diodes is much smaller than that of germanium diodes, (approx. one thousandth), therefore silicon diodes can be used more successfully at high temperatures (150º - 200ºC) than germanium diodes (80º - 100ºC).

Reverse Breakdown Voltage

If the reverse bias voltage is increased, eventually the diode breaks down and current flows in the wrong direction through the diode.  This causes permanent damage and the diode has to be replaced.

The breakdown voltage can have any value from a few volts, up to 1000V for silicon diodes and 100V for germanium, depending on the construction and forms of material used.The maximum reverse voltage is an important diode characteristic.  Under normal conditions this value should not be exceeded.

Graphical Representation

graphical representation of the operating characteristics of a typical silicon and germanium diode

Aircraft Aerodynamics Airflow types

Steady Streamline Flow

The flow parameters (eg speed, direction, pressure etc) may vary from point to point in the flow but, at any point, are constant with respect to time.  This flow can be represented by streamlines and is the type of flow which it is hoped will be found over the various components of an aircraft.  Steady streamline flow may be divided into two types:

Classical Linear Flow.  The flow found over a conventional aerofoil at low incidence in which the streamlines all more or less follow the contour of the body and there is no separation of the flow from the surface.

Controlled Separated Flow or Leading Edge Vortex Flow.  This is a half-way stage between steady streamline flow and unsteady flow described later.  Due to boundary layer effects, generally at a sharp leading edge, the flow separates from the surface;  the flow does not then break down into a turbulent chaotic condition but, instead, forms a strong vortex which, because of its stability and predictability, can be controlled and made to give a useful lift force.  Such flows are found in swept and delta planforms particularly at the higher incidences.

Unsteady Flow

 In this type of flow the flow parameters vary with time and the flow cannot be represented by streamlines.

Two-Dimensional Flow

If a wing is of infinite span, or, if it completely spans a wind tunnel from wall to wall, then each section of the wing will have exactly the same flow pattern round it except near the tunnel walls.  This type of flow is called two-dimensional flow since the motion is confined to a plane parallel to the free stream direction.

As the air flows round the aircraft its speed changes.  In subsonic flow a reduction in the velocity of the streamline flow is indicated by an increased spacing of the streamlines whilst increasing velocity is indicated by decreased spacing of the streamlines.  Associated with the velocity changes there will be corresponding pressure changes.

As the air flows towards an aerofoil it will be turned towards the low pressure (partial vacuum) at the upper surface;  this is termed ‘upwash’.  After passing over the aerofoil the airflow returns to its original position and state;  this is termed ‘downwash’ as shown.  The reason for the pressure and velocity changes around an aerofoil is explained in later paragraphs.  The differences in pressure between the upper and lower surfaces of an aerofoil are usually expressed as relative pressures by ‘-‘ and ‘+’.  However, the pressure above is usually a lot lower than ambient pressure and the pressure below is usually slightly lower than ambient pressure (except at high angles of attack), ie. both negative.

Three-Dimensional Flow

The wing on an aircraft has a finite length (ie a wing tip) and, therefore, whenever it is producing lift the pressure differential tries to equalise around the wing tip.  This induces a span-wise drift of the air flowing over the wing, inwards on the upper surface and outwards on the lower surface, producing a three-dimensional flow.

Because the effect of the spilling at the wing tip is progressively less pronounced from tip to root, then the amount of transverse flow reduces towards the fuselage.  As the upper and lower airflows meet at the trailing edge they form vortices, small at the wing root and larger towards the tip.  These form one large vortex in the vicinity of the wing tip, rotating clockwise on the port wing and anti-clockwise on the starboard wing;  viewed from the rear.  Tip spillage means that an aircraft wing can never produce the same amount of lift as an infinite span wing.  If the wing has a constant section and angle of incidence from root to tip then the lift per unit span of the wing may be considered to be virtually constant until about 1.2 chord distance of the wing tip.

The overall size of the vortex at the trailing edge will depend on the amount of the transverse flow.  Therefore, the greater the force (pressure difference) the larger it will be.  The familiar pictures of wing-tip vortices showing them as thin white streaks, only show the low pressure central core and it should be appreciated that the influence on the airflow behind the trailing edge is considerable.

Aircraft Maintenance Engineering Licences

The primary duty of all licensed engineers is to ensure that the items for which they are responsible are in an airworthy condition.  Whenever work is carried out on an aircraft, it is the duty of every licensed engineer to consider the effect such work may have, directly or indirectly on items that are the responsibility of other licensed engineers.  In all cases where an overlap of responsibility occurs, the engineer primarily responsible for the items must call in engineers appropriately licensed in like categories.  A Certificate of Release to Service must be signed by all engineers concerned, each assuming responsibility for those aspects of the matter which are covered by his licence.

Authorised engineers cannot use their authorisation on any aircraft other than the companies’ aircraft that granted that authorisation unless authorisation has been granted by the other company in writing.

Aircraft maintenance licence can be issued for aeroplanes and helicopters of the following categories:

·         Category A
 
·         Category B1
 
·         Category B2
 
·         Category C

Categories A and B1 are subdivided into subcategories according to the aircraft type its being used:

·         A1 and B1.1 Aeroplanes Turbine

·         A2 and B1.2 Aeroplanes Piston

·         A3 and B1.3 Helicopters Turbine

·         A4 and B1.4 Helicopters Piston