Presentation on theme: "Uncontrolled copy not subject to amendment"— Presentation transcript:
1Uncontrolled copy not subject to amendment AIR NAVIGATIONPart 4COMPASSESVersion 1.01
2LEARNING OUTCOMES On completion of this unit, you should: Be able to carry out calculations to determine aircraft distance, speed and timeUnderstand the principles of vectors and the triangle of velocities to establish an aircraft’s track and ground speed
3LEARNING OUTCOMES Understand the principles of the 1 in 60 rule Understand the types of compass systems used for air navigation, how they work and their limitationsKnow the hazards that weather presents to aviation
5You will have learnt about the difference between and YOU WILL HAVE GOT LOST using the Silva, a simple hand held compassTRUE NORTHMAGNETIC NORTH
6To understand aircraft compasses, their strengths and weaknesses we need to look into thesubject a little deeper
7Shape of the magnetic field around a magnet The first thing you need to understand is theshape of the magnetic field around a magnet
8The Earth’s magnetic field, follows the same pattern as the field round a bar magnet but needs a little explaining
9The red end of a magnet (known as the North Pole) is in fact a north-seeking pole Therefore, as opposites attract it can be seen that if the red end of our compass needle is to point to the North Magnetic Polethen in reality the North Magnetic Pole must, in magnetic terms be a south pole
10At our latitude, the lines of force point down at an angle (known as the angle of dip) of 65º; once the angle exceeds 75º (which occurs about 1200 miles from the Poles) the directional force becomes so weak as to render magnetic compasses virtually useless.Looking at the diagram on the left the lines of force are only parallel to the surface of the Earth at the Equator. Indeed, at the poles the lines of force are vertical!A compass needle will try to follow the lines of force but is constrained by the construction to stay almost horizontal The end result of this is that the more vertical the Earth’s field, the weaker the directional force on the horizontal compass needle becomes.
11We will now look at Aircraft Compasses There are 2 main types
12In an aircraft, the simplest form of compass is the Direct Indicating Compass (shown right), which looks very similar to the car compass, which can be boughtfrom accessory shops.N3330
14The Direct Indicating Compass The points of the compass are printed around the equator of the ball, & the heading is shown against an index mark on the bowl. The magnet is hidden in the ball.The Direct Indicating Compass (DIC), like the hand held Silva compass, has a magnet suspended in liquid, which helps to dampen any movementIt has the appearance of a squash ball inside a goldfish bowl.on gliders the compass is on the cockpit coming
15The Direct Indicating Compass The DIC has several serious limitations, so it is normally used as a standbyThose limitations are:
16The Suspended Magnet Will Only Give A Correct Reading In Steady Straight & Level Flight. During Turns & Acceleration The Magnet Is Swung To One Side And Gives False Readings
17The DIC is located in the cockpit, and there it is affected by the magnetic fields emanating from both the metal the aircraft is made fromand from the various electrical circuits in the aircraft. These othermagnetic fields badly affect the accuracy of the DIC.
18The driving power of the horizontal portion of the Earth’s magnetic field is only strong enough to turn a compass needle; it doesnot have sufficient torque to drive repeaters at other crew positions inthe aircraft
19The DIC only indicates magnetic heading, modern aircraft may require True or Grid headings
20At high magnetic latitudes (above 70º North or South) the DIC becomes sluggish and unreliable because the angle of dip is so steepand the directional force is so weak.
21It is very simple and therefore reliable Advantages of the DICIt is very simple and therefore reliableIt is very cheap and lightweightIt does not require any form of power and so will continue to work even after a total power failure in the aircraft.
22To overcome the limitations of the DIC, the Gyro Magnetic Compass was invented
23It’s made up of the following components: Gyro Magnetic CompassIt’s made up of the following components:
24A Magnetic Detector Unit, which electrically senses the direction of Earth’s magnetic field and is normally situated in the wing tip
25of any manoeuvres the aircraft may make A Gyroscope, which continues to point to a fixed point in space, regardlessof any manoeuvres the aircraft may makeFRAMEROTORY AXISZ AXISA gyroscope
27An Error Detector that senses any difference between the gyro and magnetic headings and applies a correction to the gyro at a pre-set rate
28rotation of the Earth and the aircrafts flight path around the Earth A controller or computer that applies corrections to the gyro to correct for therotation of the Earth and the aircrafts flight path around the Earth
29A display or displays to show the aircraft heading at required positions in the aircraft. Various amplifiers and motors to control the system and in some GMCs a roll cut out switch to minimise the effect of a turn on the Magnetic Detector Unit
30The basic principle of the GMC is that it uses the long-term accuracy of the detector unit combined with the short-term accuracy of the gyro.
31magnetic detector, which is correct during straight and level flight and is more accuratethan the DIC because being situated in the wing it is less affected by the deviating forces from other extraneous magnetic fields in the aircraftWhat this means isthat the gyro, which is connected to the compass, is constantly corrected by themagnetic detector, which is correct during straight and level flight
32(which is unaffected by turns) is more accurate If a roll cut out switch is used no error is fed from the magnetic detector to the gyro in the turn, if a roll cut out is not present, the error correction rate is low enough to only make a small effect which is removed when the wings are levelledDuring a turn, the gyro(which is unaffected by turns) is more accurate
33A gyro magnetic system has considerably more torque than a DIC and can therefore provide outputs to repeater units in other positions in an aircraft and/or computers in the aircraft. The output to these repeaters can be easily modified so that they can display either true or magnetic heading
34As the gyroscope is a manufactured item, it cannot be perfect Gyro ErrorsTo overcome this the gmc has developed as a system where the gyro heading can be relied on for short period ( about 10 minutes )As the gyroscope is a manufactured item, it cannot be perfectOver a period of time it will become inaccurate ( this is called gyro wander ).It can then be reset by reference to the magnetic detector
35Inertial Navigation, GPS and Beyond Throughout the UK the variation errors on maps & charts are reasonably accurate, but if we go into polar regions we face 2 problems
36Problem 1Variation values are unreliable and as large as 180 degrees between true & magnets polesTRUENORTHMAGNETIC
37Problem 2The second problem is that as the compass nears the magnetic pole the compass detector will try to point at it. this is called dip.
38The Inertial Navigation System (INS) eliminates this problem and can A modern aircraft with a heading error of one degree can easily have position errors in the order of 6 miles/hour, which nowadays is notacceptable.The Inertial Navigation System (INS) eliminates this problem and canalign itself with True North without the need for variation
39A typical inertial navigation system can achieve positional accuracies of one mile/hour. Whilst this accuracy may appear good, it is still a long way short of the latest development in navigationtechnology.
40Global Positioning System (GPS), can Using Ring Laser Gyros or Fibre Optical Gyros to feed an Inertial Reference System, which is paired with aGlobal Positioning System (GPS), canproduce a position, which is accurate to within 5 metresThe ultimate aim is toachieve millimetre accuracy, we are not there yet, but it will happen.