RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 1 COMPASSES Aim: To understand the principles and considerations of compass operations. Application: - Navigation flying - When DG fails Definitions: Simple Bar Magnet - a magnet freely suspended horizontally will swing so as its axis points roughly north/ south. Magnetic Field - Earth acts like a large weak magnet Magnetic Variation - the angular difference between the magnetic poles and true poles of the Earth. - Isogonal are lines that join places of same magnetic variation. - Agonic is the line that joins places of zero magnetic variation. Easy way to remember relationship: VARIATION WEST, MAGNETIC BEST VARIATION EAST, MAGNETIC LEAST Brief 27 RMIT Flight Training Compasses RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 2 Deviation - combined metal and electrical components of an aircraft create a magnetic field which interferes with the compass - deviation cards are provided for correction Principles: - a direct reading magnetic compass is filled with a liquid which; supports its weight, reduces friction, and dampens oscillations - Lubber line shows heading on compass card - aircraft turns around magnet which is aligned north / south Brief 27 RMIT Flight Training Compasses RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 3 Considerations: Magnetic Dip - at equator, lines of magnetic force are parallel to Earth’s surface - approaching magnetic poles, lines of magnetic force dip toward them and the compass aligns to these lines Magnetic dip is zero at equator and maximum at the poles. Minimising Dip - by placing the pivot point above the CoG, it opposes the dip force - the greater the dip force, the greater distance of the CoG outwards, and the greater the restoring force In the southern hemisphere, the compass CoG is north of the pivot line. Acceleration Errors - due to CoG displacement, acceleration errors occur. As aircraft accelerates, pivot point moves with the aircraft, but the magnet is left behind causing errors, until it is realigned. Brief 27 RMIT Flight Training Compasses RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 4 - this error is not apparent when flying north or south, due to pivot point and CoG of compass being aligned in the direction of acceleration/deceleration. In southern hemisphere: SAND HDG Acceleration Error Deceleration Error Northerly None None Easterly Apparent turn to S Apparent turn to N Southerly None None Westerly Apparent turn to S Apparent turn to N Turning Errors - the centripetal force of the turn pulls the pivot point of the compass into the turn, but the magnet is left behind again In southern hemisphere: ONUS Brief 27 RMIT Flight Training Compasses RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 5 Airmanship: - Always avoid placing ferrous materials near the compass - Apply SAND and ONUS rules where applicable - Always apply magnetic variation when flight planning - Always apply compass deviation Brief 27 RMIT Flight Training Compasses