AIR NAVIGATION Part 1 Distance Speed & Time
LEARNING OUTCOMES On completion of this unit, you should: • Be able to carry out calculations to determine aircraft distance, speed and time • Understand the principles of vectors and the triangle of velocities to establish an aircraft’s track and ground speed
LEARNING 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 limitations • Know the hazards that weather presents to aviation
RECAP • Latitude/Longitude grid divides the surface of the Earth into degrees and minutes • One minute of latitude represents one nautical mile (nm) • 1 degree of latitude (60 minutes) equals 60nm
As a complete circle is 360° then 360 x 60 gives the circumference of the Earth as 21600 nm (approx 25000 statute miles).
Lines of Longitude are sometimes referred to as MERIDIANS • When recording your position – the line of Latitude must be given first. • The starting point goes through Greenwich and is referred to as the: “Prime Meridian”
Finding Distance Between 2 Points • Use a ruler and dividers • If you do not have any equipment, using the marks along the edge of any piece of paper
Change of Latitude • If two places are on the same meridian then it is possible to calculate the distance between them rather than having to measure it • For example Torrejon airfield (near Madrid in Spain) is due south of RAF St Athan. These two latitudes are N40º29’ and N51º24’ • How would we calculate the distance between them?
Calculation N 51º 24’ First Latitude: Second Latitude: N 40º 29’ Subtracting gives: 10º 55’ To convert 10º 55’ into nautical miles: 10º multiply by 60 = 600 Add the 55’ = 655 nm
Aircraft Speed • The speed for cars, motorcycles and other land-based vehicles: • Miles per hour • For aircraft, the speed is a measure of: • Nautical Miles per hour – (Knots)
Aircraft Speed • We cannot use a speedometer to record aircraft speed. • The aircraft flies through the air. • We use an instrument called an Air Speed Indicator (ASI)
A simplified ASI Aircraft Speed • ASI measures the dynamic air pressure • Dynamic Air Pressure is the pressure caused by forward motion of the aircraft
A simplified ASI Aircraft Speed • In forward flight the pressure above the diaphragm will consist of Dynamic + Static. • Below, the pressure is just Static • The two static pressures cancel out and the diaphragm will move due to the dynamic pressure.
Aircraft Speed • The movement due to dynamic pressure is amplified and displayed on the instrument as Indicated Air Speed (IAS), reading in knots.
Corrections • The reading on the ASI can be in error because of two errors, namely Pressure Error and Instrument Pressure. • Instrument error is caused by poor manufacturing tolerances when the instrument was built.
Corrections • Pressure Error previously known as position is caused by sensing incorrect values of static pressure due to the position of the static vents relative to the airflow around the aircraft. • Both errors can be measured by testing the aircraft under controlled conditions and a calibration card with the combined errors is displayed in the cockpit next to the instrument.
Calibrated Air Speed • Once the two errors have been accounted for, we are left with Calibrated Air Speed (CAS), formerly known as Rectified Air Speed (RAS). • IAS ± Pressure Error ± Instrument Error = CAS • Thus an IAS of 118 kts with a correction on the calibration card of +2 kts would give a CAS of 120 kts.
True Air Speed (TAS) • To obtain True Air Speed (TAS) from CAS you need to correct for air density changes caused by changes in temperature and altitude. • This can be done by calculation or by Navigation Computer.
TAS • If you are flying at speeds greater than 300 kts, then you need to apply a correction for Compressibility Error, which is caused by air becoming compressed in the Pitot Tube. CAS ± Density Error + Compressibility Error = TAS
Units of Time • Time is probably the only example of scientific measurement where every nation uses the same units. • Everyone is familiar with days, hours and minutes; it is only necessary to ensure that you use hours when working with knots as this speed is nautical miles per hour.
Units of Time • In military and commercial aviation the 24 hour clock is used, set to Greenwich Mean Time GMT or Coordinated Universal Time (UTC) as it is now known. • UTC can also be known as Zulu Time • Summer Time or Daylight Saving Time is always ignored.
Calculation of Time of Flight(Still Air) • If a car travels 120 miles at 60 mph, it will take 2 hours to complete the journey. • This is calculated using the distance speed time formulae
Provided 2 quantities are known From Speed Distance and Time The 3rd one can be calculated using the following formula
Distance Speed Time Calculation Triangle (Still Air)
DISTANCE (D) = SPEED (S) TIME(T) DISTANCE (D) = TIME (T) SPEED (S) = DISTANCE SPEED (S) x TIME (T)
Example: How fast must we go to cover 1500 nm in 5 hours? Quantities known are: Distance Time
DISTANCE (D) = SPEED (S) TIME(T) Therefore we use the following formulae: 3 Therefore: 1500 nm 300 S (Knots) = = 5 hours 1
Check of Understanding One degree of latitude represents: 1 nm 6 nm 60 nm 360 nm
Glasgow is due north of Plymouth (approximately on the same meridian). If Glasgow is latitude 55°50’ and Plymouth is latitude 50°25’ what distance are the two places apart?: 450 nm 525 nm 275 nm 325 nm
55° 50’ - 50° 25’ 55 – 50 = 5 5 x 60 = 300 50 – 25 = 25 300 + 25 = 325nm
In the RAF, aircraft speeds are generally expressed in: metres per second miles per hour nautical miles per second Knots
An ASI has an instrument correction factor of +3 kts and a pressure correction factor of -1 Kts. If the instrument reads 130 kts what is the CAS? 130 Kts 132 Kts 133 Kts 134 Kts
IAS ± Pressure Error ± Instrument Error = CAS 130 kts + 3 kts – 1 kts = CAS 133 kts – 1 kts = CAS 132 kts = CAS
A Tornado is flying at a TAS of 400 kts. How far will it travel in 2 hrs? 200 nm 200 Km 800 nm 800 Km
= DISTANCE SPEED (S) x TIME (T) D = 400 kts x 2 hrs D = 400 x 2 = 800 Kts = Nautical Miles per hour 800 Nautical Miles 800 nm