# Air Navigation Distance Speed and Time Lecture One

- Slides: 38

Air Navigation Distance, Speed and Time Lecture One

RECAP Latitude and Longitude lines divide the surface of the Earth into degrees and minutes. Lines of latitude run North or South of the equator One minute of latitude represents one nautical mile (nm) 1 degree of latitude (60 minutes) equals 60 nm

RECAP 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 called MERIDIANS The starting point goes through Greenwich and is referred to as the “Prime Meridian”

Distances Between Points Nautical maps have no scales on the borders, so we use the nm (nautical mile) scale shown along a meridian, close to the area of interest on the chart.

Distances Between Points If two places are on the same meridian (longitude) then it is possible to calculate the distance between them, rather than having to actually measure it.

Distances Between Points For example: Torrejon airfield (in Spain) is due south of RAF St Athan (in Wales) on the same meridian These two latitudes are N 40º 29’ and N 51º 24’ So how do we calculate the distance between them?

Meridian Calculation First Latitude Second Latitude Subtraction gives ? N 51º 24’ N 40º 29’ 10º 55’ Remember – 60 degrees in a minute Because 29’ is more than 24’ 60º minus 29’ = 31’ Plus 24’ = 55’ As we carry 1 across, we now have 51º - 41º = 10º Now convert 10º 55’ into nautical miles: 10º multiply by 60 = 600 Add the 55’ = 655 nm

Aircraft Speed We measure speed for land vehicles in Miles per hour For aircraft, the speed is measured in Knots - Nautical Miles per hour (remember 1’ of latitude is 1 nautical mile (nm)) We show the of speed land vehicles on a Speedometer For aircraft we use an Air Speed Indicator (ASI)

Aircraft Speed The Air Speed Indicator (ASI) measures the Dynamic Air pressure caused by forward motion of the aircraft. This is achieved by measuring the air captured in the Pitot Tube A Pitot Tube

Aircraft Speed A simplified ASI Dynamic + Static Diaphragm Static Pitot Tube Airstream Static Tube The pressure of the surrounding air (“Static Pressure”) and the pressure in the Pitot Tube (“Pitot Pressure”) are measured and compared. In forward flight the pressure above the diaphragm is both Dynamic + Static. But below the diaphragm is just Static. The two static pressures cancel each other out, and so the diaphragm moves up and down due only to the Dynamic Air Pressure.

Aircraft Speed The movement due to dynamic air pressure is amplified and displayed on the instrument as Indicated Air Speed (IAS) in knots. The two static pressures cancel each other out, and so the diaphragm moves up and down due only to the Dynamic Air Pressure.

Speed Corrections ASI readings can be wrong for two reasons – Instrument Errors and Pressure Errors. Instrument Error caused by poor manufacturing tolerances when the instrument was built. Pressure Error (previously known as position error) caused by sensing incorrect values of static pressure due to the static vents position relative to the airflow around the aircraft.

Speed Corrections 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 (Indicated Air Speed) of 118 kts with an overall correction of +2 kts would give a CAS (Calibrated Air Speed) of 120 kts.

Speed Corrections To obtain True Air Speed (TAS) from CAS you need to correct for air density changes caused by temperature and altitude. If flying at speeds greater than 300 kts, you also need to apply a correction for Compressibility Error, caused by air being compressed in the Pitot Tube. When these two corrections have been made, we are left with True Air Speed (TAS) CAS + - Density Error +- Compressibility Error = TAS Thus a CAS of 120 kts with a correction of -2 kts would give a TAS of 118 kts.

Speed Corrections SUMMARY Dynamic air pressure is displayed as Indicated Air Speed (IAS) Calibrated Air Speed (CAS), (formerly known as Rectified Air Speed (RAS)) is calculated as IAS + - Pressure Error +- Instrument Error = CAS True Air Speed (TAS) is calculated as CAS + - Density Error +- Compressibility Error = TAS

Calculation of Time In aviation the 24 hr clock is used, set to Greenwich Mean Time GMT or Coordinated Universal Time (UTC) (also known as Zulu Time) Summer Time or Daylight Saving Time is always ignored. We are all familiar with days, hours and minutes; But when working with aircraft knots, it’s only necessary to use hours, as this speed is nautical miles per hour.

Calculation of Time 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 If 2 quantities are known the 3 rd can be calculated Speed (S) x Time (T) = (Distance) Distance (D) Time (T) Distance (D) Speed (S) = Time (T) = Speed (S) Speed Time

Calculations How far (in nm) will we cover at 270 knots in 3 hours? (S) 270 kts x (T) 3 hours = Distance (D) 270 multiplied by 3 = 810 nm How long will it take to travel 960 nm at 160 knots? (D) 960 nm (S) 160 knots How fast must we go to cover 1500 nm in 5 hours? (D) 1500 nm = Time (T) 960 divided by 160 = 6 hrs Distance Speed (T) Time 5 hours = Speed (S) 1500 divided by 5 = 300 kts

Check Understanding Distance on the earth’s surface is measured in nautical miles. Which of the following is true? 1 nm = 1/10, 000 of the distance from the North pole to the equator 1 nm = 1 min of Longitude 1 nm = 5280 feet 1 nm = 1 min of Latitude

Check Understanding The latitude of a point is its distance, measured in degrees and minutes: North or South of the Equator East or West of Greenwich From the North pole From the South pole

Check Understanding One degree of latitude represents: 1 nm 60 nm 360 nm

Check Understanding The distances on a navigation chart can be measured with dividers. What scale will then be used to convert that distance to nautical miles: 1: 50, 000 scale The minute scale along a parallel of latitude Any meridian scale off any chart The minute scale along a meridian, close to the area of interest on the chart

Check Understanding 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? 275 nm 325 nm 55° 50’ - 50° 25’ 50’ – 25’ = 25 55° – 50° = 5 5 x 60 = 300 450 nm 300 + 25 = 325 nm 525 nm

Check Understanding What does an ASI measure? Dynamic air pressure caused by forward motion of the aircraft Overall speed of the surrounding air Dynamic air pressure caused by speed of the outside air Forward thrust of the aircraft engines

Check Understanding Oslo Airport (Norway) is due north of Braunschweig (Germany). If their latitudes are 59 53 N and 52 20 N respectively, how far are they apart: 445 nm 453 nm 59° 53’ - 52° 20’ 53’ – 20’ = 33 59° – 52° = 7 7 x 60 = 420 454 nm 420 + 33 = 453 nm 554 nm

Check Understanding In the RAF, aircraft speeds are generally expressed in: Knots Miles per hour Metres per second Nautical miles per hour

Check Understanding What is shown on an ASI? Indicated Air Speed in knots True Air Speed in knots Dynamic Air Speed in knots Nautical miles per hour

Check Understanding Which of the following statements about Calibrated Air Speed (CAS), (formerly known as Rectified Air Speed (RAS)) is true? CAS = IAS before corrections CAS = IAS after corrections CAS always equals IAS CAS is always less than IAS

Check Understanding Universal Time (UT) is used as the standard in military and commercial aviation. What other name is this known as: British Summer Time European Daylight Time Greenwich Mean Time Local Time

Check Understanding Calibrated Air Speed (formerly known as Rectified Air Speed (RAS)) equals Indicated Air Speed with corrections for what? Altitude and Pressure error Instrument and Pressure error Air Density and Pressure error Compressibility and Pressure error

Check Understanding How fast must an aircraft fly to cover 1200 nm in 3 hours? Speed (S) 400 kts = Distance (D) Time (T) 600 kts Speed (S) = (D) 1200 kts divided by (T) 3 hrs = 400 1200 kts 3600 kts = 400 kts

Check Understanding An ASI has an instrument correction of +3 kts and a pressure correction of -1 kts. If the ASI reads 130 kts what is the CAS? IAS ± Pressure Error ± Instrument Error = CAS 130 Kts kts + 3 kts – 1 kts = CAS 132 133 Kts CAS = 132 kts 134 Kts

Check Understanding A Hercules is flying at a groundspeed of 210 kts. How far will it travel in 3 hours: 70 nm 210 nm (S) x Time (T) Distance (D) 630 = Speed nm Distance (D) = 210 kts (S) x 3 hours (T) 820 nm = 630 nm

Check Understanding True Air Speed equals Calibrated Air Speed with corrections for what? Compressibility and Altitude error Air Density and Pressure error Compressibility and Pressure error Air Density and Compressibility error

Check Understanding A Tornado is flying at a TAS of 400 kts. How far will it travel in 2 hrs? (D) = Speed (S) x Time (T) 200 Distance nm Distance (D)Km = 400 kts (S) x 2 hours (T) 200 800 nm = 800 nm 800 Km

Check Understanding What time is used as standard in military and commercial aviation? The local time of the country flying over British Summer Time European Standard Time Greenwich Mean Time

Check Understanding A Tornado is flying at a TAS of 320 kts. How long will it take to travel 80 nm? Time (T) 15 mins = Distance (D) Speed (S) 30 mins 1 Time (T) = (S) 320 kts divided by (D) 80 nm = 4 2 hours 4 hours = 15 mins

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