AIR NAVIGATION Part 1 Distance Speed & Time. LEARNING OUTCOMES On completion of this unit, you should: –B–Be able to carry out calculations to determine.

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 Lines of Longitude are sometimes referred to as MERIDIANSto as MERIDIANS

• When recording your position – the line of When recording your position – the line of Latitude must be given first.Latitude must be given first.

• The starting point goes through Greenwich The starting point goes through Greenwich and is referred to as the:and is referred to as the:

“ “Prime Meridian”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

First Latitude: N 51º 24’

Second Latitude: N 40º 29’

Subtracting gives: 10º 55’

To convert 10º 55’ into nautical miles:

10º multiply by 60 = 600Add 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)

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.

A simplified ASI

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 thefollowing formula

SpeedSpeed TimeTime

DistanceDistance

Calculation Triangle (Still Air)

SPEED (S) =DISTANCE (D)

TIME(T)

DISTANCE (D)

SPEED (S)TIME (T) =

DISTANCE = SPEED (S) x TIME (T)

Example:

How fast must we go to cover 1500 nm in 5 hours?

Quantities known are:

Distance

Time

SPEED (S) =DISTANCE (D)

TIME(T)

Therefore we use the following formulae:

Therefore:

S (Knots) =1500 nm

5 hours1

3

= 300

Check of UnderstandingOne 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?:

525 nm

275 nm

450 nm

325 nm

55° 50’ - 50° 25’

55 – 50 = 5

50 – 25 = 25

5 x 60 = 300

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