PILOT NAVIGATION

Part 1

Senior/Master Air Cadet

Learning Outcomes

Understand the affects of weather on aviation

Know the basic features of air navigation and navigational aids

Understand the techniques of flight planning

Units

Units

At school you will have been taught that in the modern world everything is

measured in metric units

However in real life many people use non metric measures. An example of this is the

use of miles & mph in the UK

Units

This lesson will look at the units commonly used for:

pressure

vertical distance

speedweather fuel

aircraft weight

Vertical Distance and Speed

In aviation horizontal distances are measured in nautical miles and speed in knots

These units are based on the length of a “GREAT CIRCLE” on the surface of the

earth

One minute of arc measured at the centre of the earth equals 1 nm on the earth's surface

Vertical Distance and Speed

In the vertical axis the majority of countries use feet to measure height or altitude. Only the former communist countries use metres

Vertical Distance and Speed

It must be noted that many countries who use feet have changed their maps to show

elevation in metres, - such as the UK OS maps

Vertical Distance and Speed

Great care is needed because an aircraft flown in thousands of feet can be in a very

dangerous position

!if a navigator reads a mountain top at 2000’ when it is 2000 metres which is

about 6000’!

Vertical Distance and Speed

Terrain clearance is done with great care and is the pilots/navigators

number one priority.

When calculating the safety altitude there should be no doubt.

Vertical Distance and Speed

Vertical Speed uses the same units as Vertical Distance

Feet/ Min

Vertical Distance and Speed

Vertical speed indicators which show rate of climb or descent are calibrated in thousands of feet/ min for most military

aircraft

Meteorological Units

World-wide the met office has changed to metric units

With the major exception of the USA

However even the met office must continue to use feet for altitude &

knots for windspeed

Aircraft & FuelStrictly speaking aircraft & fuel should be

measured by mass

In practical terms it is weight (the effect of gravity on mass ) that we use

For aircraft the units used depend on the country of manufacture

Most US aircraft (70% of the worlds total ) use pounds or imperial tons. The rest use

kilograms (kg) or metric tonnes

Aircraft & Fuel

For fuel the situation is more complicated

In theory it should be measured by mass as the amount of thermal energy in one unit of fuel relates directly to its mass!!!

Aircraft & Fuel

You cannot measure fuel mass when an aircraft is in flight, so it is measured by

volume,

as in cars.

In cars we measure the fuel volume in litres (or gallons) & then calculate the fuel use in

km per litre (or mpg )

Aircraft & Fuel

However the use of volume in the air is not accurate enough as the type of fuel

& the temperature affect the mass per unit volume

AVTUR

AVGAS

In other words the density of fuel varies from type to type

Aircraft & Fuel

Even if one type of fuel is always used, its density will change with temperature changes.

AVTUR

AVGAS

On modern aircraft the fuel is indicated inlbs or kgs and the measuring systemcompensates for density and temperature.

Aircraft & Fuel

Different types of fuel each have a Specific Gravity ( SG )

This is a measure of the ratio between the weight of the fuel and the

weight of the same volume of water

Water has a SG of 1.0A typical jet engine fuel has a SG of 0.80

Aircraft & Fuel

This means a litre of jet fuel weighs 80% of the weight of a litre of water

Conversion is done by calculator, a Dalton computer or the chart in the RAF flight

information handbook

Pressure

Various gasses & fluids in aircraft are pressurised

Several different units are used to express pressures - according to the

country of origin

Pressure

Our main concern is the pressure in the atmosphere

The higher we go the less air there is & so pressure reduces as we gain height

Pressure

Pressure is measured in Pounds Per Square Inch PSI, Inches of mercury (US) , in MM of

mercury, or millibars (MB)

Millibars is in general use outside the USA

The average pressure at sea level is 1013 MB

Most modern electronic altimeters can switchbetween millibars and inches of mercury.

Pressure

20039,000

25034,000

30030,000

40024,000

50018,000

70010,000

1013SEA LEVEL

AIR PRESSUREIN MB

ALTITUDEIN FEET

Note: An aircraft at 34000’ has a quarter the amount of oxygen of one at sea level, which is why the aircraft is pressurised to about 5000ft.

If it was not for pressurisation all aboard would be unconscious

Pressure

At a cruising altitude of 39,000 feet, a Boeing 767's cabin will be pressurized to an altitude

of 6,900 feet¹

¹Commercial Airliner Environmental Control System: Engineering Aspects of Cabin Air Quality.

http://www.boeing.com/commercial/cabinair/ecs.pdf

Conclusion

Aviation is the only major area of science still using such a wide variety

of units

There is a slow movement to metrication

Until countries agree to common units to their instruments, and whilst longitude &

latitude remain so will the confusion

Check of Understanding

In aviation how are horizontal distances and speeds measured?

horizontal distances are measured in nautical miles

speed in knots

Check of Understanding

How do the majority of countries measure vertical

distances?

the majority of countries use feet to measure height or altitude

Check of Understanding

In what units do Vertical Speed Indicators their informationdisplay?Vertical speed indicators which show rate of climb or descent are calibrated in thousands

of feet/min for most military aircraft

Check of Understanding

In practical terms, in what units are aircraft and fuel measured?

In practical terms it is weight

Check of Understanding

What is Specific Gravity?

a measure of the ratio between the weight of the fuel

and the weight of the same volume of water