Altimetry

Question 1

How do you calculate pressure lapse rate?

Answer 1

Pressure lapse rate = 96T / P

However, generally use 27ft/hPa or 8m/hPa

Question 2

What is a regional QNH?

Answer 2

The lowest pressure forecast within a given area

It is issued 2 hours before it becomes valid for 1 hour

Question 3

When moving from an area of high pressure to low pressure - what will be the effect on your altimeter?

Answer 3

Moving from HIGH - LOW

Reading will be HIGHER than you actually are (OVER-read)

Question 4

When moving from an area of low pressure to high pressure - what will be the effect on your altimeter?

Answer 4

Moving from LOW - HIGH

Reading will be LOWER than you actually are (UNDER-read)

Question 5

What is the equation to calculate Surface Pressure Variation?

Answer 5

TA = IA + 27(QNH - subscale)

Question 6

What is the difference between Indicated and True altitude?

Answer 6

Indicated - what the altimeter reads

True - Actual distance above MSL

Question 7

What is the equation to find Vertical Pressure Distribution?

True altitude within a pressure column

Answer 7

TA = IA + (4 x ISA dev. x Thickness) / 1000

Question 8

What is the equation to use when there is a combined subscale and temperature error?

Answer 8

TA = IA + (4 x (IA/1000) x ISA dev.) + (27(QNH - subscale))

Question 9

What is the basic way to correct for Temperature error?

Answer 9

For every 10°C deviation to the ISA temp - apply a correction of 4% to the IA

Question 10

What effect does moving from warm to cold air have on the altimeter?

Answer 10

Moving from warm - cold

You’d be UNDERdressed (lower than you think)

Question 11

What effect does moving from cold to warm air have to altimeter?

Answer 11

Moving from cold - warm

You’d be OVERdressed (higher than you think)

Question 12

What pressure setting should you set on take off and the initial climb?

Answer 12

Airfield QNH should be set whilst climbing up to the transition altitude

Question 13

What is the first point in flight that you change from QNH to S.P.S?

Answer 13

Set 1013 hPa at the transition altitude

Question 14

When should you change back onto QNH after operating at flight levels?

Answer 14

As you descend through the transition level, you should set QNH

Question 15

What pressure setting should you use during approach and landing?

Answer 15

Normally set QNH

But light aircraft may operate QFE

Question 16

What is the transition layer and how it is adjusted?

Answer 16

This is the gap between transition altitude and level.
You should not fly in this level - only descend through it
They are normally 500ft thick, or less, but are adjusted to reflect the MSL pressure at that time.

If layer exceeds 500ft or becomes less 0ft, it is reset to minimum

Question 17

When is QNH greater than QFF?

Answer 17

When the following are BOTH either correct or incorrect:

• Airfield is above MSL
• Temperature is greater than ISA temperature

Question 18

What are the 3 factors that affect density?

Answer 18

Pressure (Increase = ρ increase)
Temperature (Increase = ρ decrease)
Humidity (Increase = ρ decrease)

Question 19

What is the ideal gas equation?

Answer 19

Constant = PV/T

Question 20

What are the density values in the hottest and coldest places on Earth?

Answer 20

Hottest: 1.1 kg/m^3

Coldest: 1.5 kg/m^3

Question 21

What are the least favourable conditions for aircraft performance?

Answer 21

High altitude
High temperature
High humidity

Question 22

Give the equation for calculating density altitude

Answer 22

DA = PA + 120 x ISA dev.

Question 23

Define Density altitude

Answer 23

Density altitude is the ISA altitude corresponding to a measured density at a location.

It is also the Pressure density corrected for Temperature errors