Meteorology - Altimetry

Question 1

What does an altimeter’s barometric setting control adjust?

Answer 1

The altimeter reads heights above differing datums.

Question 2

What are the four main datum settings for altimeters?

Answer 2

QFE, QNH, Standard pressure, QFF.

Question 3

What is QFE?

Answer 3

A reading of station pressure taken on an accurate barometer and adjusted for any difference in height between the barometer and the field elevation. If QFE is set, altimeters should read zero on the ground at the airfield.

Question 4

What is QNH?

Answer 4

The station pressure (QFE) adjusted down to mean sea level (MSL) using ISA values for the temperature between the station and MSL. An altimeter with QNH set should read station elevation on the ground at the station.

Question 5

What is the standard pressure setting?

Answer 5

1013.2 hPa above a set altitude called the transition altitude. Heights indicated are called pressure altitudes, and they may also be given in an abbreviated form by knocking off the last two zeroes, called a flight level (FL).

Question 6

What is QFF?

Answer 6

The station pressure adjusted down to MSL using the actual station temperature and the actual lapse rate, taking into account any surface inversions, in the calculation. It is not set on aircraft altimeters but used by the meteorological service as an accurate value for MSL pressure. QFF is displayed on surface isobar charts.

Question 7

What is QNE?

Answer 7

Not a pressure setting but the touchdown height indicated on the altimeter if the standard setting, 1013 hPa, is set. Used at high airfields where QFE would be too low to set on the altimeter subscale.

Question 8

How is the adjustment of the station pressure to sea level under QNH and QFF known?

Answer 8

The “reduction of measured pressure to sea level.”

Question 9

What happens if the temperature at the station and lapse rate to sea level are in accordance with the ISA?

Answer 9

QNH = QFF.

Question 10

How does QNH compare to QFF for stations above sea level if it is warmer than ISA?

Answer 10

QNH is greater than QFF.

Question 11

How does QNH compare to QFF for stations above sea level if it is colder than ISA?

Answer 11

QNH is less than QFF.

Question 12

How does QNH compare to QFF for stations below sea level if it is warmer than ISA?

Answer 12

QNH is less than QFF.

Question 13

How does QNH compare to QFF for stations below sea level if it is colder than ISA?

Answer 13

QNH is greater than QFF.

Question 14

What changes when the pilot changes the altimeter sub-scale setting?

Answer 14

The reading on the altimeter changes because the altitude is being referenced to a different datum.

Question 15

What is the altitude change per hPa for altimetry problems?

Answer 15

27 ft (8 m) per hPa.

Question 16

What does using QFE indicate?

Answer 16

Height above an airfield.

Question 17

What does using QNH indicate?

Answer 17

Altitude above mean sea level.

Question 18

What does using the standard setting indicate?

Answer 18

Pressure altitude or flight level.

Question 19

What is barometric error?

Answer 19

If an incorrect pressure setting is left on the altimeter, it will indicate higher or lower than it should.

Question 20

How does temperature affect true altitude?

Answer 20

In high temperatures, true altitude is greater than indicated altitude. In low temperatures, true altitude is lower than indicated altitude.

Question 21

What is the altitude error due to a temperature deviation from ISA?

Answer 21

4% of height difference for every 10°C of temperature difference or 4x(Height in Thousands of Feet) x ISA Deviation

Question 22

What is QFF?

Answer 22

Station pressure adjusted to mean sea level using actual station temperature and lapse rate.

Question 23

What happens when QNH is set below the correct value?

Answer 23

The altimeter will under read, indicating a lower altitude than actual.

Question 24

What is the mnemonic for barometric error?

Answer 24

“High to low, careful go” means the altimeter will over read when flying from high to low pressure.

Question 25

How do surface pressures affect altimeter readings?

Answer 25

Incorrect surface pressures lead to barometric errors: over reading when flying from high to low pressure, and under reading when flying from low to high pressure.

Question 26

What is the general altitude correction for temperature error?

Answer 26

4% of the height difference for every 10°C of temperature difference or 4x(Height in Thousands of Feet) x ISA Deviation

Question 27

How is barometric error calculated?

Answer 27

Barometric error can be calculated separately and should be dealt with first.

Question 28

What is QNH?

Answer 28

The station pressure (QFE) adjusted down to mean sea level (MSL) using ISA values for the temperature between the station and MSL. Altimeters use ISA values for internal calculations, so an altimeter with QNH set should read station elevation on the ground at the station.

Question 29

What is QFE?

Answer 29

A reading of station pressure taken on an accurate barometer and adjusted for any difference in height between the barometer and the field elevation. If QFE is set, altimeters should read zero on the ground at the airfield.

Question 30

What is the standard pressure setting for altimeters?

Answer 30

1013.2 hPa

Question 31

What happens if QFE is set on an altimeter?

Answer 31

The altimeter should read zero on the ground at the airfield.

Question 32

What is the pressure altitude?

Answer 32

Altitude indicated by a barometric altimeter when set to standard pressure (1013.2 hPa).

Question 33

What is the effect of temperature deviation from ISA on true altitude?

Answer 33

In cold air, true altitude is lower than indicated altitude. In warm air, true altitude is higher than indicated altitude.

Question 34

What is the rule of thumb for altitude correction in temperature deviation?

Answer 34

4% of height difference for every 10°C of temperature difference.

Question 35

What mnemonic helps remember altitude changes due to temperature deviations?

Answer 35

“High to low, careful go.”

Question 36

How to calculate the altitude error for barometric pressure difference?

Answer 36

Altitude error (in feet) = Pressure difference (in hPa) x 27 ft/hPa.

Question 37

What is density altitude?

Answer 37

The altitude in the International Standard Atmosphere (ISA) at which the air density would be equal to the actual air density at the place of observation.

Question 38

What is the relationship between QNH and QFF?

Answer 38

QNH is calculated from station pressure on the assumption that ISA conditions apply. QFF is the actual sea level pressure. QNH will differ from QFF when the temperature is different from ISA.

Question 39

What happens to QNH compared to QFF in colder air?

Answer 39

QNH will be less than QFF (higher).

Question 40

What happens to QNH compared to QFF in warmer air?

Answer 40

QNH will be more than QFF (lower).

Question 41

What is the effect of temperature on QNH when setting on the ground?

Answer 41

The altimeter is calibrated to ISA. If QNH is set, it reads correctly on the ground but may need temperature correction when airborne.

Question 42

How is density altitude calculated?

Answer 42

Density altitude = Pressure altitude + (120 x Temperature deviation from ISA).

Question 43

What should be considered in high wind conditions over high ground?

Answer 43

Airflow over high ground can affect atmospheric pressure, causing the altimeter to over-read. Additional height margins should be allowed.

Question 44

What is the drift and altimeter error when flying from high to low pressure?

Answer 44

The altimeter will over-read, leading to starboard or right drift in the northern hemisphere.

Question 45

How does the altimeter error affect terrain clearance in high to low pressure flight?

Answer 45

True altitude decreases; therefore, it’s important to calculate the minimum useable Flight Level accurately.