02. Altimetry

Question 1

Altimetry

“The reading of a station pressure taken on an accurate barometer and adjusted for any difference in height between the barometer and the field elevation. Should read zero on the ground on the altimeter”

This is a definition of which pressure setting

Answer 1

QFE

1

Question 2

Altimetry

“The station pressure adjusted down to mean sea level (MSL) using ISA values for the temperature between the station at the field elevation and MSL. Since the altimeter uses ISA values, it should read the same as teh station elevation on the ground from MSL”

This is a definition of which pressure setting

Answer 2

QNH

1

Question 3

Altimetry

“Above a set altitude call the transition altitude, aircraft operate with on this pressure of 1013 hPa, sometimes referred to as the ‘pressure setting’”

This is a definition of which pressure setting

Answer 3

STANDARD PRESSURE

1

Question 4

Altimetry

“the station pressure adjusted down to MSL using actual station temperature and the actual lapse rate”

This is a definition of which pressure setting

Answer 4

QFF

2

Question 5

Altimetry

The RPS pressure setting is given to pilots based on what 2 parameters

RPS - Regional Pressure Setting

Answer 5

1. LOWEST forecast for region
2. 1 HOUR AHEAD

2

Question 6

Altimetry

What does ASR and RPS stand for

Answer 6

1. ALTIMETER SETTING REGION
2. REGIONAL PRESSURE SETTING

2

Question 7

Altimetry

Airfield QFE pressure is given from what point

Answer 7

HIGHEST POINT ON AIRFIELD SURFACE

2

Question 8

Altimetry

If the touchdown point of a runway is significantly below the airfield QFE, what may be issued to pilots

Answer 8

TOUCHDOWN QFE

2

Question 9

Altimetry

“the touchdown height that will be indicated on the altimeter if the standard setting of 1013 hPa is set on the altimeter. Used at high airfields where QFE woudl be too low a value to set on the altimeter subscale”

This is a defintiion of what pressure setting

Answer 9

QNE

3

Question 10

Altimetry

For stations ABOVE sea level;

1. If temperature is warmer than ISA, then QNH is MORE or LESS than QFF
2. If temperature is COLDER than ISA, then QNH is MORE or LESS than QFF

Answer 10

1. MORE
2. LESS

QNH vs QFF MEMORY AID

QNH vs QFF DIAGRAM

3

Question 11

Altimetry

For stations BELOW sea level;

1. If temperature is warmer than ISA, then QNH is MORE or LESS than QFF
2. If temperature is COLDER than ISA, then QNH is MORE or LESS than QFF

Answer 11

1. LESS
2. MORE

QNH vs QFF MEMORY AID

QNH vs QFF DIAGRAM

3

Question 12

Altimetry

If your QNH was 1011 and you are instructed the pressure setting is now QNH 1025 and you change the kollseman window to the new pressure setting, has your altitude INCREASED or DECREASED

Answer 12

INCREASED

REMEMBER

“WIND ON pressure, WIND ON altitude”

“WIND OFF pressure, WIND OFF altitude”

MEMORY AID

4

Question 13

Altimetry

When flying high pressure to low pressure, what will the altimeter read

Answer 13

OVER READ
Lower than you think you are

REMEMBER
* HIGH to low, watch out BELOW

MEMORY AID

6

Question 14

Altimetry

An aircraft departs an airfield (point A) at an altitude of 1500 m with the QND set to 1010 hPa. It arrives over a hill top (point B) where teh QNH is 990 hPa. The hill is 1000 m high. Assuming 8m per 1 hPa, what will the obstacle clearance be at point B

Answer 14

340 m

REMEMER
Always read the question. Typically you will give answers in feet, but in this instance, it is asking for meters!

• Determine the barometric error
• 1010 (point A as set on the subscale) - 999 (point B, where the obstacle is)
• 1010 - 990 = 20 hPa
• 20 hPa x 8m per hPa
• 20 x 8 = 160 m
• Since QNH was set at point A, and we know QNH for the airfield is taken from the station at the airfield, the QNH altitude at the airfield must be the same as the airfield altitude, in this instance 1500 m
• The QNH at point B is LESS than at point 1, meaning we need to WIND OFF PRESSURE (which means WIND OFF ALTITUDE)
• 1500 - 160 = 1340m
• The elevation of the hill is 1000 m and the aircraft is flying at 1340 m
• 1340 - 1000 = 340 m Clearance

8

Question 15

Altimetry

An aircraft is at FL40. What is the AMSL altitude if QNH is 998 hPa

Answer 15

3595 ft

REMEMBER
Always read the question. The first piece of information is that the aircraft is flying a FLIGHT LEVEL i.e. FL40. This means it must be using the standard pressure setting of 1013.
* Determine the barometric error
* Aircraft is flying a FLIGHT LEVEL so is on the standard pressure setting, which is 1013
* The QNH at the given point is 998 hPa
* 1013 - 998 = 15 hPa
* the QNH is LESS than the standard pressure setting, so WIND OFF pressure, WIND OFF altitude
* Determine how much altitude to we will lose
* 15 hPa x 27 ft per hPa (NOTE typically exams are now using 30 ft per hPa)
* 15 x 27 = 405 ft
* 4000 - 405 = 3595 ft

DIAGRAM

9

Question 16

Altimetry

If the OAT temperature differs from ISA, this will cause either an OVER READ or an UNDER READ on the altimeter. This is known as what

Answer 16

TEMPERATURE ERROR

10

Question 17

Altimetry

If the OAT is warmer than ISA, the true altitude will be MORE or LESS than indicated altitude

Answer 17

MORE
TRUE > indicated

TEMPERATURE ERROR

ERROR CORRECTION

10

Question 18

Altimetry

If the OAT is colder than ISA, the true altitude will be MORE or LESS than indicated altitude

Answer 18

LESS
true < INDICATED

For every

TEMPERATURE ERROR
​
ERROR CORRECTION
​
10

Question 19

Altimetry

What is the equation for working out temperature error correction

Answer 19

4 x HEIGHT (in thousands of feet) x ISA DEVIATION
for every 1℃ ISA Dev, 4ft for every 1000 ft

NOTE height is the indicated altitude AFTER any barometric correction plus or minus the elevation of teh station at which the pressure is measured

ERROR CORRECTION

11

Question 20

Altimetry

You are flying at FL200. OAT is -40℃ and the pressure at sea level is 1033 hPa. What is the true altitude

Answer 20

19,308 ft
DIAGRAM

BAROMETRIC ERROR
* Start with the barometric error.
* We are flying a flight level, meaning we must be flying the standard pressure setting, which is 1013
* The given QNH is 1033
* 1033 - 1013 = 20 hPa
* Given QNH is ABOVE the SPS setting, WIND ON pressure, WIND ON altitude
* 20 hPa x 27 ft per 1 hPa = 540
* Flying at FL200 so 20,000 ft
* 20,000 + 540 = 20,540 ft is aircrafts altitude
ISA DEVIATION
* Determine ISA deviation with 15 - (alt x 2) - OAT
* 15 - (20 x 2)
* 15 - 40 = -25℃
* Given OAT is -40℃
* -25℃ - -40℃ = -15℃
TEMPERATURE ERROR CORRECTION
* Use the formaul 4 x height (thousands of feet) x ISA deviation
* 4 x 20.54 x -15 = -1232.4
* The OAT is COLDER than ISA, so the altimeter will OVER READ - example
DETERMINE TRUE ALTITUDE
* 20,540 - 1232 = 19,308 ft

12

Question 21

Altimetry

QNH is calculated from station pressure, so therefore assumes what conditions.

Answer 21

ISA

13

Question 22

Altimetry

As a result of QNH being calculated from an airfield station based on ISA conditions, and QFF being calculated based on actual sea level pressure, what does this mean in regards to the relationship between QNH and QFF if the temperature deviates from ISA conditions

Answer 22

QNH and QFF WILL DIFFER

QNH vs QFF MEMORY AID

QNH vs QFF DIAGRAM

13

Question 23

Altimetry

What is the equation to determine QFE from QNH

If QFE is 1001 at 200 ft, what is QNH

Answer 23

QNH = QFE + (ELEVATION / 27 )
QNH = QFE (hPa) + (elevation / 27 ft per 1 hPa difference)

If QFE is 1001 at 200 ft, what is QNH
* 1001 + (200 / 27)
* 1001 + 7
* QNH = 1008 hPa

15

Question 24

Altimetry

What is the equation to determine QFE from QNH

If QNH is 1008 at an airfield elevation of 200 ft, what is QFE

Answer 24

QFE = QNH - (ELEVATION / 27 )

If QFE is 1001 at 200 ft, what is QNH
* 1008 + (200 / 27)
* 1008 - 7
* QNH = 1001 hPa

15

Question 25

# Altimetry If you fly from a *higher* temperature to a *low* temperature, the altimeter will *OVER READ or UNDER READ*

Answer 25

OVER READ ## Footnote **REMEMBER** * When temperature is LOW, aircraft is LOWER than you think you are [DIAGRAM](https://drive.google.com/file/d/1L3TEu-_OW5HD9VKgptFtx07SlBoXVY-T/view?usp=drive_link) 15

Question 26

# Altimetry If you fly from a *low* temperature to a *high* temperature, the altimeter will *OVER READ or UNDER READ*

Answer 26

UNDER READ ## Footnote **REMEMBER** * When temperature is HIGH, aircraft is HIGHER than you think you are [DIAGRAM](https://drive.google.com/file/d/1L3TEu-_OW5HD9VKgptFtx07SlBoXVY-T/view?usp=drive_link) 15

Question 27

# Altimetry An aircraft is making a precision approach to an aerodrome at 4000 ft amsl at an OAT of -30℃. The decision altitude (DA) is QNH based at 4200 ft, the decision height (DH) is 200 ft above touchdown. Use the [table](https://drive.google.com/file/d/1LTgwsVXFEURXY5Cou-nAyWJcM-TmWYcN/view?usp=drive_link) to determine the correct indicated altitude of decision height.

Answer 27

4240 ft ## Footnote [TABLE](https://drive.google.com/file/d/1LTgwsVXFEURXY5Cou-nAyWJcM-TmWYcN/view?usp=drive_link) * We are given the selected datum and height above to making a decision at decision height * Datum for DA is 4200 ft and the decision height (DH) is 200 ft above * Find the 200 ft column, cross reference to the given tempreature of -30℃ * We can see that we need to add 40ft * 4200 + 40 = 4240 ft 16

Question 28

# Altimetry In the Northern Hemisphere, what direction does the wind flow in a *low pressure* system

Answer 28

ANTI-CLOCKWISE ## Footnote 17

Question 29

# Altimetry In the Northern Hemisphere, what direction does the wind flow in a *high pressure* system

Answer 29

CLOCKWISE ## Footnote 17

Question 30

# Altimetry An aircraft travelling from a *high pressure* system into a *low pressure* system in the Northern Hemisphere will experience drift in which direction

Answer 30

STARBOARD or RIGHT ## Footnote **REMEMBER** * Wind in your back, LOW is to the LEFT or; * Face the wind, you are *LEFT HIGH AND DRY* (indicating high is to your left* [EXAMPLE](https://drive.google.com/file/d/1zsrYt82HUJ1wj10CatRjijfjYDUXuo4o/view?usp=drive_link) 17

Question 31

# Altimetry You are intending to fly along an airway where the minimum flight level given on teh chart is FL150 The highest ground you will over is 12,000 ft and the MOCA on the chart is shown as 14,000 ft. The temperature at FL150 is -25℃ and the local QNH, measured at an airfield where the elevation is 1500 ft AMSL is 993 hPa. What is the lowest usable Flight Level to conform with the published MOCA? | MOCA = Minimum Obstacle Clearance Altitude

Answer 31

FL160 ## Footnote **BAROMETRIC ERROR** * Calculate the barometric error first. We are at FL150 ⋉ 15,000 ft. FL means we are on SPS of 1013. * Local QNH is given as 993. We know that 993 is LOWER than 1013, WIND OFF pressure, WIND OFF altitude * (1013 - 993) x 27 ft per 1 hPa * 20 x 27 = 540 ft * WIND OFF pressure, WIND OFF altitude, 15,000 - 540 * 14,460 ft **ISA DEVIATION** * OAT is given as -25℃ * calculate what temperature would be under ISA conditions at same altitude * 15 - (alt x 2) ⋉ 15 - (15 x 2) * 15 - 30 = -15℃ * Deviation is (-15) - (-25) * 10℃ ** TEMPERATURE CORRECTION** * OAT is *COLDER* than ISA, so we are *OVRE READING*, meaning we must subtract * Use formula to determine the correction, 4 x (alt in thousands of feed) x ISA deviation * ⋉ 4 x 14.46 x (-10) * -519 (rounded up) **TRUE ALTITUDE** * We now subtract the temperature correction from the barometric error corrected altitude * 14,460 - 519 * ** 13,941 ft** This altitude is *BELOW* the MOCA of FL140. We are already flying at FL150 so it means we must fly at a *HIGHER* FL to maintain the MOCA FL160 18

Question 32

# Altimetry Why does airflow over high ground affect the atmospheric pressure? What is the affect? | Think similar to airflow over the top of a wing

Answer 32

1. BERNOULLI EFFECT 2. DROP IN PRESSURE ## Footnote * As air stream is forced up and over the hill top, the air streams get closer together * Bernoulli's therom tells us that P1 + P2 = P3, i.e. dynamic pressure + static pressure = Total pressure * The pressure remains constant through a tube, so when the tube becomes thinner, the flow at which it passes through will speed up * Static pressure will drop in other words, and dynamic pressure will increase 19

Question 33

# Altimetry *"The altitude in the ISA at which the air density would be equal to the actual air densitiy at the place of observation"* This is a definition of what altitude

Answer 33

DENSITY ALTITUDE ## Footnote * If I am flying at 15,000 ft, is the density equivilant to what we expect it to be at 15,000 ft? * The density may be more, or it may be less * Therefore, if the outside density was equivilant to 13,000 ft, despite being at 15,000 ft, the aircraft would perform as though it were at 13,000 ft. 19

Question 34

# Altimetry What is the equation to determine the pressure altitude

Answer 34

PA = Elevation + (( 1013 - QNH) x27 ) ## Footnote **REMEMER** * Pressure altitude is SPS setting, hence 1013 * If QNH was given at airfield as 1009 hPa at 1500ft * PA = 1500 + ((1013 - 1009) x 27) * 1500 + (4 x 27) * 1500 + 108 * ** PA = 1608 ft**

Question 35

# Altimetry What is the equation to determine Density Altitude

Answer 35

DA = PA + (ISA Deviation x 120) ⋉ DA = PA + (Ambient Temp - 15) ## Footnote **REMEMBER** * Pressure altitude is SPS setting of 1013 hPa, therefore a Flight Level * Aircraft at FL100 and OAT -35℃, what is DA * 10,000 + (ISA DEV x 120) * ISA Dev is 15 - (alt x 2) - OAT * 15 - (10 x 2) - -35℃ * 15 - 20 = -5℃ * -5℃ - -35℃ * -30℃ * 10,000 + (ISA DEV x 120) ⋉ 10,000 + (-30℃ x 120) * -3600 ft * DA = 10,000 - 3600 *** 6400 ft** 20

Question 36

# Altimetry What is the equation to determine Density Altitude using an ISA deviation