02. Altimetry Flashcards
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
QFE
1
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
QNH
1
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
STANDARD PRESSURE
1
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
QFF
2
Altimetry
The RPS pressure setting is given to pilots based on what 2 parameters
RPS - Regional Pressure Setting
- LOWEST forecast for region
- 1 HOUR AHEAD
2
Altimetry
What does ASR and RPS stand for
- ALTIMETER SETTING REGION
- REGIONAL PRESSURE SETTING
2
Altimetry
Airfield QFE pressure is given from what point
HIGHEST POINT ON AIRFIELD SURFACE
2
Altimetry
If the touchdown point of a runway is significantly below the airfield QFE, what may be issued to pilots
TOUCHDOWN QFE
2
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
QNE
3
Altimetry
For stations ABOVE sea level;
- If temperature is warmer than ISA, then QNH is MORE or LESS than QFF
- If temperature is COLDER than ISA, then QNH is MORE or LESS than QFF
- MORE
- LESS
Altimetry
For stations BELOW sea level;
- If temperature is warmer than ISA, then QNH is MORE or LESS than QFF
- If temperature is COLDER than ISA, then QNH is MORE or LESS than QFF
- LESS
- MORE
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
INCREASED
Altimetry
When flying high pressure to low pressure, what will the altimeter read
OVER READ
Lower than you think you are
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
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
Altimetry
An aircraft is at FL40. What is the AMSL altitude if QNH is 998 hPa
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
9
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
TEMPERATURE ERROR
10
Altimetry
If the OAT is warmer than ISA, the true altitude will be MORE or LESS than indicated altitude
MORE
TRUE > indicated
Altimetry
If the OAT is colder than ISA, the true altitude will be MORE or LESS than indicated altitude
Altimetry
What is the equation for working out temperature error correction
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
11
Altimetry
You are flying at FL200. OAT is -40℃ and the pressure at sea level is 1033 hPa. What is the true altitude
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
Altimetry
QNH is calculated from station pressure, so therefore assumes what conditions.
ISA
13
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
QNH and QFF WILL DIFFER
Altimetry
What is the equation to determine QFE from QNH
If QFE is 1001 at 200 ft, what is QNH
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
Altimetry
What is the equation to determine QFE from QNH
If QNH is 1008 at an airfield elevation of 200 ft, what is QFE
QFE = QNH - (ELEVATION / 27 )
If QFE is 1001 at 200 ft, what is QNH
* 1008 + (200 / 27)
* 1008 - 7
* QNH = 1001 hPa
15
Altimetry
If you fly from a higher temperature to a low temperature, the altimeter will OVER READ or UNDER READ
OVER READ
Altimetry
If you fly from a low temperature to a high temperature, the altimeter will OVER READ or UNDER READ
UNDER READ
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 to determine the correct indicated altitude of decision height.
4240 ft
- 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
Altimetry
In the Northern Hemisphere, what direction does the wind flow in a low pressure system
ANTI-CLOCKWISE
17
Altimetry
In the Northern Hemisphere, what direction does the wind flow in a high pressure system
CLOCKWISE
17
Altimetry
An aircraft travelling from a high pressure system into a low pressure system in the Northern Hemisphere will experience drift in which direction
STARBOARD or RIGHT
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*
17
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
FL160
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
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
- BERNOULLI EFFECT
- DROP IN PRESSURE
- 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
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
DENSITY ALTITUDE
- 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
Altimetry
What is the equation to determine the pressure altitude
PA = Elevation + (( 1013 - QNH) x27 )
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**
Altimetry
What is the equation to determine Density Altitude
DA = PA + (ISA Deviation x 120)
⋉
DA = PA + (Ambient Temp - 15)
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
Altimetry
What is the equation to determine Density Altitude using an ISA deviation
