Pressure Altimetry Question
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Pressure Altimetry Question
Feel like I'm over complicating this question but I'm losing my mind trying to figure it out. Please help!
The elevation of aerodrome P is 330 feet and aerodrome Q elevation is 270 feet. The pilot of an aeroplane at P sets the altimeter to read aerodrome elevation and then flies to Q without resetting the subscale. Aerodrome Pressure (QFE) at P on departure is 1003 hPa. On landing at Q the altimeter reads 240 feet. What is the QNH at Q?
The elevation of aerodrome P is 330 feet and aerodrome Q elevation is 270 feet. The pilot of an aeroplane at P sets the altimeter to read aerodrome elevation and then flies to Q without resetting the subscale. Aerodrome Pressure (QFE) at P on departure is 1003 hPa. On landing at Q the altimeter reads 240 feet. What is the QNH at Q?
Joined: Aug 2018
Posts: 85
Likes: 14
From: With hardship in an old Astra
Apollofield… my apologies for this rather long-winded reply. Hopefully the following background will just be revision, but I am including it for the sake of completeness... in the hope that part of it might help to trigger a useful “light-bulb” moment.
I have deliberately answered the question only in text... as while a diagram might save 1000 words, it will not necessarily be valid in all cases (e.g. when flying towards low pressure instead of flying away from it.) Therefore, I suggest that getting a good grasp of the underlying concepts will lead to more success than by trying to apply a rigid formula or diagram to all circumstances… or by trying to memorise a set of “rules of thumb” and their exceptions.
An altimeter is essentially an aneroid barometer, and directly measures the local air pressure at the static vent on an aircraft. (The selection of this particular location, is intended to remove the adverse pressure effects, which would otherwise occur in a dynamic airflow.)
The air pressure at a specific location is quoted here in hPa (although other units can be found in use elsewhere, e.g. inHg, mmHg, etc.)… and is effectively the accumulated "weight" force exerted on an observer, by the entire mass of air contained within an imaginary vertical column, with a unit cross-sectional area, and which extends vertically above the observer at that location… to the very top of the atmosphere.
If the observer then ascends towards the top of the atmosphere, the height of the imaginary overhead column is reduced, and the total “weight” of the air in that column is then also reduced. Similarly, the observed air pressure will increase, as the imaginary air column becomes taller, in the case of the observer descending in the atmosphere.
The observed local air pressure at a particular point on the Earth’s surface, will change continuously with time… albeit slowly (assuming fairly calm weather conditions). These changes are ultimately the result of accumulated changes in air temperature, due to local variations in the solar energy absorbed by the spinning Earth and its varyingly moist and turbulent atmosphere. As changes in air pressure will generate wind… then the existence of strong winds in a region, will serve as an important warning to the danger of a more rapidly changing air pressure.
When in flight, changes in location over the surface of the Earth will also lead to small variations in air pressure… while the variations observed as a result of any changes in altitude above sea-level, will generally be much more dramatic.
Parameter values for an ideal “standard” atmosphere have been defined, in which a specific variation in the local pressure while descending or ascending in a column of air (extending from sea-level up through the atmosphere), is given equivalence to a specific difference in altitude. A rounded value of 1 hPa change in air pressure per 30 ft change in altitude, is used here.
The absolute air pressure sensed at the static vent, is of relatively little immediate use to a pilot during a flight. However, the altitude above an arbitrary reference elevation is of much greater value… and an altimeter is therefore calibrated in the number of feet above a REFERENCE elevation. However, the reference elevation is entered and updated on the instrument scale, in the form of an equivalent air pressure (as expected in a standard atrmosphere), using either:
1) An air pressure reading taken by an observer at the same reference elevation, and at the same general time and place. (The observer can be external, e.g. in ATC... but can also be the pilot, who would achieve this by adjusting the reference pressure until the altitude is shown as zero feet... i.e. zero feet above the reference pressure corresponding to the current elevation at the current time and place.)
2) The equivalent air pressure, which would be expected at the reference elevation (in an ideal, standard atmosphere)… as calculated from an air pressure reading taken by an observer at a different known elevation... but at the same general time and place.
The two primary reference elevations in general use (disregarding the concept of Flight Levels), are:
1) Sea-level (local to the aerodrome). The air pressure at this reference elevation is referred to as the “QNH”… and will change (usually slowly) with time. (The air pressure at the local sea-level below the aircraft will also change with position, as the distance from the aerodrome increases after departure.)
Note that if the aerodrome is situated far inland, then it might help to visualise excavating away the high ground on which the aerodrome is built, and then dredging a channel all the way to the coast to allow the sea to flood in. The QNH is then the air pressure measured down at the mean water-level in the channel, immediately under where the airfield used to be.
2) Aerodrome elevation. The air pressure at this reference elevation is referred to as the “QFE”… and will also change (again usually slowly) with time. When the airfield is at an elevation which is above mean sea-level, then the QFE pressure setting will be lower than the QNH… as there will be less “weight” of air above the airfield than there will be above a point (lower down) at the sea-level.
The solution to the question requires the use of a common datum. As air pressures will change with time and location, the most sensible datum to use is then the elevation above mean sea-level. (Barring any seismic upheavals, this should remain constant over time at a given location!)
1) The aircraft departs from the aerodrome at P, with an aerodrome elevation of 330 ft indicated on the altimeter. This is the altitude above the elevation corresponding to the reference pressure… so the reference pressure is therefore that expected at a point 330 ft below the aerodrome… i.e. at sea-level.
2) The QFE at P is 1003 hPa. This is the air pressure at the (published) aerodrome elevation.
3) As the aerodrome elevation is 330 ft above sea-level, then the QFE will be 330 / 30 = 11 hPa less than the QNH… so the QNH at P is 1003 + 11 = 1014 hPa. (This higher pressure is the pressure down at the sea-level that would be exposed, if the high ground on which the aerodrome is built had been removed.)
4) On landing at Q, the altitude is indicated as 240 ft. Therefore, the altimeter is 240 ft above the elevation now corresponding (at the new location at Q) to the reference air pressure setting of 1014 hPa (which remains as the altimeter setting, having not been changed during the flight).
5) As the published elevation of the aerodrome is 270 ft above sea-level, then the sea-level must be 270 - 240 = 30 ft BELOW the elevation which corresponds to the reference air pressure.
6) Using the variation with altitude of 1hPa per 30 ft, then the air pressure at sea-level at Q (i.e. the QNH which could be measured directly at sea-level if the high ground on which the aerodrome is built had been removed) is greater than the existing altimeter setting by 30 / 30 = 1 hPa.
7) The QNH at Q is therefore 1014 + 1 = 1015 hPa.
8) As the QNH at Q is higher than the QNH at P, then the aircraft has been flown into a region of higher pressure.
9) Note also (although not part of the original question), that the QFE at Q is 1015 – (270 / 30) = 1006 hPa. This is expected to be different from the QFE at P, as the two aerodromes are:
a. At different elevations above sea-level.
b. At different locations around the Earth’s surface.
c. Having their aerodrome pressures observed at two different times (assuming the flight between them was not short).
Hopefully all this will be of some use...
I have deliberately answered the question only in text... as while a diagram might save 1000 words, it will not necessarily be valid in all cases (e.g. when flying towards low pressure instead of flying away from it.) Therefore, I suggest that getting a good grasp of the underlying concepts will lead to more success than by trying to apply a rigid formula or diagram to all circumstances… or by trying to memorise a set of “rules of thumb” and their exceptions.
An altimeter is essentially an aneroid barometer, and directly measures the local air pressure at the static vent on an aircraft. (The selection of this particular location, is intended to remove the adverse pressure effects, which would otherwise occur in a dynamic airflow.)
The air pressure at a specific location is quoted here in hPa (although other units can be found in use elsewhere, e.g. inHg, mmHg, etc.)… and is effectively the accumulated "weight" force exerted on an observer, by the entire mass of air contained within an imaginary vertical column, with a unit cross-sectional area, and which extends vertically above the observer at that location… to the very top of the atmosphere.
If the observer then ascends towards the top of the atmosphere, the height of the imaginary overhead column is reduced, and the total “weight” of the air in that column is then also reduced. Similarly, the observed air pressure will increase, as the imaginary air column becomes taller, in the case of the observer descending in the atmosphere.
The observed local air pressure at a particular point on the Earth’s surface, will change continuously with time… albeit slowly (assuming fairly calm weather conditions). These changes are ultimately the result of accumulated changes in air temperature, due to local variations in the solar energy absorbed by the spinning Earth and its varyingly moist and turbulent atmosphere. As changes in air pressure will generate wind… then the existence of strong winds in a region, will serve as an important warning to the danger of a more rapidly changing air pressure.
When in flight, changes in location over the surface of the Earth will also lead to small variations in air pressure… while the variations observed as a result of any changes in altitude above sea-level, will generally be much more dramatic.
Parameter values for an ideal “standard” atmosphere have been defined, in which a specific variation in the local pressure while descending or ascending in a column of air (extending from sea-level up through the atmosphere), is given equivalence to a specific difference in altitude. A rounded value of 1 hPa change in air pressure per 30 ft change in altitude, is used here.
The absolute air pressure sensed at the static vent, is of relatively little immediate use to a pilot during a flight. However, the altitude above an arbitrary reference elevation is of much greater value… and an altimeter is therefore calibrated in the number of feet above a REFERENCE elevation. However, the reference elevation is entered and updated on the instrument scale, in the form of an equivalent air pressure (as expected in a standard atrmosphere), using either:
1) An air pressure reading taken by an observer at the same reference elevation, and at the same general time and place. (The observer can be external, e.g. in ATC... but can also be the pilot, who would achieve this by adjusting the reference pressure until the altitude is shown as zero feet... i.e. zero feet above the reference pressure corresponding to the current elevation at the current time and place.)
2) The equivalent air pressure, which would be expected at the reference elevation (in an ideal, standard atmosphere)… as calculated from an air pressure reading taken by an observer at a different known elevation... but at the same general time and place.
The two primary reference elevations in general use (disregarding the concept of Flight Levels), are:
1) Sea-level (local to the aerodrome). The air pressure at this reference elevation is referred to as the “QNH”… and will change (usually slowly) with time. (The air pressure at the local sea-level below the aircraft will also change with position, as the distance from the aerodrome increases after departure.)
Note that if the aerodrome is situated far inland, then it might help to visualise excavating away the high ground on which the aerodrome is built, and then dredging a channel all the way to the coast to allow the sea to flood in. The QNH is then the air pressure measured down at the mean water-level in the channel, immediately under where the airfield used to be.
2) Aerodrome elevation. The air pressure at this reference elevation is referred to as the “QFE”… and will also change (again usually slowly) with time. When the airfield is at an elevation which is above mean sea-level, then the QFE pressure setting will be lower than the QNH… as there will be less “weight” of air above the airfield than there will be above a point (lower down) at the sea-level.
The solution to the question requires the use of a common datum. As air pressures will change with time and location, the most sensible datum to use is then the elevation above mean sea-level. (Barring any seismic upheavals, this should remain constant over time at a given location!)
1) The aircraft departs from the aerodrome at P, with an aerodrome elevation of 330 ft indicated on the altimeter. This is the altitude above the elevation corresponding to the reference pressure… so the reference pressure is therefore that expected at a point 330 ft below the aerodrome… i.e. at sea-level.
2) The QFE at P is 1003 hPa. This is the air pressure at the (published) aerodrome elevation.
3) As the aerodrome elevation is 330 ft above sea-level, then the QFE will be 330 / 30 = 11 hPa less than the QNH… so the QNH at P is 1003 + 11 = 1014 hPa. (This higher pressure is the pressure down at the sea-level that would be exposed, if the high ground on which the aerodrome is built had been removed.)
4) On landing at Q, the altitude is indicated as 240 ft. Therefore, the altimeter is 240 ft above the elevation now corresponding (at the new location at Q) to the reference air pressure setting of 1014 hPa (which remains as the altimeter setting, having not been changed during the flight).
5) As the published elevation of the aerodrome is 270 ft above sea-level, then the sea-level must be 270 - 240 = 30 ft BELOW the elevation which corresponds to the reference air pressure.
6) Using the variation with altitude of 1hPa per 30 ft, then the air pressure at sea-level at Q (i.e. the QNH which could be measured directly at sea-level if the high ground on which the aerodrome is built had been removed) is greater than the existing altimeter setting by 30 / 30 = 1 hPa.
7) The QNH at Q is therefore 1014 + 1 = 1015 hPa.
8) As the QNH at Q is higher than the QNH at P, then the aircraft has been flown into a region of higher pressure.
9) Note also (although not part of the original question), that the QFE at Q is 1015 – (270 / 30) = 1006 hPa. This is expected to be different from the QFE at P, as the two aerodromes are:
a. At different elevations above sea-level.
b. At different locations around the Earth’s surface.
c. Having their aerodrome pressures observed at two different times (assuming the flight between them was not short).
Hopefully all this will be of some use...
Thread Starter
Joined: May 2024
Posts: 20
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From: UK
Apollofield… my apologies for this rather long-winded reply. Hopefully the following background will just be revision, but I am including it for the sake of completeness... in the hope that part of it might help to trigger a useful “light-bulb” moment.
I have deliberately answered the question only in text... as while a diagram might save 1000 words, it will not necessarily be valid in all cases (e.g. when flying towards low pressure instead of flying away from it.) Therefore, I suggest that getting a good grasp of the underlying concepts will lead to more success than by trying to apply a rigid formula or diagram to all circumstances… or by trying to memorise a set of “rules of thumb” and their exceptions.
An altimeter is essentially an aneroid barometer, and directly measures the local air pressure at the static vent on an aircraft. (The selection of this particular location, is intended to remove the adverse pressure effects, which would otherwise occur in a dynamic airflow.)
The air pressure at a specific location is quoted here in hPa (although other units can be found in use elsewhere, e.g. inHg, mmHg, etc.)… and is effectively the accumulated "weight" force exerted on an observer, by the entire mass of air contained within an imaginary vertical column, with a unit cross-sectional area, and which extends vertically above the observer at that location… to the very top of the atmosphere.
If the observer then ascends towards the top of the atmosphere, the height of the imaginary overhead column is reduced, and the total “weight” of the air in that column is then also reduced. Similarly, the observed air pressure will increase, as the imaginary air column becomes taller, in the case of the observer descending in the atmosphere.
The observed local air pressure at a particular point on the Earth’s surface, will change continuously with time… albeit slowly (assuming fairly calm weather conditions). These changes are ultimately the result of accumulated changes in air temperature, due to local variations in the solar energy absorbed by the spinning Earth and its varyingly moist and turbulent atmosphere. As changes in air pressure will generate wind… then the existence of strong winds in a region, will serve as an important warning to the danger of a more rapidly changing air pressure.
When in flight, changes in location over the surface of the Earth will also lead to small variations in air pressure… while the variations observed as a result of any changes in altitude above sea-level, will generally be much more dramatic.
Parameter values for an ideal “standard” atmosphere have been defined, in which a specific variation in the local pressure while descending or ascending in a column of air (extending from sea-level up through the atmosphere), is given equivalence to a specific difference in altitude. A rounded value of 1 hPa change in air pressure per 30 ft change in altitude, is used here.
The absolute air pressure sensed at the static vent, is of relatively little immediate use to a pilot during a flight. However, the altitude above an arbitrary reference elevation is of much greater value… and an altimeter is therefore calibrated in the number of feet above a REFERENCE elevation. However, the reference elevation is entered and updated on the instrument scale, in the form of an equivalent air pressure (as expected in a standard atrmosphere), using either:
1) An air pressure reading taken by an observer at the same reference elevation, and at the same general time and place. (The observer can be external, e.g. in ATC... but can also be the pilot, who would achieve this by adjusting the reference pressure until the altitude is shown as zero feet... i.e. zero feet above the reference pressure corresponding to the current elevation at the current time and place.)
2) The equivalent air pressure, which would be expected at the reference elevation (in an ideal, standard atmosphere)… as calculated from an air pressure reading taken by an observer at a different known elevation... but at the same general time and place.
The two primary reference elevations in general use (disregarding the concept of Flight Levels), are:
1) Sea-level (local to the aerodrome). The air pressure at this reference elevation is referred to as the “QNH”… and will change (usually slowly) with time. (The air pressure at the local sea-level below the aircraft will also change with position, as the distance from the aerodrome increases after departure.)
Note that if the aerodrome is situated far inland, then it might help to visualise excavating away the high ground on which the aerodrome is built, and then dredging a channel all the way to the coast to allow the sea to flood in. The QNH is then the air pressure measured down at the mean water-level in the channel, immediately under where the airfield used to be.
2) Aerodrome elevation. The air pressure at this reference elevation is referred to as the “QFE”… and will also change (again usually slowly) with time. When the airfield is at an elevation which is above mean sea-level, then the QFE pressure setting will be lower than the QNH… as there will be less “weight” of air above the airfield than there will be above a point (lower down) at the sea-level.
The solution to the question requires the use of a common datum. As air pressures will change with time and location, the most sensible datum to use is then the elevation above mean sea-level. (Barring any seismic upheavals, this should remain constant over time at a given location!)
1) The aircraft departs from the aerodrome at P, with an aerodrome elevation of 330 ft indicated on the altimeter. This is the altitude above the elevation corresponding to the reference pressure… so the reference pressure is therefore that expected at a point 330 ft below the aerodrome… i.e. at sea-level.
2) The QFE at P is 1003 hPa. This is the air pressure at the (published) aerodrome elevation.
3) As the aerodrome elevation is 330 ft above sea-level, then the QFE will be 330 / 30 = 11 hPa less than the QNH… so the QNH at P is 1003 + 11 = 1014 hPa. (This higher pressure is the pressure down at the sea-level that would be exposed, if the high ground on which the aerodrome is built had been removed.)
4) On landing at Q, the altitude is indicated as 240 ft. Therefore, the altimeter is 240 ft above the elevation now corresponding (at the new location at Q) to the reference air pressure setting of 1014 hPa (which remains as the altimeter setting, having not been changed during the flight).
5) As the published elevation of the aerodrome is 270 ft above sea-level, then the sea-level must be 270 - 240 = 30 ft BELOW the elevation which corresponds to the reference air pressure.
6) Using the variation with altitude of 1hPa per 30 ft, then the air pressure at sea-level at Q (i.e. the QNH which could be measured directly at sea-level if the high ground on which the aerodrome is built had been removed) is greater than the existing altimeter setting by 30 / 30 = 1 hPa.
7) The QNH at Q is therefore 1014 + 1 = 1015 hPa.
8) As the QNH at Q is higher than the QNH at P, then the aircraft has been flown into a region of higher pressure.
9) Note also (although not part of the original question), that the QFE at Q is 1015 – (270 / 30) = 1006 hPa. This is expected to be different from the QFE at P, as the two aerodromes are:
a. At different elevations above sea-level.
b. At different locations around the Earth’s surface.
c. Having their aerodrome pressures observed at two different times (assuming the flight between them was not short).
Hopefully all this will be of some use...
I have deliberately answered the question only in text... as while a diagram might save 1000 words, it will not necessarily be valid in all cases (e.g. when flying towards low pressure instead of flying away from it.) Therefore, I suggest that getting a good grasp of the underlying concepts will lead to more success than by trying to apply a rigid formula or diagram to all circumstances… or by trying to memorise a set of “rules of thumb” and their exceptions.
An altimeter is essentially an aneroid barometer, and directly measures the local air pressure at the static vent on an aircraft. (The selection of this particular location, is intended to remove the adverse pressure effects, which would otherwise occur in a dynamic airflow.)
The air pressure at a specific location is quoted here in hPa (although other units can be found in use elsewhere, e.g. inHg, mmHg, etc.)… and is effectively the accumulated "weight" force exerted on an observer, by the entire mass of air contained within an imaginary vertical column, with a unit cross-sectional area, and which extends vertically above the observer at that location… to the very top of the atmosphere.
If the observer then ascends towards the top of the atmosphere, the height of the imaginary overhead column is reduced, and the total “weight” of the air in that column is then also reduced. Similarly, the observed air pressure will increase, as the imaginary air column becomes taller, in the case of the observer descending in the atmosphere.
The observed local air pressure at a particular point on the Earth’s surface, will change continuously with time… albeit slowly (assuming fairly calm weather conditions). These changes are ultimately the result of accumulated changes in air temperature, due to local variations in the solar energy absorbed by the spinning Earth and its varyingly moist and turbulent atmosphere. As changes in air pressure will generate wind… then the existence of strong winds in a region, will serve as an important warning to the danger of a more rapidly changing air pressure.
When in flight, changes in location over the surface of the Earth will also lead to small variations in air pressure… while the variations observed as a result of any changes in altitude above sea-level, will generally be much more dramatic.
Parameter values for an ideal “standard” atmosphere have been defined, in which a specific variation in the local pressure while descending or ascending in a column of air (extending from sea-level up through the atmosphere), is given equivalence to a specific difference in altitude. A rounded value of 1 hPa change in air pressure per 30 ft change in altitude, is used here.
The absolute air pressure sensed at the static vent, is of relatively little immediate use to a pilot during a flight. However, the altitude above an arbitrary reference elevation is of much greater value… and an altimeter is therefore calibrated in the number of feet above a REFERENCE elevation. However, the reference elevation is entered and updated on the instrument scale, in the form of an equivalent air pressure (as expected in a standard atrmosphere), using either:
1) An air pressure reading taken by an observer at the same reference elevation, and at the same general time and place. (The observer can be external, e.g. in ATC... but can also be the pilot, who would achieve this by adjusting the reference pressure until the altitude is shown as zero feet... i.e. zero feet above the reference pressure corresponding to the current elevation at the current time and place.)
2) The equivalent air pressure, which would be expected at the reference elevation (in an ideal, standard atmosphere)… as calculated from an air pressure reading taken by an observer at a different known elevation... but at the same general time and place.
The two primary reference elevations in general use (disregarding the concept of Flight Levels), are:
1) Sea-level (local to the aerodrome). The air pressure at this reference elevation is referred to as the “QNH”… and will change (usually slowly) with time. (The air pressure at the local sea-level below the aircraft will also change with position, as the distance from the aerodrome increases after departure.)
Note that if the aerodrome is situated far inland, then it might help to visualise excavating away the high ground on which the aerodrome is built, and then dredging a channel all the way to the coast to allow the sea to flood in. The QNH is then the air pressure measured down at the mean water-level in the channel, immediately under where the airfield used to be.
2) Aerodrome elevation. The air pressure at this reference elevation is referred to as the “QFE”… and will also change (again usually slowly) with time. When the airfield is at an elevation which is above mean sea-level, then the QFE pressure setting will be lower than the QNH… as there will be less “weight” of air above the airfield than there will be above a point (lower down) at the sea-level.
The solution to the question requires the use of a common datum. As air pressures will change with time and location, the most sensible datum to use is then the elevation above mean sea-level. (Barring any seismic upheavals, this should remain constant over time at a given location!)
1) The aircraft departs from the aerodrome at P, with an aerodrome elevation of 330 ft indicated on the altimeter. This is the altitude above the elevation corresponding to the reference pressure… so the reference pressure is therefore that expected at a point 330 ft below the aerodrome… i.e. at sea-level.
2) The QFE at P is 1003 hPa. This is the air pressure at the (published) aerodrome elevation.
3) As the aerodrome elevation is 330 ft above sea-level, then the QFE will be 330 / 30 = 11 hPa less than the QNH… so the QNH at P is 1003 + 11 = 1014 hPa. (This higher pressure is the pressure down at the sea-level that would be exposed, if the high ground on which the aerodrome is built had been removed.)
4) On landing at Q, the altitude is indicated as 240 ft. Therefore, the altimeter is 240 ft above the elevation now corresponding (at the new location at Q) to the reference air pressure setting of 1014 hPa (which remains as the altimeter setting, having not been changed during the flight).
5) As the published elevation of the aerodrome is 270 ft above sea-level, then the sea-level must be 270 - 240 = 30 ft BELOW the elevation which corresponds to the reference air pressure.
6) Using the variation with altitude of 1hPa per 30 ft, then the air pressure at sea-level at Q (i.e. the QNH which could be measured directly at sea-level if the high ground on which the aerodrome is built had been removed) is greater than the existing altimeter setting by 30 / 30 = 1 hPa.
7) The QNH at Q is therefore 1014 + 1 = 1015 hPa.
8) As the QNH at Q is higher than the QNH at P, then the aircraft has been flown into a region of higher pressure.
9) Note also (although not part of the original question), that the QFE at Q is 1015 – (270 / 30) = 1006 hPa. This is expected to be different from the QFE at P, as the two aerodromes are:
a. At different elevations above sea-level.
b. At different locations around the Earth’s surface.
c. Having their aerodrome pressures observed at two different times (assuming the flight between them was not short).
Hopefully all this will be of some use...
Joined: Aug 2018
Posts: 85
Likes: 14
From: With hardship in an old Astra
(I tried to post this further comment about an hour ago… but the original seems to have become lost in the ether. So, at the risk of duplication...)
rudestuff… that would only apply if the aircraft departed from P with the aerodrome QFE set as the reference pressure… in which case the altimeter would have shown zero ft, instead of showing the “aerodrome elevation” (i.e. 330 ft), as specifically mentioned in the original question.
Alternatively, starting from the QFE at P (stated as 1003hPa), then the QFE at Q will be 2 hPa greater due to the aerodrome elevation being 60 ft lower, and a further 1 hPa greater, due to having flown into a region of greater pressure (as established from the earlier analysis of the change in QNH). Hence the QFE at Q is 1003 + 2 + 1 = 1006 hPa… rather than your figure of 1004 hPa. (However, the original question only asks for the QNH at Q.)
apollofield… you’re very welcome, and good luck with the rest of the course.
Perhaps I should also tighten-up on my earlier terminology, in case I have introduced some unnecessary confusion. The datum which I am suggesting to use, is of course the mean sea-level (which is a fixed vertical reference), and not (as I had it typed) “the elevation above mean sea-level”… which is a variable quantity which is then derived in relation to the fixed datum, and which will obviously change as the aeroplane climbs and descends during the flight.
rudestuff… that would only apply if the aircraft departed from P with the aerodrome QFE set as the reference pressure… in which case the altimeter would have shown zero ft, instead of showing the “aerodrome elevation” (i.e. 330 ft), as specifically mentioned in the original question.
Alternatively, starting from the QFE at P (stated as 1003hPa), then the QFE at Q will be 2 hPa greater due to the aerodrome elevation being 60 ft lower, and a further 1 hPa greater, due to having flown into a region of greater pressure (as established from the earlier analysis of the change in QNH). Hence the QFE at Q is 1003 + 2 + 1 = 1006 hPa… rather than your figure of 1004 hPa. (However, the original question only asks for the QNH at Q.)
apollofield… you’re very welcome, and good luck with the rest of the course.
Perhaps I should also tighten-up on my earlier terminology, in case I have introduced some unnecessary confusion. The datum which I am suggesting to use, is of course the mean sea-level (which is a fixed vertical reference), and not (as I had it typed) “the elevation above mean sea-level”… which is a variable quantity which is then derived in relation to the fixed datum, and which will obviously change as the aeroplane climbs and descends during the flight.
