Cathay Question - Can any do this one?
Thread Starter
Join Date: Jan 2000
Location: England
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
Cathay Question - Can anyone do this?
1. Given an indicated altitude of 10,000ft, and an actual OAT of -20oC, you set your altimeter to a local station setting of 29.62”. If the station elevation is 2,500ft, what is your actual altitude?
9,250ft
9,550ft
9,850ft
Wondered if anyone knows how to do this one? I've run out of brain cells?
Cheers
9,250ft
9,550ft
9,850ft
Wondered if anyone knows how to do this one? I've run out of brain cells?
Cheers
Last edited by Katanaman; 21st Sep 2007 at 14:28. Reason: spelling mistake
Only half a speed-brake
With the risk of looking stupid, I give it a try.
ISA at 10k should be -5°C. So it is a ISA - 15 situation.
What is the danger? Because of the colder air mass, different presure levels are "squeezed" closer to the ground than would normally be expected. Therefore, although your altimeter displays pressure level (that's what altimeters do) normally assiociated with alttitude of 10 000 ft, you are in fact lower than indicated.
If 10000 is any sort of minimum altitude, this must not happen. For cold weather operations, it is an every day practice to compensate for negative ISA deviations based on actual ISA dev and height above pressure reporting station.
I think that the following question is equivalent:
Pilot at height of 7000 ft above station applies temperature correction for ISA -15°C. Then he comes to a consclusion, that in order to maintain the prescribed minimum safe altitude, he needs to fly not below 10 000 ft as indicated on his altimeter. What is the minimum altitude he is trying to observe:
a)... b)... c)...
Of course normal use is reversed, i.e. MNM Alt is known, so is your height and ISA dev, you look into a compensation chart and find out the minimum indicated altimeter value you need to keep.
Does it make any sense?
FD.
ISA at 10k should be -5°C. So it is a ISA - 15 situation.
What is the danger? Because of the colder air mass, different presure levels are "squeezed" closer to the ground than would normally be expected. Therefore, although your altimeter displays pressure level (that's what altimeters do) normally assiociated with alttitude of 10 000 ft, you are in fact lower than indicated.
If 10000 is any sort of minimum altitude, this must not happen. For cold weather operations, it is an every day practice to compensate for negative ISA deviations based on actual ISA dev and height above pressure reporting station.
I think that the following question is equivalent:
Pilot at height of 7000 ft above station applies temperature correction for ISA -15°C. Then he comes to a consclusion, that in order to maintain the prescribed minimum safe altitude, he needs to fly not below 10 000 ft as indicated on his altimeter. What is the minimum altitude he is trying to observe:
a)... b)... c)...
Of course normal use is reversed, i.e. MNM Alt is known, so is your height and ISA dev, you look into a compensation chart and find out the minimum indicated altimeter value you need to keep.
Does it make any sense?
FD.
Last edited by FlightDetent; 21st Sep 2007 at 14:22. Reason: 7500 changed to 7000 to be more exact and a second set of typos
well he is using QFE isn't he? station level pressure? right?
So if his Altimeter is showing 10,000', then he is 10,000 above the station, which is 2,500' above MSL. So without taking the cold temp into account isn't he 12,500 above MSL.
From the Cathay manual: "published procedural altitudes, including MSA's, should be corrected using the table below"
ISA add 0%
<ISA-15 add 10%
<ISA-30 add 20%
The temp at 12,500' should be about -10c, so we have a deviation of ISA-10
Therefore take 0% off 12,500' and he is close to 12,500' above MSL.
Also don't forget when we say "altitude" we generally mean above MSL
When we want to talk about above the ground we say "height"
Another clue.
So if his Altimeter is showing 10,000', then he is 10,000 above the station, which is 2,500' above MSL. So without taking the cold temp into account isn't he 12,500 above MSL.
From the Cathay manual: "published procedural altitudes, including MSA's, should be corrected using the table below"
ISA add 0%
<ISA-15 add 10%
<ISA-30 add 20%
The temp at 12,500' should be about -10c, so we have a deviation of ISA-10
Therefore take 0% off 12,500' and he is close to 12,500' above MSL.
Also don't forget when we say "altitude" we generally mean above MSL
When we want to talk about above the ground we say "height"
Another clue.
Last edited by ACMS; 21st Sep 2007 at 16:00.
really?
Station level pressure is QFE pal. Always has been and always will be.
If you want above MSL then use the term QNH.
He used the term "local station setting" to which I see him meaning QFE
Now if he means local QNH, then the answer is different, and there would have been absolutely no point mentioning elevation of the ground or the pressure setting would there Mr expert.( apart from injecting a red herring )
Station level pressure is QFE pal. Always has been and always will be.
If you want above MSL then use the term QNH.
He used the term "local station setting" to which I see him meaning QFE
Now if he means local QNH, then the answer is different, and there would have been absolutely no point mentioning elevation of the ground or the pressure setting would there Mr expert.( apart from injecting a red herring )
Join Date: May 2005
Location: F370
Posts: 199
Likes: 0
Received 0 Likes
on
0 Posts
Temperature Correction
Guys,
Ignore the altimeter setting, it is the local setting and not introducing any errors.
When making temperature corrections, you need to correct the height [B]of the column of air[B]. In this case the elevation is 2500' and you are at 10,000' so the column is 7500' tall(ISA). Using Cathay's 10% correction for the actual ISA-15, that means that the true height of the column is 10% less = 6750'. Adding the elevation back on means that your true altitude is 9250'.
Ignore the altimeter setting, it is the local setting and not introducing any errors.
When making temperature corrections, you need to correct the height [B]of the column of air[B]. In this case the elevation is 2500' and you are at 10,000' so the column is 7500' tall(ISA). Using Cathay's 10% correction for the actual ISA-15, that means that the true height of the column is 10% less = 6750'. Adding the elevation back on means that your true altitude is 9250'.
Join Date: Apr 2000
Location: 30 West
Age: 65
Posts: 926
Likes: 0
Received 0 Likes
on
0 Posts
Who Cares ?
Work for an airline that has more to it than asking questions from the Ark !
We all did these once, passed them and got a licence.
If CRM has any meaning whatsoever, the days of questions like this should be confined to the Squadron archive.
Q1 Do you have a valid licence
Q2 Do you have a reasonably rounded CV
Q3 Are you someone who we could spend 8 hours in a cockpit with, then retire to the bar with
Q4 When can you start
I thank you
Work for an airline that has more to it than asking questions from the Ark !
We all did these once, passed them and got a licence.
If CRM has any meaning whatsoever, the days of questions like this should be confined to the Squadron archive.
Q1 Do you have a valid licence
Q2 Do you have a reasonably rounded CV
Q3 Are you someone who we could spend 8 hours in a cockpit with, then retire to the bar with
Q4 When can you start
I thank you
All major Airlines ask some pretty crazy questions but the info is out there for those that do the research. If I asked that question and the guy had no idea I'd not form a very good opinion of his desire to join my airline. It's not necessarily the question but whether you have bothered to do the research.
That's one aspect they're looking for.
And besides, it is important information to use in cold weather. I hope I never fly into Fairbanks with you in bad wx and fly into a hill.
cheers
That's one aspect they're looking for.
And besides, it is important information to use in cold weather. I hope I never fly into Fairbanks with you in bad wx and fly into a hill.
cheers
Only half a speed-brake
Station level pressure is QFE pal. Always has been and always will be.
If you want above MSL then use the term QNH.
He used the term "local station setting" to which I see him meaning QFE
Now if he means local QNH, then the answer is different, and there would have been absolutely no point mentioning elevation of the ground or the pressure setting would there
If you want above MSL then use the term QNH.
He used the term "local station setting" to which I see him meaning QFE
Now if he means local QNH, then the answer is different, and there would have been absolutely no point mentioning elevation of the ground or the pressure setting would there
The question is
a) a height-altitude excercise
b) ISA correction exercise
c) combined
I heard that CX (apparent author of the question) uses QFE. Do not know if it is true, but I assume so. That's why I initially considered "local station setting" which is a rather vague description to mean QFE. But again, it reads "given indicated altitude 10k you set 29,62" this would make little sense to me, as QFE read-outs are definitely height. Also, 10k on QFE of station that is elevated 2500 would have you in altitude 12k-ish as you calculate, which comes nowhere near the answers suggested.
Hence my second assumption, that the only option available is for "local station setting" not to mean "local pressure readout" (the numbers just do not match the answers) but QNH instead which is the common practice at many places. Station elevation is indeed required because ISA dev correction is applied only to the altitude difference between reporting point and the user (=height).
This all certainly brings back JAA ATPL memories where precise answers are often required on loosely defined data. For all I know the author perhaps meant "your INDICATED ALT is 10 k (on some QNH obviously) then you reset altimeter to OFE of 29,62 what is your real ALT"
The bad taste is still there. 
Ironically, even if the situation is analysed, we still have different formulae that give different answers. Thanks heavens my company provides a compensation table constructed by trained proffesionals and does not expose me to the peril of pilot calculations.
FD
Join Date: Nov 2001
Location: NY
Posts: 277
Likes: 0
Received 0 Likes
on
0 Posts
Relationship of true and calibrated (indicated) altitude:
TA= CA + (CA-FE)*(ISADEV)/(273+OAT)
where
TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
CA= Calibrated altitude= Altitude indicated by altimeter when set to the
altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air
column between the station and the aircraft (in C)
OAT= Outside air temperature (at altitude)
The above is more precise than provided by the E6B or similar.
i think this is the formula i used. it works out if it is
TA= CA + (CA-FE)*(ISADEV)/(273+OAT)
where
TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
CA= Calibrated altitude= Altitude indicated by altimeter when set to the
altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air
column between the station and the aircraft (in C)
OAT= Outside air temperature (at altitude)
The above is more precise than provided by the E6B or similar.
i think this is the formula i used. it works out if it is
FLIGHT DETENT: Ahh No mate. WE NEVER EVER USE QFE, WE ARE REQ'D TO USE QNH
Although quite a few places have both QNH and QFE on their ATIS. ( MNL is one place )
Station level pressure is QFE, so the question is really quite simple. ( i think anyway )
The Altimeter indicates the number of feet above whatever subscale setting you have. If it's QFE then it tells you how many feet you are above that pressure level. 30' per hpa is the ISA rule.
The wording in the question needs to be clarified by the original poster.
Then we'll all know
Yes thank hevens we don't need to use this crap in CX either.
We only need to remember to correct if it's damn cold outside, and we have a nice reference table in our VOL 2 Part 2 for that info.
Although quite a few places have both QNH and QFE on their ATIS. ( MNL is one place )
Station level pressure is QFE, so the question is really quite simple. ( i think anyway )
The Altimeter indicates the number of feet above whatever subscale setting you have. If it's QFE then it tells you how many feet you are above that pressure level. 30' per hpa is the ISA rule.
The wording in the question needs to be clarified by the original poster.
Then we'll all know
Yes thank hevens we don't need to use this crap in CX either.
We only need to remember to correct if it's damn cold outside, and we have a nice reference table in our VOL 2 Part 2 for that info.
Guys it is NOT QFE. Using a bit of common sense will tell you that.
1. The words used are "local station setting." This is more likely to mean QNH adjusted to read correctly on the ground at that airfield. Local QNH is adjusted in this way so that pressure altitude is correct relative to the aerodrome. This is in contrast to an area QNH setting.
2. The "local station setting" given is 29.62". This converts to about 1003 hPa (what I'm used to using.) Which, if it was QFE, gives a QNH of about 1086 hPa. This is well outside the range of normal.
3. The question talks about altitude. Altitude is measured AMSL not above ground.
4. The available answers are close to the result you get assuming QNH, not QFE. If you get an answer of around 12,000' and the available answers are between 9,000' and 10,000' then it is likely you've got something wrong. In this case it is assuming QFE is used.
Posters above say that Cathay like you to use 5% per 10 degrees ISA deviation. I leanred to use 4% in my ATPL but the results are close enough for it to not matter.
10,000 - 2,500 = 7,500 difference between your altitude and the accurate "local station" altitude
ISA is -15
Using 4%/10 ISA difference gives 6%
It is cold therefore the altimeter over-reads so we take 6% off 7500 (or multiply by 0.94.)
7500 * 94% = 7050
7050 + 2500 = Actual altitude of 9550.
So answer is B.
Using 5% per 10 ISA deviation then the percentage difference becomes 7.5%
7500 * 92.5% = 6937.5
6937.5 + 2500 = 9437.5
So the answer is still B.
1. The words used are "local station setting." This is more likely to mean QNH adjusted to read correctly on the ground at that airfield. Local QNH is adjusted in this way so that pressure altitude is correct relative to the aerodrome. This is in contrast to an area QNH setting.
2. The "local station setting" given is 29.62". This converts to about 1003 hPa (what I'm used to using.) Which, if it was QFE, gives a QNH of about 1086 hPa. This is well outside the range of normal.
3. The question talks about altitude. Altitude is measured AMSL not above ground.
4. The available answers are close to the result you get assuming QNH, not QFE. If you get an answer of around 12,000' and the available answers are between 9,000' and 10,000' then it is likely you've got something wrong. In this case it is assuming QFE is used.
Posters above say that Cathay like you to use 5% per 10 degrees ISA deviation. I leanred to use 4% in my ATPL but the results are close enough for it to not matter.
10,000 - 2,500 = 7,500 difference between your altitude and the accurate "local station" altitude
ISA is -15
Using 4%/10 ISA difference gives 6%
It is cold therefore the altimeter over-reads so we take 6% off 7500 (or multiply by 0.94.)
7500 * 94% = 7050
7050 + 2500 = Actual altitude of 9550.
So answer is B.
Using 5% per 10 ISA deviation then the percentage difference becomes 7.5%
7500 * 92.5% = 6937.5
6937.5 + 2500 = 9437.5
So the answer is still B.
Only half a speed-brake
Howdy
I dare to disagree. If the Q stated "pressure indicated at station level" as you propose, there would be little to quarrel about. Unfortunately it does not (I think anyway). It says "local station setting" and that is rather open to discussion once we agree to the fact that all of the results assuming QFE are way off from the answers available.
Exactly! Though I am afraid that Katanamaran is just quoting a study-book. Of which we need the editor to give him a good engine wash-up for not being up to the task (academically-wise) and sawing havoc in the inspiring minds of his readers.
FD.
Exactly! Though I am afraid that Katanamaran is just quoting a study-book. Of which we need the editor to give him a good engine wash-up for not being up to the task (academically-wise) and sawing havoc in the inspiring minds of his readers.
FD.
Yes I tend to agree it must mean QNH, I guess "local station setting" means QNH. On first look it sounded way to close to QFE.
Thank god I'm already employed, I'm not sure I'd pass the tech crap now
cheers
Thank god I'm already employed, I'm not sure I'd pass the tech crap now
cheers
short flights long nights
Is it actually important.???? I agree with ACMS..there is no way I could get thru this now..I am glad I have a good job!!!!
SpaceRanger
Join Date: May 1999
Location: Samsonite
Posts: 213
Likes: 0
Received 0 Likes
on
0 Posts
Answer
This is what it is all about.
Altimeters are pressure sensors, calibrated with a feet scale according to ISA scale.
As mentioned in another reply, if it is colder than ISA, the isobars will be closer, resulting in your being lower than what your altimeter reads. Someone heard the saying From high to low, lookout below ? This goes for both low pressure (if flying on 1013) as well as for temperature, both for QNH/1013 (QNE).
If the station is at the same level as yourself, there will be no correction, but as the station is below you, your altitude is only correct at the station level. As mentioned above, your height will deviate with 4% for each 10 degree ISA deviation. The ISA temp is around -5 at FL100 (-4.8 if you want to be anal), and since they say it is -20, you are 15 below ISA. These 15 degrees have to be corrected for the air between the aircraft and the station, in this case 7500 feet. So the calculation is
10000 - 0.06 x 7500 = 9550'
It is confusing to many - was to me for a long time as well - but if you read my explanation above and supply it with sketches of various scenarios, you should be able to see the logic.
If flying FL, remember to correct for QNH also, as well as up to 2000 feet for mountaineous areas, the latter, if we are discussing miminum safe altitudes.
Regarding the QFE/QNH questions raised above ... first of all, at 2500 feet, it is highly unlikely a QNH, as a QFE around 2500 feet should be in the order of 27½ inches, +/- local pressure. But, whether or not it is QNH or QFE does not matter in this question, as the QNH is irrelevant in this case. If the question stated you were in FL100/flying on QNE (which is 1013/29.92), then you would have to correct for it. High to low, look out below.
Altimeters are pressure sensors, calibrated with a feet scale according to ISA scale.
As mentioned in another reply, if it is colder than ISA, the isobars will be closer, resulting in your being lower than what your altimeter reads. Someone heard the saying From high to low, lookout below ? This goes for both low pressure (if flying on 1013) as well as for temperature, both for QNH/1013 (QNE).
If the station is at the same level as yourself, there will be no correction, but as the station is below you, your altitude is only correct at the station level. As mentioned above, your height will deviate with 4% for each 10 degree ISA deviation. The ISA temp is around -5 at FL100 (-4.8 if you want to be anal), and since they say it is -20, you are 15 below ISA. These 15 degrees have to be corrected for the air between the aircraft and the station, in this case 7500 feet. So the calculation is
10000 - 0.06 x 7500 = 9550'
It is confusing to many - was to me for a long time as well - but if you read my explanation above and supply it with sketches of various scenarios, you should be able to see the logic.
If flying FL, remember to correct for QNH also, as well as up to 2000 feet for mountaineous areas, the latter, if we are discussing miminum safe altitudes.
Regarding the QFE/QNH questions raised above ... first of all, at 2500 feet, it is highly unlikely a QNH, as a QFE around 2500 feet should be in the order of 27½ inches, +/- local pressure. But, whether or not it is QNH or QFE does not matter in this question, as the QNH is irrelevant in this case. If the question stated you were in FL100/flying on QNE (which is 1013/29.92), then you would have to correct for it. High to low, look out below.
Last edited by TheDrop; 23rd Sep 2007 at 07:30.
Join Date: Dec 2000
Location: very close to STN!!
Posts: 523
Likes: 0
Received 0 Likes
on
0 Posts
funny how the yanks manage---
most flights in and out of fairbanks are by yanks, whose ATP exam, funnily enough, does not consist of 14 separate exams-primarily designed to pay for the BMW's of the school administrators and test creators.
"from high to low, look out below"
one then assumes that the pilot should be able to discern whether the "low" is pressure or temperature.
pilot's memory should serve well enough to remember the last walk around and whether the big coat was required or not---
if coat was required--then cold temp is the factor.
reach for handy chart and convert--
really not rocket science like some are trying to make it appear.
and i've seen in -45 there. and when you are that far down the scale, it doesn't really matter which scale you are using--C or F.
"from high to low, look out below"
one then assumes that the pilot should be able to discern whether the "low" is pressure or temperature.
pilot's memory should serve well enough to remember the last walk around and whether the big coat was required or not---
if coat was required--then cold temp is the factor.
reach for handy chart and convert--
really not rocket science like some are trying to make it appear.
and i've seen in -45 there. and when you are that far down the scale, it doesn't really matter which scale you are using--C or F.