After reading my instruction manual 4 or 5 times and scratching my head, I found your video and the light came on. Not only did I understand what I was looking for which was how and why temperature and pressure affect the altimeter, but I accidentally came to clearly understand the reasoning and effect of setting the altimeter to 29.92 above 18,000 feet. Thank you.
Excellent explanation, being a low time VFR pilot its a challenge to retain all the information necessary to feel and be safe, This will certainly help me.. Thanks for the post.
As always, love the videos, they’ve certainly helped me with my IFR training. Not just getting an answer correct, but understanding WHY the answer is correct is so much more important for long term memory.
The best way to find true altitude is with a measuring tape. What to do when flying ?.In practice approx pressure estimates are used in the sub scale of the altimeter. The altimeter reads the height above that setting . When using flight levels above 18000 feet in USA or 10000 feet in Australia, the same pressure setting is always used, 1013 mb or 29.92 in/Hg in the sub scale of the altimeter. .No matter what the real pressure, aircraft will all clear mountains when using flight levels.
I see this video was posted 8 years ago, and you answered a question just one month ago. That is awesome! Your video is excellent, as is your teaching style. Very well done!
OMG you are a genius! I was so confused on the "low to high clear blah blah" stuff that it seemed I'd never get it. This is such a logical explanation, thanks a million!
+reza farivar We at passfaaexams.com/ always find a way to explain concepts at the Correlation level rather than the Rote. Understanding Altimeters by correlating them to our brain's perceptions is definitely a better way of learning how they operate.
very good, studying for the EASA ATPL 15yr pilot and had a brain fart, this was great to get me back on track, you never stop learning in this game. you sound like click and clack!
Given: Aircraft at FL150 overheard an airport. Elevation of airport 720 FT. QNB 1003 hPa, OAT at FL 150 is -5 degrees Celsius. What is the true altitude of the aircraft (Assume 1hPa = 27FT)
Great explanation.. and maybe I'm thinking about it too much. But if field elevation is 650 feet above sea level, and pressure drops with altitude, wouldn't the field pressure be lower than 29.92 and not higher (30.57)? Or should I just assume there is a high pressure weather system rolling through, or it's a hot day? It may seem like a smart-ass question, but I'm really just trying to wrap my brain around it! Thanks!
Beware at 4:15...he mistakenly says standard pressure is 27.92 In-Hg ....the correct Standard Pressure value is 29.92 In-Hg. He may have just been going too fast.
Many of our customers come to us after having already purchased some other material and all of them tell us the exact same thing: "Wish I had this explanation first." passfaaexams.com/
I struggled with understanding the physics behind the temperature one too. Here's what I came to: Cold air is heavier, denser, and slower moving, so it all sinks to the ground and drops off very quickly as you rise. Hot air is lighter with more energy, so it is able to climb higher and spread out more. So the difference in pressure between 100 feet of hot air is covered in say 90 feet of cold air. So in this example, when your altimeter says 1000 agl, you're really at 900. (Actually I doubt it would be linear like that, I'm pretty curious if anyone knows)
Temp mainly changes near ground level only a small percentage of air, higher altitude its more predictable and constant its about 2000 FT were air can be unpredictable, altimeter works by measuring the total weight of air above you, these differences in temp are so minutely small its not worth calculating, and max allowable altimeter error is +/-75 feet so if your flying with a friend and you had +75 and he had -75 error thats 150 ft error between you too
cold air contracts .. and as the whole column is sitting on the surface, the "typical" pressure /altitude curve shrinks towards the ground. now if you're in the same physical elevation, the ambient air is all of sudden thinner, i.e. altimeter reads higher. aircraft don't have this luxury - they are flown by the pilot, who looks at the arrows and maintains the apparently steady altitude / flight level. however, this level is now CLOSER to the all the layers below. hence, you are descending, relative to the zero level, while maintaining your precious isobar line (vertically, rather than horizontally). maintaining the same elevation, cooling air means rising altimeter. maintaining steady altimeter readout means you fly closer to the ground.
Da se visinomjer namjesti na 0=15 C stupnjeva kao pocetna temperatura? 0 je prava visina=15 C stupnjeva je pocetna temperatura? Svakih 333,3 metara temperatura pada za 1 C stupanj? 😀
I thought low pressure was warm air and high pressure was cold air. So why would it not be you thought u were lower in altitude when you go to low pressure area?
I got the hot to cold and high to low, i just didn't understand why the indicated altitude increased as you increased the setting in the kollsman window, thanks for clarifying.
Lets say you are flying at 1000 feet with a standard pressure of 28.92 the altimeter correctly reads 1000 feet. now you fly into an area with the pressure at 29.92 without resetting the altimeter. the altimeter would read 0 as the pressure has increased and the altimeter is fooled into thinking that you are closer to the ground. If you now reset the altimeter to the correct pressure of 29.92 the altimeter would read 1000 feet correctly again. Hope this helps.
Hello, please help me to solve step by step the exercise: "Pressure altitude 10000 ft, Temp -30 C. What is the True Altitude?". Am not interested in a solution like: "Ok, on your Pathfinder computer put the PA and the Temp and ask for the result". I work with a Jeppesen Techstar but it asks to put the Corrected Altitude in order to give me the True Altitude. What to do in order to find the CA?
@@xXivanhornetXx Well we sell the software all over the world as it is not based on question and answer memorization but rather on teaching you the material and that does not change no matter whether you live in America or Europe. We have many UK and German customers.
Please...there is a science to all of this. Please think about this... Altimeters measure air density, they are NOT made to measure temperatures. Air is more dense near the ground and become less with higher altitudes. Right? 1. Cold air is MORE dense than hot air....that also means that warm air is LESS dense than cold air. 2. Dry air is MORE dense than hot air...that also means that humid air is LESS dense than cold air. Try to take off on a hot and humid day. The heat does not mean your closer to the ground, it means the air is less dense and you have a high density altitude. Really! If you are measuring air density (altimeter) and increase the density, the altimeter will register a lower altitude. Why? The air is more dense. The saying is true when you fly from a high to low PRESSURE areas. BUT flying from a warm (less dense) into cold air (more dense) DOES NOT MAKE THE ALTIMETER RISE! That is wrong! If you don't believe me, put the numbers in a E6B and try it out. You'll see that higher temperatures make for higher density altitudes. Likewise, cold air does not make the altimeter rise. Cold and dry air makes for the lowest density altitude.
Sir do we have to keep adjusting our altimeter setting along flying into an area of low/high pressure in order to maintain our true alititude or there is another method to do so our indication would be true ? Because as explaned above on the video a long you are flying into an area of cold weather the pressure decreases gradually and the altimeter reads higher (obvious) , how can we make the altimter indicates true altitude along flying into an area of hot/cold weather ? Thank you sir for your efforts
no you would not When Flying you fly at say FL250 25,000 ft you would have your altimiter at MSL 29.92 In-Hg or 1013.2 hPa STD Setting So you would never change your setting during cruise as this would be dangerous to you and other aircraft flying near by as those settings are Standard and all aircraft uses it as you would not want to have a mid air collision because you had a different altimeter setting, your true altitude changes constantly as you move into low/high pressure but your Indicated Alt would stay the same, only time you change it is when coming in to land or take off or if you need to calculate altitude of a obstacle like a mountain and this can be done if you have the current local pressure of Sea Level, Normally Temp is not really calculated as it is not changing your readings much only a few feet and also allowable altimeter error is +/-75 feet so your altitude could be off by 70 feet or could be 150 feet different from another guy nearby
Yes you should ALWAYS change your altimeter when flying from low to high pressure or vise versa. That's why you get the weather and altimeter settings when flying from one airport to another. If you do not adjust for the pressure change, your altimeter will show you to be at a different altitude than you TRULY are.
Great video - thank you!! The only thing that was not clear to me without additional research was that in the US altimeters are set to Standard Datum Pressure (29.92'' Hg) when they reach FL18 and thereafter (therealoft? :) ). Above that they fly at assigned PRESSURE ALTITUDES and not TRUE ALTITUDES (altitude above MSL). If this is true, that would be necessary to know to solve the last problem in your video. Without it, I did not have an initial point of reference and could not figure it out without advancing the video. You did eventually mention that "an airplane flying @ FL29 would have an altimeter setting of 29.92. It also flies at PRESSURE ALTITUDE." That cleared things up for this particular problem, but did not provide the explanation why it was set to 29.92. For a beginner such as myself, that would have been useful information. I hope you don't take my comment as criticism but rather as feedback, because, as other people have stated before me, your video does clear things up a lot. So thanks again, and fly safe!
Sorry, after re-watching it for the fourth time, I did finally catch the "if you were following procedure" note in the example preceding the last problem, but again, it was a rather general comment, and I still think that it would make things a little more clear if you stated that FL18 is the altitude where you do it. Maybe I'm just slow... :)
why do small planes deep-in-free-fall when they travel from high Plato to a low-land/gorge area ?? Why doesn't it fly over it without that pain ? Anyone, please explain . .
what I don't understand when we fly from hot to cold WX as I know we fly from low pressure to high pressure , when I read in Jeppesen it said that when we fly from hot to cold its that we fly from high pressure to low pressure . what I know is that cold weather has more dense which is more pressure than hot weather ?!
Please somebody help me understand. It is supposed that the colder air is more dense. Should not the altimeter read lower when flying into lower temperature, since the air is colder, therefore denser? I don't understand
isnt it dangerous not to know the true altitude? isn't it risky to have to do these calculations in flight? why don't they have an instrument that simply measures the barometric pressure surrounding the airplane as it changes and keeps changing? This video definitely explains but why all of this in the first place??
zane forbes your altimeter would indicate a higher altitude than your true altitude. However, temperature does not effect the altimeter as much as the pressure does and thus, you should understand the relationship between pressure, temperature and density. :)
That all makes sense until you consider density altitude where a higher temperature results in the airplane thinking that it is at a higher altitude. In that case wouldn't going from cold to hot make you tend to fly the plane lower and so you would again need to look out below?
I think you might be a little confused sir... Density altitude has nothing to do with how an altimeter reads. It affects airplane performance not altimeter performance and this video discusses how an altimeter reacts to temperature and pressure changes not how the airplane reacts.
I am indeed a little confused... Let's see now. If an airplane is parked at a high elevation airport and I'm watching the altimeter from dawn until noon of a what is becoming a very hot day, the indicated altitude will progressively LOWER as the outside air temperature increases because the airplane, like me, would be thinking it is going down to a lower altitude where the air is warmer. But at the same time, the performance of the airplane would diminish due to the less dense warmer air.
The is the problem: "because the airplane, like me, would be thinking it is going down to a lower altitude" . The Altimeter, like you, would be thinking it is going down to a lower altitude, not the airplane.
Given: Aircraft at FL150 overheard an airport. Elevation of airport 720 FT. QNH 1003 hPa, OAT at FL 150 is -5 degrees Celsius. What is the true altitude of the aircraft (Assume 1hPa = 27FT)
Your idea is awsome, new way of learning
After reading my instruction manual 4 or 5 times and scratching my head, I found your video and the light came on. Not only did I understand what I was looking for which was how and why temperature and pressure affect the altimeter, but I accidentally came to clearly understand the reasoning and effect of setting the altimeter to 29.92 above 18,000 feet. Thank you.
This guy is great. His funny descriptions are exactly what’s going around in my head when I’m looking at these instruments.😂
Excellent explanation, being a low time VFR pilot its a challenge to retain all the information necessary to feel and be safe, This will certainly help me..
Thanks for the post.
Best video about this subject !! Loving the comparison with our body which will finally make me remember it forever
That's how we teach every concept at: passfaaexams.com/
The only video that set my mind right on this.
Thank you very much for making this video!
Great explanation!!! This helps me tremendously with a concept I struggle to remember.
As always, love the videos, they’ve certainly helped me with my IFR training. Not just getting an answer correct, but understanding WHY the answer is correct is so much more important for long term memory.
The best way to find true altitude is with a measuring tape. What to do when flying ?.In practice approx pressure estimates are used in the sub scale of the altimeter. The altimeter reads the height above that setting . When using flight levels above 18000 feet in USA or 10000 feet in Australia, the same pressure setting is always used, 1013 mb or 29.92 in/Hg in the sub scale of the altimeter. .No matter what the real pressure, aircraft will all clear mountains when using flight levels.
Great Video. Thanks
Such a good explanation!
Best videos for a refresher on TH-cam.
Thank you for explaining this difficult subject in a way easy to understand .
Very helpful. Non of my instructors have successfully presented this concept.
I see this video was posted 8 years ago, and you answered a question just one month ago. That is awesome! Your video is excellent, as is your teaching style. Very well done!
Thank you sir. After making sure our students ace their exams, our second priority is customer and student service satisfaction!
passfaaexams.com/
OMG you are a genius! I was so confused on the "low to high clear blah blah" stuff that it seemed I'd never get it. This is such a logical explanation, thanks a million!
+reza farivar We at passfaaexams.com/ always find a way to explain concepts at the Correlation level rather than the Rote. Understanding Altimeters by correlating them to our brain's perceptions is definitely a better way of learning how they operate.
very good, studying for the EASA ATPL 15yr pilot and had a brain fart, this was great to get me back on track, you never stop learning in this game.
you sound like click and clack!
Given: Aircraft at FL150 overheard an airport. Elevation of airport 720 FT. QNB 1003 hPa, OAT at FL 150 is -5 degrees Celsius. What is the true altitude of the aircraft (Assume 1hPa = 27FT)
This is a great video! This concept makes a lot more sense now after having it explained this way. Thanks!
Thank you for great videos. If you can bump up your volume a bit so we can hear well. Thank you.
Great explanation.. and maybe I'm thinking about it too much. But if field elevation is 650 feet above sea level, and pressure drops with altitude, wouldn't the field pressure be lower than 29.92 and not higher (30.57)? Or should I just assume there is a high pressure weather system rolling through, or it's a hot day? It may seem like a smart-ass question, but I'm really just trying to wrap my brain around it! Thanks!
You're a genius. I just understand it with the lateral pressure and temp. changes.
Beware at 4:15...he mistakenly says standard pressure is 27.92 In-Hg ....the correct Standard Pressure value is 29.92 In-Hg. He may have just been going too fast.
Standard pressure at 2000ft
Great Explanation. Relate-ability is key!
You're a good teacher.
You’re the best, blessings to you
Thank you so much for this! You've made it so clear. Now I do it this way ! I'm still learning more but thanks for your explanation!
Wish I had this explanation first.
Many of our customers come to us after having already purchased some other material and all of them tell us the exact same thing: "Wish I had this explanation first." passfaaexams.com/
Hey man this is great this makes so much more sense, Thank you!!
I struggled with understanding the physics behind the temperature one too. Here's what I came to: Cold air is heavier, denser, and slower moving, so it all sinks to the ground and drops off very quickly as you rise. Hot air is lighter with more energy, so it is able to climb higher and spread out more. So the difference in pressure between 100 feet of hot air is covered in say 90 feet of cold air. So in this example, when your altimeter says 1000 agl, you're really at 900. (Actually I doubt it would be linear like that, I'm pretty curious if anyone knows)
Temp mainly changes near ground level only a small percentage of air, higher altitude its more predictable and constant its about 2000 FT were air can be unpredictable, altimeter works by measuring the total weight of air above you, these differences in temp are so minutely small its not worth calculating, and max allowable altimeter error is +/-75 feet so if your flying with a friend and you had +75 and he had -75 error thats 150 ft error between you too
cold air contracts .. and as the whole column is sitting on the surface, the "typical" pressure /altitude curve shrinks towards the ground. now if you're in the same physical elevation, the ambient air is all of sudden thinner, i.e. altimeter reads higher. aircraft don't have this luxury - they are flown by the pilot, who looks at the arrows and maintains the apparently steady altitude / flight level. however, this level is now CLOSER to the all the layers below. hence, you are descending, relative to the zero level, while maintaining your precious isobar line (vertically, rather than horizontally). maintaining the same elevation, cooling air means rising altimeter. maintaining steady altimeter readout means you fly closer to the ground.
Da se visinomjer namjesti na 0=15 C stupnjeva kao pocetna temperatura? 0 je prava visina=15 C stupnjeva je pocetna temperatura? Svakih 333,3 metara temperatura pada za 1 C stupanj? 😀
I thought low pressure was warm air and high pressure was cold air. So why would it not be you thought u were lower in altitude when you go to low pressure area?
I got the hot to cold and high to low, i just didn't understand why the indicated altitude increased as you increased the setting in the kollsman window, thanks for clarifying.
Lets say you are flying at 1000 feet with a standard pressure of 28.92 the altimeter correctly reads 1000 feet. now you fly into an area with the pressure at 29.92 without resetting the altimeter. the altimeter would read 0 as the pressure has increased and the altimeter is fooled into thinking that you are closer to the ground. If you now reset the altimeter to the correct pressure of 29.92 the altimeter would read 1000 feet correctly again.
Hope this helps.
Hello, please help me to solve step by step the exercise: "Pressure altitude 10000 ft, Temp -30 C. What is the True Altitude?". Am not interested in a solution like: "Ok, on your Pathfinder computer put the PA and the Temp and ask for the result". I work with a Jeppesen Techstar but it asks to put the Corrected Altitude in order to give me the True Altitude. What to do in order to find the CA?
Best explanation ever, really thanks A LOT
Thank you Sir. All of our explanations are this simple: passfaaexams.com/
@@GianLucaNoia I'm EASA, but thanks anyways!
@@xXivanhornetXx Well we sell the software all over the world as it is not based on question and answer memorization but rather on teaching you the material and that does not change no matter whether you live in America or Europe. We have many UK and German customers.
Please...there is a science to all of this. Please think about this...
Altimeters measure air density, they are NOT made to measure temperatures. Air is more dense near the ground and become less with higher altitudes. Right?
1. Cold air is MORE dense than hot air....that also means that warm air is LESS dense than cold air.
2. Dry air is MORE dense than hot air...that also means that humid air is LESS dense than cold air.
Try to take off on a hot and humid day. The heat does not mean your closer to the ground, it means the air is less dense and you have a high density altitude. Really!
If you are measuring air density (altimeter) and increase the density, the altimeter will register a lower altitude. Why? The air is more dense. The saying is true when you fly from a high to low PRESSURE areas. BUT flying from a warm (less dense) into cold air (more dense) DOES NOT MAKE THE ALTIMETER RISE! That is wrong! If you don't believe me, put the numbers in a E6B and try it out. You'll see that higher temperatures make for higher density altitudes. Likewise, cold air does not make the altimeter rise. Cold and dry air makes for the lowest density altitude.
Great way of explaining it!
Sir do we have to keep adjusting our altimeter setting along flying into an area of low/high pressure in order to maintain our true alititude or there is another method to do so our indication would be true ?
Because as explaned above on the video a long you are flying into an area of cold weather the pressure decreases gradually and the altimeter reads higher (obvious) , how can we make the altimter indicates true altitude along flying into an area of hot/cold weather ?
Thank you sir for your efforts
no you would not When Flying you fly at say FL250 25,000 ft you would have your altimiter at MSL 29.92 In-Hg or 1013.2 hPa STD Setting So you would never change your setting during cruise as this would be dangerous to you and other aircraft flying near by as those settings are Standard and all aircraft uses it as you would not want to have a mid air collision because you had a different altimeter setting, your true altitude changes constantly as you move into low/high pressure but your Indicated Alt would stay the same, only time you change it is when coming in to land or take off or if you need to calculate altitude of a obstacle like a mountain and this can be done if you have the current local pressure of Sea Level, Normally Temp is not really calculated as it is not changing your readings much only a few feet and also allowable altimeter error is +/-75 feet so your altitude could be off by 70 feet or could be 150 feet different from another guy nearby
Yes you should ALWAYS change your altimeter when flying from low to high pressure or vise versa. That's why you get the weather and altimeter settings when flying from one airport to another. If you do not adjust for the pressure change, your altimeter will show you to be at a different altitude than you TRULY are.
Great video - thank you!!
The only thing that was not clear to me without additional research was that in the US altimeters are set to Standard Datum Pressure (29.92'' Hg) when they reach FL18 and thereafter (therealoft? :) ). Above that they fly at assigned PRESSURE ALTITUDES and not TRUE ALTITUDES (altitude above MSL). If this is true, that would be necessary to know to solve the last problem in your video. Without it, I did not have an initial point of reference and could not figure it out without advancing the video. You did eventually mention that "an airplane flying @ FL29 would have an altimeter setting of 29.92. It also flies at PRESSURE ALTITUDE." That cleared things up for this particular problem, but did not provide the explanation why it was set to 29.92. For a beginner such as myself, that would have been useful information.
I hope you don't take my comment as criticism but rather as feedback, because, as other people have stated before me, your video does clear things up a lot. So thanks again, and fly safe!
Sorry, after re-watching it for the fourth time, I did finally catch the "if you were following procedure" note in the example preceding the last problem, but again, it was a rather general comment, and I still think that it would make things a little more clear if you stated that FL18 is the altitude where you do it. Maybe I'm just slow... :)
This is gold
why do small planes deep-in-free-fall when they travel from high Plato to a low-land/gorge area ?? Why doesn't it fly over it without that pain ?
Anyone, please explain . .
what I don't understand when we fly from hot to cold WX as I know we fly from low pressure to high pressure , when I read in Jeppesen it said that when we fly from hot to cold its that we fly from high pressure to low pressure .
what I know is that cold weather has more dense which is more pressure than hot weather ?!
Make more videos on different types of altitude please!
Really confiusing at all
Please somebody help me understand. It is supposed that the colder air is more dense. Should not the altimeter read lower when flying into lower temperature, since the air is colder, therefore denser? I don't understand
Treat temperature and density as two completely separate factors and pretend they have nothing to do with each other.
This video is wonderful. Thank you very much.
lovely explanation thanks
I always enjoy your videos but I had to write to say this one had me laughing about the high to low guessing part.
excellent thanks
If this helped please consider taking a look at our full software at: passfaaexams.com/
isnt it dangerous not to know the true altitude? isn't it risky to have to do these calculations in flight? why don't they have an instrument that simply measures the barometric pressure surrounding the airplane as it changes and keeps changing? This video definitely explains but why all of this in the first place??
perfect explanation
What would happen to the TRUE ALTITUDE if we flew into an area of cold weather ? Would it increase or remain the same ?
zane forbes your altimeter would indicate a higher altitude than your true altitude. However, temperature does not effect the altimeter as much as the pressure does and thus, you should understand the relationship between pressure, temperature and density. :)
high to low watch at below
Very helpful. Thank you!
Thank you GLN !!
Genius
Thankyou!
You are welcome Sir. For more awesome simplifications visit: passfaaexams.com/
wow this is really smart analogy
good job! very helpful
EXCELLENT!!!
Great
fantastic video!!!!!!
That's a beautiful tip, thanks!
I set my altimeter to 0 .. I fly from. “sea level “..I’m a low lander! ;)
Brilliant! Thanks
Altié-Meter is lyké a Barrow-Metré
I like it!
Excellent explanation!!
8:28 where did he get those extra 500 feet from?
I guess you already know, right? it has been four years 😂
@@nickde6339 It's been 7 years now. I am taking flight lessons for my Private Pilot License and I understand this fully.
please fix the audio, i can barely hear it
A very great way to teach it!!!!!
Awsome
why is the audio so low?
Can't hear it. Volume is too low.
I'm not even deaf.
wow thx so much dude i was watching this video when i came across the same question you pointed out in example 2 lol xD.
Good video but volume is super low
5:15 Bingo! Thank you!!!
thks!!!!
That all makes sense until you consider density altitude where a higher temperature results in the airplane thinking that it is at a higher altitude. In that case wouldn't going from cold to hot make you tend to fly the plane lower and so you would again need to look out below?
I think you might be a little confused sir...
Density altitude has nothing to do with how an altimeter reads. It affects airplane performance not altimeter performance and this video discusses how an altimeter reacts to temperature and pressure changes not how the airplane reacts.
I am indeed a little confused...
Let's see now. If an airplane is parked at a high elevation airport and I'm watching the altimeter from dawn until noon of a what is becoming a very hot day, the indicated altitude will progressively LOWER as the outside air temperature increases because the airplane, like me, would be thinking it is going down to a lower altitude where the air is warmer.
But at the same time, the performance of the airplane would diminish due to the less dense warmer air.
The is the problem: "because the airplane, like me, would be thinking it is going down to a lower altitude" .
The Altimeter, like you, would be thinking it is going down to a lower altitude, not the airplane.
AHA! Got it.
Thank you very much.
No Problem sir.
We don't call ourselves Pilot Training Solutions for nothing!
passfaaexams.com/
So simple to memorise it now... This shit "from high to low, watch below" makes no sense to me
This is just one of the many simple explanations we provide in our full software: passfaaexams.com/
I feel like an idiot I can’t understand this
From 1928 so sad technology hasn’t moved with the Altimeter we are living in the donkey ages
It works, why change?
Given: Aircraft at FL150 overheard an airport. Elevation of airport 720 FT. QNH 1003 hPa, OAT at FL 150 is -5 degrees Celsius. What is the true altitude of the aircraft (Assume 1hPa = 27FT)