View Full Version : Altimetry - me dim, a Q
I am much confused but beginning to see the light with regards to altimetry. If someone could be kind enough to explain the solution to the below Q, it would go some way to completing my understanding of some fundamental concepts in altimetry.
I have worked on it and think I have the right answer - an explanation will confirm it and assist my general understanding. Thanks very much in advance.
Minimum safe altitude in an area is 12,000ft. QNH at a local airfield is 1030hPa.
Average temp of atmosphere is ISA+10. What will altimeter read at this safe
7th Apr 2004, 12:50
using a 'pressure correction to elevation 'chart you'll find that,for a QNH of 1030 the necessary correction to obtain pressure altitude is -500',so I guess the correct answer to your question is b)11520
Thanks Alex, I would appreciate the details of the calculation without the chart - this being the crux of my problem. But thanks for you reply.
7th Apr 2004, 13:04
well, Cron ,it's something like this:
thessur it's 1030,higher than 1013 ,right.So,the real pressure altitude it's lower,cause the pressure decrease with increasing altitude.
1030 it's lower than 1013.Imagine you move down on a ladder to the step marked 1030.
The difference can be calculated .1030-1013= 17
Then 17 * 30 =510 (it's aprox 30' for each HPA)
Thus resulting 510' difference with a - sign,as I told you above
Thanks Alex, is the ISA-10 part a red herring?
7th Apr 2004, 13:27
Sorry, but I disagree. In fact, I think the question is wrong. First of all, there is one vital piece of information missing, and that is what datum is set on the altimeter. Without that piece of knowledge, you can't answer the question.
If 1013 is set, then alex has started off on the correct lines - but when do you ever fly around at MSA with 1013 set? Even then, to get a true altitude you would need to take the temperature into account. But it's so rare to care about the temperature when you're on 1013 (because you're normally well above MSA, so temperature errors don't matter) that none of the textbooks will tell you how to do this.
If either QNH or QFE were set, you would still need to take the temperature into account. But here there is another piece of information missing, and that is the elevation of the airfield whose QFE or QNH you are using. You need this because that is your datum altitude - the temperature correction will be a percentage of your height above this datum.
So I don't believe it's possible to answer the question, on two counts.
Thanks FFF, I feel better already, can I push you to bung in some likely 'missing' values - and proceed from that?
8th Apr 2004, 08:54
There could be several questions. For example:
You are flying at 12,000' on a QNH of 1030hPa. If you set your altimeter to to 1013hPa, what altitude would it indicate? The answer is then as per Alexban's reply.
More likely would be to turn it the other way around. You are flying at 12,000' on the standard pressure setting. At what altitude are you flying if the QNH is 1030hPa? This time, the answer is as per Alexban's reply, but add the 510' instead of subtracting it, so the answer is 12510', which isn't even close to any of your options.
Or you could turn it into a temperature correction question by saying that you are at 12,000' on the QNH. The tempareture is ISA-10 degrees. (Temperature correction questions always seem to be colder than ISA, because this is the dangerous one!) The QNH of the local airport is 1030mb (this is a red herring), the altitude of the local airport is 5000' (this isn't a red herring!). What is your true altitude. For this question you need to work out your height above the datum. The datum is 5000', your height above this is 7000'. The error is 4% of this (4% for every 10 degrees), i.e. 280', so the answer is (12,000'-280'=) 11,720'.
Note, though, that in every case I'm telling what your altimeter setting is - the crucial thing which is missing from your question. If you want to add anything about MSA, then go ahead, but that would be a complete red herring - the altimeter doesn't care about any nearby mountains or masts.
I'm sure some of the current groundschool students or instructors will have plenty of "real" feedback questions along these lines.
This is a simple "True Altitude" question.
The Safety Altitude is measured from Sea Level.
The QNH provided is of course the sea level pressure.
However, the temperature of the column of air between sea level and the aircraft does not conform to the International Standard Atmosphere (ISA) as it is ISA +10. Thus the altimeter which is calibrated using the ISA will not show the true altitude.
Remember for temperature - "high to low look out below" because the altimeter will over read.
In this case, the temperature is higher than standard so the altimeter will under read. So the answer is either a or b.
Use your Whizz wheel to tell you what the indicated altitude would be when the true altitude is 12000ft and the temperature is +1C (ISA+10).
8th Apr 2004, 10:18
I disagree. The QNH is not the sea level pressure! It is normally assumed to be so, but it is fact:The pressure 'reduced' to mean sea level, assuming ISA temperature profile from the station/airfield to MSL(Reference (http://homepage.ntlworld.com/booty.weather/metinfo/isa.htm), but I've aded the emphasis.)
Normally this difference does not have much practical impact. But when we're specifically talking about a case when the temperature does not fit the ISA profile, the difference is important. That's why you can't perform a temperature correction calculation like this without knowing the elevation of the airfield or station whose QNH you are using.
Also, although I don't have a whiz wheel with me to check this, I think that the only time the whiz wheel allows you to set a temperature/altitude combination is for TAS/IAS conversions, not for temperature corrections to altitudes? Hence why we had to learn the rule of thumb that I quoted above (and also, coincidentally, on the QFF thread which is also running in this forum) for our ATPL exams.
Thank you, all contributors.
You are correct with your definition of QNH.
However, if increase the pressure to sea level assuming ISA and then take it back up past the station elevation assuming ISA (which is what the altimeter does), this makes little difference. So both the adjustment ot arrive at QNH and the subsequent reading on the altimeter (indicated altitude) all assume ISA....it's apples and apples.
You are given information that the actual temperature is ISA - 10. Yes this could vary with height (not a constant lapse rate) but if ignore any such variation, you can take the true elevation of the station, reduce it to sea level using the actual temperature and then bring it up to the true altitude of 12,000 (the MSA) again using the actual temperature. You thus have your true altitude.
In both cases the lapse rate and pressure change from the station to sea level is the exact same as the lapse rate and pressure change from sea level back up to the station (but the sign changes) so who cares how high the station is.
Even the basic whizz wheel - the CRP1 which PPLs use has a facility for calculating true altitude.
I don't see why we need to make a simple whizz wheel question too complicated.
8th Apr 2004, 11:28
Hmm, will have to check my whizz wheel tonight...
As for your counter-argument, you say:However, if increase the pressure to sea level assuming ISA and then take it back up past the station elevation assuming ISA (which is what the altimeter does), this makes little differenceYes, that's true. But if the temperature is not ISA, you can increase the pressure to "sea level", but this is not the true sea level.
Station elevation is 300 feet. Assume 30 feet per millibar at ISA temperature, and an average temperature of ISA-10 degrees.
Measure the QFE as 1003mb.
Now we can calculate the QNH as being 1013mb [1013 + (300/30)].
Next, dig a hole, 300' deep. (Or alternatively, assume the station is on the top of a cliff.) Go down to sea level, and measure the pressure. It will not be 1013mb. You will reach 1013mb 12 feet above sea level (calculated using the rule of thumb of 4% per 10 degrees, since I don't have a whizz wheel with me at work). The pressure at sea level, therefore, will be slightly higher. (This pressure is the QFF.)
With QFF set on your altimeter, I agree completely with what you're saying. But with QNH set, what you are calculating is not your height above sea level, but your height above this 12 foot point. If you want to get the correct answer, you have to bear in mind that the only point at which the altimeter will read the correct altitude is at 300' - the altitude of the station. Again, let's give it some numbers... assuming you are flying at an indicated altitude of 900', with QNH set. According to you, the error is (using my rule of thumb again) 4% of 900, 36 feet - your true altitude is 564'. According to me, the error is 4% of 600, 24 feet - true altitude of 576'. Drawing a diagram of this situation will prove that 576' is, in fact, the correct answer. 564' would be the correct answer only if the question stated that you were flying on the QFF - but when does anyone ever fly on the QFF? Note also that I could not have got to my answer without knowing the station elevation of 300'.
EDIT - have just thought of an even easier way of explaining why DFC can't be right (thanks to a response to your duplicate question on the Rotorheads thread, which I just stumbled across).
Assume you are at 12,000', as per the original question. You have QNH set. But you are actually on the ground, at an airfield with elevation of 12,000'. The temperature error must be zero in this case, because QNH is the value which you set to get your altimeter to correctly read airfield elevation when on the airfield. Using DFC's theory would have your altimeter reading some huge temperature error, despite the fact that you're on the ground. You can not do temperature correction problems without konwing the elevation of the station.... QED!
9th Apr 2004, 19:44
Cron - Just check the wording of the question – it makes all the difference! Does it state 12000’ is a safe elevation or safe altitude? If it is an altitude, then the question is a simple matter of temperature error correction (I suspect this is the case). If it states elevation, then the problem requires correction for QNH and temperature, and as FlyingForFun has stated, there is not enough information to complete the question.
FlyingForFun – It is clear that you have a firm grasp of altimetry principles. However, I’m sure we’ve all seen these simple questions in our training days that aim to explore our understanding of a single concept i.e. temperature error. I don’t honestly believe that the question is as deep as you are thinking, although I fully agree with your argument in principle.
Cron – hope you haven’t been too confused by all of this, I’ll try and clear things up a bit. Until you get your head around it all, altimetry can be quite confusing! You will appreciate that altimeters are calibrated to read correctly in an ISA atmosphere, such that setting an accurate QNH on an ISA day, the altimeter will read your true altitude (ignoring instrument error throughout). However, when the atmosphere deviated from ISA (most of the time!), you have to be aware of the effects on the altimeter reading. Temperature variations cause differences to occur between pressure altitude (altimeter reading) and true altitude (as most books will explain) – an easy to remember phrase has been coined “high to low, watch out below”. In explanation, as the temperature falls, true altitude becomes less than pressure altitude (altimeter reading) which can lead to dangerous situations developing. MEF’s/MSA’s on charts are all ELEVATIONS – actual heights AMSL. Thus, in cases of extreme cold, you cannot expect to fly around at a pressure altitude (altimeter reading) of that MSA and maintain 1000’ terrain separation, as your true altitude will be less than your indicated pressure altitude. Simple corrections can be made on the Whizz Wheel, or by using the rule of thumb FlyingForFun suggested – 4% of your altitude per 10*C variation. Thus, in your case, I would assume that then answer is (b). 4% of 12000’ is 480’. The temperature deviation is positive, so true altitude will be greater than pressure altitude (indicated altitude).
FlyingForFun’s argument is very correct, but only applies to problems where you have to correct for temperature and pressure errors i.e. converting elevation figures into altitudes, considering temperature and pressure datum variations from ISA. These problems are more ATPL type stuff though.
To summarise briefly, QNH’s are measured for aerodromes by measuring the surface pressure at that aerodrome (QFE). Taking aerodrome elevation into account, the QFE is then interpolated down to MSL using ISA variation to given QNH i.e. it is assumed that the atmosphere between the aerodrome and sea-level is ISA. Thus, the QNH calculated by the aerodrome may not be the actual pressure at MSL, but as the altimeter itself assumes ISA too, all is well, and the aerodrome true altitude (elevation) will be displayed on the altimeter (whilst on the ground at the ARP). True pressure at MSL is often calculated by aerodromes too, this is called QFF, as is passed to meteorological services usually for predictions/synopsis etc. It is deduced by interpolating QFE to sea-level, taking into account the atmospheric variation from ISA. However, we never use QFF in aviation as it could lead to large true alt/pressure altitude indication errors at the aerodrome (as explained above), which is not acceptable when flying instrument approaches etc.
The problem then comes when calculating a safe pressure altitude above a given elevation, given that a non-ISA atmosphere exists. You have to consider pressure error (due to QNH/RPS not being 1013) and temperature error, but due to the above explanation, temperature error only exists between the point at which the QNH was calculated and your true altitude. It all gets a bit confusing initially, and I won’t bore you with the detail as I suspect you don’t really need to know.
Anyway, hope this has shed some light on the whole situation or at least helped in some way. Otherwise, please let me know if I’m incorrect in any way!
9th Apr 2004, 22:50
just have a short question, not sure if i`m right!
Let`s assume you sit in an airplane on the airport`s surface at ISA with the correct QNH, consequently the altimeter will show the correct airport elevation.
You are sitting there for 2 h ,or whatever, and the temperature will increase to ISA+10!
Will the altimeter change or will it remain constant? And Why?
10th Apr 2004, 10:01
As far is I’m concerned, no (I stand to be corrected!). As long as atmosphere deviations from ISA don't cause the surface pressure to change at the airfield, then the altimeter reading will remain the same.
Remember, the creation of QNH assumes an ISA atmosphere from the airfield elevation down to MSL. The altimeter is calibrated to assume an ISA atmosphere from MSL to the airfield elevation (or any elevation, for that matter) - hence no error at the airfield. You have to appreciate that QNH is a "fictional" pressure at MSL that is created to help us, in that it eliminates temperature error to the altimeter setting point.
Hope this helps,
10th Apr 2004, 12:38
Thanks a lot for your replies!!
Just to summarize:
Only on the ground, provided you have the correct QNH, the altimeter will read the correct height regardless of temperature. But as soon as you leave the ground the temperature becomes significant and my altimeter will be susceptible to temperature induced errors.
(Flying from warm to cold my true altitude will reduce)
Please correct me if I`m wrong , I don`t want to learn s.th. wrong.
10th Apr 2004, 13:32
8th May 2004, 13:00
I just found the original question with no ambiguity any more!
Hope this clears things up!
You are planning to fly across a mountain range. The chart recommends a minimum altitude of 12000 feet above mean sea level. The air mass you will be flying through is an average 10°C warmer than ISA. Your altimeter is set to 1023 hPa (QNH of a nearby airport at nearly sea level). What altitude will the altimeter show when you have reached the recommended minimum altitude?
a) 12210 feet
b) 11520 feet
c) 11250 feet
d) 11790 feet
Answer b) is correct