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!
Rgds,
Cuban_8