Fresh Air Matters... with Capt. Yaw
What is an ISA? No, it is not an Independent Savings Account (well it could be, but not in this column!). We are concerned with the International Standard Atmosphere, or more specifically the ICAO (International Civil Aviation Organisation’s) version of it.
It is a lie. A blatant, misleading ‘standards’ lie. The ISA is hypothetical and probably has never really existed for more than a nano-second or two, yet the ISA is the basis on which much of aviation and meteorology is based. For the record, it states that the ‘Standard’ atmospheric condition is one where, at mean sea level, the temperature is 15 Celsius, the atmospheric pressure is 1013.25 milli-bars or Hecto-Pascal’s (same pressure measurement units, different names), one cubic meter of air has a mass of 1.225 kilogrammes and for every kilometre you ascend from the surface the temperature will drop by 6.5 Celsius, or in other words 1.98 Celsius drop per thousand feet ascended. Oh, and it assumes that one hecto-Pascal difference equates to a change in altitude of about 27 feet at sea level… Who cares? YOU should!
You see, it is the ‘basis’ of calculation, albeit purely hypothetical. So many instruments are ‘calibrated’ to this mythical ISA, which only exists in the mind of a man (or woman), in a white coat, in some lab somewhere. Furthermore, the rate of change of temperature changes as you move up through the atmosphere, as does the height represented by each Hecto-Pascal or milli-bar change.
It was decided, by some really smart people at ICAO, that all aircraft should ‘measure’ their height, speed, performance estimates, etc. against this ‘standard’. It means that all the altimeters (should) read the same in a particular ‘space’ (but none are really telling the truth); it means that all the airspeed indicators have the same error in the same air (unless they are True Airspeed Instruments which adjust for temperature and pressure). It also means that, despite kicking off from a hypothetical atmospheric lie, we are all calibrated to that same lie, making us safer.
A bit heavy? Wait a minute…. There is reason to ‘fly the lie’. The ‘lie’ is close enough to the truth to make sense, and for most situations we can make informed decisions that are close enough to the reality to remain safe. Without it we would probably make more mistakes and have more accidents – and the work of the Air Traffic Controller would be one of more guesswork than science!
Terrible isn’t it? Our aviation safety is based on ‘a statement of fact’ that is clearly not the case. But we need it – and we must ALL accept it or we are in trouble up there!
You have lived this same sort of lie all of your life. ‘There are 365 days in a year and 24 hours in a day’. Poppycock, and we prove it to be poppycock every 4 years… when we have a leap year, recognition that the year is actually 365 days, 5 hours, 48 minutes and 46 seconds long. If we did not have leap years we would have shifting seasons – as the year ‘shifted’ one point diddly days every four years. Our calendar dates back a long way (to about 46BC and the time of Julius Ceasar – hence the ‘Julian’ Calendar), and we have fiddled with it a lot. The original concept of a leap year every four years worked great until 1582 AD, when the Pope (Gregory XIII) decided that the accumulated error, of nearly 13 days, had to be stopped … The Pope decreed that all years perfectly divisible by 4 were to be ‘leap years’ … except for century years which normally would not be a leap year, unless divisible by 400.
Yes, it does sound complicated, and why the Pope got involved is beyond me too! What it boils down to, is that we are all being ‘sort of lied to’, in one way or the other, every day, and we need to accept certain ‘lies’ and base our planning and forecasts on a ‘common lie’. You see, today is not really today, if we correct the calendar… but since we are all working from the same ‘reference point’ and using the same ‘rules’ we are able to use the situation to work together.
The questions have to be asked a) ‘is there any harm in the ‘lie’?’; b) ‘who is benefiting from the lie?’; c) ‘is the real truth going to change anything?’; d) ‘is there a better way?’; and most importantly e) does everybody agree to the ‘lie’?’. If the answers are NO, NOBODY, NO, NO and YES, then it appears to work.
Of course, if you need to ‘lie’, and it really is necessary, you can change the facts to make your ‘lie’ the truth… and that has been done too! Until the first of July 1959 the inch was not a standard measurement – it varied from country to country, and it did not enjoy a direct relationship to the internationally standardised Metric measures. So, the US and the Commonwealth got together and they actually changed the measurement of the inch! Standards people decided that one inch would equate to exactly 25.4mm. Why the proposal for a 1inch = 25mm concept was not accepted has been a burden to school children, designers and seamstresses around the world ever since. (By the way there are still at least three different measurements for the ‘foot’ going around!)
We all use numbers to represent things. How tall are you? How old are you? What is your collar size? What is your weight? How far do you live from here? The answers are all numbers – none of which make sense without a UNIT of measure. Unless we can all agree on the units, and the basis on which they are referenced, we are simply talking gobbledygook to each other.
I remember being told by a policeman, at the scene of an accident, that the road was twelve wide. I asked ”twelve what?”, to be told “inches!”. Of course, it was a ‘tip of the tongue answer’ but one that could have created a major issue in the courts – since there are not any cars that can fit onto a twelve inch wide road! Common sense prevailed and the measure was taken as metres. Much more sensible!
In industry we use millimetres and metres. In schools, for some totally unknown reason, they insist on teaching predominantly using ‘centimetres’ – a measurement that you will be hard pushed to find on any engineering drawing! It is actually a waste of brain cells to learn centimetres – millimetres and metres are so much more practical and common use! (For the record, I once ordered a 3m x 1m banner to be delivered a 3ft x 1ft banner – when I stopped shouting, I started laughing – a lot – then I understood, the ‘art studio’ only had an imperial measuring tape… my mistake…)
I guess the bottom line of all of this ‘hypothetically referenced numerical representation of our world’ is simple; we all need to be on the same page, using the same units and to ensure that we are all understanding one another, if not, mistakes and accidents will, and do, happen.
Capt. Yaw is Chief Flying Instructor and Chief Engineer at WAASPS, and lead Pilot with Medicine on the Move, Humanitarian Aviation Logistics (www.waasps.com www.medicineonthemove.org e-mail email@example.com)