Found this a while back - maybe from Pprune ?:
Not my words. But thanks to the author anyway.
Pressure sensitive altimeters do not make allowance for ISA deviation and therefore Indicated Altitude = True Altitude ONLY on an ISA day. Since ISA days are as rare as rocking horse poop an altimeter will only read the published figure on the ILS chart (at the OM or DME check HT) that often as well....virtually never.
A pressure sensitive altimeter reads the height of a theoretical column of air at the top of which is the altimeter and the bottom of which is sea level....assuming QNH set not 1013.2mb...by measuring and comparing the local air pressure to the MSL datum pressure.
On a warmer than ISA day the column is taller (air less dense) and on a colder than ISA day it is shorter (air molecules more densely packed)
The magnitude of the difference between Indicated and True altitude depends on how far above the datum the altimeter is.
On warmer than ISA days the aircraft will be flying higher than indicated on the altimeter and as this is safer it has always been ignored in Australia because that is the predominant condition. This has led to a degree of ignorance on the whole subject in Australia. It follows that on a colder than ISA day the aircraft will be lower than indicated on the altimeter and this can be very dangerous but because the difference is very small at typical Cat 1 minimas there is a level of complacence.
Now consider the case of the ILS. The glideslope DME Ht check occurs at a predetermined distance along a, for all intents and purposes, angled 'surface' set a 3 odd degrees....say 5 DME/1500'. Lets 'freeze' the aircraft at that point. On a warmer than ISA day what will the altimeter read, higher or lower....and on a colder than ISA day?
By how much?
Can you reasonably do any meaningfull altimeter check without knowing?
Will the error be the same at the minima?
Lets look at the colder day as that is the dangerous one.
I have said above that on a colder than ISA day the TRUE altitude will be less than INDICATED altitude. Viewed from the pilot's perspective 'frozen' on the ILS above the altimeter is OVERREADING....it might be saying 1580' for instance on a really cold day. Once upon a time not so long ago the AIP prescribed adding that difference to the minima...you will shortly understand why that is dumb....the AIP doesn't say that anymore because at some point CASA learned how dumb it was....but I digress!
Clearly once you pass the OM or DME/HT check the only reference you have for height above the runway threshold elevation is the altimeter...at the 5 DME point above you WERE at 1500' (lets assume you were bang on glideslope and a MSL runway)
irrespective of what the altimeter said. As you approach the DH/DA your only reference is the altimeter...lets freeze the aircraft at the Cat 1 minima of 200'.
What is your TRUE altitude?
Here is a little 'rule of thumb' formula which will tell you. The most important thing to remember is the temperature datum is the airfield that gave you the temp via ATIS or whatever and that temperature as ISA devn is what counts NOT the ISA devn at cruise alt.
+ or - 4' per degree of ISA devn(ht/1000).
So applying this formula to the above ILS...lets assume temperature 0 degree giving an ISA devn of 15.
- 4 x 15(1.5)
-60 x 1.5= -90'
So if on the day of the above ILS it was 0 C on the ground and the altimeter said 1590' at 5 DME then the altimeter is accurate.
At the minima.
-4 x 15(.2)
-60 x .2 = -12'
At the minima if you descended to 200' Indicated altitude your TRUE altitude would be 188'.
If you followed the AIP as it was writ 20 yrs ago you would have descended to an indicated alt of 290' which would have been a TRUE alt of -60 x .29 (call it .30) which = 272' and possibly not got visual...you might even do it twice and then divert to your alternate for no good reason at all.
You can see that if it was 30C on the ground the whole situation reverses and becomes safer...at the OM you would be indicating 1410' for a true alt of 1500' and at the minima indicating 200' gives a TRUE alt of 212'.
Now apply the same formula to a MSA of 7000' on a 0C day and how much terrain clearance do you really have? On a 30C day? And the MDA on an NDB or VOR or GPS approach?
Now you will be able to predict what the altimeter will say at the OM or DME/Ht Check before Topd and having allowed for known altimeter tolerance (checked against the airfield reference point at point of deparature) be confident that when you call the altimeter check on the ILS you actually understand what you just said and it wasn't just ignorant BS.
Hope this helps.
DogGone
