If it hit the fix at 5,000’ which was also the charted value, then conditions are ISA, so it is at a real 5,000'. As soon as they vary from ISA, the indicated crossing altitude will differ: less in warm conditions and more in cold conditions.

As the atmosphere warms and cools, the “feet” that your altimeter indicates also expand and contract and are only equal to the reference foot in ISA conditions. An ILS glide slope, however, follows the same path through the sky irrespective of temperature. It would do the same even if there was no atmosphere at all.

The fix crossing altitude stays the same in absolute terms, as it is defined as a distance from the glide slope transmitter. What you see on your altimeter as you cross this fix depends on the composition of the atmosphere below.

In terms of separation from other aircraft, when you are on the ILS you are following a fixed path no matter whether it’s Monday or Thursday or if it’s 40degs or -40degs or if you have 950hPa or 1050hPa on the sub scale. Other aircraft who are using their altimeters on a common pressure setting will be separated from each other but not necessarily from you.

Right. Which is why when cleared for an ILS approach, until you reach the Final Approach Segment (glideslope intercept altitude), you should be complying with published crossing altitude restrictions. Even if meeting those restrictions takes you above the glide slope.

So what if you are on radar vectors on an intercept heading and cleared fir the ILS from 5000'? Do the lower procedural minimum alts still apply? AGP 13 is a prime example.

Apologies, LEMG ILS-Z 13.

Procedurally the approach starts at 6000 with step downs to 5000 and 4200, thereafter the the ILS glide. With vectors they'll clear you from 5000.

You don't descent below 5,000 until passing D16.5 GMM (or 17.4 AGP) then either descend to 4,200 to intercept the GS or stay at 5,000 to intercept the GS. You have this option because there are no step-down fixes between D16.5 and the official GS intercept point at D11.7. If the GS is not available then after D16.5 you descend to 3,400 until D9.9, then follow the LOC-only profile.

How unprofessional is this
 

Take a look at skyvector.com and get out the KLAX TAC chart. Gives you a good idea of the B airspace. I think you'll be well within the airspace but separation to other (controlled traffic) may not suffice.

https://www.airnav.com/depart?http:/...026PRMAAUP.PDF

Here's the current link. It's the ATTENTION ALL USERS page for the PRM approaches in ATL.

aterpster seems to have a good grip on the topic at hand. How is it that in ATL, you can always descend with the glideslope, but at other places (like ORD) it's sometimes inadvisable?

Interestingly, we joined the ILS at 10k going into ORD a few weeks ago. The FD rocked the wings a few times when transitioning from NAV to LOC, but otherwise flew it beautifully.

That is what "no fly list" and union "pro standards" are for. Hope he gets his ass chewed. Don't accept the visual.

Thanks for the replies ......any idea how to work this out?
 

RMC, somewhere in ICAO Doc 8168 I once found the equation to calculate the altitude corrections for non-ISA temperature.
Do not take my word for it without checking, IIRC the DELTA needed to get the true geometrical HEIGHT above the measuring point were more or less the same for the warm temperatures as they were for the cold ones.

The catch was the reference "zero" - it is SL @ 15 DEG centigrade. I.e.

For COLD temperatures the correction down to 0°C (ISA -15) is negligible.
For HOT temperatures, in a similar fashion, until you hit ISA +15 the difference is not that relevant.

If you only have the COLD tables, to get a taste about the HOT deviation you may attempt :

OAT 40 deg at SL = ISA+25
----- reverse -----
ISA -25 = -10 deg Centigrade
==> use the values for -10 degrees and apply them in the opposite sense, to get a taste of the HOT air deviation.

You can get the formula from the generic Barometric formula with lapse rate:
https://en.wikipedia.org/wiki/Barometric_formula

But don't fret about the large equation, for temperature correction you can assume most stuff to be constant.
You assume p1=p2 (because that is what your altimeter converts to an altitude) and then you can throw out the exponent and a lot of other stuff.

You end up with this equation:
h1/t1 = h2/t2

This assumes temperatures in Kelvin. For temperatures in C and solving for h2 the formula becomes:

h2 = h1 * ((273.15+t1)/(273.15+t2))

Your example:
h1 = 5000 ft
t1 = 15 C (standard ISA)
t2 = 25 C
result:
h2 = 5173 ft

t2 = 30 C results in h2 = 5260 ft

t2 = 35 C and h1 = 8000 ft results in h2 = 8555 ft


h1 is the altitude your altimeter is reading (it assumes ISA, so t1 = 15C)

h2 is the actual altitude you are at assuming a sea level temperature of t2.

Relevant pages attached. The whole equation from the middle of the first page could be expressed as

-(ISA_dev) * Fn_error[height, elevation].

Meaning that the Height Error Function provides a difference between the reading and the true height. Which is then scaled in direct proportion to the ISA_dev to obtain the result. Whether hotter or colder than ISA makes no difference to the number, only reverses its polarity.