Crossing Height Tolerances ILS.

[novicef]

Does anybody know what the tolerances are for crossing heights. For instance if you are to cross the Outer Marker at 1200Ft. What tolerances are allowed above and below that height bearing in mind a pressure altimeter with it's errors involved and possible a slight change in QNH.

[mauswara]

I work on +/- 100 feet @ ISA.+/- 150 feet up to ISA + 20.I seem to recall an article on this very subject in a past crash comic.cheers maus.

[Tofasttofly]

There is no tolerance stipulated by CASA.

[kellykelpie]

If you are reading high - add the height to the minima. If you are low no need to correct i.e. if 100 ft high at the outer marker, minima becomes DA + 100ft.

[Dookie on Drums]

Jeppesen Terminal 2.7.3.1

"The final approach segment contains a fix at which the glide path/altimeter relationship should be verified. If the check indicates an unexplained descrepancy, the ILS approach should be discontinued."

There is no need to adjust your DA according to the above.

Cheers. :ok:

[Chimbu chuckles]

Kellykelpie that is what the DCA/CAA rules said donkey's years ago and they were flat out wrong.

Do a search as there was a thread covering this stuff on D&G just in the last few weeks...all you need to know in that thread.

Do NOT mindlesly add such a discrepancy to the DH.

Here is the link. http://www.pprune.org/forums/showthread.php?t=233215

[KRUSTY 34]

Correct as usual C.C.

If there is an unexplained discrepancy at the check height, then our ops mannual requires changing the minima to the localiser approach minima, thereby discontinuing the ILS approach.

Hope this helps.

[singleseater]

The ILS glideslope is a slope in space and has no relationship to the pressure altimeter. It is completly unaffected by temp.
However, the crossref you use to check the glideslope is another thing
DME V's alt will be affected unless you can use rad alt over the airport ref, IE totally flat ground on the approach not generally a happening thing.
So temp will affect the alt you expect to see on the press alt at the ref points, either markers or more hopefully dme checks. These need to take into account for the temp variation.
Warm, alt under reads, no lose of terrain clearance so no correction
Cold, alt over reads, must correct to maintain terrain clearance. Correction will depend on temp but as a guideline
Enroute MSA should be corrected for ISA -15 or greater
MDA/DH should be corrected for ISA-30 or greater. Make sure u tell the man.

Radar vector mins should only be corrected after ATC informed, he may have already compensated and other traffic may be infringed if u do not do as you are told. But u must be aware that the mins are affected.
If u think temp b 4 the approach, there should be no UNEXPLAINED alt deviations

[kellykelpie]

Thanks for the amazingly academic reply to errors at the outer marker (not talking cold wx ops) Chimbu. I've got some leave coming up so I will put some time aside to read your thesis. In the mean time I will discuss this with our training department - but what would they know?

[Chimbu chuckles]

Edit

I wrote a long explanation of true altitude vs Indicated altitude and then realised that I was too tired to be doing such at 0300 LT so scrapped it.

I was getting confused...which in this subject is easy to do as I will explain later when I have had a sleep...suffice to say KK above is even more confused:ok:

[kellykelpie]

Understood and thanks

[haughtney1]

All the check relates to is a Gross error check, my personal warning bell goes off if the difference is equal to or greater than the approach minima.

i.e. CAT 1 200ft
CAT11 100ft..etc etc etc

Its also there to improve your SA on approach, as you would expect to check Alt and DME as well:ok:

[Riddick]

Would you really discontinue an approach if the OM over read. My god, or change to a Localiser minima.

I've always used this at the OM, "If your high you die, so add it onto the DA minima", and if your low dont worry about it.

Another good saying KISS,("Keep It Simple Stupid").

Riddick;)

[desmotronic]

The flight tolerance at the om is half full scale deflection on the glideslope.

[M.25]

The flight tolerance at the om is half full scale deflection on the glideslope..........

So if you cross the OM up to half scale above glideslope, you would expect your indicated altitude to read slightly higher than the altitude published on your chart, and if it does, you could consider this an explained discrepancy - and continue the approach. Same goes if you are up to half scale below the glideslope on crossing the OM.
If you cross the OM exactly on glideslope, you would expect that the indicated altitude would read quite close to the published altitude. If it does not, this is an unexplained discrepancy, and the approach should be discontinued. (You may have forgotten to set the QNH, your static port may be obstructed etc etc) You can then scratch your head at a safe altitude.:ok:

[Chimbu chuckles]

Kellyk and others I am going to try and take this back to first principles...for those readers who are perhaps just starting out flying.

This can be a confusing subject depending on whether you are looking at the problem from the point of view of Indicated Altitude or True Altitude. Also comparing altimeter corrections at a fixed point in space defined by a distance up a sloped electronic beam to a baro only derived DH or MDA can be tricky to word correctly...as I found last night when re reading my post when I should have been asleep...I think what I wrote was correct but managed to even confuse myself.:uhoh:

SO!

A pressure sensitive altimeter measures a theoretical column of air that, if it were not for terrain, would go all the way to sea level. They do that by comparing the air pressure at the altitude you're at to the air pressure at Mean Sea Level as reported by ATC. In designing the inner workings of the altimeter certain assumptions are made about the rate of change of pressure with altitude. As with all other performance related things in aviation those assumptions are based on the theoretical International Standard Atmosphere. They are calibrated to ISA and, as their name implies, they are pressure sensitive NOT temperature sensitive.

But as pressure is effected by temperature it follows that any time the temperature is different from ISA the altimeter will have an error.

The magnitude of that error is dependant on,

1/. How much variation from ISA exists and,
2/. How far above the temperature datum the altimeter is.

In warmer than ISA conditions the column of air will be taller because all the little air molecules are further apart, in colder than ISA the reverse is true.

That is really important to remember later.

By how much?

+/-4' per degree ISA Devn(ht/1000)

Remember that ht is the height above the temperature datum...which is usually the airfield you're landing at....it is NOT altitude.

This is easy to grasp when talking only about flying at an Indicated Altitude but can seem arse about face when you apply it to a G/S check..

Why do we do an altimeter cross check?

1/. To confirm we are on the correct G/S signal lobe,
2/. Gross error check for correct QNH set in the subscale.

It is possible to intercept a higher or lower lobe and fly down an ILS with normal indications while being higher or lower than you should be...as an Air NZ crew found to their horror at Tonga (I think it was).

The easiest way to get your head around it is to imagine doing an approach to a sea level airport over water and compare what your altimeter will read compared to a radalt. The ILS check is at 6DME/2000’.

Lets freeze the motion on the ILS at 6 DME. You're exactly 'on slope', the altimeter has 0 PEC and you're on the correct lobe.

This is a fixed point in space as defined by a slant range (DME Distance) up along a very narrow electronic beam. Your RADALT will say 2000' because your true altitude is 2000’.

Remember though that the height of that theoretical column of air representing a 2000’ change of pressure varies with temperature. In ISA that column of air will closely match your radio altitude. Imagine now if we apply heat to that column of air. It will get taller but the aeroplane is stuck at a fixed point in space defined by the distance along that narrow, sloped radio beam. In ISA+ the pressure altitude of 2000’ is above the aircraft by some amount….our Altimeter will Indicate some lower figure. Our Altimeter is under-reading.

In ISA- that column of air will shrink and take up less space but the aeroplane is still frozen at that fixed point in space so now a pressure altitude of 2000’ will be below the aircraft by some amount and as a result our altimeter will read a higher altitude and is over-reading.

We can calculate how much using the above formula.

For argument sake it is 30C on this day…ISA+15.

-4x15(2.0) = -120’

Our altimeter will indicate 1880’ at 6 DME but we are at 2000’.

This is safe.

In ISA-15

+4x15(2.0)=+120’.

At 6 DME our altimeter will indicate 2120’ but we are actually at 2000’

Less safe but we do NOT simply add that 120’ to the DH the way CAA used to suggest.

Approaching the DH we are now only interested in true altitude as a raw number not compared to a fixed point in space because we are only referencing barometric altitude.

At 200’ DH in ISA-15 conditions we see via the formula that the difference between True Altitude and Indicated Altitude is,

+4x15(.2)=12’

If we added that 12’ correction and descended to 212’ Indicated Alt our True Alt would be 200’ as confirmed by the radalt.

If we simply added the error at the OM we would descend to an indicated alt of 320’ and a true altitude of 300 odd feet. And that would be what the radalt would read when you carried out a completely unnecessary missed approach if the cloudbase was at 250’.

[novicef]

Chimbu thanks for that.

[mustafagander]

I love all that theoretical stuff, but how do we know when we are EXACTLY over the OM and ought to check the altitude? In my experience the OM indications are fairly broad.

BTW, is it true that each alternate G/S lobe is reversed? i.e. the correct one reads normally, the ones next each side read fly up for fly down and v.v.?

[Chimbu chuckles]

OM are not as accurate as DME ht checks but you're not really looking at super precision...just having a reasonably accurate idea what it should say and then conducting a meaningfull G/S check as opposed to a meaningless one.

It's more about just knowing how your altimeter works and applying that to your work environment at appropriate times. I never both calculating a true alt at the DH and my calc at the G/S check is the result of 10 seconds thought after I get the ATIS. I do apply the formula to high MORAs in winter if it is REALLY cold.

I have not heard that about false G/S signals.

[haughtney1]

Some interesting info here

Erroneous ILS signals (http://www.caa.govt.nz/safety_info/ILS_4_october_2000.htm)

ILS System Overview (http://www.allstar.fiu.edu/aero/ILS.htm)