Pressure error correction/ special wx min

[gravitysucks]

Many, many beers ago I remember something about adding 50 ft to an ILS DA by way of Pressure error correction, something to do with safeguarding against altimeter errors at the important end of an ILS.

Also remember something about Special Alternate Weather Minima, 2 little asterix in the minimums section of the approach plate, allowing a 100 ft reduction in MDA??

I've been flying in Blighty for some years now, where neither of these apply. Returning to Godzone soon, and trying to get back on speed with the Aus rules.

Can someone explain the above for me. (Small words please and no joined up writing!!)

[Icarus2001]

How about you go searching the documents yourself? Try the AIP for both queries.

The AIP is available on www.airservicesaustralia.com look for on line documents.

Pressure error correction(AOM)... page ENR 1.5-12

Special Alternate Weather Minima... Page ENR 1.5 -31

[ITCZ]

One has been away for a while, hasn't one. One has confused four different topics as two related situations.

1......

Special Alternate Weather Minima (lets call it SPAM) is not anything to do with a 100' reduction. SPAM relates to precision approaches.

SPAM allows for lower alternate minima than the standard alternate minima when planning (pre-flight and in-flight) to an aerodrome that has an ILS runway available, provided that your aircraft has duplicated ILS equipment, and is explained in Jepp Terminal 6.2 (page AU-26).

2......

The 100' reduction pertains to MDA and alternate minima for aerodromes with published non precision approaches at some locations when an actual aerodrome QNH is obtained. The 100' reduction is explained in Jepp Terminal 5.3.2 (page AU-25).

The 100' equates to an allowance for slightly more than a 3 HPa drift, or error, between actual aerodrome QNH and the forecast aerodrome QNH which may be greater than 9 hours old. Remember that a TAF usually has four QNH's forecast spaced at 3 hourly intervals. The last QNH given on the TAF will be one that is predicted at a time ten hours after the forecaster issued it.

While we are at it, something that was added since you were last in Oz was the ability to use a Forecast Area QNH and add 50' to the Forecast Aerodrome QNH minima, that is, the non-100'-reduced-shaded-box-minima.

3.......

The 50' addition to an ILS DA relates to Aerodrome Operating Minima, and is described in Jepp Terminal 2.6.2 (page AU-17). The requirement is that operators establish operating minima for each aerodrome, and particularly that operators apply aircraft Pressure Error Correction (should be a table in your Aircraft Flight Manual) or in lieu of this, add 50' to the published DA.

To be precise, the 50' is not 'by way of PEC,' it is only used if you don't have a PEC figure and a way of saying that if you had a PEC figure it would certainly be less than +50 feet.

I am not sure that I should take you to task on one last bit where you suggest that the PEC is a safeguard against altimeter errors.

4.....

The primary safeguard against altimeter errors in the CAT I precision approach is the 'outer marker check' or FAP as described in Jepp Terminal 2.7.3.1 (ILS Altimeter Checks, page AU-17). That is the point at which you check your altimeter, and adjustments to DA are made for very cold days.

I believe (but cannot refer to my notes, I'm on an overnight) that the PEC corrects for pressure effects due to installation and configuration of the altimeter equipment on a particular aircraft, and is therefore analogous to IAS v EAS/CAS discussions about airspeed indicating systems, not as a 'safeguard against altimeter errors at the important end of an ILS.'

Avoid that phrase in job interviews! You'll have to unfreeze that frozen northern hemisphere brain, dear fellow! :ok:

[OzExpat]

There was a time when those of us in "the trade" referred to the Special Alternate Minima by the more accurate phrase "Special Low Alternate Minima", or SLAM. But, for some reason, this acronym never made it into official publications... :}

[gravitysucks]

Icarus and ITCZ many thanks, it's appreciated. Off just now for some anti-freeze!!
Regards, GS.