ILS App. Outer marker - glide slope, altimeter checked!

[Kaptin M]

During an ILS approach, the outer marker is used as a check of the pressure altimeter, assuming one is on loc and on slope...agreed?

I understand that the nominated height is predicated on an ISA day.

If the height over the OM is, say, 100 feet low - let's use the figure on the Jepps as 1500', and the actual on the Press Alt reads 1400 - by how much should the minima be adjusted? Let's use a minimum of 300'.

Should it be adjusted by 100', or 1/5 of 100'? [ie. 300' is 1/5 of the check height of 1,500', therefore the 100' difference @ 1,500'= 20'@300']

[HIGH n MIGHTY]

At the outer marker, if your altitude is higher than the check altitude specified on the chart, you add the difference between the two PLUS the aircrafts pressure error correction .. if not specified, the PEC is assumed to be 50ft...Therefore the lowest minima should be at all times 270ft, unless the PEC shows otherwise..If your altitude shows lower than the specified altitude at the OM, you simply use 220ft ( standard or whatever the chart specifies ) and add the aircraft PEC...IF THE AIRCRAFT IS HIGHER...YOU ADD......IF THE AIRCRAFT IS LOWER...YOU LEAVE THE MINIMA...!! But remember the aircraft PEC..!!!

[Shanwick Shanwick]

If the MDA for the approach in question is 300'you CAN NOT descend below below 300' without the required visual reference under any circumstances.

Your pressure altimeter will read 1400 +/-100 at the outer marker/FAF unless it's buggered.

[quid]

Kaptin M-

I believe it's the other way around. If you are reading 1400 and the crossing altitude is published as 1500 then you're 100 ft. to the good (safe side). Assuming the same 100 foot error exists at minimums, then when you're reading 200, you are actually at 300.

The barometric (pressure) altimeter is only calibrated for ISA. If the temperature is colder, then the altimeter will read higher than actual. Bad. That is why Jepp publishes an Altimeter Correction chart. You enter it with two values, the reported temp. and the altitude above the reporting station (airport). I don't have the chart handy, but the magnitude of the error is signifigant. Approximatly 6000' above a -30C airport, the altimeter may be reading about 1000' lower than actual. The PEC pales in comparison.

BTW, I think an Outer Marker is much too inaccurate to see a 20-30 (or even 100') error. If you're using the swing of a Compass Locater, or a DME it will be more accurate.



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[bookworm]

The temperature error is proportional to the height above the measuring station. The rule of thumb I use is that a 20 degC cooler day than ISA (say -5 degC at sea level) gives a 7% overread of altitude.

So at 1500 ft on glideslope you'll see 1600 ft on the altimeter on an ISA-20 day. When you get to 300 ft on the glideslope, the altimeter will read 330 ft. So add 30 ft to your normal decision height.

I'd suggest making the correction based on the temperature, not the altimeter reading at OM crossing. 100 ft is only half a dot at the OM. If you see 100 ft off at OM, it may be the glideslope indicator.

It's also worth bearing in mind that the altimeter setting is rounded to the nearest millibar (30 ft), a typical static calibration correction (PEC) is 30 to 50 ft at approach speed, and the barometer itself may be good only to +/- 50 ft anyway.

So don't sweat it! But I suppose on an ISA-20 day that's unlikely... :)


[This message has been edited by bookworm (edited 29 August 2000).]

[pterodactyl]

Kaptin M and ors,
Outer Marker crossing altitude checks and Altimeter correction in cold have been covered at length in previous posts. Suggest you use search function. You will find a great deal of discussion on these topics. :)


[This message has been edited by pterodactyl (edited 01 September 2000).]

[Checkboard]

From the Tech Log Archive, in the Altimeter Correction Cold WX OPS (http://www.pprune.org/ubb/NonCGI/Archives/Archive-000002/HTML/20000604-3-000466.html)

Variations from ISA temperature start becomming really applicable at or below ISA-15°. If it is not represented graphically in your Ops manual, you can use the 'rule of thumb':

+4 feet for each -1°C temperature variation from ISA, per 1000' altitude above the airport (QNH datum)

The airport QNH is adjusted so that it is accurate at the field, it is only the airspace above the field elevation that needs to be corrected for density variation.

e.g. For a 200' Cat I minima at, say ISA-20°, this produces a correction of (4 x 20 x 0.2) 16ft. Round it up to 20ft. and, remembering the rhyme: "ISA low? Watch out below!", set the published Decision Altitude PLUS 20ft. on the minima bug.

At an OM crossing height of say 1300ft AGL the variation is much larger (4 x 20 x 1.3 =)104ft. so your crossing height, on glide slope, would be more like 1404ft. indicated above the field elevation.

This assumes that the ISA-20° is constant throughout the atmosphere, and not affected by an inversion, which would mess up the calculations.

quid, the outer marker is just a warning to check the altimeter as the signal does occur over a 150 feet or so. When the marker beacon sounds, you perform the altimeter check at the DME distance shown on the chart - acurate to 10 or 20 feet.

As the QNH is rounded to the nearest hpa, then it will be acurate to within 15 feet.

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Tech Log forum moderator

[pterodactyl]

Another useful link more specifically dealing with Outer Marker checks below:
http://www.pprune.org/ubb/NonCGI/Forum3/HTML/000119.html

[This message has been edited by pterodactyl (edited 01 September 2000).]

[Kaptin M]

Thanks Checkers, and Ptera, much appreciated.
K.M.