Lagoon

I'm confused about a question I came across:

Approximately what altitude is the glideslope centerline at the MM of a typical ILS?

The correct answer is 200 feet.

I thought it would be higher than that. Does anyone have more input to explain the answer?

I know the answer is available in the AIM, but some background or context would be helpful.

Thanks.

Adlerdriver

Minimums on a category-1 ILS are typically 200' above the runway. The MM coincides with this point on the approach where you transition to visual and land or go missed approach.

PerfInit

Lagoon, You have been asking some good instrument questions. Have you read the FAA's free publication titled "Instrument Procedures Handbook"? Google it, free PDF download. Also lots of helpful info in the AIM, also free download and recently revised.

Lagoon

So Decision Altitude is also the Middle Marker?

Lagoon

I'll look through IPH. It seems to be a good resource.

Adlerdriver

No. Decision Altitude is just that, an altitude. If you're flying the glide slope within tolerances and you hit DA, you take the appropriate action, either transitioning to land or missed approach regardless of the status of the MM. The MM (if it's available) can maybe provide you a back up warning but it's really not part of a normal ILS approach.

Marker beacons are not required to fly an ILS and from what I'm seeing in the US, they are being phased out. I still see them in Europe. ILS to 14L in Cologne and 25L in Brussels (as examples) both have MM. On both those approaches, the MM corresponds with the missed approach point for the LOC approach to those runway. Again, both could provide a potential backup notification that you're at the missed approach point but on both approaches, DME is required and really would be the primary method of MAP identification.

So, the answer to your original question simply relates to the fact that a MM is usually positioned at a point along the ILS that equates to 200' HAT on the glide slope.

badflaps

In the olden days with raw data, 200' was all she wrote. Not that anyone paid any attenion.

Subieguy14

Here you go, This is basically the know it all guide to Instrument flying.

http://pilotscafe.com/download/IFRQuickReviewSheets.pdf

i believe its page 6 that has the ILS system.

HuggyU2

It's been a long time since I've seen a marker beacon. Even longer since I actually used one..

tomgoodman

If you fly a really old airplane, you might see an "airways" marker beacon, which was used enroute.