Marker Beacon Sensitivity

[Capt. Greaseon]

In the AIM is states that when flying an ILS approach :

Aircraft with airborne marker beacon receivers with a selective sensitivity feature should always be operated in the "low" sensitivity position for proper reception in of the ILS marker beacons.

Just wondering if anyone knew the specific reason for using the "low" setting ?

[Tinstaafl]

High setting was for use when fan markers were used along airways to mark one's progress along the airway ie pre-DME.

Hi setting (high sensitivity) was used to allow for the resulting weaker signal cruise altitude caused.

[pigboat]

Tinny, that's what we were taught. Our drill was high marker, high mute - low marker, low mute.

[quid]

Some CAT II decision heights are predicated on IM crossings. (Those that are RA not authorized.) In these cases, the LOW setting will give a more accurate indication of DH.

[Hew Jampton]

the LOW setting will give a more accurate indication of DH Erm, I think not. How can an azimuth position indicator such as an MKR indicate height?

[mono]

HJ,

'cos you are using it in association with the glide path signal. It's an admitedly basic triangulation of marker distance and GP angle.

If for example you receive an middle marker signal at 4000ft above airfield elevation when established on the GP, you know you have captured a false beam.

Selecting LO changes the reception cone from something like 2nm diameter to approx 1/2 to 3/4 nm.

Marker signals are also used in some autopilot systems to start a gain reduction program which prevents LOC and GP over shoot due to beam convergence.

[quid]

HJ-

You're not confusing the sometimes co-located "compass locater" (NDB) with the marker beacon, are you? There is no azimuth signal from a marker beacon.

[Hew Jampton]

quid and mono

No confusion here. An MKR cannot, as one of you claims, indicate DH. Only an altimeter, radio or pressure, can do that. It is not correct to infer from receiving an MKR signal that the aircraft is at a particular height.

An MKR does not give an azimuth signal per se, as does an NDB, but it does indicate position in the azimuth plane by virtue of the fact that if an aircraft receives it, the aircraft is approximately overhead it. Technically it's a position line rather than a position because it's a fan marker but when this position line is combined with the runway extended centreline (LOC) it gives an azimuth position.

At that position the aircraft's actual height from the altimeter, radio or pressure, is compared with the nominal MKR crossing height on the approach plate for, among other things, a gross error check for correct GP lobe and correct altimeter subscale setting. Same if altitude is used.

[forget]

Looks like Marker Receivers, probably the oldest piece of avionics aboard, are headed for a 'second wind'. See www.airspecinc.com

[quid]

HJ-

The inner marker certainly does give a decision height. If you are established on a 3 degree glide slope, when you cross the inner marker, you are at 100 ft.

We are not permitted to use a barometric altimeter to establish DH on a CAT II approach. I checked with 3 Air Carrier Inspectors from the DTW FSDO and they don't know of any carrier that is. (That doesn't mean there aren't, just that they don't know of any.)

In any event, at my airline, if the CAT II approach shows "RA not authorized", our DH is crossing the inner marker.

At your airline, if the Radio Altimeter is inoperative, are you permitted to shoot a CAT II? If so, do you use a barometric altimeter to determine DH?

(I can only speak to the rules in the US, other states may be different.)

[Captain Stable]

Sorry, quid, but you have it the wrong way round.

Crossing the Inner Marker you should be at 100'. You don't use the marker to confirm your height. You use it to cross-check glideslope indications, the same way you check your height (or altitude when operating QNH) crossing a locator NDB or at a 4 DME fix, or whatever.

You should not under any circimstances whatsoever use any variety of "fix" heights/altitudes as accurate indications of azimuth position when flying a Cat II/III approach. In LV approaches, everything works off the RadAlt. If that is inop, then LV approaches are not authorised.

Conversely, on Cat I approaches everything is off the servo altimeter and you may not use the RadAlt for DH.

[Hew Jampton]

If you are established on a 3 degree glide slope Too much of an 'if' for me and seemingly too much of an 'if' for the airline and regulatory authority who developed All Weather Operations, starting with the Trident Autoflare and who have more experience of AWO than possibly many other countries put together. Using IM passage to indicate DH in the absence of the RA is not accepted, not that there are many IMs in service anyway; none in the UK (I think) and very few in Europe, mainly a handful in France. Cat II on a baro not accepted here either (I think, without checking); I only mentioned pressure altimeters to keep it generalized to all markers.

Edited because my post crossed with Capt Stable's. Although I agree with the good Captain that one should not use an MKR to infer height (because that's all one can do, infer it, not measure it), it seems that at least one State (ICAO, not United)might permit it. Rather them than me!

[777AV8R]

Correct me if I am wrong...but...I believe SEA-TAC Rwy 16L CAT II, uses innermarker for CAT II DH as the uneven terrain leading up to the escarpment, precludes the use of Rad Alt for the approach. It's been a while since I shot one there, but the last time that I did.....it was on Inner Marker.

[quid]

777-

It's RWY 16R that's CAT II with RA Not Authorized. 16L doesn't have an IM, and that's the reason it can't be used for CAT II.


CS and HJ-

Nope. Not wrong. It's just done differently here in the US. Without going into a debate about low vis experience, we have to accept the fact that thousands upon thousands of CAT II approaches have been shot (without accidents) to many of the following "Radio Altitude Not Authorized" airports. And these a just some of them:

KMPS 30L
KPIT 10L
KPIT 10R
KSEA 16R, and many more. In all these cases, the Jepp chart shows "Radio Altitude Not Authorized".

We've already agreed that baro altimeters can't be used to determine DH. If you can't use baro, and the RA is not authorized, then what DO you use to determine DH? Here in the US it's the IM.

Unlike your reports of very few IMs in Europe, here in the States I can't find a CAT II approach without one. Many of them don't have OM or MMs, but they all have IMs.

The following is a direct quote from our Ops. Specs. Para. CO59, (g), (2), (a). (This sections governs CAT II Ops.)

"For ILS operations, a precision or surveillance radar fix, a designated NDB, VOR, DME fix, or a published GSIA fix may be used in lieu of an outer marker. Except for CAT II instrument approach procedures designated as "RA NA" (radar/radio altimeter not authorized) operative radar/radio altimeters may be used in lieu of an inner marker. A middle marker is not required."

....."may be used in lieu of an inner marker". This statement alone infers that the normal DH is predicated on IM crossing. The RA may be used "in lieu" of the IM. It's our procedure to go miss at the first indication of either the IM or RA if the runway is not in sight.

So, I'll sign off. I guess we'll just have to agree that different states have different rules (and I made that disclamer in a previous post) and let it go at that.

[777AV8R]

Thanks 'quid'..couldn't remember which runway it was...and you are correct on the other airports as well.