Hold Short

[bjwoody61 3]

Had something interesting happen yesterday. I was at KSEA flying to KPDX. Got Clearence to expect 34R. Requested taxi and got "Taxi to 34R using taxiway Bravo Charlie hold short 34R and hold short 34R". Got to ILS Hold Short and was cleared to cross 34R. got up to the Hold short and was instructed to contact tower. Why the instruction to cross active runway? What's the difference between ILS Hold Short and Hold Short.

[Dave-Pilot2ATC 870]

Sounds like a bug.  The ILS Hold Short is typically used in instrument approach weather conditions with aircraft on final to avoid interference for the ILS signal for landing aircraft.  In P2A, it should have been ignored.

I'll look into the bug.

Thanks for the report.

Dave

[Chock 17,769]

Whenever the visibility is below a certain level (think it is 800 feet RVR if I recall corectly, might be wrong about that though), or when an aircraft is on finals at or past the OM, you will be asked to hold 'short of the ILS critical area'.

This is because anything within that area can interfere with the integrity of the ILS signal if you get too near the ILS antennas (this is a bit like if you waved something like an electric razor near the antenna of an FM radio - you'll hear it interfering with the signal because the razor probably won't be shielded well enough to prevent RF interference). The metal of your aircraft's fuselage (particularly if it has a very tall tailplane), your engine's ignition systems, your radios and other electrical gizmos on your aeroplane could all potentially cause the ILS transmitter's signal to degrade or even deviate a lot. So they have to keep you away from the antenna when something is coming in for a landing.

So, 'hold short' just means stop before you get to the normal hold point, 'hold short ILS' means stop at the specific marker on the taxiway for the ILS critical area. Some runways simply have one marker which serves both purposes, but if there are two markers, you can identify them this way: A normal hold point marker has three 'tabs' sticking out of it (one on the left side of the line, one on the right, and one in the middle), an ILS hold marker has multiple tabs all the way across it. So you'd be okay at the normal hold point if you were number two to line up for take off, since the plane in front of you on the runway ain't using the ILS, but when something is coming in and using the ILS, you need to be out of the RF interference area. I doubt it has a bearing on matters in FS, but that's what it is.

 

 

[Dave-Pilot2ATC 870]

Chock,

Thanks for the details.  I believe the Tower or Ground will issue an ILS Hold Short when the Ceiling is below 800 ft and visibility is below 2 miles if aircraft are using the ILS. 

Dave

[bjwoody61 3]

thanks Chock for the explanation. Had no idea. Learn something every day