Expert: Paul Ruderham - 3/3/2006

Question
Hi there, I am a third year student studying Aviation Technology at the University of Leeds, England.

I am currently writing my dissertation which is mainly focused on Instrument Landing Systems.

At the moment I am focusing on Range Information and in particular, Marker Beacons. I am somewhat confused by the conflicting information that is available to me in books and on the internet and was hoping that you could clear up the following for me....

Everywhere I look commonly states that the Outer Marker transmits the Morse code sequence for the letter 'O' (- - -) at a rate of two dashes per second. No problem there.

Now, with reference to the Middle Marker, some places state that the marker transmits the Morse code sequence for the letter 'M' (which is - - ) , BUT other places state that the Middle Marker transmits a dot-dash sequence at a slightly quicker and higher pitch than the Outer Marker.

Finally, the Inner Marker suposedly transmits a Morse code sequence for the letter 'A' (which is . - ) but I have read elsewhere that the Inner Marker transmits dots only at a rate of 6 dots per second (this would make more sense as the Morse code for 'I' is ..)

If you could clear all this up for me I would be very greatful. Any other information you think might be relevant to my dissertation would also be very much appreciated. I would also like to ask for your permission to include your name in my acknowledgments section.

Thanks a lot.

Chris

Answer
Hi Chris,

The outer marker is usually 3.5 to seven miles from the ILS's runway threshold. It illuminates a purple light on your aircrafts marker beacon display and gives a three dash morse code identifier that you can hear through your aeroplane's audio switching panel or cabin speakers.

The middle marker is usually about 3500 feet from the threshold, causing an amber beacon light to shine, and has a dot-and-dash identifier. Frequently activation of the middle marker nearly coincides with the decision height (DH the altitude at which you must decide to land or not) and published minimums for the runway to which you're flying.In other words being at the middle marker often means you are at 200 feet agl and one-half mile from the runway threshold. If you don't see the runway when the middle marker goes off, it's probably time to execute a missed approach.
Inner markers (white light, continuous dots) are found with ILS approaches that can be flown to lower minimums, such as Category II ILS's, where decision heights can be as low as 100 to 150 feet agl. Inner markers are located right at he runway threshold.
Other fixes along the approach path can be locater outer markers (outer markers that also give out NDB signals that you can select on your airplane's ADF receiver), intersections from nearby VOR radials, or distance information from localisers that have DME capability.

I hope this helps you. You can have my permission to include my name in your acknowledgements section.

regards
PR