Lead radial amount and rate of turn

3 Answers

1. Best Answer
   
   

   
   

   Skyfox on Jun 05, 2016

   [For reference, in the version of the Instrument Flying Handbook I have, FAA-H-8083-15B, the sentence referenced in the question here is on page 9-19.]
   The full paragraph states:

   The technique for intercepting a localizer from a DME arc
   is similar to intercepting a radial. At the depicted lead radial
   (LR 223 or LR 212 in Figures 9-19, 9-20, and 9-21), a
   pilot having a single VOR/LOC receiver should set it to the
   localizer frequency. If the pilot has dual VOR/LOC receivers,
   one unit may be used to provide azimuth information and the
   other set to the localizer frequency. Since these lead radials
   provide 7° of lead, a half-standard rate turn should be used
   until the LOC needle starts to move toward center.

   By my deductification, I think the reason for half standard rate is to prevent rolling out of the turn too early and never getting on course with the localizer. Since the localizer provides a very narrow window (in degrees) for full scale needle deflection, at a standard rate turn from the DME arc starting at the 223° lead radial off the OMN VORTAC, for the average slow moving airplane the heading may roll out to 70° well before actually getting to the localizer. By turning at half standard rate it’s a more gradual turn and provides easier interception of the localizer. Since the paragraph above says, “a half-standard rate turn should be used until the LOC needle starts to move toward the center,” that seems to imply that once the needle starts to move it’s then at pilot’s discretion to tighten up the turn as necessary (including going full standard rate in the turn) to correctly intercept and line up with the localizer. Of course, for a much faster airplane like a jet, even a standard rate turn will quickly start to intercept the localizer.

   This of course is just my guess about it all.

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2. 
   

   
   

   Steve on Jun 05, 2016

   So the basic idea of lead radial is to establish us on the localizer course, not to the FAF?

   Steve.

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3. 
   

   
   

   Skyfox on Jun 14, 2016

   Right. Get established on the localizer and that will take you to the FAF. Getting established ahead of time gives you a little bit of extra time for making sure everything is set up correctly with your nav instruments, throttle, airspeed, etc., and in the case of a full ILS, getting established and stabilized in a descent that follows the glideslope before you actually cross the FAF. Then when you do cross it, your workload is easier because you’ve already gotten so many things taken care of. Reaching the localizer right when you intercept the glideslope or FAF brings a lot of stuff together for you to do all at once.

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