keithl... A fix is normally used at the end of the outbound leg to reduce the dimensions of the protection area. Thus it is a mandatory turn point. Timing is not quite so simple because there is no real guarantee as to how far away from the NDB you are at the end of the time period. This is obviously a function of how the wind affects your progress - a tailwind will take you further away, while a headwind will mean you are much closer to the NDB.
Thus it is necessary to adjust the timing, in order to compensate for the effect of a known wind. The whole idea of this is to ensure that you will be far enough away that the reversal maneouvre gives you a fair chance of intercepting the FAT. The first paragraph of
DFC's post has said the same thing that I've been trying to say.
Indeed,
FOUR REDS is correct about the application of drift. I've had to contend with some pretty significant drift on a NDB procedure and have only ever applied enough correction to adjust for that and, therefore, remain on course within the legal tracking tolerances. As I commence the reversal, I slow down a bit more than usual so that I won't overshoot the FAT too much. It soon becomes apparent if I'm going to overshoot it, so I simply keep the turn coming around, to re-intercept the FAT because that is what I must do - and I know that the procedure will protect me while doing so.
This might also satisfy
bookworm's concern about being outside the legal tracking tolerance after overshooting the FAT. Sure, you might be 5-degrees of the FAT, but you are continuing to turn, to re-establish that track. The sooner that you can establish yourself within 5 degrees of the track, the sooner you can start descent in the final segment. It's not perfect, of course, but there isn't a lot more that a designer can build into this type of approach.
One advantage that I have in making the correction on the o/b is that I have course guidance and, within the accuracy of the instrument, I know where I am with respect to the published track. I can make sure that I'm no more than 5 degrees off.
This is at the expense of exploring the outer limits of the outbound protection area on the up-wind side. There is comparitively less protection here because Pans Ops assumes that the pilot will track outbound as accurately as possible. There is basically more protection for the reversal because of the vagaries of wind, the different TAS and bank angles of different aircraft, etc. All of this implies that there is less accuracy in this maneouvre and hence the protection area makes a lot of worst-case assumptions to provide protection.
By contrast, if I fly the published o/b and accept the drift in the turn, the first I'll know about how far off the FAT I am is when I see the needle indicate it at the completion of the turn. For all I know, it may be 10 degrees off.
I'd venture to suggest that you will have a pretty reliable indication of this situation when you're halfway through the reversal turn. I certainly watch the needle movement through the turn, to get the earliest possible indication of overshoot, or even undershoot. For an overshoot, I then have the flexibility of going to 30-40 degrees of bank quite safely at this stage, to minimise the amount of overshoot. I know that some companies limit bank angle to 25 degrees, but I am not so limited.
I guess it has to be said that, if you're getting significant cross-wind on the o/b leg and then not have enough time to reach MDA after re-intercepting the FAT, a go-around will be necessary. Indeed, if the cross-wind is truly significant (I would classify 50+ knots that way), there's a pretty good chance that the cross-wind on the landing runway could be very close to the maximum for the aircraft anyway. In that event, I'd be looking for an alternate aerodrome or, at least, an alternate procedure to another runway.
I wonder if we're starting to get too technical in responding to the initial enquiry?
