ICAO holding patterns vs FMS

[D-IFF_ident]

Can anyone give me directions to a source document explaning ICAO holding regulations, rather than an explanation of procedures? I ask out of curiousity with regard to all the nice shiney FMSs installed on many of our aircraft. By my understanding the FMS 800, for example, will fly a holding pattern very nicely, whether it be a timed hold or a DME based hold. But I also understand that the FMS 800, for example, will apply one times the drift to an outbound heading, intercept the inbound heading and not correct for head/tail winds on a timed leg outbound. So, what I want to know is: Do ICAO regulations allow single-drift holding patterns, or should we be using radio navaids and a stopwatch, applying 3 times the drift outbound etc, and not coupling the FMS in the hold, apart from GPS/RNAV holds of course. Hmm?

[BEagle]

Don't forget that the FMS800 merely navigates the a/c to remain on the defining course. Because there is no feedback from track error in the turn to modify the autopilot bank angle in old a/c such as the Vickers FunBus, having dutifully tracked the holding course, when it reaches the turn it will merely command a standard turn at 25-ish deg AoB, to roll out on the reciprocal track. It will then sniff out that track, commanding turns as required to achieve that. Timing isn't really relevant, as it will know when it gets to the end of the outbound track.

Unless the a/c has been modified to have full authority over roll angle with the AP NAV mode slaved to FMS, it will only 'hold' as best it can - and that will also depend upon the accuracy of the hold pattern inserted.

The RAF FIH gives a good description of the requirements for holding; however a modern FMS-equipped a/c will have an AFS system which allows for the a/c characterstics and is able to process all values of position, wind, cross-track error in order to fly a truly accurate hold.

My preference with the FMS800 was to let it navigate to the entry fix, then deselect NAV mode and fly the a/c around the hold using the 'traditional' 3xdrift outbound and 'due allowance' for head/tailwind, using HSI/CDI beam bar information and the RDU display with the AP in MAN mode. Then reselect NAV outbound on the procedure until setting up for an Auto ILS or full manual approach. I didn't like the 'half and half' way of using the AP on an instrument approach in a hybrid style.

Those who had a good grasp of the system found the VC10K installation much simpler to operate than the VC10C installation which put the CDI into True whenever the source switch was at INS.

[FFP]

D-IFF,

Sounds like you've got an FMS to play with now ?!?!?!

Check your PM's :ok: