about "fly heading"
 

as a controller ,when instructing an aircraft to exit a orbit or holding and vector it to a new heading, if i should use the phraseology: " xx, fly heading XXX"? in this case， i want to know how the pilot will turn to the new heading , in the shortest direction or in the oribit direction ?

If the turn direction is not specified, the pilot is expected to turn the shortest amount of degrees. Actually I don't know if this is mandatory, but for sure an ATC expects the pilot to do that and pilots try to comply with it.
As you can imagine during a holding it can be chosen to continue the turn for 200 degress rather than going back and change the direction to make 160 degrees. Probably it's a matter of shortest time and workload rather than lower amount of degrees...

I think what I wrote can be considered the "accepted rule", is there a written one?

A few variables, and it might be assumed by some that the pilot will take the shortest turn.

However reversing a turn in an orbit to roll out on a specified heading does not always make sense. For ease of flying, passenger comfort etc, the crew may well elect to continue the turn until the specified heading, even if it is the long way round.

In a published hold, it would be fair to assume that the pilot will continue to turn in the published direction of the hold.

However, these are assumptions... if you are in any doubt why not use specific phraseology?

In a hold with a specified datum "Hold cancelled, leave XXX (datum) heading xxx degrees" - aircraft will continue in the same direction

In an orbit you could use 'Continue right/left turn onto heading xxx degrees"

In both of the above, you and the pilot are in no doubt as to what way you want them to turn.

If you want them to reverse the turn onto a heading, use "turn left/right heading xxx degrees" - you could even prefix it with "Stop turn/orbit...".

Bear in mind that unless you have a very good reason for wanting to reverse the turn, the chances are the pilot would rather continue in the original direction.
By the time the turn has been stopped and reversed, it will take almost as long to get to your heading, it will take up more airspace, and the exit point will be less predictable.

Maybe not 100% ICAO standard phraseology, but it is unambiguous.

ok, i think i see, by the way, can you make an example about the icao phraseology :FLY HEADING (three digits), WHEN ABLE PROCEED DIRECT
(name) (significant point);

Do not confuse Pilot Navigation and Controller Navigation. It can either be 'Fly Heading' (Controller Nav) or 'Proceed Direct X' (Pilot Navigation), not both. Nothing to prevent 'Fly Heading, intercept track to X'. 'When able' should not be necessary - these days a pilot should always be able to immediately 'fly direct'. In the case of 'FLY HEADING (three digits), WHEN ABLE PROCEED DIRECT (name) (significant point)' there is no control of predicted track so I think such phraseology is rubbish, ICAO or not.

ok, it is excerpted from icao standarad phraseology. i wonder what does it mean by "name " and "significant point" ,does they mean different point?

Why not say "continue right/left to heading XXX" ??

HD, what happens if they have already gone through the heading specified? Doesn't happen often, but occasionally due to radar lag I have heard "turning back to xxx".

Talkdownman, not necessarily - exceptions to everything! When giving a reroute or weather avoidance I will sometimes say "fly heading xxx, when able route to <waypoint>". Would this also be used to VFR traffic avoiding cloud??

mxwbuaa, I think they both mean the same point, however several points could be given as a routing.

OK, fair enough, but I would never issue a heading for wx avoidance, I'd put the nav straight back in the cockpit and go vertical.
VFR - I leave nav in cockpit unless pilot requires nav assistance then only advisory headings. I'm not into vectoring VFR whether on the radar or in the cockpit...

Lemme guess - you have no experience of working in Class B or C approach control then?

This could get interesting :}

Then there is "steer heading........" and "roll-out heading........".

You guess correctly. I don't think we have any Class B or C approach control here in the UK. Come to think of it we don't have that much VMC either...

I don't think they have irony in the USA either :E

They will when Jellystone Park erupts! :ok:

What about saying "turn left/right to heading xxx". And if you have trouble discerning left and right, you are in the wrong profession....

already covered by this:

Ah, but how do you know what the Drift Angle is...? Mode S?