turning at a waypoint before or after...
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turning at a waypoint before or after...
Just want to clarify a point...
Controller asked us the other day once round the hold leave 'Dayne' on a heading of 030 degrees.
The pilot anticipated the turn just prior going overhead onto the heading. I dont want to split hairs here (nor in a busy cockpit enviroment) but I thought one goes overhead first then start your turn. I can see the argument anticpating the turn means if you get it quite inaccurate that you may end up turning too early and thus cause an annoyance to ATC.
..and we dont want to do that!
Controller asked us the other day once round the hold leave 'Dayne' on a heading of 030 degrees.
The pilot anticipated the turn just prior going overhead onto the heading. I dont want to split hairs here (nor in a busy cockpit enviroment) but I thought one goes overhead first then start your turn. I can see the argument anticpating the turn means if you get it quite inaccurate that you may end up turning too early and thus cause an annoyance to ATC.
..and we dont want to do that!
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I think this is one of those situations where the pilot thinks the controller works with far more accuracy than he actually does.
You know, even if the turn is a massive right 90 or something, it doesnt make a blind bit of difference whether you turn at the waypoint, a a mile before it, or a mile after it.
Remember, the heading itself is not a GPS track - its a heading! a rough guess, of where the controller would like to see you move along the screen ...
Accuracy in lateral navigation is limited from an ATC point of view - and is naturally so because of radar limitations, wind (which is generally an unknown quantity to us), aircraft characteristics (interia of a 747 turn vs. a 318 for example)
That's why we use vertical separation heaps more than lateral!
You know, even if the turn is a massive right 90 or something, it doesnt make a blind bit of difference whether you turn at the waypoint, a a mile before it, or a mile after it.
Remember, the heading itself is not a GPS track - its a heading! a rough guess, of where the controller would like to see you move along the screen ...
Accuracy in lateral navigation is limited from an ATC point of view - and is naturally so because of radar limitations, wind (which is generally an unknown quantity to us), aircraft characteristics (interia of a 747 turn vs. a 318 for example)
That's why we use vertical separation heaps more than lateral!
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There are a few younger ATCOs who don't know the difference between
'Leave (fix ABC) heading XYZ'
and
'After (fix ABC) fly heading'.
If instructed to do the former (as you were in your example), then one can and would expect the pilot to manoeuvre the aircraft (within reason) to ensure that the aircraft will fly over the fix on, or as near as, the instructed heading. If this heading is vastly different from the normal holding pattern track, then it will make a difference.
If the instruction is the latter (after fix fly heading), then you fly the normal hold, then turn onto the desired heading.
However, even in the LTMA, we won't worry about the odd mile off track - any more than 3 or 4 in the holding pattern and you may well be quizzed!
As for the comment about accuracy of lateral navigation, radar, unknown aircraft characteristics and unknown effects of wind that was made above, that's not correct in the UK, and certainly not at the levels and speeds you are talking about (i.e. in DAYNE)
'Leave (fix ABC) heading XYZ'
and
'After (fix ABC) fly heading'.
If instructed to do the former (as you were in your example), then one can and would expect the pilot to manoeuvre the aircraft (within reason) to ensure that the aircraft will fly over the fix on, or as near as, the instructed heading. If this heading is vastly different from the normal holding pattern track, then it will make a difference.
If the instruction is the latter (after fix fly heading), then you fly the normal hold, then turn onto the desired heading.
However, even in the LTMA, we won't worry about the odd mile off track - any more than 3 or 4 in the holding pattern and you may well be quizzed!
As for the comment about accuracy of lateral navigation, radar, unknown aircraft characteristics and unknown effects of wind that was made above, that's not correct in the UK, and certainly not at the levels and speeds you are talking about (i.e. in DAYNE)
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A mile or so won't make a big difference to us. The instruction to leave a fix on a heading will be done for sequencing, not for separation purposes. Some aircraft will turn a mile or so early, others run over the fix and turn, so we're not relying on it for separation. (Maybe other ATCOs can recall what aircraft type had the reputation for turning early at waypoints - either the E145 or A320).
030 off DAYNE is just a downwind heading so the real fine-tuning will be done with the turns to base and final.
030 off DAYNE is just a downwind heading so the real fine-tuning will be done with the turns to base and final.
One difference which can affect this situation is aircraft type. As has been previously stated it's splitting hairs and in the great scheme of things it won't make a significant difference.
But on the B737-800 I fly at the moment let's say you are going to XXX (no holding) and the route is programmed to fly to YYY with a large track change then the FMS will work the geometry and start the turn early so as to roll out on the next track without an intercept or overshoot. So let's say we are asked to "Leave XXX heading ZZZ" we would usually select this heading ZZZ as the a/c starts the programmed track change to YYY. Depending on the Heading change versus the programmed track change you may or may not overfly the fix on the new heading - are you with me so far?
When I flew the A320 track changes at waypoints were more or less the same (although I recall that it would use about 5 degrees of bank and therefore start the turn earlier and cunningly cut the corner to save fuel) but the pilot could also select an option to "overfly" the waypoint before turning if one so wished.
Phew, I knew there was a reason why I woke up early for my dawn patrol this morning!
But on the B737-800 I fly at the moment let's say you are going to XXX (no holding) and the route is programmed to fly to YYY with a large track change then the FMS will work the geometry and start the turn early so as to roll out on the next track without an intercept or overshoot. So let's say we are asked to "Leave XXX heading ZZZ" we would usually select this heading ZZZ as the a/c starts the programmed track change to YYY. Depending on the Heading change versus the programmed track change you may or may not overfly the fix on the new heading - are you with me so far?
When I flew the A320 track changes at waypoints were more or less the same (although I recall that it would use about 5 degrees of bank and therefore start the turn earlier and cunningly cut the corner to save fuel) but the pilot could also select an option to "overfly" the waypoint before turning if one so wished.
Phew, I knew there was a reason why I woke up early for my dawn patrol this morning!
Last edited by fireflybob; 23rd Jan 2009 at 19:14.
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I agree with above, I've noticed the A320 family particularly the Shamrock's cut the corner. For me, it's most noticeable at EXMOR when they're coming up from the South then cutting left to STU for EIDW.
Doesn't bother me, I know they're going to do it and if there's a separation issue they'll be on a heading anyway. Plus I know I can ask them to overfly the point before commencing the turn should it be needed.
As to the original question, if I ask an aircraft to leave a point heading it generally to set them up with some form of separation before I actually need it so I've got time to fiddle with it if needs be.
In my head that all made sense. so hopefully it does here
Doesn't bother me, I know they're going to do it and if there's a separation issue they'll be on a heading anyway. Plus I know I can ask them to overfly the point before commencing the turn should it be needed.
As to the original question, if I ask an aircraft to leave a point heading it generally to set them up with some form of separation before I actually need it so I've got time to fiddle with it if needs be.
In my head that all made sense. so hopefully it does here
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As a driver its a bit embarrasing when it happens, classic one being Detling -Alkin, especially when keeping the speed up. My machine can really go for it very early leading to me to sheepishly cough to it and asking Thames if another 10 deg right would be in order . Me at the same time wondering if the controller is secretly thinking if I have no idea what my aircraft is doing 
