Mdi
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Join Date: Feb 2000
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Mdi
Can any of you ATCO chaps explain MDI (minimum departure intervals) please. I know that it requires there to be a minimum time interval between departures, but when does it apply, how does it interfere with slots and so on?
I heard it refered to at Glasgow this morning, where there were also LVPs in force as well: it sounded like an ATCOs nightmare.
I heard it refered to at Glasgow this morning, where there were also LVPs in force as well: it sounded like an ATCOs nightmare.
Join Date: Dec 1999
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At Heathrow:
In normal circumstances, we have either 1 minute (wheels up) or 2 minute route separations. if the initial SID tracks of successive departures diverge by 45 degrees or more, then it's wheels up, if they follow the same inital track or diverge by less than 45 degrees it's two minutes.
For example, our standard deparature interval for a BPK followed by another BPK is two minutes.
When either LTCC or LACC put on MDIs, either due weather, staff shortage etc, then the two minutes gets changed to another interval. Usually it's four minutes, but I've seen 10 minutes.
When weather hits it's not unusual to see virtually every route given an MDI: BPK, WOBUN one every four minutes, CPT and SAM every six minutes, DVR/DET one every five minutes. It gets really difficult to try and keep an expeditious departure rate when that happens. Even with only one route being regulated in this way, the overall deparure rate tends to go down, because you have to conentrate on hitting the 'one every four minutes', sometimes to the detriment to the overall departure sequence, because Delivery will be starting one every four minutes, and you'll very soon get a queue at the hold!
Regarding slots, they in effect go out the window. As long as you call to start in good time to make the slot, and do not depart earlier than the slot tolerance, you can depart long after the slot has expired, at least at Heathrow. The problem for us comes when the MDI gets cancelled. Delivery might have five or ten aircraft that called up to make their slot, but have been delayed because of the MDI. However, the MDI is cancelled, and then you have five or ten aircraft that are now well outside their slot times and no longer can depart outside it, because there is no MDI in force any more. Things can get a bit awkward because Flow/TC/AC often believe that getting the MDI cancelled is doing us a favour, getting rid of delays, and can't see that cancelling it might cause delays in it's own right
In normal circumstances, we have either 1 minute (wheels up) or 2 minute route separations. if the initial SID tracks of successive departures diverge by 45 degrees or more, then it's wheels up, if they follow the same inital track or diverge by less than 45 degrees it's two minutes.
For example, our standard deparature interval for a BPK followed by another BPK is two minutes.
When either LTCC or LACC put on MDIs, either due weather, staff shortage etc, then the two minutes gets changed to another interval. Usually it's four minutes, but I've seen 10 minutes.
When weather hits it's not unusual to see virtually every route given an MDI: BPK, WOBUN one every four minutes, CPT and SAM every six minutes, DVR/DET one every five minutes. It gets really difficult to try and keep an expeditious departure rate when that happens. Even with only one route being regulated in this way, the overall deparure rate tends to go down, because you have to conentrate on hitting the 'one every four minutes', sometimes to the detriment to the overall departure sequence, because Delivery will be starting one every four minutes, and you'll very soon get a queue at the hold!
Regarding slots, they in effect go out the window. As long as you call to start in good time to make the slot, and do not depart earlier than the slot tolerance, you can depart long after the slot has expired, at least at Heathrow. The problem for us comes when the MDI gets cancelled. Delivery might have five or ten aircraft that called up to make their slot, but have been delayed because of the MDI. However, the MDI is cancelled, and then you have five or ten aircraft that are now well outside their slot times and no longer can depart outside it, because there is no MDI in force any more. Things can get a bit awkward because Flow/TC/AC often believe that getting the MDI cancelled is doing us a favour, getting rid of delays, and can't see that cancelling it might cause delays in it's own right
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Yep. I was assuming both aircraft were jets, which are all in the same group, except for maybe a few slower Citations, who are in their own slower grioup with Saab 2000s, then slower again are F50s, ATRs etc. The 1 or 2 minutes gets modified depnding on which speed group they both belong to.
