Thursday, 9 July 2009

Slot City

I'm on the 4th of 6 early starts. Each day I'm awoken from a deeper sleep by the alarm clock at o'dark hundred and each day the disappointment gets greater and greater as I realise I am actually working, rather than forgetting to turn the alarm clock off or it being a bad dream.

It's a case of dej√° vu each morning, the quiet roads on the way to work, the same aircraft sitting awaiting departure each morning, and equally unfortunate bleary eyed crews turning up and exchanging sympathies about how many earlies we're doing. Greeting most pilots in the morning, after the usual mutual moaning of fatigue the next question is always 'Is there any slot?'

Early morning is a busy time for both airports and the airspace around them. With a wave of departures all wanting to leave at the same time and follow the same pieces of airspace to the same destination, or destinations close by, their flow has to be regulated. That's where the black art of slots come into play in Europe.

I don't work in flight planning ops or air traffic, so the following simplified explanation will probably leave some banging their head and comparing me to an uninformed journalist trying to report on some minor aviation incident, but alas, I'll have a go.

Each flightplan filed in Europe goes through a computer known as CFMU, or Central Flow Management Unit. I imagine it as a massive super computer stored in a darkened room with it's own micro-climate, with many operatives nurturing it and keeping it happy. In return, it will crunch many numbers and try and attain the most efficient use of airspace around Europe, keeping things moving and preventing bottlenecks at busy spots. To attain this regulated flow, it devises a time for each aircraft to depart/arrive/pass overhead a point, and this results in a Calculated Take Off Time, or CTOT. Since modern flight planning is so precise, sectors of many hours can usually be accurate to a minute or so easily, it churns out a CTOT for a flight so that it should progress smoothly.

This CTOT, or slot as it's commonly referred to, is the time the aircraft should be taking off at. This puts added pressure on everyone, if the slot is missed it could be a long time before the flight gets another one. Although the CTOT is an actual time, there is a discretion of -5 to +10 minutes that provides a window that the aircraft can depart in. However, it's to be used only by the local Air traffic controllers, to allow them to work the aircraft into the flow of traffic on the ground, since it won't always be possible to have it at the runway at that exact time. For us the ground crew, we're working to get the aircraft off stand 10, 15 or 20 minutes before the CTOT, depending on the ramp traffic and taxi time.

So on any early morning, it's not unsual to have most of the departures having slots. This messes up the usual order of departures we would be expecting, as a flight due to depart at 0630 may have a slot of 0720, but a flight due to depart at 0635 may have a slot of 0642. It's also not unsual to have a slot that results in the aircraft having to depart the gate early in order to make it. Slots often tend to change as well, as the traffic flow changes. In the space of the hour leading up to departure, it can be common to have the slot to change three or four times, jumping forward or even backwards.

Finally getting around to taking a picture now and again, without giving too much away about where I work. Below is one I took recently, the panel located on the back of the nose gear strut of a 767. You can see some of the buttons I talked about in a post last year when discussing the headset conversation. Don't be tempted to push the large red or black button...unless you really need to! You'll just create endless amounts of paperwork for yourself, and a no tea and biscuits meeting with the manager.

The headset lead plugs into the jack on the bottom left, titled Flight Interphone. The wheel well light switch controls a light inside the wheel well so it can be easily inspected, and the switches relate to the APU, so that it can be shut down and the fire bottle discharged should anything happen.

2 earlies to go...then some proper sleep!


Wayne Conrad said...

The gear handle in a plane is shaped like a gear, and the flaps handle shaped like a flap. Because to mistake one for the other is really bad.

So here we have a small black button that is OK to press, not far from a large red button that is usually bad to press, and a little farther away from a large black button that is usually really, really bad to press. And the only thing different between "OK" and "really, really bad, most of the time" is the size.


dpierce said...

Great read as always! Do you know if slots are assigned after the flight plan is filed, or do they try to pre-assign things based on an estimated schedule?

They really should put a safety cover over those "bad" buttons.

Anonymous said...

In Australia we use CTMS, which sounds similar to your CTOT. It is only really designed for airports that need slots, like Sydney.

Dispatcher said...

Slots are assigned once the flight plan is filed as far as I know. Sometimes you will see two flight plans for one flight, each with a different route under a different callsign to try and find the route with the least delay.

Because airspace around certain parts of Europe is so congested, even if its a quiet departure airport and the destination is equally quiet with traffic, slots still appear to control the flow through the busy sectors enroute. Yesterday I had one such flight that had a slot due to three restrictions enroute, nothing to do with problems at departure or arrival aerodromes.

abouagela said...

just what to know what is the meaning of EO 18:30, you could see this for departure messages like AD18:20 and EO 18:30, please any one could explain that for me.

A Esarwi

Dispatcher said...


I can't say I've come across it before. Only thing I can think of is perhaps it's an estimate airborne time. At some of the busier airfields, the time between pushback and airborne could be 30mins easily, perhaps it's a way of letting the airline and downroute station know the flight is underway, but just not airborne yet?

Maybe someone else can shed some light on it


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