Separation between departing a/c

[Moorsel]

Dear all,
I am doing some comparative studies on Minima separation between departing a/c (from the same runway).

I was really surprised both reading the UK regulation ( CAP 493 par. 8.6 ) and reading the FAA regulation, from par. 3.9.1to par. 3.9.10.

http://www1.faa.gov/ATpubs/ATC/CHP3.HTM

Just starting with the British case. I am wondering which separation you apply between departing a/c - (same tracks) from LHR . I am sure that the separation is not 5 minutes as reported in CAP 493. I understand that this rule could be overridden by much more practical rules (e.g. assigning an initial FL to the second a/c). So my questions would be: what is the typical interval in minutes or seconds between 2 medium a/c departing from LHR from the same rwy and on the same departing track ? Which separation do you apply?

Now I would have some questions for our American friends. Do you really do what it is reported in your regulation? It seems to me that when a second a/c takes off from the same runway, you may have some seconds during which you have got neither a radar nor a vertical separation. How can you safely get a radar or vertical separation between 2 departing a/c with such a tight separation minima?
Do you apply such regulation in all weather conditions and in all airports/runways?

Many Thanks for your answers

Francesco Moorsel

[Gonzo]

At LHR, two aircraft departing on the same SID from the same runway - two minutes.... and no change in assigned level (alt. 6000)

[HEATHROW DIRECTOR]

Moorsel.. Further to Gonzo's answer... remember that in the London TMA (as well as most others) aircraft are under radar control immediately they leave the ground and 2 minutes will provide 3nm radar separation. A/c on different tracks from Heathrow can go as low as 1 minute apart - less if the weather is good enough for the Tower Controller to maintain visual contact with the a/c until they are separated. This probably applies to the majority of busy airporst around the world.

[Scott Voigt]

Hi Francesco;

In the US we do indeed flush them off the runway <G>... If they are like types such as two MD80's or B737's etc. all we need to have is one aircraft at least 6000 ft. down the runway and the nose wheel coming up. When that happens, and if the aircraft are going to diverge within one minute after departure, we can launch them using visual separation. It is the "they are never going to hit rule <G>." By the time the second aircraft comes off the runway, you have the first aircraft in radar contact in most cases and will soon be seeing the second one. After the first minute goes by, they will be established on headings that will give them 15 degrees divergence and other forms of separation can be disregarded... In other words, the tower can flush a LOT of aircraft off othe the airport.

I hope that answers your questions, if not though please ask for more clarification.

regards

Scott

[Moorsel]

Thank you Scott, very kind of you.
Indeed I would be delighted to get some more explanations. I put my comments/questions below your sentences.

Francesco

*****
Scott: " In the US we do indeed flush them off the runway <G>... If they are like types such as two MD80's or B737's etc. all we need to have is one aircraft at least 6000 ft. down the runway and the nose wheel coming up."

Francesco: In fact my understanding is that you only need 6000 ft on the same rwy to clear for take off the second a/c of the same category ( as you say either MD80's or B737's). If the runway is shorter, then it is enough that the first departing a/c has overflown the thresold. It seems to me that this rule applies both in IMC and VMC.

******
Scott: " When that happens, and if the aircraft are going to diverge within one minute after departure, we can launch them using visual separation."

Francesco: I think that this is valid also in Europe. The point is that we use the wording "immediately after take off".

******
Scott: "It is the "they are never going to hit rule <G>."

Francesco: In fact my understanding is that the Tower ATCO has always a good rule to launch the second a/c when the first has left 6000 Ft, but excluding wake vortex issue.

******
Scott: " By the time the second aircraft comes off the runway, you have the first aircraft in radar contact in most cases and will soon be seeing the second one. After the first minute goes by, they will be established on headings that will give them 15 degrees divergence and other forms of separation can be disregarded..."

Francesco: Who instructed the second a/c in a 15 degree divergent route after take off? Perhaps the TWR ?

******
Scott: In other words, the tower can flush a LOT of aircraft off the airport.

Francesco: Yeah ! I think I have got the meaning ! As you said " the TWR launches a/c". I remember that when I was an air force officer I was part of a military exercise which simulated the airbase under attack. Our duty as military ATCOs was to launch as many a/c as possible in half an hour ! That's more or less the duty of American TWRs during the departure wave !

Saying that in a more formal way it seems to me that "the minimum interval between two successive departing a/c is equal to the Runway Occupancy Time of the first taking off a/c, provided that there is no wake vortex or other specific issues to consider.

According to the situation, the TWR may add some safety margins to the above rule. In that case a typical instruction is the assignmenet of a divergent route ( say 15 degrees) after take off before passing the a/c to the TRACON.

Am I right?

Thanks
Francesco

[Scott Voigt]

Francesco;

You have it all correct. The tower controller either through a letter of agreement with the approach control, via instructions from the approach control, or due to airspace being delegated to the tower as IFR airspace that they are responsible for, can issue the needed headings to get the separation that they need.

regards

Scott

[FWA NATCA]

Moorsel,

Everything here in the U.S. works pretty much as Scott stated. Basically for large aircraft the seperation is 6000' and airborne, and you turn the subsequent aircraft 15 degrees (fan the departures). As with everything within ATC, common sense does apply.

If the departure airport only has a VFR tower, then the norm is 45 degrees or more between departures. Again the VFR tower will receive departure instructions from the overlying Radar facility that fans multiple departures.

Not to confuse you, during some special events (such as the EAA Air Venture Air Show) we receive a special wavier that halfs the required seperation requirements for VFR's arrivals and departures. IFR's are still handled utilizing normal seperation. You can find the NOTAM information at http://www.eaa.org

Mike
NATCA FWA

[rej]

I used to enjoy watching the students at CFSACO sweating over the most expetitious way of launching multiple IFR deps in the non-radar sims. IDS, OTL and then throw in the VFR controller's IFR wake turbulence sep - mmmmm heaven!!

[Moorsel]

Thank you FWA NATCA.

I understand that operations in US are very different from Europe. If you do not mind I would ask some further clarifications.

You said:

" Basically for large aircraft the seperation is 6000' and airborne, and you turn the subsequent aircraft 15 degrees (fan the departures). "

Comments/Questions:

1. Separation on the runway is 6000 feet. OK, fair enough.

2. Then when the a/c is airborne, it will receive an heading of 15 degrees (at least). OK. I would imagine that at some airports you might have some problems with the radar terrain clearance. As you say, common sense applies.

3. Have you got noise restrictions at busy airports and are they constraining your dep. procedures?


Many thanks
Francesco Moorsel

[Scott Voigt]

Hi Francesco;

We do have noise abatement at some airports, but it is usually not nearly as stringent as most of Europe. There are always exceptions though... Terrain can cause problems too, but it appears that most of our main airports don't have to much of a constraint there either.

One thing that helps us out at some airports is that aircraft normally are going in different directions, so we may fan out some a little at the first, but then after airborne we can spread then even further apart by going north, south, east and west with them. If we are in the middle of a departure push, and there is room on the taxiways to do so, we will try to taxi aircraft so as to stagger the departures so that no two aircraft are going out the same departure gate if at all possible when following. This allows for more flexibility by the departure controller. As always there are some airports that can be more flexible than others. The DFW ( Dallas / Fort Worth ) airport is one of those that is extremely lucky. Lots and lots of concrete, not obstructions to speak of and lots of ways to get aircraft out and airborne...

regards

Scott