Intersection Departures

[Slipped_Surly_Blonde]

Heres a quick question for all those in the know regarding wake turbulence and intersection depatures....

For a medium aircraft departing from a runway intersection behind a heavy using the full length, is the time delay always three minutes or does it depend on how far the intersection is from the end of the runway??

If so how far does the interesction have to be inset before it is deemed an intersection and whereis it written?

i.e. using H/G on 08L or B/C on 26L at LGW.

Couldn't find it in any of the manuals on the flight deck .... you guessed - another long sector!!

Cheers!

SSB;)

[Rainboe]

Isn't it 2 minutes for full length and 3 minutes from an intersection? I may well be out of date, after 8 years flying the preceding big one, one doesn't really care anymore about those that follow!

[BOAC]

Cannot help with a reference, SSB, but somewhere in the darkest recesses of the brain I recall that 'B' and 'H' at LGW do not 'count' as displaced for wake purposes - although it is always your prerogative to do so.

I think 'B' and 'H' are 300mtrs or so (don't have the charts to hand) and I recall a figure of 760mtrs as the 'definition'.

[Spitoon]

ICAO Doc 4444 says
"5.8.3 Departing aircraft
5.8.3.1 A minimum separation of 2 minutes shall be applied between a LIGHT or MEDIUM aircraft taking off behind a HEAVY aircraft or a LIGHT aircraft taking off behind a MEDIUM aircraft when the aircraft are using:
a) the same runway;
b) parallel runways separated by less than 760 m (2 500 ft);
c) crossing runways if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1 000 ft) below;
d) parallel runways separated by 760 m (2 500 ft) or more, if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1 000 ft) below.
5.8.3.2 A separation minimum of 3 minutes shall be applied between a LIGHT or MEDIUM aircraft when taking off behind a HEAVY aircraft or a LIGHT aircraft when taking off behind a MEDIUM aircraft from:
a) an intermediate part of the same runway; or
b) an intermediate part of a parallel runway separated by less than 760 m (2 500 ft)."

Unfortunately I don't think ICAO says what intermediate means. But in the UK if there are two runway entry points within a short distance the CAA may approve procedures so that they can be considered to be the same for departure separation purposes.

[Slipped_Surly_Blonde]

Thanks guys

Saw LGW tower give a Dash-8 lined up at Bravo take-off clearance after 2 mins behind a Jumbo so I guess that answered my question!! Sincerely hope you were able to cut and paste all that Spitoon!

SSB

[Slipped_Surly_Blonde]

In fact I'd be even more impressed if you had typed it quickly from memory!

SSB

[BOAC]

The main problem with wake turb is flying into it:) If you can rotate and climb before/above the wake (as modified by wind) you will be OK. Hence the Dash rotes very early, well before the main vortices fron the Jumbo start (at rotate)

[Rainboe]

Still to be watched for is the case of an intersection take-off following a preceding big one, and then experiencing an early engine failure, so you cannot rely any longer on outclimbing the vortex and turning performance may be limited. That is probably why the vortex separation is so restrictive for an intersection departure.

[dv8]

Almost on the same topic Can I clarify a point

If you are the same weight category is there no vortex time limit
i.e will be cleared for take off as soon as the preceding is airborne?

[Rainboe]

I believe not. For a 747, there is no bother following another 747. I think it's the same for littler ones.