Wake Vortex separation

[Nubboy]

Possibly an old chestnut but I'm still confused.

Given line up and wait clearance at FRA 07L from an intersection after a Lufty 744. My go and I start the watch and suggest 3 minutes. My LHS (a senior trainer) says Yep and on we go. Given take off clearance after 2 minutes and we requeat a further minute. The tower says no and pull us off.

En route we look up the minima in the rule book and find 2 minutes or 5 miles with Radar, or 3 minutes if an intersection depature with a Medium behind a Heavy without Radar. Up to now we've always used 3 mins regardless of Radar or procedural separation. Have we missed something? How does Radar stop you hitting his wake vortices?

Thanks in advance.

[cossack]

I can only speak for the UK not FRA.

If the intersection concerned is considered by ATC and notified as a "same departure point" for vortex wake, then the 3 minutes can be reduced to 2, but only if you accept it. The intescection would need to be less than (approx) 400m from the full length departure point.

I bet getting you off the runway wasted more time than waitng the extra minute! :)

It is good ATC practice to ask if you can accept the reduction in time prior to issuing a line up clearance.

If it wasn't an intersection notified for this reduction, then ATC were wrong and you were right.

[HEATHROW DIRECTOR]

A lot depends on when you time it from. Most tower controllers would clear you to go so that there were 3 minutes between the time you lifted off and the previous departure did the same. I know that some crews start the watch from the time the one ahead is actually airborne... and given that an average jet takes about 40-50 seconds to lift off you immediately have an extra minute wasted. I suggest you trust ATC to do the timing and go when the Tower clears you..

[Gonzo]

At LHR, intersection departures from block 102 for 09R, 86 for 27L and 18 for 27R are, according to our procedures, not considered to be intersections for the purpose of wake vortex. However, in my opinion it shows an appreciation of a/c types and general awareness if the before we launch you we just confirm that you're happy to take the 2 mins behind a heavy, or if say, you're a 767, 1 minute. I have to agree with the above, though, getting you to vacate may have satisified the controller's immediate perceived annoyance, but it must have lost him/her more time! All I can suggest is that next time ask for 3 mins when it's clear you're going to go behind a heavier a/c from an intersection.

Gonzo

[Bear Cub]

Director....my understanding is that the vortex starts (at least in anger) at the point of rotation. Nose wheel off = vortex.

If the second departure was from an intersection then surely it is closer to this point and should leave a bigger gap.

I just read (yesterday) a stunning research report on wake vortex and many of the "rules adopted so far" were shown to be incorrect - according to a study conducted at LHR.

[Gonzo]

Incorrect? Like the rule where a L behind a H at Heathrow is 7 miles, whereas everywhere else in the UK (except Gatwick?) it's 8 miles? Discuss........

Gonzo

[Nubboy]

Thanks for the replies Gents.
Yes we did start the watch when the Lufty 744 started his roll, not at rotatiion. Yes, it did take as long to clear the runway as to have waited and gone, but 40 tonnes of Fkker rotating where 400T on 744 had gone didn't seem a good idea.
Although we don't do FRA regularly, as opposed to other German airports, they seem to have given me more surprises than most. The best was changing ILS from 25L to 26L and insisting it was our equipment that had failed as they had a signal good indication in the tower. Further digging on the ground revealed a technician had changed them over without authorisation. Never mind, keeps you on your toes. Good job there's not too many new carriers with low experience levels.