Ask the staffer
Dec 23, 2017, 2:59 am
#2836
Join Date: Nov 2011
Location: LHR Air Traffic Control
Programs: BAEC Silver
Posts: 875
Dec 23, 2017, 3:29 am
#2838
Join Date: Sep 2014
Location: Cumbria
Programs: BAEC GGL/CCR, Hilton Diamond, Starbucks Gold
Posts: 4,510
Dec 23, 2017, 5:06 am
#2839
Join Date: Aug 2008
Posts: 2,065
Last edited by Waterhorse; Dec 23, 2017 at 11:35 am
Dec 23, 2017, 8:33 am
#2841
Join Date: Feb 2014
Location: PHX
Programs: AA Aluminum, WN B+
Posts: 929
Heathrow Tower, I'm curious under your new procedures what would be the separation between a Cessna 172 behind a 737 on parallel runways? Years ago when I was a student pilot I took off from PHX at least 4 minutes after a 737 on a parallel runway. Tower turned me left to fly across midfield on course. Everything was smooth until I crossed the wake, then WHAM! Our heads were in the ceiling. Smooth for the rest of the flight.
The two runways (7L and 7R) are much closer than the runways at Heathrow; basically just a taxiway separates them.
The two runways (7L and 7R) are much closer than the runways at Heathrow; basically just a taxiway separates them.
Dec 23, 2017, 9:21 am
#2842
Join Date: Aug 2008
Posts: 2,065
Not a pilot, so these will be very much newbie questions, but Aob = angle of bank, is that correct?
Am I to understand that basically, what you describe is the aircraft involuntarily being banked by 20 degrees (and turning as a result) due to the wake turbulence? And when you speak of control authority, you refer to the fact that the FO could make no further(?) input to correct it?
Or have I misunderstood?
Thanks
Am I to understand that basically, what you describe is the aircraft involuntarily being banked by 20 degrees (and turning as a result) due to the wake turbulence? And when you speak of control authority, you refer to the fact that the FO could make no further(?) input to correct it?
Or have I misunderstood?
Thanks
Dec 23, 2017, 9:44 am
#2843
Join Date: Nov 2011
Location: LHR Air Traffic Control
Programs: BAEC Silver
Posts: 875
Heathrow Tower, I'm curious under your new procedures what would be the separation between a Cessna 172 behind a 737 on parallel runways? Years ago when I was a student pilot I took off from PHX at least 4 minutes after a 737 on a parallel runway. Tower turned me left to fly across midfield on course. Everything was smooth until I crossed the wake, then WHAM! Our heads were in the ceiling. Smooth for the rest of the flight.
The two runways (7L and 7R) are much closer than the runways at Heathrow; basically just a taxiway separates them.
The two runways (7L and 7R) are much closer than the runways at Heathrow; basically just a taxiway separates them.
Two things regarding your particular case....Firstly, 4 mins is in excess of any separation proscribed, so this is illustrative that wake encounters can still occur even with the correct separation. Secondly, the USA has a more liberal attitude to wake turbulence separation. In the UK ATC is not permitted to reduce or ignore wake separation standards. In the US, on paper, ATC can ask pilots if they are happy to ignore/reduce wake separation. In practice, this usually ends up with ATC giving traffic information with a caution: “You are number two to a 737 in your one o’clock, three miles, caution wake turbulence” and that is seen as ATC fulfilling their responsibility.
Dec 23, 2017, 10:04 am
#2844
Join Date: Jan 2017
Location: Aberdeenshire
Programs: BAEC Silver
Posts: 153
Would a smaller aircraft still encounter the turbulence following a large aircraft on take of if it had a shorter take off distance and faster rate of climb?
Is this taken into account in the minimum wake seperation calculations?
Is this taken into account in the minimum wake seperation calculations?
Dec 23, 2017, 10:18 am
#2845
Join Date: Aug 2008
Posts: 2,065
Wake vortices descend and when they hit the ground they spread out from the point of origin. They are pushed along in the wind so the worst case is a gentle crosswind which will keep the upwind vortex over or in the flight path longer.
The vortices are only a real issue for an airborne aircraft. A lighter or smaller aircraft will normally get airborne before a heavy/larger one and will therefore normally be airborne above the danger. This is why intersection departures need greater separation. Heavier aircraft produce stronger vortices - generally, though the 757 is a known producer of strong vortices. The stronger the vortex the greater the danger, particularly for lighter, slower aircraft as they have less total energy to use to escape the vortex. Inertia can be a life saver in a strong vortex.
Encounters close the ground are the most dangerous as you have less energy and less altitude to play with. Most encounters are brief and not dangerous, merely a sharp jolt. However in the wrong circumstances they can be startling and spectacular. The worst I ever experienced was in training during a stream formation landing, at about 200agl I was turned on my side - c70 AOB in about 1/2 second. Hence we pilots are slightly distrustful of reductions in separation from scientists that can prove all sorts in the lab. Real life can bite and current separation rules still lead to vortex encounters - thankfully most of the non threatening type
The vortices are only a real issue for an airborne aircraft. A lighter or smaller aircraft will normally get airborne before a heavy/larger one and will therefore normally be airborne above the danger. This is why intersection departures need greater separation. Heavier aircraft produce stronger vortices - generally, though the 757 is a known producer of strong vortices. The stronger the vortex the greater the danger, particularly for lighter, slower aircraft as they have less total energy to use to escape the vortex. Inertia can be a life saver in a strong vortex.
Encounters close the ground are the most dangerous as you have less energy and less altitude to play with. Most encounters are brief and not dangerous, merely a sharp jolt. However in the wrong circumstances they can be startling and spectacular. The worst I ever experienced was in training during a stream formation landing, at about 200agl I was turned on my side - c70 AOB in about 1/2 second. Hence we pilots are slightly distrustful of reductions in separation from scientists that can prove all sorts in the lab. Real life can bite and current separation rules still lead to vortex encounters - thankfully most of the non threatening type
Last edited by Waterhorse; Dec 23, 2017 at 11:34 am
Dec 23, 2017, 10:36 am
#2846
Join Date: Nov 2011
Location: LHR Air Traffic Control
Programs: BAEC Silver
Posts: 875
Just to add to Waterhorse's excellent contributions, when an aircraft has a wake encounter, the pilot will file a report. In the UK we have a great relationship with most airlines so that, once we request it, they will send the flight data (such as would be examined after a crash) to us so we can actually see the effect of the vortex to avoid any subjective assessment. For example, we've had reports of a 40 degree AOB encounter, but on examination the aircraft was already established in a 30 degree bank in a left turn, so the wake added 10 degree rolling moment.
The strength of wake is assessed in terms of a rolling moment coefficient.
I'm confident that the UK has one of, if not the, best database of wake encounter information.
The strength of wake is assessed in terms of a rolling moment coefficient.
I'm confident that the UK has one of, if not the, best database of wake encounter information.
Dec 23, 2017, 1:20 pm
#2847
Moderator, Emirates
Join Date: Oct 2012
Location: Where My Heart Is
Programs: BAEC Silver, FB Platinum, KQ Asante Gold, Shebamiles Blue, Emirates Blue
Posts: 3,386
More complicated usually means less safe - particularly when human interaction happens. Even under current rules things happen. I was turned through 20 Aob at 50 feet the other early. I was in an A321 following an A319. We had standard separation but it was an ideal day for a vortex encounter - wind less than 5 knots and coming from 30 degrees off runway heading. It didn't last long but we had run out of control authority and had to hang there for 2-3 seconds in a very poor place. My FO was distracted by the event for the whole flight after that. He had run out of experience and ability. Let's hope this commercially driven reduction in separation doesn't hurt people in the real world. Modelling cannot recreate real life fully.
S
Dec 23, 2017, 1:36 pm
#2848
Join Date: Aug 2008
Posts: 2,065
It was a Manchester but the weather this week does seem ideal for a wake encounter
Dec 24, 2017, 3:27 am
#2849
Suspended
Join Date: Nov 2011
Posts: 536
We could have a huge matrix that covers all eventualities, ensuring there's (almost) never a wake turbulence encounter - but as Heathrow Tower has already said it's impossible to expect a controller to remember tens or hundreds of thousands of variable combinations. The regulations are - by necessity - a simplification. As soon as you simplify things (excluding, of course, doing something silly like specifying 10 minutes between all aircraft) you will allow more encounters to occur, it's impossible not to.
Dec 24, 2017, 3:52 am
#2850
Join Date: Aug 2008
Posts: 2,065
Sadly I live in the real world where errors cost lives. I'm sure you work very hard abmbd are very conscientious but as I described under the current rules I had a nasty wake encounter a couple of weeks ago and now, mainly for commercial reasons we are facing reduced separation minima. You'll have to forgive my skepticism