Jetliner wake turbulence

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Warmi

Line Up and Wait
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Warmi
Today I was flying below busy class C Midway airspace here around Chicago at around 1700 MSL - and while I was enjoying myself, there were multiple commercial jetliners both descending and ascending all around me - the closest I ever got to one was about 1800-2000 feet vertical separation and about 3-5 miles horizontal but I am wondering what would you guys consider minimum safe distance for a small GA plane in a scenario like that …
 
I used to do pipeline patrol. There's a fuel line up the east side of KMDW that runs north to the river that I used to patrol.

I got a lot closer than "miles". Seemed safe at the time. It probably wasn't, but I'd go for standard book separation now.

--Matt
 
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Was thinking about that yesterday - my delta flight flew over the practice area at TTA at about 5000’, ~3 miles west of the airport. I’m sure that was plenty of clearance, but I’ll bet it looked really big to any students training.
 
Used to land interspersed with the big guys at IAD all the time. My favorite was the Concord. You follow the advice of staying out of their path. Not really a problem because I always want to land past the touchdown point anyhow to minimize time to the first turn off.
 
You should be OK with 1000' separation. You may get rocked a little, but the wake should be dissipated enough that its not going to flip ya, flip ya for real.
 
https://www.faa.gov/training_testing/training/wake/media/08APP3C.PDF

In the LA Basin, I'm often faced with decisions on wake turbulence from airlines on approaches. An important factor to consider is winds aloft. If the wind is pretty calm, the wake is going to sink directly (more or less) down and not dissipate as much. You need to visualize where the wake is going after the airliner passes. It will drift with the wind, and sink at about 400 feet per minute. While 1,000 ft separation is unlikely to result in a big upset, it is still possible to get some significant induced roll at that separation. Crossing directly under the airliner is better than passing ~2 minutes behind it at that separation.
 
1 mile horizontal and 1000 vertical as long as we are both with the same controller.

Closer around airports.
 
This one appears to be 500' vertical separation at 9,500'. I would have thought that would have been recoverable.
 

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I start paying attention at less than 2000’. I’ll course correct if it’s 1000’ or less.
 
And for takeoff following a big aircraft, I use 3 minute separation between departures.
 
Anyone who flies the the track system in the North Atlantic can tell you that 1000 and even 2000 feet vertically is not a safe margin. Vortices are funny things and weather conditions can have a large effect on time to dissipate. Don’t fly under large aircraft if avoidable and if landing behind them stay above their flight path and land beyond their touchdown point.
 
Wake turbulence is a real deal. I have had heavies rolled 30 degrees by RJ's. The guidance provided works. Stay above the flight path, land longer. Stay up wind of the flight path. The thing that makes it work, is that the wake descends. IMG_0442.JPG IMG_0446.JPG
Those are pictures on approach into Paris. The plane in front of us going through the clouds on final. I can't remember what it was in front of us, but a transport catagory aircraft.
 
Seems like the 1000’ can be an ATC allowance, meaning, one could get that separation through normal vectoring. Of course you have options to give a wider berth. As usual particulars factor in, are you crossing at an 90 degree angle with altitude below? Or are you generally parallel & close to the ground?

There have been a handful of wake turbulence fatals while under ATC control, in recent memory. Two that easily come to mind is one off Milwaukee, into the lake, another at Reno, parallel runway. Both of these accidents had very experienced pilots in the cockpit.

Just a reminder to be vigilant, current environmental factors are always a player.
 
With 2000’ of vertical separation I don’t worry about it. At 1000’ I’m expecting it. This is in the work jet.

Fun flying, if I was within 1000’ I’d change course. Probably at 2000’ too.
 
Lake Hood sits adjacent to (less than 1/2 mile) an international airport with significant 747-777 freighter traffic. Nearby Elmendorf AFB traffic crosses the northern departure/approach route at 90° with 500-1000’ of vertical separation. We get plenty of wake turbulence advisories from tower and try to give the bigger planes like the KCs, AWACs, and C-141s plenty of space. Fighters will rock your world, too. Regardless of what “rule” you use for clearance, there are always exceptions to a rule. I have two friends, very experienced pilots, who’ve encountered wake turbulence that bent their wing spars after making time and space for the crossing traffic. One a Cessna 180, the other a 182. They both described it as violent. Both at below 2000’ and two miles offshore over an ocean inlet. Be safe out there.
 
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