Wake Turbulence - 75 vs. 74

[davethelimey]

Was intrigued to see in the (latest) 380 post that a 757 produces more wake turbulence than a 747. How, bearing in mind the difference in size, is this possible? Wing shape?

If someone could provide some answers, and possibly of how wake turbulence is produced and manifests itself I would be most grateful.

Cheers.

[NigelOnDraft]

I was also interested in that comment. I doubt it is "true" as a general statement - after all, there are many factors to generating wake turbulence.

What is true is that the 757 is towards the top end of the weight category for a "medium", and also has caused a number of wake events. As such, some operators choose to treat it as a "heavy" and require 2 minutes after it's departure etc.

I stand by to be corrected on the overall statement that a 757 generates more wake than a 747...

NoD

[davethelimey]

I don't suppose you'd mind defining a "wake event" for me?

Just filling in the gaps:D

[Notso Fantastic]

I saw the comment and wrote it off as incorrect. The only wake encounters I have had in a 747 have been from other widebodies. I think the big cause of wakes are wing area and wing loading. As one goes up, the other goes down. It's only when you have a large wing area and a large wing loading that wake danger intensifies, so you need a large heavy aeroplane- big though the 757 is, it just doesn't cut it against a heavy 747, and it has a large wing reducing wake problems. Simplistic, but I think it all simply boils down to weight. But I will now be corrected..............

[Bus429]

Maybe I should have stated that the 757 is classified, for the purposes of wake, as a "heavy".

Sorry for confusion
A study (http://128.173.204.63/courses/cee5614/cee5614_pub/acft_classifications.pdf)

[Brian D.]

Robert Dorsett - 1994

Everyone's saying something a bit different, but the bottom line is that
the 757's wing-flap geometry can produce *relatively* heavy wake vortices,
but not *spectacularly* heavy for airplanes of its weight. The *risk* of
the vortices comes from the 757's performance: it can out-climb just about
every other transport out there by a factor of 1.5 to 2. This means that
classic restrictions on vortex separation may not work: you can't stay "at
or above" another airplane's wake if you can't stay at or above the other
airplane.

NOAA Testing - 1990

Nearly 250 flybys were flown passed the INEEL Test Range 200-ft tower. The vortices were measured by an array of 100 high-speed hot-film anemometers and visualized with an array of colored smoke-emitting grenades. The highest velocities of any previously measured aircraft were generated by the B-757. However, when normalized relative vortex circulation is compared, the vortex strengths of the B-757 were comparable to other aircraft in its weight category. Anecdotal evidence indicated a relationship of atmospheric stability to vortex longevity. However, statistical relationships could not be conclusively drawn.

Linky to NASA report - 1998

http://techreports.larc.nasa.gov/ltrs/PDF/1998/cr/NASA-98-cr206933.pdf

[secret agent 86]

I can't say that I've ever experienced a 747 wake, but about a month ago departing YYZ, encountered a 757 wake that required full aileron to control it. The 757 was 8 miles ahead...most uncomfortable...I'll treat them with more respect in the future.

[Willit Run]

I was going into AMS 2 days ago in a classic 747 and we were right behind a NW 747 and hit their wake. HOLY COW! It was violent and short lived (thank god). Not something I want to do again! We rolled 30 degrees, like right now!

[NigelOnDraft]

I don't suppose you'd mind defining a "wake event" for me? Well, it might just be a comment on the RT, and/or a report filed. Or it might be the aeroplane rolling upside down, crashing and killing all on board (which has happened), or anything in between...

Maybe I should have stated that the 757 is classified, for the purposes of wake, as a "heavy". Sorry - disagree, at least in the UK. It's a medium. Was when I flew it, and still is when I takeoff behind it... (in another medium).

As I said earlier, certain airlines specify it shall be treated as a heavy. However, it is not in that weight category...

NoD

[Jerricho]

Ah, the good old 757.

In the UK, as NOD indicates, it is treated as an "upper medium" aircraft (The Uk doesn't have the standard ICAO Heavy, Medium and Light classifications). This creates an interesting situation. Heathrow approach treats it as a medium when following a heavy, thus requiring 5 mile vortex separation. When it is being followed by 737/A320 aircraft 4 miles is used as a votex separation. And when a 757 is being followed by a heavy, it isn't considered to require vortex separation thus 3 miles (or 2.5 depending) is required.