Wake turbulence - routine or something to worry about?
 

A few years ago I was flying from BFS to LTN, to be precise on Easyjet Flight 168 on 22nd December 2005. Somewhere over the Liverpool area or thereabouts we encountered wake turbulence. The effect was quite dramatic, difficult to describe, but maybe the equivalent of 20 seconds of severe ordinary turbulence crammed into 2 or 3 seconds. Or a bit like being dropped from a height onto a bouncy surface.

I have to commend my fellow passengers as their universal reaction was stunned silence. After what seemed like five or ten minutes (no doubt to give him time to check everything was OK) the pilot announced that we had passed through the wake of a 747.

Is this routine, or was it a near miss? Is this level of wake turbulence dangerous or well within the safety parameters of a 737?

It is a relatively common occurence. The wake can exist quite a considerable distance behind the aircraft that is creating it. You can compare it to passing through the wake of another boat or ship. Where it is a large vessel, you don't want to get too close or the turbulence can be quite uncomfortable.

With jet aircraft moving at around 8.5 miles a minute, two aircraft crosing each others path at right angles might do so with only a seperation gap of 30 seconds or so. It is quite likely in those circumstances to encounter a couple of bumps (tram tracks) as you cross over the wake. Normally it is quite a light transistion, but sometimes as you discovered it can be more noticable.

The only time I've encountered it was flying BA (it was actually FlyBe at that time, I think) from Bristol to Paris one morning. It was very bad, and unexpected. The four of us pax drinking champagne never spilt a drop: the three pax drinking coffee spilt it all over themselves: there has to be a moral to that......

One of the fun things to do in a light aircraft is a Rate Two 360 degree turn to try and catch your wake. However, with commercial aircraft separation is very much the order of the day and certainly when I've been in a light aircraft at an airfield that uses heavies, ATC make very sure there is good separation and I'm sure the same is true for all commercial heavy operations.

Commercially, as a passenger, I've experienced wake turbulence a few times on take-off and once in the cruise. Take-off incidence was a sort of fast ripple effect whereas the cruise instance was a real bump, almost like driving over a large brick in a car.
In recent times, I don't think there has been any instances of a commercial a/c breaking up because of wake turbulence but there was the sad case of American Airlines 587 which crashed into Queens, New York after an overly aggressive control input (during wake turbulence) caused failure of the airframe.

NTSB Press Release

In the cruise it isn't a problem, it is a problem near the ground where there isn't time to recover control (i.e. take-off and landing), and it has caused a few crashes in the past. Air Traffic Control and pilots now apply strict separation standards between departures and arrivals to ensure that it doesn't cause a problem now.

As I understand the problem of avoiding a wake, the plume of wake sinks down at a variable rate, depending on winds and air pressure. Thus a 744 might have (say) gone over your head three minutes earlier at (say) 1,000 above you and then the wake 'settles' down through the sky and ... Bump. You may or may not be in the 'right' place to encounter it.

And some of us may have enjoyed doing circuits at Kai Tak in HKAC's C152 or C172, being slotted in by ATC between the jumbos. Wake avoidence was down to us. Fun that was!

As I've reported in other sections of this website - and been derided for exaggerating - Mrs. Frequent Flyer and I were well and truly jolted out of our complacency on a BA Embraer 145 over the sewage works in Stockport while approaching MAN.
The aircraft seemed to roll on to its side one way producing gasps and screams from the passengers, right itself and then roll on to the other side. This produced more gasps and screams from those who felt they were going to land well and truly in the proverbial in every sense.
The pilot regained control of the plane and climbed as quickly as possible.
Someone in the cockpit also spoke to the cabin crew member at the front of the aircraft and I heard her reply: "No. There have been no injuries." However, had everyone not been using their seatbelts in preparation for landing there would undoubtedly have been injuries given the violence of the motion.
The pilot then explained we had been caught in the wake vortex of a much larger aircraft in front.
It was a sobering experience.

http://burland.de/Images/Wake.jpg

Here's a pretty good (if tragic) indication of wake movements.

Wind moves JL47's wake into the (tighter) path of AA587, dropping 500ft in the process.

Rest of the story here