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The first photo in this article appears to depict three Airbus A350s flying closely in tandem at approximately the same altitude.
I thought that a large jetliner would leave a trail of wake turbulence behind it that would be unsafe for several minutes. Is there something special about the Airbus A350 that makes this formation possible? Am I misinterpreting the photo?
The V formation in the second photo seems much more reasonable.
You are right that wake turbulence would be a danger when flying behind another aircraft like that.
The perspective of the photo makes it hard to see the actual position of the trailing aircraft relative to the ones in front of it. Pilots do this sort of thing in airshows when they fly in formation. The trailing aircraft are generally slightly below the ones in front of it. This allows good visibility of the plane they are following. At the higher speeds that these jets are flying at, the wake vortex will not drop enough to impact an aircraft fairly close behind and below.
See this old photo of the Thunderbirds. From this perspective, it is easier to see that the trailing aircraft is also below the flight path of the leading aircraft.
Yes, it does appear that the airplanes behind the lead in a formation would be affected by wake turbulence. In reality, there are several factors which contribute to make it not so risky.
Airplanes make the formation on high altitudes. Wake Turbulence becomes a bigger issue during takeoff and landing phases. As mentioned on Wikipedia:
Wake turbulence is especially hazardous in the region behind an aircraft in the takeoff or landing phases of flight. During take-off and landing, aircraft operate at high angle of attack. This flight attitude maximizes the formation of strong vortices. In the vicinity of an airport there can be multiple aircraft, all operating at low speed and low height, and this provides extra risk of wake turbulence with reduced height from which to recover from any upset.
Wake turbulence travels like this (source):
As you can see, based on weather and wind, the following airplanes would know what should be their best position in reference to the lead. They avoid this situation (source):
Airplanes in formation do not fly on the same altitude but are either slightly above or below the lead. This is very hard to figure out when you are watching from ground, or in several pictures.
What I'm going to say is my own opinion. I don't claim it's true, that's just my understanding of wake turbulences... I'm eager to learn, so feel free to correct me if I'm wrong :)
I'm assuming wake turbulence is moving air (airflow) caused by an aircraft flying ahead, wether airflow moving down, up, sideways or spiralling - I understand the pressure difference - Without going deep in particles motion (air molecules) I think you just have to consider four things :
your position behind a the leading aircraft : right behind its fuselage, right behind one of its wing tip, beyound its wingtip trail, from the axis of the leading aircraft.
your distance behind the leading aircraft : a few feet just behind (without colliding with) or miles away ?
wake size increases as you go far behind the aircraft producing the wake turbulence, but its strength decreases accordingly.
The point 3 tells you where NOT to position your aircraft behind another. Especially at a (relative) location where the wake is still powerfull, and its size is large enough to affect your controls surfaces and wing envelope, and send your aircraft in an uncomfortable motion, brutal gain or lost of lift, differential lift, etc.
So, right behind another aircraft, you can fly without been affected (much) by the wake turbulence, because the wake size is not large enough to send your aircraft sideways, however, you still encounters slight turbulences, especially if you fly just right behind the wingtips. Worst situation is when you fly several hundreds feets behind the aircraft : large volume of strong spiralling airflow that you can't visualize. Miles aways, the airflow velocity is assumed to have dissipate enough because of its lost of strength due to expansion and air resistance.
Of course, the wake turbulence of a Cessna is not the same of an A380. Weaker and smaller for a Cessna, that dissipates pretty quickly. This leads me to introduce point four :
However, that doesn't mean there aren't risks. Main risk in such close formation is vibration. Test pilots are trained to identify their origin and deal with.
And also, I don't have a comparable aircraft in size, so I'll take a DC10, or a B52 : I think you can't put such earlier generation of aircraft right behind an A350 (or 787) because of Wing technology (this related to the question "does the A350 has something special ?") Old aircraft have much less flexible wings, that makes them really sensible to brutal airflow. I'm not a physicist, but more flexible wings better distribute the load factors along said wing, considerably reducing vibrations and fatigue.
That's why wake turbulences are more dangerous at lower speed : the wake can grow enough to compromise the flight enveloppe of the following aircraft even at the tightest separation. That's why wake turbulence are a serious issue at lower altitudes where the moving airflow is really dense and affect surfaces greatly. Logically, wide wakes should dissipate more quickly in dense atmosphere, but the (tiny) spiralling vortex can last several minutes and is still very dangerous especially for smaller aircraft.
Fighter jets in airshow and flight formation :
a) They fly at high speed => not enough expansion to affect nearby followers
b) tandem formation requires lining up with leading aircraft => good distribution in differential lift.
c) in tandem formation, followers are usually below (rarely above) the preceding aircraft.
Finally, a Cessna control surfaces (and flaps) are more sensible than the ones that equip a 777. Never get a Cessna below and behind a 777 in a five to ten minutes timespan unless you know where the wake turbulence path has drifted and/or landed.
Side note : I'm not a pilot. I'm just interrested in how an aircraft flies. There are tons of documents out there about wake turbulences, wake vortices, stall schemas, etc. Some are contradictory, some are complementary. At a molecule level, the magic of an airflow looks like a perfect example of the theory of chaos. But the above is the summary of what I found here and there. That's no science, just opinion.
