Tailwind

[lucavigg]

I've noticed a lot of questions about tailwind during landing and takeoff, but I was wondering about limits during the cruise.

The Jetstream has been very changeable over the last few days. I noticed that westbound tracks were going way way north a few days ago and east bounds were using the tailwind to their advantage. Some flights arriving at LHR 80 minutes early.

A colleague is returning from Miami on Wednesday / Thursday and the forecast shows tailwinds of 180-200 knots in places.

Would speeds of that kind be safe for a majority of transatlantic aircraft or would they keep the eastbound tracks away from the excessive winds?

L

[mixture]

Would speeds of that kind be safe for a majority of transatlantic aircraft

If it wasn't safe, they wouldn't fly it.

IIRC, its relative airflow that matters in relation to airframe integrity, which is why airframe limits are specified on an Indicated Airspeed basis rather than True,Ground or Calibrated Airspeed.

If you think about throwing a tennis ball inside a moving car, the ball will move at speed X but combined with the underlying speed of the car it will have moved at the vector sum of the two speeds and thus covered a larger distance in a shorter amount of time than it would have ordinarily managed to do at its original speed.

In much the same way that if you put a tennis ball infront of a hairdryer pointed vertically upwards, the tennis ball will settle at a fixed location close to the hairdryer... it won't go rushing off into the distance.

At least that's the way I understand it, no doubt in true PPRuNe style, someone vastly overqualified who got a 1st in Aeronautics at University will be along shortly to give you the fully technical explanation with fancy formulas included .... in the mean time, here's a video of a man in a white coat playing with his balls.... :O

_oZYArx5oM4

As it happens, I flew the Miami route with a 200kt tailwind fairly recently and it seems I survived to tell the tale, as did the aircraft....the only bad part of the whole flight was having to hold until the Heathrow NIMBY 6am limit passed so we could land ... :)

[Otto Throttle]

The strength of the jetstream is not an issue from a point of view of aircraft limitations. However, turbulence associated with the jetstream is a concern for the pilots. Very rapid changes in the speed and direction of wind can create very turbulent and unpleasant flying conditions, which in the most extreme cases can also be very dangerous, and naturally pilots will choose to avoid flying in those conditions regardless of any benefits in terms of time and fuel burn. This turbulence can occur when aircraft fly into or out of the stronger winds of the jetstream, or even when in the jetstream itself. Strangely, the actual strength of the jetstream does not necessarily correspond to the severity of any turbulence.

The type of weather forecasts which pilots receive before a flight are usually quite good at predicting the likelihood and the severity of any turbulence, but are not infallible. In the cruise, pilots rely heavily on reports from other aircraft in the same operating area to monitor any turbulence and plan their routing and altitude accordingly. Operations across the ocean have additional complications however as communication with Air Traffic Control units is very difficult and as such, pilots have limited options for altering their planned flight details.

[wiggy]

What Otto and mixture said.

One way of thinking of it is that once you're airborne your're "embedded" in a block of air, and you just fly in it, regardless of whether it's stationary or moving at 200 knots. It's a bit like swimming lengths in a swimming pool moving on the back of a lorry....:sad::sad: You'd chug along swimming through the water as normal, and it would feel the same to you, regardless of whether the pool is doing 10 or 100 mph.....(I'm assuming a very smooth driver and a very smooth road:} :}"......

Basically we'll happily take any tail wind/avoid a headwind but if there's significant turbulence associated with the strong winds we might think again, plan accordingly.

[ExXB]

Wiggy,
I understand that turbulence is worse from crosswinds than from tail or headwinds. Or is this just another myth I picked up somewhere or other?

[mixture]

I understand that turbulence is worse from crosswinds than from tail or headwinds. Or is this just another myth I picked up somewhere or other?

Turbulence in air either comes from changes in wind or thermals....

Therefore have a steady state wind (cross,tail or head) then you've got no turbulence. Have gusts, updrafts, wake or something else messing around with the air you are flying in then you'll have turbulence.

Its a bit like driving on the road...normally you feel nothing, hit a pocket of disturbance in the road (speed bump, pothole etc) and you'll feel a jolt.

Simples. :ok:

[wiggy]

I understand that turbulence is worse from crosswinds than from tail or headwinds

I can't think of a reason why the individual "bumps" :hmm: would be any different, but if you cut though a jet stream at right angles then in theory you'd be through the turbulence (most usually at it edges) quicker than if you were stuck paralleling the 'stream.

[lucavigg]

So a steady, fast jet stream with little variation of speed would be ok but if the turbulence reported within it were too strong, those daily transatlantic tracks would go nowhere near it?