744 Turbulence penetration

[Wycombe]

Hope someone can answer this question from a PPL with a "big aeroplanes" question (just to satisfy personal curiosity, that's all):

Earlier this week I flew transatlantic (Carribean -> UK) on 744 with a well-known UK airline.

Mid-Atlantic (just SW of the Azores, according to Skymap), we encountered what I would describe as moderate turbulence (I'm guessing associated with a jetstream) - there were some fairly sharp vertical oscillations (causing CC to make for their seats pretty sharpish :o )) and some lateral jolts aswell. Some pax around me were visibly unnerved by it, although my kids slept right through it :).

What I noticed on Skymap is that our Groundspeed reduced significantly (580mph down to 480mph), only creeping slowly up again after we were back in smooth air.

I'm thinking that the speed reduction was due to turbulence penetration. Just wondered what the limits are, as I would imagine the 744 needs to still move pretty fast at this alt. (FL360)??

[Intruder]

Turbulence speed is .82 - .85 Mach. Normal cruise speed is in that range, so there should have been no significant change in indicated Mach.

The change in ground speed may have been due to reduction in tailwind or increase in headwind.

[skyvan]

Also be aware that very often turbulence is associated with a change in wind speed and direction, you may have just run into a 90kt headwind.

Also, in that area (Azores/Cape Verde Islands area) we commonly notice wind changes, from a tailwind to a headwind (heading Eastwards). I fly JFK to Johannesburg, and have experienced 150kts tailwind to 300nm from Ilha da Sal, with a change then to 40kts head on from Sal, with the associated turbulence. So that could well be part of what you have had happen to you.

[BlueEagle]

The last company I worked for we normally flew the B744 at .86Mach at FL330 and up. If we thought it might get turbulent or experienced unexpected turbulence we would reduce speed to .83M, if it was moderate turbulence we might reduce even further, depending on our weight and the low speed buffet boundary. A reduction of 85kts(100mph approx.), does sound quite a lot.

[sky9]

Boeings recommendation on most aircraft is to maintain normal speed in light or moderate turbulence. "Normally, no changes to cruise speed are required when encountering moderate turbulence" If you are going to reduce speed due to turbulence you should really be 4,000ft below optimum altitude to preserve your buffet boundary.

Passengers and some pilots tend to overestimate the level of turbulence experienced.

[BRISTOLRE]

Similar but surely common experience happened to me on a UA777 SFO-LHR a few weeks ago travelling as SLF.
Over Northern Canada 60N 80W we picked up a jetstream for about 2 hours, it became uncomfortable after a while.
Having said that was seated back row.

Ground speed on skymap showing a +31kts tailwind component jumping to +220kts, boy you could feel it.
There was a lot of vertical movement and sometimes violent sideways jolts. I guess crew tried to make most of it and ride it out.

Ironically, by entering Scottish airspace we hit a +46kt headwind...

We maintained FL360 most of the way, then had a medical emergency onboard due a pax getting stressed about the buffeting. Hence had priority to land at LHR.
Quick flight from the West Coast!

Anyone else had such a high tailwind component and such a rough ride under these conditions?

[Intruder]

I fly the Pacific more than the Atlantic (first NAT track in over 2 years was last week!). However, I have seen winds to 220 knots over japan and the western Pacific. The jet stream max typically may be anywhere from FL330 to 370.

It is common to experience turbulence (officially "light" to "moderate" most times, but feels like "severe" to the SLF) when near the jet stream, especially when crossing it perpendicularly. Often, once established within the core of the jet, the ride is smooth. A lot depends on the wind shear (variation of wind with altitude) at/near your current FL.

[Wycombe]

Thanks all for the input so far - interesting stuff.

Intruder - you're probably right that head/tail winds may explain a large part of the groundspeed fluctuation - what caught me out was probably the fact that the speed seemed to go roughly back to where it was after the air smoothed out (but I guess what you see on skymap is a culmination of contributory factors).

sky9 - I know it's common for pax to over-estimate turb levels. I've flown hundreds of hrs all over the World as pax (civ & mil) and would say this was amongst the roughest I've encountered. I've also read about "severe", and it was definately nothing like that.

BlueEagle - I've heard about the relatively narrow safe speed range you guys can find yourself in according to weight & buffet boundary (also had it explained to me once, back in the good old days when it was possible to get on the jumpseat!) - that was really what prompted my original question.

Cheers all.