Turbulence Speed on B757

[Airbus_Driver]

Was on the North Atlantic Track the other day and we experienced some moderate turbulence. Since we are required to maintain Mach .80, no one ever slows down regardless of the turbulence encountered due to traffic encroachment in a non-radar environment. Yet, over land in a radar environment, nearly everyone I fly with slows to turbulence penetration speed of Mach .78.

I was told by the Captain that slowing to Mach .78 only helps reduce the buffer for Critical Mach and thus Mach buffeting.

Is this true? Isn't there any structural risks when not slowing? I would appreciate any technical insight.

[stilton]

Probably like yourself, our turbulence speed is 290/.78 on the 757.

Having said that I would always prefer to be a little fast in turbulence, especially if it gets worse .8 Mach is a nice comfortable speed in the 75.

You are not going to break anything and this Mach will give you a good margin above low speed stall if you start losing speed with wave action for example.

Going too fast is easy to fix, but if you run out of thrust at altitude with steadily decreasing speed you only have one option :E

[John Farley]

I suspect that the clue why people are not worried about holding .8 is in the relatively small IAS difference between .78 and .8. - at a guess some 5-7 kts depending on your FL

IAS of course is what structural loads are about

[BOAC]

As John says, slowing to .78 from .8 is no big event ATC wise, and if it lasts long enough so that you cannot 'compensate' later to keep waypoint ETAs within limits, you just tell ATC and issue revised ETAs. They will let you know if that creates a problem and if it is that bumpy, in all probability everyone else is doing the same. The faster you hit the bumps the greater the airfame loading, as he says. Not really a 'structural risk' as such, just common sense.

[safetypee]

… prefer to be a little fast in turbulence …
… any structural risks when not slowing?

There’s turbulence and turbulence. Being a bit fast in cobblestones might be more comfortable, but a critical point might be if the turbulence is an indicator of a more severe condition.

A personal, pre-planned encounter with a Cb in a fully instrumented civil aircraft flying at turbulence speed resulted in reaching both limiting positive and negative ‘g’ values within a 10 sec period. The vertical air currents were estimated between 3-6000 ft/min which also caused large roll angles, one of which ‘spat’ the aircraft out of the side of the cloud at stick shake alpha – total duration 30 sec. This test condition was not repeated!

A more realistic operational (unplanned) encounter involved a very smooth clear night climbing out of Greenland ~FL200, when a severe up-gust above a lenticular cloud produced a harsh pitch up of about 1.5g briefly. Full forward stick was required to maintain some control of the decreasing speed whilst balancing the duration of flight at less than 1g.

Speed limits are published for good reason.

[stilton]

Your point is well taken, I have been in wave action, however, where we lost so much speed we had to ask for an immediate descent as we had no more thrust available.

I stand by my assertion you are better off a tad fast, its only a few knots and well worth having.

[BOAC]

There’s turbulence and turbulence. - that's the key, stilton. forgive the pun, but you are talking 'chalk and cheese' here. Wave is rarely 'turbulent' in the true meaning of the word. It is just a disturbance in the airflow. I don't think the OP was referring to wave!

The factors behind the choosing of a 'turb speed' are many, and centre around a combination of protection from excess structural loading combined with retention of sufficient margin of performance for up and down speed changes. I was told the DC-10 (I think it was) actually had a higher 'turb speed' than its normal cruise. My only airliner is the 737 where it was lower.

In true 'turbulence, ie 'mod' or worse, I have always had an eye on the 'magic' buffet boundary markers on the EFIS 737s to ensure I was 'in the middle'. In strong wave you should ideally pitch your speed into the middle of your buffet boundaries (if you can ever identify the 'normal' air!) and this will give you the best speed margins. It is, however, rare to 'run out of power' and if you do, you were at the wrong cruise level as you found.:)