B777-300ER Turbulence Speed Schedule

[Airman2000]

Hello guys,

I'm new to type and some things are getting me confused.

The limitations chapter of the FCOM says:

"The turbulent air penetration speed (in severe turbulence) is defined as: 270 knots
below 25,000 feet, 280 knots or 0.82 Mach whichever is lower at 25,000 feet and
above. Maintain a minimum speed of 15 knots above the minimum maneuvering
speed when below 0.82 Mach."

Now suppose the following scenerio: You`re cruising at FL310 (most usual intial level for our operation) and suddenly you encounter severe turbulence. Now suppose that 0.82 is equivalent to 300kt at that level and the minimum maneuvering speed is 270kt. In my interpretation of the FCOM, I should use an IAS of 285kt for turbulence penetration (0.82 is greater than 280kt / 270kt+15 is equal to 285kt). Is that the right way of thinking?

I`m asking that because in 100% of times I see my fellows just use 0.82 without considering whether it is higher than 280kt or not. I tried to ask a
captain and he told me he should use 0.82 because at that level he is flying Mach instead of IAS.

Any light on the matter will be highly appreciated by this apprentice.

Happy landings!

[Stel]

Airman 2000, you are correct in your assumptions.
Usually the crossover during the climb (in ISA conditions) from 280kts to 0.82 Mach occurs at approximately FL345; possibly slightly earlier if you are complying with the +15kts requirement. Once 0.82 Mach is equivalent to an IAS of less than 280kts, or the possibly slightly increased IAS you were flying, IMN should be flown; the severe turbulence IAS restriction (and the +15kts above minimum manoeuvre speed requirement) are no longer applicable.
For example if in the climb in severe turbulence conditions:
· Maintain 270kts up to 25,000’ (15kts requirement never an issue at these levels)
· Maintain 280kts, or higher above 25,000’ (ensure a minimum of 15kts above minimum manoeuvre speed)
· When IMN reaches 0.82, disregard IAS constraints and fly IMN (15kts above minimum manoeuvre speed additive no longer required).
Remember also that the turbulent air penetration speed/Mach no requirements are for severe turbulence, something that is not that often encountered.

[FullWings]

Remember also that the turbulent air penetration speed/Mach no requirements are for severe turbulence, something that is not that often encountered.
Agreed. From the severe turbulence definition we use:

Turbulence that causes large, abrupt changes in attitude or altitude. Aircraft may be momentarily out of control. IAS fluctuates by more than 25kts. Occupants are forced against seat belts or shoulder harness. Unsecured objects are tossed about. Food service and walking are impossible.
Which is a pretty rare scenario, thankfully, from where I sit. (Couldn't handle the lack of food service!)

Also, why fly at this speed? Passenger comfort? No, because you'd slow down for that. Structural reasons? Again, you'd fly as slowly as sensible to reduce the dynamic loading. Manoeuvre margin? Yes, so as to avoid Vmo/Mmo and stalling. It's got to be pretty bad to run into both problems in the same encounter unless you're up in the top corner of the flight envelope...

If it was really important, we'd fly all the time at 270/280/M.82, just in case.

[GE90115BL2]

Boeing say above 25,000' maintain 280 KIAS OR .82 WHICHEVER is less PROVIDING you keep 15 kts above min man spd when below .82

Simple really, just assess the IAS/MACH you are at and slow down to whichever one is less while respecting the above comments.

When IMN reaches 0.82, disregard IAS constraints and fly IMN (15kts above minimum manoeuvre speed additive no longer required

I don't agree, in the ER cruising at about FL310 you indicate around 305 kts and mach .83 If severe turb is encountered you should try to slow to 280 kias ( or min man +15 ) and not .82 . So in this case your comment about disregarding IAS is wrong.

So I agree with the original posters comment.

You`re cruising at FL310 (most usual intial level for our operation) and suddenly you encounter severe turbulence. Now suppose that 0.82 is equivalent to 300kt at that level and the minimum maneuvering speed is 270kt. In my interpretation of the FCOM, I should use an IAS of 285kt for turbulence penetration (0.82 is greater than 280kt / 270kt+15 is equal to 285kt). Is that the right way of thinking?
correct.

[slamer.]

Next time your in the sim, ask them to demo severe turb.....:eek:

[GE90115BL2]

good idea. :ok:

Might break the sim or at the very least rough you up a bit !!

[Stel]

GE90115BL2

You have taken my statement out of context; my example was for a climb, 270kts transitioning to 280kts above 25,000'and when IMN reaches 0.82 (having climbed up to that point at 280kts) disregard the IAS constraints and fly IMN; obviously as you climb your IAS will decrease and IMN becomes the limiting speed reference.

If you are in the cruise or descent, totally different scenario.

[Airman2000]

Thank you guys for your help!

I was not very aware of the fact that you only got to respect the min maneuver speed + 15kt when you are below 0.82.

In my company, my fellows have the habit of flying the turbulence speed schedule in any turbulence which would require the fasten seat bealts to be turned on, even though the schedule is for severe turbulence. It looks like they are trying to be conservative and to get a softer ride.

Happy landings!

[john_tullamarine]

Generally not a problem to be a bit conservative .... and it makes for a much more pleasant cup of coffee.

[FlareArmed]

Temperature does not affect the CAS/MACH crossover. It always occurs at the same altitude. 280/0.82 is about 34500' as stel states, but ISA deviation is not relevant.

[GE90115BL2]

I thought you basically said once the system swaps from IAS to MACH forget the IAS and fly .82 as the turb penetration speed?
This is not correct for all ops. Normal cruise in the ER as you know is around FL300 initially at about .82 to .83 indicating about 300 to 310 KIAS ( from memory) now in this case should you encounter turb you most definately don't fly at .82 and disregard 280 KIAS. You slow below .82 to 280 or min man +15.

[Old Smokey]

And FINALLY someone got it right, dead right!:ok:

Well done GE90115BL2 :ok:, I'll fly with you any time:ok:

Regards,

Old Smokey

[Airman2000]

I thought you basically said once the system swaps from IAS to MACH forget the IAS and fly .82 as the turb penetration speed?

This is what 99% of the guys in my company are doing and that fact was bugging me, because I could have misinterpreted the manual. But you show me that I was not mistaken. IMHO the guys have a big trouble with the english language, which is the manual's language.

Happy landings!

[GE90115BL2]

Thanks..........

I'll admit that when you sit here at home trying to fathom it all out it can be a pain...................trying to remeber what IAS matches what IMN at which FL !!

But when you are sitting in the jet it's quite easy to do.

------------Look at the ASI, determine which is the lower speed for you .82 or 280. If that is below .82 then don't go any slower than min man ( top of yellow band ) +15 kts.....................SIMPLE.:ok:

At low levels it'll be about 280 kts, high levels .82

Enjoy the worlds best electric Jet, she's a beaut:ok::)

[MrBernoulli]

Enjoy the worlds best electric Jet, she's a beaut

Yes, but don't let the Scarebus wallahs hear you say that .............. :ok:

[Airman2000]

Enjoy the worlds best electric Jet, she's a beaut

For sure!!! I can't even miss the table!!! Simply wonderful ship! :ok:

[Jr. Airman]

FlareArmed

Temperature does not affect the CAS/MACH crossover. It always occurs at the same altitude. 280/0.82 is about 34500' as stel states, but ISA deviation is not relevant.

Apologies for going off topic. Maybe i'm missing something here, could you please explain how temperature doesn't affect CAS/MACH crossover? Mach from memory is TAS/LSS and speed of sound is a function of temperature. And CAS is purely impact pressure right?

[john_tullamarine]

.. how temperature doesn't affect CAS/MACH crossover?

You have part of the story under control ..

(a) speed of sound - a = a0 √[T/T0] - depends on OAT

(b) Mach is a function of altitude and CAS. The following might be a tad convoluted because of the limitations of presentation .. however, if you transcribe it to paper it shouldn't be too difficult to see the details (assuming my bracket counting capabilities are OK) ...

(M^2)/5 = [1/δ [(1 + 1/5(Vc/a0)^2)]^3.5 - 1 ] + 1]^(1/3.5) -1

... if you define two of these variables, the equation gives you the other, independent of OAT

... the CAS/MACH crossover. It always occurs at the same altitude

.. providing we are looking at the one pair of CAS/M values.

[Jr. Airman]

john_tullamarine ... many thanks!

[unb5]

Dear sir, you seem to know your thing so I pose this question to you or to any other knowledgeable person out there. Is there the same rule for the B757 ? Its Vb speed is 290kts/.78 mach so do we use this speed at all levels above 10,000 feet and is there any documentation to support this ? Thanks

[TwinJock]

The way that I remember it - below FL350, I open the speed window and set 280 IAS, above FL350 open the speed window and set M0.82!

Once the coffee has settled down, and only below FL350, I ensure that the speed is 15Kts above the amber band!

Have to keep it simple - after 8 hours staring into the darkness things must be easy!

[Old Smokey]

Pedantic Mode ON

The Crossover Pressure Height for 280/0.82 is 34928 ft.

Why is Turbulence Penetration Speed 270 CAS at 25000 feet, but 280 CAS at 34928 ft? ..... Because EAS is within 1 knot of 263 EAS (Ve) which is what we need. (Aircraft need EAS, not CAS).

Why, at whatever level we must ensure no lesser speed than Vls+15? ..... Because Vls provides 1.3G protection, insufficient for Moderate turbulence. Buffet protection increases at the rate of 0.1G per 7 Kt above Vls, so to provide for 1.5G (moderate turbulence) we should fly at no less than Vls+14 (Nicely rounded up to +15).

Pedantic Mode OFF

Regards,

Old Smokey