Turb Penetration Speed

[Fatfish]

Jets decending on arrival in Mod CAT (not CBs), what would be a better speed? 300 knots for quicker penetration at lower AA (angle of attack) OR a clean 220 knots at a higher AA? Which would be a softer ride? OR it makes no difference. :confused:

[Slasher]

I think its only the 757 where the Vturb gives a better ride as
such but I could be mistaken. Every aircraft I have ever gone
through turbulence in is as rough as guts at any speed.

[wingtip777]

another question about this: manual says penetration speed .82\280, sometimes your cruis speed is less than .82\280. would you increase to .82 for penetration? nomally the low speed makes airplane suffer minimum load factor, right?

[punk666]

B737 Vturb speed is .76/280 for climb and decent and for cruise you use the FMC calculated N1.

I found some captains still use .76 in the cruise if its light to moderate turbulence.

[ImbracableCrunk]

B737 Vturb speed is .76/280 for climb and decent and for cruise you use the FMC calculated N1.

I found some captains still use .76 in the cruise if its light to moderate turbulence.

Ooh, yay. One of my favo(u)rite topics.


Per Boeing 737 AFM:

The recommended procedures for flight in severe turbulence are:
1. Air Speed
Target air speed should be approximately 280 KIAS or 0.76 Mach,
whichever is lower. Severe turbulence will cause large and
often rapid variations in indicated air speed. DO NOT CHASE THE
AIR SPEED.

Note that there's no mention of phase of flight, just that the target speed is 280/.76.

Also, check out the airspeed unreliable/turbulence penetration speed in the Performance Handbook or QRH. Those are the same numbers that are in the FMC for Turb N1.

[Fatfish]

Note its an arrival phase. Turb penetration for efficient flt is clearly spelled out in the books. However this is just for comfort in a descent. :confused:

[ImbracableCrunk]

Duly noted. There was a discussion of this about 4 months ago. And before that, I'm sure. Some posters even went in to some physics formulae.

[punk666]

If you look under the adverse weather section in the Supplementary Procedures of FCOM1

PHASE OF FLIGHT AIRSPEED
CLIMB: 280 knots or .76 Mach

CRUISE :Use FMC recommended thrust settings. If the
FMC is inoperative, refer to the Unreliable
Airspeed page in the Performance–Inflight
section of the QRH for approximate N1 settings
that maintain near optimum penetration
airspeed.

DESCENT: .76 Mach/280/250 knots. If severe turbulence is
encountered at altitudes below 15,000 feet and
the airplane gross weight is less than the
maximum landing weight, the airplane may be
slowed to 250 knots in the clean configuration.

I know the AFM supersedes the FCOM but why the difference then?

[ImbracableCrunk]

I know the AFM supersedes the FCOM but why the difference then?

I would imagine that it has to do with power being fixed in climb and descent and variable in cruise.

Maybe the AFM was a simpler version and the FCOM version has a little more technique?

[FullWings]

Another thing to note is that the turbulence penetration speeds are for severe turbulence. e.g. +/- 25kts IAS or more, "forced against straps", "momentarily out-of-control", blah blah...

If you encounter turbulence like this, then keeping the speed above the stall and below Vmo/Mmo could be regarded as a major achievement, so aiming for turb. speed is a good idea.

If you're knowingly penetrating an area where such conditions have been reported, then whatever the manual says... But I think most of us would do our level best to avoid it, if possible!

(I get, well, not annoyed, that's too strong, more :rolleyes: when people slow down at the first little ripple - if you're that worried, why not do the entire flight at turbulence speeds, just in case?)

[de facto]

Actually on the 737 , .76 in cruise will give you more or less turb n1... .77 in cruise at high levels will give u a speed close to VB,therefore no need to continiously monitor the airspeed during light turbulence,as some would ,flying at .80 :hmm:

[Slasher]

Fully agree with you Full Wings esp the last part - there is a
lot of throttle bashing these days from the automatons, who
I believe just aren't trained in turbulence recognition and
flying (with and without AP) on both Boeings and Airbooses.

As you say Vturb is of course only for severe turbulence, but
there are times I'd rather be just a tad too fast at relatively
high altitude than too slow, to cater for the possibility of a
sudden high TAT rise when within large areas of embedded
active CuNim (my flying is mainly sub tropical/tropical).

That's assuming of course one is stuck at a high level without
enough comfort-zone "fat" on one's ASI.

For planned turbulence I use not above optimum in the 320,
and used 2000ft below optimum in the 737 when I was on it
(even 4000ft below in some extremes).

[Capn Bloggs]

The slower the better ride. I thought the effect was the square of the speed change but was corrected in an earlier thread about this.

I see no reason not to slow down a bit if it gets bumpy. The TPS is for severe turbulence, not coffee-spilling. If you've got 60 knots on the bottom foot, slowing down will smooth out the ride, and if climbing, will reduce your time in the bumps. Win win.