View Full Version : turbulence penetration speed

3rd May 2002, 01:14
A colleague and I have been having a discussion on how and when turbulence penetration speed is applied.

We both have differing opinions.

I say when encountering moderate to severe turbulence and he says when the ride becomes uncomfortable.

It was during this discussion that we realised neither of us really new or understood its application.

We would appreciate some input from some of the "experts"out there.

3rd May 2002, 03:44
This is how I see it:

Turbulence is defined in the Australian AIP as either light, moderate or severe. From the AIREP format, the following definitions are given:

Moderate Turbulence: There may be moderate changes in aircraft attitude and/or altitude, but the aircraft remainsunder positive control at all times; usually small variations in air speed; changes in accelerometer readings of 0.5g to 1.0g at the aircraft's centre of gravity; difficulty in walking; occupants feel a strain against seatbelts; and lose objects move about.

Severe Turbulence: Abrupt changes in aircraft attitudce and/or altitude; aircraft may be out of control for short periods; usually large variations in air speed; changes in accelerometer readings greater than 1.0g at the aircraft's centre of gravity; occupants are forced violently against seat belts; and lose objects are tossed about.

All very techo, so in the air I applied the following definitions:

Light Turbulence: I start to worry about my coffee spilling, I put the "fasten Seatbelt" sign on.

Moderate Turbulence: I gulp my coffee, and instruct the Flight Attendants to be seated, I begin to ask/look for clearance to smoother air.

Severe Turbulence: I slow the aircraft to the turbulence penetration speed, thank god everyone is strapped in, and demand clearance to smoother air.

3rd May 2002, 05:48
This I understand, but the question remains how does this relate to Vb, turbulence penetration speed?

3rd May 2002, 08:13
As a general rule that has served me well in over 30 years in heavy jet transport aeroplanes......slow to the recommended speed at the onset of moderate or greater turbulence.
However (and this was a BIG problem in the early jet aeroplanes ie; B707, early models especially), do NOT get too slow.
The results are...not good.
And get to know the cruise buffet onset chart. Vital information there.

3rd May 2002, 08:34
Olderair the answers vary from pilot to pilot. What would only have me only flicking on the seat-belt sign may have another selecting turbulence speed. I dont think theres any written difinitivley (attitude\altitude\IAS changes etc) with regards to when you must or should fly at Vb.

In my own experience (usualy CB penetration) I reduce or increase speed to Vb if my eye-balls start (or expect to start) bouncin up and down out of sync with the instrument panel.

I have heard some types (ex: 757) Vb gives a smoother ride to the bums down the back even in only moderate turbulence.

3rd May 2002, 09:08
I believe Vb is determined during certification to give the best margin between the likelihood of stalling and exceeding Vmo through degredation of control during turbulence.

In the 737 (3/4/5) I normally slow to a lower cruise Mach number in bumps, (providing their is sufficient margin in minimum IAS/Mach), because the ride dramatically improves at .7M and below. I think Checkboard has it right, with one big CAVEAT - don't forget that sometimes you will need to ACCELERATE to Vb!

Young Paul
3rd May 2002, 09:46
I heard that severe turbulence was defined as when the F/O stopped eating his passenger meal .....

seat 0A
3rd May 2002, 09:57
My sense of smell usually has me stopping eating any passenger-meals ;)

Vb was recently adjusted on the 737 PG from M.70 to M.73
I always thought M.70 was a bit low.
I normally slow down to Vb in moderate turb, when the speed starts to jump up and down. In the 737 NG this tends to happen sooner then on the PG. Both because the speed indicator is more jumpy and because econ cruise speeds on the NG are much closer to MMO.

4th May 2002, 02:05
How about if you have the AP on when encountering turbulence.?

Do you disconnecr it and attempt to fly the aircraft or as I recall in other similar threads, most pilots leave it connected

It seems on the surface that the speed discussion seems to assume that the decision is reactionary after encountering turbulence, yet so is the decision above about whether to disconect the AP.

If you leave the AP connected and functioning why bother to change speed.?

4th May 2002, 02:57
Some autopilots have a turbulence mode...works GOOD.:D

4th May 2002, 03:22
Some personal definitions of turbulence :

Light : A half full cup of coffee starts to spill over the lip.

Moderate : All the coffee spills.

Severe : Were's that cup gone ?

On a more serious note. There are often subtle clues to the onset of "some" turbulence, other than the obvious nasty coloured bits on the radar.

If you are under a Cirrus layer and that layer has curling threads coming down or up from it. There's a fair chance of CAT around.

An early indication of mountain / standing wave is often found by watching the N1/EPR changing. The rougher "cobblestones" are on the back (downwind) of the lift, so you get more warning running downwind than into it. (ie you get the lift = EPR down, then the rattle.) Remembering always that the strongest waves are often "Blue" (clear air). The lenticular are on the top and are a good indication if you can see them. Just downwind of the lenticular is (theoretically) the worst place.

The best thing you can do when you get a "cobblestone" rattle going upwind, is get the speed back to Turb PDQ. Flying into wind a good "rattle" is nearly always followed by a lift surge. The boundary is often sharp and you can get a disturbing increase in IAS as you cross into it. But don't be too sharp on the throttles - what goes up, must come down. You just get the power all the way back and you hit the sink. Equally nasty and potentially terrifying. Nothing spools up slower than a big fan at F350.

An idea, since that is all I can offer. Consider the use of speedbrakes in the lift/IAS surge to control an increasing speed. Leaving the engines spooled up for the sink that is sure to follow. Mercifully we meet high altitude wave infrequently, but the fact that it is waving at high altitude means it is rarely benign

At turbo prob altitudes you can usually bet on a lee wave front being formed if the wind is within 30 degrees of normal to a mountain range. However with the amount of lateral shear that goes on above that it is often a difficult one to forecast. Also no-one supplies TeFi Grams any more, so spotting any associated stability layer is impossible. I personally just watch out for wave <300 nm downwind of any significant ranges. If you can "sniff it out" early and get the seat belts on before you hit it, you win. Sometimes you get it wrong - but it is on the safe side.

Any change in OAT means possible CAT about. The rule I follow, that seems to work is "follow the temperature". OAT UP = climb for smoother air. However I suspect this might be like coin flipping at times. Any better ideas on this "method" would be much appreciated.

Now this is a strange one that I have only just started noticing. Perhaps because we are at last getting most of the "fag smoke" out of the filters. After an hour or so of clear air cruise, the filters are obviously very dry. When you get into a layer of higher humidity - not even cloud - then you get a faint smell of what I can best describe as "warm bakalite / circuit boards". This always seems to preceed a bump.

And as for the speed. Get back to Recommended Turb Speed early. It gives you a better set of margins, a slightly softer ride and does no harm.

Some links on Wave that you might find interesting. Mostly culled from Gliding links.

Nasa Met - Mountain Wave (http://www.awc-kc.noaa.gov/awc/help/mwaveinfo.html)

DG Gliders Wave Page - Good Diagrams (http://www.dg-flugzeugbau.de/foehnwellen-e.html)

MUST READ ON Mountain Wave associate CAT (http://www.etl.noaa.gov/about/review/as/ralph/i.html)

4th May 2002, 05:42
411A the best Ive had so far was the 747-300's AP in TURB mode (50% control-surface deflection limit). I cant recall a time when the thing ever gave up the ghost or didnt fly through the crap better than I/we could.

4th May 2002, 05:50
MG, also severe CAT can be foretold by a sudden rise and fall or a sudden rise or fall in the TAT by more than 5 deg. Happened a lot over the Pacific and gave a few secs warning. Worst Ive encountered was a sudden rise of 11deg and all hell broke lose 4 secs later! Like hitting a brick wall.

4th May 2002, 06:10
All very interesting guys but surely Vb is a design limitation speed.

If so, is there a definitive answer as to when this speed should be applied?

I have read that : "Vb - the design speed for maximum gust intensity. Vb is developed by the designer as a recommended turbulence penetration speed for an aircraft in severe turbulence with varying vertical gust components up to 50 feet/second for a light aircraft."

So if this is correct does it only apply in severe turbulence?
If we are in moderate turbulence do we have to slow down - comfort excluded.

Everyone seems to have differing opinions and it appears that when to fly at Vb is not taught.

4th May 2002, 06:18
And if you do find a definitive answer Olderair then let ME know what it is too!

4th May 2002, 07:34
Oldair, so far as I know, Vb is not a limit speed...as for example Vmo/Mmo is, but you would be well advised to take note/action when necessary.
We wouldn't want to chuck an engine off a pylon...now would we?
Ugh...not good:eek:

Bally Heck
4th May 2002, 07:37
To quote Mr Boeing in his 75/6 FCTM.

"Severe turbulence should be avoided if at all possible. However, if severe
turbulence is encountered, use the turbulent air penetration procedure listed in the
Operations Manual. Turbulent air penetration speeds provide high/low speed
maneuver margins in severe turbulent air."


4th May 2002, 08:41
"I refer the honorable gentlemen to the point I raised a while ago"

Heck - do we need John Tullamarine right now!?

Where are you, John?

MY tuppence worth again - I fly Vb whenever I have concern for control of the aircraft in turbulence (no, I don't mean co-pilot flying:eek: )

5th May 2002, 10:00
With due deference to those who know the subject, I think we need to be careful with our definitions here. The historical airworthiness requirement is to establish a speed to fly in rough air which will provide protection from stalling, on the one hand, and structural damage on the other. The definined gusts from which protection is sought are 66fps at Vb, 50fps at V/Mmo and 25fps at Vd (these will doubtless have been tweaked and changed into metric with the onset of JAR). The (old) definition of Vb, therefore, is the design speed for maximum gust intensity - ie the max speed at which an aircraft type can encounter a gust of 66fps and not break up. Vb is, therefore, one of the parameters used to establish Vra.
As for when to reduce to turbulence speed, I think passenger comfort will normally dictate that moment before any concerns for structural integrity come to the fore, but I'm not a freight dog!
Perhaps it's time for JT or NW to weigh in on this one!
Cheers all,

Weight and Balance
6th May 2002, 01:31
To follow up on mcdhu's post:

For little (FAR23) planes, Vb is also the speed at which a gust that produces limit structural loads also brings you to the brink of a stall. Above Vb, a gust can increase alpha, and thus wing lift, until you bend or break things, without stalling. Below Vb you will stall before exceeding the limit load. This makes Vb (or less) a safe speed when you might encounter really big gusts. I'll have to go back and read FAR/JAR25 to see if this is the same for the big stuff.

Of course, there is more to safe flying than just avoiding bending things. A stall, especially in turbulence, could lead to a close encounter of the terrain kind. This often exceeds limit loads ;) .

7th May 2002, 02:10
If your experience indicates some increased chance of encountering turbulence, some preparation should be made. With increasing possibility of severe turbulence, greater preparation should be made, including adjusted to turbulence penetration speed. If unavoidable severe turbulence is actually encountered, you need to be already at the recommended speed / altitude with cockpit and cabin prepared.

7th May 2002, 03:13
Absolutely right, autoflight, the 6 "P's" are appropriate...
Prior Planning Prevents Pi@@ Poor Performance.