TUBBULENCE PENETRATION 737/280KTS/m.76

[JOE MAXY]

Hi Guys
I have a question. When we encounter severe turbulence in the cruise we follow boeings turbulence penetration technique. i.e cws pitch (do not use ALT HOLD), maintain wings level and the desired pitch attitude. Do not chase the airspeed or the altitude. Disengage the A/T and set the turbulence N1 from the cruise page in the FMC.

What is our target airspeed when encountering severe turbulence in the cruise?? I was under the impression that 280kts/.76 provided the aircraft with optimal turbulence penetration regarding wingloading.

[IRRenewal]

The airspeed is whatever the turbulence N1 gives you.

I would assume it works out at roughly .76 in still air as it is normally showing a bit lower than the N1 required for econ cruise (just try it in the cruise when there is no turbulence, it won't bite. Or in the sim if you're not happy trying these things for real), but the whole point with turbulence penetration in the cruise is that there is no target airspeed. No target means no target to chase.

[EGBKFLYER]

Is that an NG? My memory says 280/ M0.73 for a 737-300 - no worries these days as with the CI we fly, cruise is about M0.73 and we descend at 250 :{

Our Part B manual says the above speed for clb/ dsc and whatever turb N1 gives you in the cruise. It's in the Supplementary Procedures under Severe Turbulence. What does your manual say?

[HiFli]

I agree wholeheartedly with IRRenewal. The 280kias/M0.76 is only applicable in the climb and descent.

[Mshamba]

Sounds like the NG as we have the same figures for. Offtopic: CI was boring recently, but since a couple of weeks our flightplans show increasing CI's so happiness is coming back a little bit. Descending with 253 (!) is so so boring... we manually changed to 290. Hows your company dealing with that?

[BOAC]

Any published 'turbulence speed' is a compromise between adquate control margins and excessive airframe strain. IF you fly NG, you can look at the 'gap' on the AI between your buffet boundary markers and check that you are in the middle. That will give you a good margin. At lower altitudes (below 20) and also at light weights, there is no harm in reducing to 250 kts say to improve comfort and maintain manoeuvre margin.

I would caution a little on what IRR has said, and if you see the speed shifting significantly out of the comfort zone it is indeed a good idea to 'chase it'. :) as Boeing say.

[Bergie]

I agree with BOAC, I would also like to add that Boeing calls them "severe" turbulence penetration speeds. By definition, unsecured articles in the cockpit will either bounce around or be dislodged laterally. ie eyeball bounce :bored:
I don't know why so many pilots slow down at the slightest hint of turbulence.

[Mshamba]

I don't know why so many pilots slow down at the slightest hint of turbulence.
"Passenger comfort" :8

[hawk37]

Seriously now, would anyone out there really aim for 280 kias if expecting, or actually in, severe turbulence? As Mshamba mentions, it seems to me pilots tend to slow down, even below 250 kias, once encountering anything above light turbulence.

[JOE MAXY]

Thank you for the replies.

So in the cruise we disengage the A/T and set turb n1 from the fmc. okay

In the climb/ descent if we encounter severe turbulence we set the speed to 280kts/.76 and also disconnect the autothrottle.

What i am having difficulty with is why in the descent/climb the turb n1 figure(which in the cruise provides near optimum penetration airspeed) cannot be selected.

[Retire2015]

In climb, leave climb thrust set.

R

[BelArgUSA]

Our friend BOAC above described best procedure and to apply for Vra/Mra.
Vra/Mra = turbulence speed or Mach number.
Yes, the 707 and 727 were at 280 KIAS/Mach .80 as well.
The 747-200/300 is 295/.835... These are "book figures".
A better speed/technique to be flown is as follows -
xxx
Obtain high speed and low speed buffet margins figures.
Place two "airspeed bugs" on such speeds or Mach number in cruise.
The higher the level, the narrower the spacing between the bugs.
Depends also what buffet margins you use - i.e. 1.3G or 1.4G.
In severe turbulence, target Vra/Mra is mid-point between bugs (bisector).
Example - 302/267 hi/low buffets = Vra 284.5 KIAS.
At lower levels, low speed buffet is replaced by "stall speed - flaps UP".
xxx
:ok:
Happy contrails

[ImbracableCrunk]

I've been flying with some pilots lately who are dialing the MCP airspeed to get the N1 to match the Turb Penetration N1 from the FMC. That's with the A/T engaged.

To me, this is silly.

This is straight from the 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 it doesn't say climb and descent only - it's for cruise as well.

IRRenewal wrote:

but the whole point with turbulence penetration in the cruise is that there is no target airspeed. No target means no target to chase. Note that Boeing calls it a "Target air speed."

Why would Boeing say climb or descend at a specific target speed, but in cruise, do whatever?

In cruise, either leave the A/T on and set 280/M.76 or turn it off and set the suggested N1 (which is the power setting for 280/M.76).

I'm all ears to differing opinions.

[Capt Chambo]

Why would Boeing say climb or descend at a specific target speed, but in cruise, do whatever?

In cruise, either leave the A/T on and set 280/M.76 or turn it off and set the suggested N1 (which is the power setting for 280/M.76).

I'm all ears to differing opinions.

Next time you have "struggled" to FL410 have a look at your airspeeds once stabilised in the cruise. I would hazard a guess that you will be indicating M0.78~M0.80, or about 250KIAS.
(These are approximate figures to illustrate a point. In real life your numbers may be slightly different depending on your operation, your aeroplane, and the prevailing ambient atmospheric conditions)

At FL410 setting 280Kts will put you into an overspeed condition.

At FL410 setting M0.76 will have you very close to Minimum Maneuver Speed.

Whereas setting 280Kts/M0.76 is fine at lower cruise levels it doesn't necessarily work for higher levels. Setting Turbulence N1 (which is usually somewhere about 1% less than your cruise N1) will usually ensure you neither overspeed, or underspeed. Boeing don't differentiate between cruising flight levels, and so don't provide recommended airspeeds.

ps. Don't forget that if you are in RVSM and the turbulence is so severe that you cannot maintain your Flight Level within tolerance you need to advise ATC. Thankfully genuine "Severe Turbulence" is comparatively rare.

[ImbracableCrunk]

Next time you have "struggled" to FL410 have a look at your airspeeds once stabilised in the cruise. I would hazard a guess that you will be indicating M0.78~M0.80, or about 250KIAS.
(These are approximate figures to illustrate a point. In real life your numbers may be slightly different depending on your operation, your aeroplane, and the prevailing ambient atmospheric conditions)

At FL410 setting 280Kts will put you into an overspeed condition.It's been about 3 years since I've gone up to 410, but I do remember that the coffin corner starts to get pretty tight.

But back to the point, the recommended speed is 280/M.76 whichever is less. At 410, one should be setting M.76. I think that's about 230kias.

At FL410 setting M0.76 will have you very close to Minimum Maneuver Speed.For some reason, the tables I'm looking at only go to FL390, but at 390, it says: Vb = M0.76. The lower and upper limits for .3G is 223kias and 252kias for a 737-9 at 150,000lbs. It looks like M0.76 puts you closer to the bottom than the top. But, that's Boeing's Vb.

Whereas setting 280Kts/M0.76 is fine at lower cruise levels it doesn't necessarily work for higher levels. Setting Turbulence N1 (which is usually somewhere about 1% less than your cruise N1) will usually ensure you neither overspeed, or underspeed. Boeing don't differentiate between cruising flight levels, and so don't provide recommended airspeeds.I'd say they do. That's why Boeing has a table called "FLIGHT WITH UNRELIABLE AIRSPEED / TURBULENT AIR PENETRATION Cruise (.76/280)" They then go on to provide N1 and pitch for various altitudes. Eg. At FL400 and 140,000lbs, N1 91.0% and +3.5 pitch.

ps. Don't forget that if you are in RVSM and the turbulence is so severe that you cannot maintain your Flight Level within tolerance you need to advise ATC. Thankfully genuine "Severe Turbulence" is comparatively rare.Agreed!

Regardless of whether the speed should be M0.76 or some other, the goal is a safe airspeed.

My greater concern is pilots using the MCP to command the autothrottle to set an N1 which a power setting for a desired speed. (Whatever speed it may be.) Convoluted!

Either set the MCP to the speed you want, or turn off the autothrottle and set an N1 that you want.

[de facto]

250 kts is ok for turbulence speed only below FL 150 and aircraft mass below max landing weight

[ImbracableCrunk]

Do the pilots who say there is no target speed have QRHs that don't mention specifically the 280/M0.76? There are different QRHs out there.

How much variation is there in AFMs?

[ImbracableCrunk]

What i am having difficulty with is why in the descent/climb the turb n1 figure(which in the cruise provides near optimum penetration airspeed) cannot be selected.

This is probably more simple than you're trying to make it: in a climb or descent, airspeed is controlled by pitch. In level flight, it's controlled by N1.

In climb, you select the climb N1, and pitch is controlled simply via FMC or MCP SPD. In our case, 280/.78. This is just like in a Cessna 172 - max power, pitch for Vy, for example.

In descent, power is of course at idle and pitch is controlled by airspeed.

The Turb N1 is only for level flight (hence CRZ page) and is power setting to get you to the Turb Penetration Speed. It's never "selectable" and doesn't show on the N1 page.

[Tmbstory]

Mshamba:

Mshamba made a good point and as well, if it is good for the VIP passengers, then it also feels better for the crew.

Tmb

[BOAC]

Regarding post #10 it is apparent that 'Joe' is not a pilot or he would know that you do not climb or descend very well with Turb N1 set.:)

[ImbracableCrunk]

Oh, I think you could descend quite well with Turb N1. It might just get a bit noisy. Clackers and such. ;)