737 Crz Cg

[flyby797]

Hi everyone,
I'm having some problems with a colleague of mine on who's right who's wrong. The topic is CRZ CG.
I'm pretending that on 737CL the correlation of %MAC and trim (on the trim wheel) may be used inflight to insert the CRZ CG in the PERF/INIT page.
My friend pretends the contrary arguing that this correlation %MAC/ trim units is ONLY usable on the ground to set the T/O trim. On his side , this correlation has disappearred on 737NG at least on the last that was delivered to us this year.
Anyone to clear this point:ok:?
Thanks

[OscarYankee]

If you are using manual loadsheets, maybe you could check this yourself? In my experience the trim indication at the trim wheel is a guideline at best. Just check the difference between the cp indication and fo indication next time you fly.
Without having any facts to base it on, I would say that the most correct "procedure" would be to extract the %mac from the loadsheet...

/OY

[flyby797]

I totally agree with you OY but another way to put it maybe: is the relation btn % MAC and CG only VALID on the ground (for TO trim) or is it valid AT ALL times?
Thanks

[Nightrider]

The %mac for T/O from the trim sheet provides an almost zero stick pressure for the N1-out climb after clean up. Thus, the pressure on the control should not be trimmed during intitial climb.
Cleaning up the aircraft, on a normal all-engine climb, will also reduce the pressure to almost zero.

The change in %mac during cruise, i.e. fuel burn-off, is calculated by the FMC and shows up in the changing values.

There is no procedure and no need for any pilot input once the aircraft is airborne. As a matter of fact, changing the trim figures in the FMC to a higher value will reduce the amber g-load protection bar and allows the computer to operate at a higher IAS / Mach number; a very dangerous idea as it has been seen several times, unprofessional and total lack of understanding of the system.

The trim indicator beside the trim wheels, as mentioned before, are rarely matching each other, and are used only to set the appropriate T/O trim setting; inflight this indication can be used as an idea about the trim situation, but, as one can read on the scale, it is only valid for T/O.

On a CAT II/III approach, after the auto-trim action, settings of up to 7 units can be observed, well outside the green band. As said, this value is only for T/O, it does not provide any valid figure for landing.

[eckhard]

It's a while since I flew the 737 but I got about 5,000hrs in them in the 80s and 90s so let's see if I can remember............

I think the MAC scale on the trim indicator is only valid for Flaps 1 and 5 (I seem to remember you had to make an extra correction for a Flap 15 take-off).

This is why it is not to be used in the cruise, as the flaps are retracted and therefore the balance of CP/CG will be different. Also, the thrust will be much less than at take-off, further altering the balance of forces in the pitch plane.

Our loadsheets had a 'fuel index' which corrected the MACZFW to give the MACTOW for various fuel loads. We used this technique in the cruise to find our current MAC.

Hope this helps!

[flyby797]

i think that i maybe didn't express clearly my concern. Nighrider, i agree with you concerning the evolution of the protection by the yellow hollow bars. And that's my initial point: you have a MACTOW on the loadsheet. Let's say around 22%. That's not the value that should be inserted on the PERF INIT page. After TO cleaning up the aircraft will advance the CG (negative moment of the movement of the gear and TEF, much lesser positive moment of the LED). So what value, besides the FMC default value, should i insert? eckard would you happen to have any value of those fuel index? for 734? Because what you wrote is axactly my concern: where do i get this CRZ CG.

[hoover1]

you put the gross takeoff %of mac in the per int page on the ground and it is calculated by the fmc for the pahes of flight and configuration. you do not use the default value. you can use it but it will not give you and acurate max and optimal cruise altitude. you use the % of mac and reference it with the manual and it will tell you where to put the trim based on what flap setting you are using.

[KristianNorway]

Hello flyby

We take the lowest MAC stated on the loadsheet, mostly being the MACTOW, and subtract 5% for the CRZ CG. This is not on the CL, though. That's the NG.
And that's conservative for our type of operation since we most often have fuel in the centre tanks, and burning away this only increases the MAC CRZ CG.
This would be the opposite of what you're theoreticizing, since this gives us more than actual load margin on the speedtapes.

The positive moment and minimum speed you experience during initial cleanup in climb, usually V2+15, and the negative moment you experience during speed buildup in climb are both accounted for by inserting the MACTOW before departure.

[john_tullamarine]

the correlation of %MAC and trim (on the trim wheel)

Where there is a need for a T/O stab trim setting, the AFM will give guidance regarding the appropriate setting. Generally, stab setting (in arbitrary OEM units) will be a function of flap setting for T/O and CG. I suggest that a look at the relevant AFM chart and the load sheet for the flight will be somewhat more useful than trying to reverse engineer the pedestal decal. One needs to keep in mind that the decals may not be installed with superhuman accuracy.

However, keep in mind that the typical trimsheet contains intentional errors introduced by the designer to counter real life errors. As a result, the trimsheet's apparent calculated CG (whether %MAC or FS) may be quite intentionally "wrong" to some extent. One needs to remember that the intent of the trimsheet designer is to keep the actual aircraft CG inside the AFM envelope .. and not to provide input data for other activities. Unless you have reverse engineered the trimsheet to figure out the error trapping techniques adopted by the designer, probably it is not a good idea to believe the apparent numbers in the manner you are suggesting.



is the relation btn % MAC and CG only VALID on the ground (for TO trim) or is it valid AT ALL times?

A very important thing to keep in mind is that %MAC and FS values are only different ways of stating the same quantity - CG. It follows that the relationship between %MAC and CG is always valid and, as an aside, is a simple arithmetic relationship -

%MAC = 100 * (FS - LEMAC)/MAC

where

FS is the CG measured from the OEM datum (in, mm, decafurlongs, whatever ..)

LEMAC is the FS of the MAC's leading edge, again measured from the OEM datum

MAC is the length of the MAC as prescribed by the OEM

The line pilot is better served by FS data as it is simple and straightforward. However, the aerodynamicist and flight test communities prefer %MAC for some engineering reasons. For the typical OEM, weight control data for the AFM generally is developed by the aerodynamics cell. Guess which measure (FS or %MAC) typically makes it to the AFM ?



The %mac for T/O from the trim sheet provides an almost zero stick pressure for the N1-out climb after clean up.

Your observation may be correct (coincidentally) for your particular Type. However, the AFM concern is with handling in the takeoff configuration. Generally, the pilot has pickled the trim substantially by the time the aircraft has been cleaned up ?



cleaning up the aircraft will advance the CG (negative moment of the movement of the gear and TEF, much lesser positive moment of the LED).

In the overall scheme of things I wouldn't worry too much about gear and flap trim changes. Assessment of handling against CG is not so critically precise in terms of the specific CG, although functionally so. Nonetheless, the pilot should aim for CG accuracy which can be defended at the Enquiry.



to have any value of those fuel index? for 734?

Suggest you be VERY careful using data from any trimsheet for some other purpose.

Trimsheet designers usually design their sheets to their own style and criteria (which is why one often can hazard a reasonable guess as to who probably designed a particular sheet). As a result, one can easily find half a dozen trimsheets for the same Type .. all of which are quite different in their apparent design detail .. but which should all produce a similar, acceptable, loading result. Specifically, the IU formula for each sheet may be totally different (apparently). Hence you MUST know what the fuel index values mean before you go off and use them elsewhere.

Two anecdotes -

(a) I can recall a light twin trimsheet of mine (Baron, as I recall) for which an owner-pilot rang up one time to query why the trimsheet IU formula didn't appear to match a simple calculation of mass * arm. After a few minutes of explanation it was obvious that his poorly remembered groundschool pendantry was incompatible with having any interest in understanding how the particular trimsheet actually worked !! As an aside, folk should keep in mind that much ground school training is directed toward basic understanding and passing the exams .. which is not always relevant to the way things might really be done by real world practitioners.

(b) a fleet of larger light twins used my trimsheets at one stage. One of the engineers revised the loading data for a specific aircraft which had been modified. Unfortunately, while he was quite competent with basic weight control work, his familiarity with the ins and outs of trimsheets was a tad lacking and he generated the trimsheet entry data from a basic mass * arm calculation which gave the wrong IU entry argument. Fortunately (my trimsheets are designed to make this error unworkable as a safeguard) the Chief Pilot caught up with me to query the unworkable IU argument. Took 30 seconds or so to calculate the correct number so that the flight wasn't delayed and he went off happy. The engineer concerned was mortified when I worked through the trimsheet calculations with him .. discussion over a few cups of coffee and he picked up the trimsheets basics without too much effort.

Point is - weight control and performance charts - a lot of background stuff which you never heard about in groundschool usually goes into these - tread warily lest the ground come and smite thee mightily. Good intentions just don't cut it with aircraft risks.


The main question seems to be along the lines of "what number should I put where in the box ?"

Answer is pretty simple - "whatever and wherever the OEM prescribes" ... this should be incorporated into your company SOPs.