737 NG Alternate FWD CofG

[Driver 170]

Operation with Alternate Forward Center of Gravity Limit for Takeoff

Any pilots carried this operation procedure out? Do you have to get permission or is it dispatch that recommends this type of loading?

Question;

A more aft CG increases the lift available at a given angle of attack due to the reduction in nose up trim required from the horizontal stabilizer.

I thought an aft CG reduces the tail down force there by decreasing the lift on the wing?

[john_tullamarine]

A more aft CG increases the lift available at a given angle of attack due to the reduction in nose up trim required from the horizontal stabilizer.

I thought an aft CG reduces the tail down force there by decreasing the lift on the wing?

Are the two statements not saying essentially the same thing ? The important thing is that the aft movements reduces the tail down load .. which increases the overall aircraft up load.

To do with scheduling for the two stall speeds .. maximum stall is for forward CG limit .. if you restrict the CG then you might be able to achieve a knot or two benefit in stall speed. This allows a revised takeoff speed schedule and improved runway performance.

[Denti]

Any pilots carried this operation procedure out? Do you have to get permission or is it dispatch that recommends this type of loading?

It was normal operation for us. Depending on %MAC we could either use it or not, in two steps. Simply check the CG on the ACARS Loadsheet, then choose the correct option in the performance program. But loading was scheduled to use that as much as possible anyway, so usually the CG was as much back as possible without the risk to run into problems due to short-term changes in passengers/cargo.

No dispatch release or permission needed. And if we didn't want to use it, we didn't. Our choice. Not much of a difference anyway.

[Driver 170]

Cheers guys i'm going through the ATPL Mass and Balance maybe that will give me a better understanding.

@Denti, you mentioned its down to pilot discretion if you wanted to use the Alternate FWD CofG, if you opted for this choice would the loaders load the cargo and bags a special way or...

[Denti]

No, there is a standard loading schedule that pretty much assures that the procedure is usable, and load distribution, based on that schedule, is decided by the centralized load center which later on does the load and trim sheet as well, based on the latest figures from the loading crew and boarding gate. Once the crew has the loadsheet, either via their iPad or ACARS, they can decide which option to use.

[Driver 170]

What do you mean by centralized load centre? So no matter what, the aircraft always gets loaded the same way (cargo and baggage), but what then sets CofG AFT or FWD option. Slightly confused !!!

Also why are some companys allowing pilots to use a set Cruise CG in the FMC?

Quote;

We use 18% CG on the PERF-INIT page for the whole flight which I find the number to be quite realistic. It makes a huge difference especially when you do a 5-6 hour sector with respect to fuel!

[Driver 170]

I thought Cruise CG is a set value once the aircraft is fully loaded with pax and bags

[Denti]

The CofG ALT option is just that, an option in the performance software. It nets usually one or two degrees higher ATM, but thats about it. All in all a minor performance impact. Some pilots did choose to calculate a bit more conservative and do not select that option, therefore keeping a slightly higher margin on the runway for the no-go case.

We do use two centralized load control centers in eastern europe that do all our load and trim sheets. They try to give loading instructions that lead to a CG that is as much aft as possible while still being operational viable, for example transfer bagagge needs to be in a different hold than local one so that through checked baggage can be processed faster to allow short hub and spoke connections.

The cruise CG thing is a totally different thing. We used to have absolutely no guidance about it at all, so every pilot did whatever he thought best. That changed when some captain as PM grew bored on a flight and decided to play with the FMC. Eventually he set the cruise CG to 36% which resulted in stall warning activation and an altitude loss of about 5000ft in the stall recovery procedure, of course without any notice to ATC in the moment.

After that we got a procedure to set the cruise CG every hour, based on our take off CG and the initial fuel load and remaining fuel at the time of recalculation. Eventually that was ditched for just leaving the pre-set cruise CG in the FMC and not messing with it at all. The pre set one is 8% CG and extremely conservative.

[Driver 170]

Thanks, But how can the AFT CG affect the no go case? I know during roll below 80kts you apply light forward pressure to the control column untill the rudder becomes effective.

For cruise CG, if you use 8% thats not going to be the exact CG number the aircraft is working under? So why not just use the exact real number? Sorry for the wording

[ATPJon]

As usual there is a bunch more to all of this than meets the eye. In cruise we want to minimize drag so an aft cg is better consistent with handling requirements.

Now for takeoff we have a far more complicated set of circumstances. First, what limits our performance on a specific takeoff? It is either accelerate go OR accelerate stop. until nowa pilot we probably don't know (and your dispatcher doesn't either because the software usually doesn't report it.) Nor do you know which of the 4 climb segments is most restrictive (2nd segment is NOT always most limiting despite what you may have been told a gazillion times!) Notice we have not mentioned obstacles (cranes, cumulogranitis, mountains, etc.) until now? The reason is the performance software looks at all the information including the obstacles and runs multiple calculations using different variables (flap settings, brake capability, V speeds, thrust, location and height of obstacles, and single engine performance assuming an engine failure at V1). Using all this plus the applicable FARs it calculates your maximum allowable takeoff weight. There are all kinds of options such as heavy duty brakes, tires, alternate cg locations, longer time limit for an engine at max thrust to name but a few things that impact performance.

The five things the software looks at first are Brake Energy Limit, Tire Limit, VMCG, how much weight can lifted off this runway (length, slope, elevation etc.), and Max Structural TO. Then the engine failure at V1 and the obstacles get mixed in.

The forward cg limit gives the rudder a longer arm thus greater moment and a lower VMCG which may allow you to lift more weight out of an airport if VMCG was driving your limit. Before you can haul it enroute you got to lift it out of there.

ATPJon

Hope this helps!

[john_tullamarine]

.. and decided to play with the FMC. Eventually he set the cruise CG to 36% which resulted in stall warning activation and an altitude loss of about 5000ft in the stall recovery procedure ...

Are you sure that playing with the FMC can produce a stall ? That would be a very persuasive and authoritarian FMC, methinks.

First, what limits our performance on a specific takeoff? It is either accelerate go OR accelerate stop

.. along with a bunch of other requirements, some of which you have nominated in your following discussion ...

The five things the software looks at ...

The software should be looking at the whole gamut of limitations ...

The forward cg limit gives the rudder a longer arm thus greater moment and a lower VMCG which may allow you to lift more weight out of an airport if VMCG was driving your limit.

That just might be arrant nonsense ? Perhaps you might have a read of FAR 25.149 https://www.ecfr.gov/cgi-bin/text-idx?SID=8faf9fb87379344cccafed99b90a7419&mc=true&node=se14.1 .25_1149&rgn=div8

A little knowledge can be a dangerous thing ...

[ATPJon]

More to consider

Fwd CG for take off requires an ADM supplement that has to be purchased for specific airframes and turned on in the software. Also keep in mind transport aircraft are certified using procedures that require staying within 35 feet of the centerline using only aerodynamic controls during VMCG testing. Since the aircraft does not leave the ground (or only briefly because the intent is NOT to go fly) during VMCG testing we don't care about drag but about control. So in this case we want a short arm for the rudder because that reduces the rudder's effectiveness giving us a worse case scenario.

There is a lot more to all this than meets the eye. Until I attended Airbus and Boeing training and worked as an Aircraft Performance Engineer I too believed a lot of the myths and half truths we as pilots are taught. My other experience includes 5,000 hours total time GA and 121 turboprops. Taught and maintained B757/767 and DC-10 aircrew material at UAL. Three years experience as an Aircraft Performance Engineer.

[ATPJon]

The 5 limits discussed are computed

John,

The 5 limits are computed and the most restrictive is applied is the point I am making. Probably very few of us have heard about these things let alone understand them or how to calculate them. The point is to raise awareness and understanding. You don't have to be a surgeon to put a band aid on a wound. But you should also be smart enough to realize a band aid will not cure a gaping gun shot wound. As pilots we have a lot of knowledge but that doesn't mean we know everything. The FARs are a mess! Very few are truly stand alone consequently we have a tangled mess. Think about fuel requirements, alternates, and who can fly with whom. 30 or 45 minutes, better than 2,000 &3 plus or minus 1 don't need one, and anybody can fly with anybody! Not quite so fast!

[john_tullamarine]

Perhaps I am to learn something here ? And that is a serious, not a frivolous, question.

First, I had no intent to cause you distress - if I did, then my apologies, good sir ... and, certainly, I have no interest in getting into a micturating contest ... quite unproductive and unbecoming.

Also keep in mind transport aircraft are certified using procedures that require staying within 35 feet of the centerline using only aerodynamic controls during VMCG testing

No problem with that other than 35 ft probably should read 30 ft ? Not of great note in the overall scheme of things, though, noting that the rules do change over time.

.. during VMCG testing we don't care about drag but about control. So in this case we want a short arm for the rudder because that reduces the rudder's effectiveness giving us a worse case scenario.

Indeed, as per the rulebook and FTG. But whence, then, comes your previous statement on Vmcg ? That was the cause of my concern.

Now, here may be the learning point which I seek - first, I have no direct experience (either as a pilot or PE) with those aircraft having a forward CG fiddle factor. I am presuming that the OEM reason for this is that the general reduced stall speed spinoffs associated with being a bit aft of the most forward limit are eminently saleable ? On the other hand, should there be a provision whereby Regulator and Applicant can negotiate a very restrictive (forward) CG envelope with attendant Vmcg recertification in the manner which your previous comment implies, then I have no knowledge of that. If that be the case, could I trouble you to provide some appropriate and authoritative reference material for my benefit ? For this, I would be more than appreciative.

I've taken the trouble to have a looksee at your profile and you appear to be appropriately qualified and experienced to comment on this topic.

PS For whatever reason, I didn't see your second post last night.

I really don't see its relevance but the Vmcg question needs to be addressed for the benefit of the new chums .. please ? The first sentence appears to make little sense in respect of the certification animal.

Probably very few of us have heard about these things let alone understand them or how to calculate them. The point is to raise awareness and understanding.

Providing some education (at an appropriate level) for the new chums is a major thrust in my approach to PPRuNe. I think we all appreciate that the Industry is a bit light on when it comes to theory training for pilots. It is for that reason that we need to get to some endpoint with the Vmcg comment from an, apparently, technically competent chap ?

[ATPJon]

No worries mate!

John,

No worries mate! Long day for me so I'm kind of tired and can't find some of my reference material. Again much of this seems contradictory or illogical to a pilot. It did to me UNTIL I went through the training and work experience as a performance engineer.

If memory serves me well FAR 25 covers certification from the perspective we are discussing. This section imposes a number of restrictions (legal term conservatisms) on the testing process and results that are incorporated into the AFM. Why? To give a margin of safety and allow for aging and errors that creep in due to differences between pilots. A small sample of these are the effect of a headwind is halved but for a tailwind is doubled. The brake pads used are worn down by 80% (only 20% remaining).

Anyone who tries to say you can discuss performance without considering obstacles, 2nd segment is always the most limiting, or you can ignore special departures if the weather is not IMC is to be ignored. More later.

The better 7/8's is ringing the dinner bell!



More later dudes and dudettes!

Alternate forward CG is an AFM supplement that is airframe specific and must be purchased. Most manufacturers treat this information as proprietary and confidential so you would have to discuss this directly with Airbus, Boeing, or your manufacturer.

[john_tullamarine]

Again much of this seems contradictory or illogical to a pilot.

It shouldn't be so if they are trained appropriately. I doubt that any of the line and supervisory pilots I have trained and signed off over the years had any problems with the material (to an appropriate level).

If memory serves me well FAR 25 covers certification from the perspective we are discussing.

Indeed .. FAR 25 is all about certification.

A small sample of these are the effect of a headwind is halved but for a tailwind is doubled.

Are you sure about that ? Perhaps you might just recheck that statement and provide a correction to it ?

The brake pads used are worn down by 80% (only 20% remaining).

Are you sure about that ? Some authoritative references would be nice to see.

Most manufacturers treat this information as proprietary and confidential

All OEMs treat just about everything as super top secret .. money rules.

.. and then we shall await your Vmcg answers and commentary with bated breath ..

[mutt]

Most manufacturers treat this information as proprietary and confidential
Aero 23 - Increase Takeoff Gross Weight Using CFM56-7B26/B2 Thrust Rating and Alernate Center of Gravity Not really as they have talked about it for years.

2nd segment is always the most limiting, or you can ignore special departures if the weather is not IMC is to be ignored.
Have you worked with 4 engine aircraft?

Also keep in mind transport aircraft are certified using procedures that require staying within 35 feet of the centerline using only aerodynamic controls during VMCG testing
(a) It must be demonstrated that, when the critical engine is suddenly made inoperative at VMCG during the takeoff ground roll, the airplane is safely controllable if the takeoff is continued. During the demonstration, the airplane must not deviate more than 30 ft. (25 ft. prior to amendment 25-42) from the pre-engine-cut projected ground track.

A small sample of these are the effect of a headwind is halved but for a tailwind is doubled.
Half the headwind and 150% the tailwind.
The 150 percent operational tailwind velocity factor, required by §§ 25.105(d)(1) and 25.125(f), affords a satisfactory method for determination of airplane takeoff and landing performance information and limitations up to a limiting tailwind velocity of 15 knots when using a flight test data base obtained under zero wind conditions.

The brake pads used are worn down by 80% (only 20% remaining).
It all depends on the date of certification, pre 1981 post 1981 amendment 25-42 or 25-92.
(1) Section 25.101(i) was added to require accelerate-stop distances to be determined with all the airplane wheel brake assemblies at the fully worn limit of their allowable wear range.

https://www.faa.gov/documentLibrary/...ular/25-7B.pdf

Have fun

[ATPJon]

Glad to be back after bunch of stuff!

Originally Posted by john_tullamarine

Perhaps I am to learn something here ? And that is a serious, not a frivolous, question.

First, I had no intent to cause you distress - if I did, then my apologies, good sir ... and, certainly, I have no interest in getting into a micturating contest ... quite unproductive and unbecoming.

Also keep in mind transport aircraft are certified using procedures that require staying within 35 feet of the centerline using only aerodynamic controls during VMCG testing

No problem with that other than 35 ft probably should read 30 ft ? Not of great note in the overall scheme of things, though, noting that the rules do change over time.
You are probably right 30 v 35 but there is a limit. The point is during certification you must keep the airplane within X feet of the centerline using aerodynamic controls only...no nose wheel steering or asymmetric thrust. Line crews want to avoid running off the side of the runway but can use the full width of the runway. So some pilots ignore/discount AFM limits because they are rules but they think they can outperform the AFM limits.

.. during VMCG testing we don't care about drag but about control. So in this case we want a short arm for the rudder because that reduces the rudder's effectiveness giving us a worse case scenario.

Indeed, as per the rulebook and FTG. But whence, then, comes your previous statement on Vmcg ? That was the cause of my concern.

Now, here may be the learning point which I seek - first, I have no direct experience (either as a pilot or PE) with those aircraft having a forward CG fiddle factor. I am presuming that the OEM reason for this is that the general reduced stall speed spinoffs associated with being a bit aft of the most forward limit are eminently saleable ? On the other hand, should there be a provision whereby Regulator and Applicant can negotiate a very restrictive (forward) CG envelope with attendant Vmcg recertification in the manner which your previous comment implies, then I have no knowledge of that. If that be the case, could I trouble you to provide some appropriate and authoritative reference material for my benefit ? For this, I would be more than appreciative.
There is NO fiddle factor. You comply with the AFM which may incorporate supplements that expand on the limits. Said supplements involve engineering and additional testing. There may also be mandatory changes to hardware and software. Accordingly the manufacturers charge big bucks to upgrade your capabilities.

I've taken the trouble to have a looksee at your profile and you appear to be appropriately qualified and experienced to comment on this topic.
Thank you for checking I get tired of comments by or from people with little or no relevant training and experience. I used to work at a major supplier of aeronautical information where if you paid me a penny for every time I was asked, "Why are you disagreeing with me? What are you talking about? I've got 300 hours and what you are talking about doesn't make sense because that's not how I do it in the C-172 or Seminole!" We were discussing a performance question about a B-757. I love working with people who ask good relevant questions AND realize when they have reached the event horizon of their knowledge.

PS For whatever reason, I didn't see your second post last night.

I really don't see its relevance but the Vmcg question needs to be addressed for the benefit of the new chums .. please ? The first sentence appears to make little sense in respect of the certification animal.

Probably very few of us have heard about these things let alone understand them or how to calculate them. The point is to raise awareness and understanding.

Providing some education (at an appropriate level) for the new chums is a major thrust in my approach to PPRuNe. I think we all appreciate that the Industry is a bit light on when it comes to theory training for pilots. It is for that reason that we need to get to some endpoint with the Vmcg comment from an, apparently, technically competent chap ?

A huge part of the problem is most pilots do not really understand performance from an engineering perspective. They know how to apply corrections but have no understanding of the processes, policies, and procedures involved. Amplifying this issue is the lack of background on the part of those teaching performance at all levels including part 121 operators. I have been through performance training at an international flag carrier and two 121 commuters as a pilot. I have also several years experience as an performance engineer and completed narrow body performance engineer training at Airbus and Boeing.

Ok off the soapbox. Thanks for listening and hope this is helpful.

ATPJon

[ATPJon]

Wanted to add a few things

1-2nd segment climb frequently but not always most limiting. If it was always most limiting we wouldn't have to worry about the other segments. Ensuring obstacles are cleared has a huge impact on how much we can weigh at take off and during climb out. And for 121 and 135 we have to clear obstacles with an engine inop. We are used to our performance with all engines operating which distorts our perception of what level of experience we anticipate during a departure. Further muddying the water is our inability to anticipate the impact of obstacles. I say this from a mental picture and not having access to tools that predict and illustrate the impact of obstacles. Go look at your AFM, does it have any tools for predicting obstacle clearance?

2-Do you believe that you do not need to follow departure procedures in VMC? If so you may be a fool! Depending on circumstances you may see an obstacle but not have sufficient performance to miss it!

3-Why do pilots love to quote TERPS? It tells us how airspace is designed and only that. Yes some criteria are flight tested but your airplane is not tested or verified to conform to TERPS. Think of it like a highway, There are tons of federal and state laws governing width, slope, construction techniques etc. But you as a driver choose how fast to drive, your driving manner (aggressive, road hog, tailgater, etc).

4-We should approach things with equal concern to be legal, safe, and smart. You can be legal but unsafe and stupid! Part 91 is full of this! Can I take off from an airport when the weather is below minimums for all approaches serving that facility? Yes! Are we legal, yes. Are we safe or smart no way!

Everyone have a great tomorrow!

ATPJon

[ATPJon]

Hi everybody I'm back after a bunch of stuff.

Have we beaten this into submission or does someone have more questions?

JohnT is there another topic you would like me to weigh in?

ATPJon