Changing stabilizer trim from recommended

[stilton]

After receipt of the loadsheet with our final numbers including, amongst other things % mac and stab trim setting some of our pilots are changing the setting away from this.

Usually adding one to one and a half units nose up to "make for an easier rotation"

Seems like a bad idea and I have always spoken up and persuaded them to reset it to the recommended value.

Bearing in mind the occasional 'rotation surprise' of nose heavy or light loading, in the latter case it would appear with this 'technique' you could be tempting a tailstrike and/or rotation to a slower than optimal airspeed.

While this recommended setting (off the loadsheet) may not always be completely accurate it seems like a good place to start!

Any thoughts?

[411A]

In our operation, the manual loadsheet is prepared by the Flight Engineer, from the load information given to him by the freight/baggage loaders.

This is far from accurate, and an additional bit of nose up trim is added by the pilots, for a normal rotation.

IF the crew positively know what they are doing, this works well.
Perhaps less well so with larger companies...IE: new guys on type, especially.

Further, computer generated loadsheets are prone to rather large errors, sometimes not fully appreciated by crew...IE: using ZFW listed as the TOW.

SQ comes to mind, in the not to distant past.

Check to be sure.
Ground folks who prepare the loadsheets don't have to fly the aeroplane when it is severely mistrimmed..according to the loadsheet.

Pilots DO.

[FlexibleResponse]

stilton,

Some very good questions and thoughts. If it was a good idea to fiddle with the trim settings, the test pilots at the factory would have made appropriate recommendations.

If you set the trim according to your Standard operating procedures and according to the Load sheet you will be legal and also probably live a long and happy life.

If you set your trim according to anything else, you may die, or worse still, live to regret your actions and the lives that you have lost as a result.

[XPMorten]

I calibrate aircraft performance for X-Plane simulator aircraft.
The way I understand it, the purpose of takeoff trim is basicly to have the correct trim on initial climb (V2+15 or whatever). So, that if you handfly
the acf it will climb out straight as a missile at the
calculated pitch without additional elevator input. To much up trim will cause a
"rollercoaster" climb and in worst case a stall.

But hey, this is simulator theory :p

Cheers,

M

[john_tullamarine]

The calculated CG and stab trim will be as accurate as the data which goes into the calculation. For a well designed trimsheet, reasonably accurate load information, and a modicum of care in executing the trimsheet, the error will be insignificant in the overall scheme of things.

Problem with ad hoc trim settings is that the certification requires a variety of stick force measures to be within acceptable limits ... the flight test program validates the AFM guidance .... the ad hoc do-it-on-the-fly stab setting MAY cause the flight test program work to be invalidated.

If there is a residual error in the stab setting, so be it. That's what pilots have muscles for ...

I know which approach I would prefer to argue in court after the accident ......

[stilton]

Thanks for the good and informative contributions.

I'm sure most of us have had rotation 'surprises' and, while the forward cg case obviously just requires a stronger pull and is usually just a minor inconvenience (unless grossly mistrimmed)

The aft cg case however can cause premature rotation and/or an unexpectedly rapid upward pitch at VR with all of the problems associated.

We take it on a certain amount of faith that the %mac and trim provided are accurate but 'surprises' still occur.

If one were to experience an aft CG surprise in addition to adding
a few units to 'make it easier to rotate' I think things could turn pear shaped quite rapidly.

I would welcome other input.

[Old Smokey]

I would think that deliberately setting the stabilizer to anything other than that computed from the load sheet is a very questionable practice.

We do live in the real world where there will always be some degree of variation, forward or aft, of the computed CG position, but these small variations should be well within the 'handling tolerances' for pilots, and are considered during flight testing.

As one poster has indicated, ideally, the Stab Trim setting is optimized for the V2+15 (or thereabouts) climb-out, and, as such, already has a degree of inbuilt 'positive rotation bias'.

If the aircraft IS correctly loaded, any additional aft trim can only result in an excessive pitch up.

If the aircraft is incorrectly loaded with a CG aft of the computed position, any additional aft trim may well lead to (and has) a serious tail scrape, and excessive pitch up after airborne, possibly all of the way to stall.

If the aircraft is incorrectly loaded with a CG forward of the computed position, OK, any additional aft trim may well compensate for the error, but how did you know that this was the case in the first place? If the aircraft is loaded within the normal deviation tolerances considered at flight testing, controllability of the aircraft is still well within pilot manageable control forces.

If an accident or incident arose from deliberate pilot mis-trim, you won't have a leg to stand on at the enquiry. Bye bye career.

Regards,

Old Smokey

[LEM]

Years ago, when we received our first ATR 75s, I used to set the trim SLIGHTLY nose heavy, as I didn't want some rookie FO to scrape my tail...

Btw, I have noticed there is a common tendency to rotate faster than the recommended 3° per second.
Better to have it slightly nose heavy and having to apply a little input maybe at the end of rotation, than the contrary.

I can't recall any accident for a nose heavy airplane, but there are for tail heavy ones... they eventually stalled... but of course these are extreme circumstances.

Today, on the B737, I always set what they write on the loadshett, and when that's incorrect, I don't know why the result is always nose heavy, luckily.

[XPMorten]

After reading about calculation mistakes/inaccuracy's etc on
the loadsheets above.

There is an "easy" way to calculate the real CG exactly.
All you would need is;

- a scale weight in the nose gear (which already is
compressable)
- your total weight after loading (taxi weight).

Then some simple mechanical formulas
programmed into the FMC - voila.

Has this ever been tried by Boeing or anyone else?
Must have....

M

[411A]

Boeing had the STANS system installed on cargo 707's many years ago, and in my experience, it worked pretty good.


Now, let us suppose that one flies a particular type a lot, and has come to notice that, with normal pax/baggage/freight loading, the 'normal' stab trim from the computer load sheet is (for the sake of discussion), 4.4 units, nose up.


Then, one fine day, the pilot notices that, with the more or less normal pax/baggage/freight load, the stab trim is proudly announced on the load sheet as.....3.1 units, nose up.

One wonders...do most pilots just blindly set this amount of stab trim for takeoff, without looking slightly deeper into the reason why the stab setting stated on the trim sheet is ah...just slightly different than normal?

And further, when one has been given the estimated payload at dispatch, and determined the fuel required for the planned flight, then finding the load sheet rather far from the estimated TOW (originally), do most here just insert the TOW into the FMC, without questioning the reason for the discrepancy?

[Old Smokey]

To add to 411A's yep.............

Boeing have a system fairly much as you describe it on the B777. It doesn't provide the Stab Trim required, but rather provides a mis-trim warning, which sends you back to re-check the numbers.

Not working too well, there's presently a Boeing bulletin regarding erroneous warnings, which trigger fairly regularly.

Practice makes perfect, they'll get it right in the end.

Regards,

Old Smokey

[Willit Run]

Our company does a lot of max weight TO's and we see more than our share of red lights at the end of the runway. If, at that time we have a nose heavy load and the trim doesn't reflect this, well, we have a few less red lights to worry about untill they get fixed and then we have some "splainin to do Lucy".

one of our problems is, we get TWO different trim settings, one from our weight and balance sheet and one from our performance sheet. The W&B seems to me the more accurate one, but some folks like the performance setting. these can differ as much as 1.2 units. thats a fair amount on a 747 but very recoverable; just puts the performance factors out the window!
Its not just a job, its an adventure!

[john_tullamarine]

... now that's interesting ... unless the performance programs are checking a network loading solution to find out the CG for tha day, I fail to see how the performance data can have any knowledge of the CG .. which usually is all that defines the stab setting for a given flap setting ?

[Willit Run]

Well, we are a bit confused too, but we take the C/G from the weight and balance and input the data to the performance program, and a new number comes out. Why they are different sometimes is a mystery, but we are trying to get an answer from the computer geeks that sell this thing. Sometimes they are off by a tenth, sometimes they are off by 1.2 units.
One thing you have to remember is, our fleet standardization stopped after the 1st plane. We have four different thrust engines, 5 different gross weights, more other small differences that i can hardly remember. This is a performance program sold by Teledyne, it fairly widely used, (so i'm told).