Occasionally, on spooling up for take off I'm receiving a red flag warning on the upper DU "Stabilizer Configuration".

 

As far as I can tell, everything has been configured correctly including trim.

 

I was not able to locate this problem here in the forum, or in the documentation.

 

Appreciate any assistance.

 

PS: this is in relation to my 777-200 which has all the relevant updates.

Yeah, I got that too, once. I cancelled the warning, took off and completed the flight without incident.

 

According to the FCOM the CONFIG STABILIZER message pops up when "Stabilizer is not within the greenband range when either engine's thrust is in the takeoff range on the ground".

 

I'm not sure if this has been implemented and I too would like to know from some knowledgeable soul if there is a particular course of action to be taken when presented with that warning.

Takeoff Stabilizer trim with a value smaller than 2.75 is very close to the edge of the greenband.

 

2.50 is within 0.01 of the edge of the green band.

2.49 gives you the config warning.

 

I usually trim to 2.55+ even if the calculated trim is 2.50.

I wish the auto load-out would push the COG further forward so as to make the takeoff trim in the 3-4 range.

Thanks Trent!

 

Interestingly enough, according to the T/O data in the CDU, we're supposed to set trim at 2.50 - and yet sometimes, this red flag comes up?

I hope I am not misquoting, I believe Ryan once commented that his Dad who flies the T7 said T/O trim is not critical and they simply make sure it's in the green band. Also, the message is advisory and you can take it with a grain of salt.

Thanks Dan.........I'll just continue onward and kill the alarm.

Thanks Dan.........I'll just continue onward and kill the alarm.

I was looking for something else and ran across a FCOM Bulletin regarding exactly this topic:  See FCOM B-73.

My strategy is simply to ensure either:

1: I manually load more stuff in the forward cargo, so the Takeoff Trim calculated is in the ~3.0 area

 

2: When the Calculated trim is 2.5 or less, always set slightly higher value than 2.5

 

If you have it set higher than the requested value, it just means there will be less back pressure on the yoke to rotate.

Also, the message is advisory and you can take it with a grain of salt.

 

Discussing "low CG%  and low Trim units":

IIRC the EICAS message "STABILIZER CONFIG"  is a red alert in combination with an aural warning and I would take it with "standing on the brakes". Luckily the config alerts come up on engine spool up just above approx. 40% N1. So you just loose some meters of RWY.

Although this is here not the real problem. If this happens in the freighter, you would have to return to your stand (which you never should have left) in order to re-balance your cargo (or fuel).

In case of a passenger flight, the "solution could be" to send quite some PAX behind the CG. But I think PAX and crew won't be too confident about that, especially all the downgraded 1st class PAX will insist to talk to the carriers head office...

(partially just kidding and please see info below!)

 

 

1: I manually load more stuff in the forward cargo, so the Takeoff Trim calculated is in the ~3.0 area

 

 

"In my experience" it is actually just the other way around, but don't worry: I get this mixed up, too.

The problem is, that with lowered CG % rates (CG moved FWD e.g. due to fuel used from left/right wing tank (AFT CG) and center tank ("on" CG") empty/unchanged on the last flight, the trim unit number decreases and goes below the smallest green tape value (above the top of the green tape; odd but corresponding to the CG movement.) So in order to get higher CG % rates (more AFT) you have to transfer cargo from the fwd cargo bay to the aft cargo bay... :blink:

The effect of changing the CG by flying the tanks empty is actually wanted. This way the stabilzer can be in a more neutral position during all phases of flight and thereby reduce fuel burn:

During climb CG AFT. During CRZ CG moving FWD due to fuel burn (lower pitch at high ALT). During DES a "FWD CG" config with more neutral pitch. But the latter can lead to "CG too low (FWD) for T/O config" situations...

 

 

EICAS message "STABILIZER CONFIG" is a red alert in combination with an aural warning

 

I don't know how I missed that, thanks Claus (ref FCOM 15.20.5). However, the Boeing Bulletin B-73 is pretty clear that stab trim at or just beyond green band is a nuisance alarm. Also, in backchecking my error I found that the green band is one of three different possible CG calculations either nose down, midband or nose up (FCOM 16.20.14) and a pressure guage on the nose gear is involved in this. Interesting. Still learning, that is good.

I get this when loading the standard default panel state (using the long state)

 

Got it pre SP when I had the trim out of the trims green band so make sure it isn't

 

 

However, the Boeing Bulletin B-73 is pretty clear that stab trim at or just beyond green band is a nuisance alarm.

IMO Boeing hereby gives the pilot the final decission (and guilt) while having thrown a strong standard warning message at him. That is o.k. for me. (There is no bad, evil or "are you nuts?" graduation for stab config warnings.)

 

 

 

the green band is one of three different possible CG calculations either nose down, midband or nose up (FCOM 16.20.14) and a pressure guage on the nose gear is involved in this

I have never heard of the "nose gear oleo pressure switch" (FCOMv2 9.20.14) and its function to cross-check computed FMC CG with the actual A/C load situation. I have also "copied" now that the "green tape" is actually called "greenband"... B)

 

"In my experience" it is actually just the other way around, but don't worry: I get this mixed up, too.

The problem is, that with lowered CG % rates (CG moved FWD e.g. due to fuel used from left/right wing tank (AFT CG) and center tank ("on" CG") empty/unchanged on the last flight, the trim unit number decreases and goes below the smallest green tape value (above the top of the green tape; odd but corresponding to the CG movement.) So in order to get higher CG % rates (more AFT) you have to transfer cargo from the fwd cargo bay to the aft cargo bay... :blink:

The effect of changing the CG by flying the tanks empty is actually wanted. This way the stabilzer can be in a more neutral position during all phases of flight and thereby reduce fuel burn:

During climb CG AFT. During CRZ CG moving FWD due to fuel burn (lower pitch at high ALT). During DES a "FWD CG" config with more neutral pitch. But the latter can lead to "CG too low (FWD) for T/O config" situations...

 

If the trim is far forward (say, any number less than 2.5) if you were to trim back into the green band (say a number like 3.5) you would need forward pressure on the yoke to "not rotate" at V1 minus (a number).

 

Yoke in neutral position = nose rotating at takeoff speed in these situations. (and possible "forward pressure" on yoke to maintain an attitude)

 

If you put more stuff in the front of the plane, then this would keep the nose down (or, for our purposes, give the calculated trim position a higher number - closer to the middle - rear of the green band).

If you put more stuff in the front of the plane, then this would keep the nose down (or, for our purposes, give the calculated trim position a higher number - closer to the middle - rear of the green band).

 

Your explanations really make a lot of sense! So I thought there must be something else going on that leads me to false assumptions while being convinced of writing what I saw in the PMDG 777-200 LR Freighter praxis...

I did a lot of FMC test inputs for load and fuel. What I noticed is that there is a sudden greenband JUMP at a certain amount of fuel entered.

Probably this greenband jump has something to do with the "nose gear oleo pressure switch" (FCOMv2 9.20.14) and its function to cross-check computed FMC CG with the actual A/C load situation and to determine a nose-down-, mid- or nose-up-greenband selection.

To recapitulate the greenband-jump-effect:

T7F (no AUX tanks) "ready for departure", display=kg, enter load level 80% and "short range" for fuel.

This should result in the following values:

Fuel=left+right 27212kg (center tank empty), Main Cargo=51201kg, FWD Cargo=19904kg, AFT Cargo=11884kg, Bulk=2359kg.

CG=28%, Trim=3.75, Greenband=3.5 to 10.

(So the trim value is close to the upper end of the greenband).

 

NOW CHANGE THE FUEL AMOUNT FROM 27212kg to 18000kg:

The greenband range is still from 3.5 to 10.

NOW CHANGE THE FUEL AMOUNT FROM 18000kg to 17000kg:

The greenband JUMPS to a range from 1 to 10.

 

If you have more than approx. 17500kg of fuel, the greenband range is only 6.5 unsits wide, with less fuel the range expands to 9.0 units.

So a weight difference of less than 1000kg (500kg per left/right tank) causes the greenband to jump.

 

So what I might actually have experienced was moving 2000kg from the FWD cargo to the AFT cargo which gave me a lower trim value(!) but now inside a greenband that at the same time had changed to the larger 1 to 10 range? 

 

Now I think that there might be

a:  an anomaly in the greenband range switch or

b:  the "nose-down-, mid- or nose-up-greenband selection"  done by the "nose gear oleo pressure switch" is simulated in a way that I can not comprehend.

 

EDIT: although switching to the larger greenband range (including lower trim values) makes actually perfect sense when moving cargo to the AFT.

That would mean that the "nose up pressure change" inside the "nose gear oleo pressure switch" would select a "nose-up-greenband", which is the larger one with the lower trim unit numbers. :blink:

I guess I need a cup of coffee now... 

Edited September 17, 201411 yr by vr-pilot