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Stab Trim - When?

Thu Feb 17, 2005 11:40 pm


During T/O and landing, when does the situation rise that crew has to manually trim the stab ( definitely A/P disengage ) ?


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RE: Stab Trim - When?

Fri Feb 18, 2005 12:26 am

Talking about aircraft other than FBW Airbus here...

Before takeoff we have set the stab trim for our weight, configuration and CG location. It should be right for the initial takeoff. However, I noticed in lots of swept wing planes that somewhere around 300-500' after liftoff some nose-down trim would be required. A possible explanation for that is that the fuel centroid location assumed in the W&B is based on level pitch attitude. When we go 20-25 degrees nose-up the fuel flows to the aft parts of the tanks including the outboard parts of the wings. This would have the effect of shifting the CG aft slightly.

During and after flap retraction some trim is going to be required.

For landing, it is a technique used by some pilots to trim nose-up throughout the flare and even until the nosewheel touches down. They feel they get smooth touchdowns that way. I choose not to do this normally, and I get about as many greasers as they do. Again, it is a matter of technique and not procedure.

Anyway, when not maneuvering, or, in other words in unaccelerated flight, you trim when you find yourself pulling or pushing against the yoke.
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RE: Stab Trim - When?

Fri Feb 18, 2005 4:13 am

Hi Slam,

Quoting SlamClick (reply 1):
They feel they get smooth touchdowns that way

Is it what THEY feel, or does it really make difference ?


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RE: Stab Trim - When?

Fri Feb 18, 2005 5:24 am

As Slamclick says, it's a matter of technique. It makes a difference in accordance to individual pilot skill, not in a technical sense. Trimming nose up you can keep the aircraft in a more or less constant nose pitch attitude, which allows for a gradual ROD during the flare (and thus a greaser). This happens because trim adjustments are quite finite, and the rate of trim change is constant, making it easier to flare accurately. But if you have steady hands, you really don't need to do this. I reckon if you overtrim the nose up pitch in the flare, you're just asking for trouble.

And Airbii don't need stab trim, as it is done automatically during most phases of the flight, except for the flare. At that point, there's no point in using manual trim anyway, just glide the aircraft down smoothly using the sidestick.
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RE: Stab Trim - When?

Fri Feb 18, 2005 5:27 am

Nose up trim is discouraged on the B757 to prevent large nose up pitching moments during the flare and thus increasing the potential for a tailstrike.

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RE: Stab Trim - When?

Fri Feb 18, 2005 6:06 am

Didn't an AA 767 crew do an emergency landing at paris once with elevator failure and landed it with the stab trim?

My old flying instructor told me that you can land a C152 using just the trim, I've never tried it hough  Smile


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