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tommchowat

Lack of pitch authority - Landing

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Hi Guys.

 

This has been an ongoing issue for a while, I'm not sure if it's a 777ism or not! Basically during landing there is a distinct lack of pitch up authority. Example, if you get slightly above the glide and pitch down to compensate, then try to recorrect with a pitch up for the flare, even full rearward movement of the controls does little to nothing, which results in a rather... Positive landing. If I disconnect early on the approach it flies fine, all control inputs are responsive, but it's only the last fifty feet or so where I've little to no control in nose up effort. I've noticed on approaches where I'm bang on the glide all the way down it remains responsive enough to start a flare at around 50 or 40 feet for a nice touch down. So I'm wondering if a nose down correction late in the approach is affecting the ability to pitch up... Seems a bit strange. Trim issue? I'm not sure where the trim is at this stage as I haven't looked but I'm assuming it should be around 5-7 units ANU. No issues during takeoff rotation or manual flight in other stages.

I seem to have made no less than four of these posts. Feel free to delete the others!


Tom

737-800 F/O

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Correct behavior, the A/P biases the trim nose up a little before 50 AGL in anticipation of flare.  FCTM recommends transition from A/P to manual landing at about 300-600 AGL... finding the reference to the trim anticipation is eluding me but check landing and flare on FCTM pg 6.8


Dan Downs KCRP

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Yeah, I don't like to spurt out stuff without backup... sorry.  I'm sure it's in the FCTM someplace.


Dan Downs KCRP

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Actually the system is supposed to introduce a nose down command at around 30 feet or so to force the pilot to flare, for me it's a lot more noticeable if I land with the auto throttle off versus leaving it on, but I know what you mean.


Bryan Richards

 

"People depend so much on automation that they forget how to get the automation to work." B.W.

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I can try it with leaving the auto throttle in I suppose, but usually when I fly an approach I disconnect the auto throttle and then the autopilot (and yes, I'm aware the purists will tell me that Boeing recommend leaving the auto throttle engaged during a manual approach but I don't do it in the 737 so won't do it here!)


Tom

737-800 F/O

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but I don't do it in the 737 so won't do it here!

 

There's something to be said for consistency. There's also something to be said for the fact that a computer can't read your mind, so the A/T is always reacting to you. While it's quick and it'll do the job, it's always "behind the plane" so to speak, when you're flying. I mix both, depending on my mood or level of laziness that day.


Kyle Rodgers

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Trim issue?

 

Hi Tom,

 

To find out why it's happening, it might be worth displaying the FBW trim speed to see where that is relative to your speed at flare. PMDG have increased out of trim feel forces by reducing elevator authority when pulling (or pushing) away from the reference speed. (This isn't how the aircraft FBW works of course). If your IAS is low compared to the trim reference you'll get less nose up elevator for a given pitch input than usual. That combined with the nose down bias for flare could well have you running out of nose up pitch authority.

 

I avoid touching the 777's trim when landing after AP disengagement and I haven't had any flare issues, even though my manual control on the ILS isn't that great and I often drift off the G/S and have to correct late.


ki9cAAb.jpg

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Tom-

 

However you elect to operate the airplane is entirely between you, your FO, your airline's training department and your airplane.  The manufacturer can make all kinds of recommendations, but ultimately your airline and your training department will tell you how they want it done in the real world- and you WILL do it that way....  Or you get to spend time sitting in front of the desk of some guy like me, explaining why you aren't.  :ph34r:

 

Since this is sim-world...  Operate it however you like it...  But I always recommend that you do it the same way EVERY time.  This is one of the things we push on REALLY hard in real world flying.  Consistency builds safety because you build experience and expectation and that helps you to immediately identify if something is going wonky.

 

The FBW airplanes (including the 777) have a little known quirk in them that many pilots are either unaware of- or don't fully understand- and that is that the mathematics used to manage the airplane wind up making compensations for ground effect that a standard airplane doesn't make.

 

After all- if you fly a 757 into ground effect- the ground effect will cause the airplane's trajectory to change, resulting in a lower rate of descent or- in the right conditions- a float.

 

The 777 (and A330 incidentally) both have pronounced control law changes that take place when the airplane enters ground effect because the control logic that is in effect right at that moment is working to maintain a constant vector path.  Thus- when the airplane enters ground effect, the airplane makes an adjustment to help it maintain trajectory, thus canceling out the ground effect altogether.  As a result- there is additional logic built into the system that essentially works to "simulate the effects of ground effect" that get taken out by the FBW operation...  and this results in a whole bunch of very complex stuff that I am not entirely clear about- but the airplane is making pitch input changes while you are starting to think about flaring...  (30-50' or so)

 

This is tough to replicate in a sim environment that doesn't use force feedback because the pressures involved in your control inputs don't ever change, unlike the airplane....

 

For this reason- you are sensing the change in control feel as reduced effectiveness...  Does that make sense?

 

If you want to feel it in another area of the envelope- take the airplane up and try to bank it steeply...  as you approach the edge of what it wants you to do- it the airplane loads in control pressure against the yoke input so that the pilot feels the controls getting heavier and pushing back on the control input.  We simulate that through effectiveness- thus creating the same result... but it "feels" like sluggishness....

 

Sorry for the lengthy answer- but i hope that helps you "feel" your way through it.  :P


Robert S. Randazzo coolcap.gif

PLEASE NOTE THAT PMDG HAS DEPARTED AVSIM

You can find us at:  http://forum.pmdg.com

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If you want to feel it in another area of the envelope- take the airplane up and try to bank it steeply...  as you approach the edge of what it wants you to do- it the airplane loads in control pressure against the yoke input so that the pilot feels the controls getting heavier and pushing back on the control input.  We simulate that through effectiveness- thus creating the same result... but it "feels" like sluggishness....

 

Yes Robert, I just had the chance to test all of these, now beautifully implemented in the latest release of the 777 I just installed in my FSX-SE!

 

How I wish the guys at my other sim - DCS World - could accept my claims to implement the same approach, specially for simmers without a FF joystick - effectiveness to replicate control heaviness ! so beautifully implemented in the PMDG 777 !


Don't know, but to me MFS looks a lot like the "Barbie" of desktop flight simulation...

Doesn't matter - I like to play with Barbies...

 

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